Category Archives: Ecology

Human population density drives extinctions

mountain gorilla

Sometimes there are scientific studies that seem to confirm the obvious. To wit: The more people that live in an area, the more species that go extinct.

No matter how superfluous it seems, it’s good that scientists undertake these studies, if only to confirm our suspicions, rule out potential confounding variables, or simply make the phenomenon feel more real. All three are the case with the recent paper on population density and animal extinctions. Jeffrey McKee, an anthropologist at the Ohio State University, first published on the relationship back in the early 2000s, and his latest confirms some of his earlier results and predictions.

McKee and his colleagues constructed a few models and fed them data for various variables, including human population density, species richness, GDP, conservation status (vulnerable, endangered, critical, etc.), precipitation, temperature, and others. The first time they ran the model, back in the early 2000s with data for the year 2000, they discovered a very strong correlation between population density and threatened species and nothing else. A later refinement found the population density-conservation status link could be refined by including GDP per unit area. The latest run, published this month using data for the year 2010, not only confirmed the predictions made in 2000 about 2010, it refined the overall predictive power of the model.

This new paper forecasts out to 2050, and the outlook is grim. By 2020, an average growing nation can expect 3.3 percent more threatened mammal and bird species. By 2050, that rises to almost 11 percent. So if a country has 114 threatened mammal and bird species today, like the United States does, it can expect to have 118 by the end of the decade and 126 by 2050. All due to population growth.

Faced with those statistics, conserving biodiversity seems like a quixotic battle. Population growth is beginning to slow, but there’s no way to halt it completely and immediately. Yes, human population growth will level off eventually, but what can we do in the meantime? One answer is to protect high-biodiversity areas from development, though that’s easier said than done. The richness that makes many ecosystems so complex also makes them attractive to humans. If development is to take place in those areas—and I see no reason why it won’t—we’ll have to take special care to support its native species.

The other option is to be more conscientious about where we develop. It’s likely that there’s a middle ground where human impacts can be balanced against losses in biodiversity. Finding it will take forethought and a bit of planning, but as I’ve said before, we can’t just plan the land we’ll occupy, we also have to plan the land we won’t.

Photo by Daniel Coomber


McKee J., Chambers E. & Guseman J. (2013). Human Population Density and Growth Validated as Extinction Threats to Mammal and Bird Species, Human Ecology, 41 (5) 773-778. DOI:

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Extinction debts catch up quickly

Chiew Larn Reservoir islands

John T. Curtis, a 20th century ecologist, drafted a simple, four-panel map that appeared in a volume of research presciently titled Man’s Role in Changing the Face of the Earth. It’s an important map for a number of reasons, not least of which was the impact it had on my life. This is how I described it in my first non-introductory post here at Per Square Mile:

It is a simple map, or rather series of maps. Four panels, four dates—from left to right: 1831, 1882, 1902, and 1950. In each successive panel, the dark swaths of ink that represented forest cover in Cadiz Township, Wisconsin, grew successively smaller and more fragmented.

Curtis's map

It’s a powerful image, one that drives home just how much we have affected this world. But like many images, there’s a lot that’s both implied and unknown. One of the unknowns of Curtis’s map was how life was faring in those small flecks floating in a sea of farm fields.

Now, we may be a step closer to understanding how dire that situation really is. A team lead by David Bickford, a professor at the National University of Singapore, recently wrapped up a 25-year study of forest patches turned into islands by the filling of the Chiew Larn Reservoir in Thailand in 1986 and 1987. The reservoir flooded nearly 64 square miles (165 square kilometers), isolating more than 100 patches of species-rich tropical forest. What had been hilltops were transformed into islands. Five to seven years after the flooding, the research team surveyed small mammal populations on 12 of the islands and 16 of the islands 25 to 26 years after.¹ None of the islands had signs of human impact.

Bickford and his team discovered that species vanished from the islands at an astonishing rate. Nearly all of the native small mammals were gone on the smaller islands (under 24 acres or 10 ha) in just five years, while on larger islands (24-138 acres or 10-56 ha), they were nearly extinct after 25 years. Their findings jibes with a message conservation biologists have been sharing for some time—the smaller the island, the fewer the species, and the longer the time since isolation from the mainland, the fewer species.

There are a number of possible reasons why small mammals disappeared from these islands, none of which are very heartening. The researchers point out that invasive Malaysian field rats were a problem on the islands, likely outcompeting or outright killing the native species. By the 25-year time point, “all islands were dominated by the invasive rodent and if not already in ecological meltdown, were well on their way to becoming Rattus monocultures,” Bickford and his team note.

But there are other possible reasons, too. Small habitat patches may not be large enough to sustain a viable population. When ranges are compressed, populations face a number of hardships, from increased competition for resources to inbreeding and intrapopulation strife that can raise stress, increase conflict, and lower breeding rates.

This study isn’t just about isolated islands in a remote corner of Thailand. It’s also about the flecks of land we cordon off every time we fell a forest, plow a field, or plat a subdivision. We’re creating small islands of habitat surrounded by seas of human dominance. Certainly some animals and plants can move between those islands, but not all do and not all at rates needed to sustain remnant populations. Some animals may be better than others at navigating human oceans, but even they may be doomed, unable to withstand competition or predation from introduced species. If we are to minimizing the impact we have on the environment—whether those be cities, farms, or even oil fields—we can’t just plan the land we’ll occupy, we have to plan the land we won’t.

  1. I would have liked to see a control transect on the mainland to see how the islands’ biodiversity compares, but they didn’t do that for whatever reason. (Perhaps they couldn’t find an area that wasn’t affected by humans.)

Image courtesy of Antony Lynam


Gibson L., Lynam A.J., Bradshaw C.J.A., He F., Bickford D.P., Woodruff D.S., Bumrungsri S. & Laurance W.F. (2013). Near-Complete Extinction of Native Small Mammal Fauna 25 Years After Forest Fragmentation, Science, 341 (6153) 1508-1510. DOI:

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Against invasive plants, underdog natives hang on

Dianella ensifolia

One of the scourges of our globalized economy is invasive species. In California, annual Mediterranean interlopers have upended the state’s once perennial grasslands. The Australian outback has been blanketed with prickly pear cacti from the American Southwest. And wattles from Down Under are a scourge in South Africa. But as widespread as invaders are, we’re only just beginning to understand how they move around the globe, establish themselves, and reshape the ecosystems they disturb.

A key unsettled debate is whether or not invasive plants change patterns of biodiversity. Some studies have found that biodiversity suffers when nonnative plants arrive and take over. Others have found the opposite, that new species add to the mix rather than deplete or homogenize it. Well, the authors of a new paper published today in Science say both answers are right. According to them, it’s all a matter of scale.

Studies on invasive plants and biodiversity can generally be classified according to scale, small and large. Small scale studies pore over tiny patches of land, generally less than 25 square meters. In those cases, researchers have generally found that plant biodiversity suffers when invasives are present. Other scientists say that, on the large end of the scale, they see no difference.

In an attempt to reconcile these consistently disparate findings, Kristin Powell of Washington University in St. Louis and her colleagues sought to bridge both scales. They set up both large (500 m2) and small (1 m2) plots in Hawaii, Florida, and Missouri, each of which has its own problematic nonnative. In Hawaii, it’s the fire tree, Morella faya; in Florida the cerulean flax lily, Dianella ensifolia; and Missouri the Amur honeysuckle, Lonicera maackii. The study subjects run the gamut from overstory tree (fire tree) to mid-story shrub (honeysuckle) to understory herb (flax lily). Each state in the study has parts that are invaded and parts that are not, a fact which Powell and her colleagues used to their advantage by surveying plots on either side.

What they found is as nuanced as you might expect from a confused and messy situation involving the natural world. On small scales, the 1 m2 plots, they found that biodiversity was, in fact, lower. In the large plots, species richness was a slight bit lower, but it was close enough to be a wash. These results essentially jibed with those found by other scientists.

If you dig a little deeper, things get more interesting. They also found that ecosystems hosting invasive plants are generally more homogenous—the patchy pastiche you would normally expect just wasn’t there. But they’re not ecological clean rooms, either. Though diversity was down, there didn’t seem to be evidence of extinctions. Native plants may have disappeared from a large number of the smaller quadrats, but they typically weren’t absent from the larger plot. The diversity was there, it was just hiding among the invaders.

That’s good news, in a way. We probably won’t lose interesting and potentially important plant species because of competition from invasive plant species. But that’s not to say the natives are free and clear, though. If their populations are suppressed too much, they could be sitting ducks for another disaster, such as a catastrophic fire or human development. They’d be one step away from being wiped off the map.

This study’s results suggest that we should reevaluate how protections like the Endangered Species Act can—or can’t—help in the case of invasives. If, as this study says, invasions seldom lead to extinctions, then protections like the ESA won’t help much against invasive species. But if invasives depress population numbers enough, native species would be vulnerable, meaning the ESA would be more relevant than ever. In those cases, we should be even more vigilant in areas overrun with invasives. On the surface, they may not look like healthy ecosystems. But lurking within are the native remnants of one. If we buy those systems enough time, they may sort themselves out.


Powell, Kristin I., Jonathan M. Chase, and Tiffany M. Knight. 2013. “Invasive Plants Have Scale-Dependent Effects on Diversity by Altering Species-Area Relationships.” Science 339: 316-318. DOI: 10.1126/science.1226817

Photo by SSKao.

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Designing systems, scientifically

Japanese countryside

A couple of months ago, I was talking with a friend about design. Well, really, about the scope of design. Many designers tend to think in terms of discrete objects—a chair, a phone, a building. This exasperated him, and for good reason. See, discrete objects aren’t the only things that are designed. Cities are, too, he argued. “You see that city over there,” he said, emphasizing his point. “People made that.”

If we were to pick up this conversation again—and we might—I’d add landscapes to the list. Our understanding of ecology has matured to the point that we are beginning to grasp our species impact on the planet. We are, and have been for thousands of years, changing our surroundings to suit our needs. Today, there isn’t a single ecosystem that’s untouched by humanity. That’s a definition of design if I’ve ever heard one.

But in many ways, we’ve been molding ecosystems haphazardly, at best. If design isn’t just the result of human activities, but is a considered plan put together with purpose and intent, then maybe we’re not designing landscapes after all.

Both sides of the argument are equally valid, I think. But one thing is clear—when we change our surroundings, we need to place more emphasis on design. A couple of recent papers have made that clear to me.

In one, Joan Nassauer and Paul Opdam wrote an op-ed of sorts in the influential journal Landscape Ecology back in 2008. Nassauer and Opdam suggested that the field of landscape ecology add design as a third tenet to the existing two, pattern and process. It would help address a shortcoming of the field. Landscape ecology was originally formulated based on the premise that we can learn how a landscape functions by studying how it is configured. If we get pattern, then we’ll get process. It’s been a phenomenally successful framework, but like many sciences, landscape ecology has stumbled when it comes to the implementation part.

That’s problematic because we humans are always mucking about with the landscape. Every time we repave a road, carve out a new subdivision, or erect a skyscraper, we’re altering the face of the Earth. And currently, we’re doing it in perhaps the most uncoordinated way possible. When set out to build a road, skyscraper, or city, our ambitions may be big, but our thinking is small. We tend to focus on the immediate impacts—how much steel we’ll need, how much land we’ll use up, and so on. We forget to consider how that creation will affect the surrounding landscape.

There are a few exceptions. Nassauer and Opdam cite two case studies, an ecological corridor network in Denmark and a watershed workshop in Iowa. Another paper by Simon Swaffield referred to an overhaul of the wetlands and waterways in and around Christchurch, New Zealand. There are others, too. When I was in graduate school, members of my lab were working on the Sierra Nevada Adaptive Management Project. The U.S. Forest Service was working with scientists and locals to come up with a way to minimize potentially catastrophic wildfires. The centerpiece of the Forest Service’s original plan was to clearcut a giant checkerboard of chevrons into 11 national forests. It’s design on a landscape scale if I’ve ever seen it. Their process is, too. Though the Forest Service wasn’t keen on the idea initially, they now are working closely with locals to ensure the final implementation is palatable to everyone. Researchers from the University of California are serving as arbiters, of sorts, making sure the science is sound and that everyone’s voice is heard. The back and forth, the multiple stages, the consideration of possible outcomes—that’s the design process.

If landscape ecology can successfully incorporate design as a third tenet, it will be in a league of it’s own. It’s a science that thinks on a system-wide scale; few such sciences worry about design. There are other systems that have design applied to them—cities are certainly one—but those design processes aren’t always informed by the sort of systems science that’s inherent to landscape ecology. The field could be a real pioneer, and in the process provide some order to slapdash overhaul we’re currently giving our surroundings.


Nassauer, J.I. & Opdam, P. (2008). Design in science: extending the landscape ecology paradigm, Landscape Ecology, 23 (6) 644. DOI: 10.1007/s10980-008-9226-7

Swaffield, S. Empowering landscape ecology-connecting science to governance through design values, Landscape Ecology, DOI: 10.1007/s10980-012-9765-9

Photo by Tim De Chant.

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If the world’s population lived like…

Shortly after I started Per Square Mile, I produced an infographic that showed how big a city would have to be to house the world’s 7 billion people. There was a wrinkle, though—the city’s limits changed drastically depending on which real city it was modeled after. If we all lived like New Yorkers, for example, 7 billion people could fit into Texas. If we lived like Houstonians, though, we’d occupy much of the conterminous United States.

Here’s that infographic one more time, in case you haven’t seen it:

The world's population, concentrated

What’s missing from it is the land that it takes to support such a city. In articles and comments about my infographic, some people overlooked that aspect—either mistakenly or intentionally. They shouldn’t have. Cities’ land requirements far outstrip their immediate physical footprints. They include everything from farmland to transportation networks to forests and open space that recharge fresh water sources like rivers and aquifers. And more. Just looking at a city’s geographic extents ignores its more important ecological footprint. How much land would we really need if everyone lived like New Yorkers versus Houstonians?

It turns out that question is maddeningly difficult to answer. While some cities track resource use, most don’t. Of those that do, methodologies vary city to city, making comparisons nearly impossible. Plus, cities in most developed nations still use a shocking amount of resources, regardless of whether they are as dense as New York or as sprawling as Houston. Any comparison of the cities in my original infographic would be an exercise in futility at this point.

But what we can do is compare different countries and how many resources their people—and their lifestyles—use. For countries, the differences are far, far greater than for cities. Plus, there’s a data set that allows for reliable comparisons—the National Footprint Account from the Global Footprint Network. Their methodology is based on peer-reviewed research by Mathias Wackernagel, the organization’s founder. It’s consistent and comprehensive. Each country’s footprint is assembled from sub-footprints, ranging from cropland to carbon to urbanization to fishing grounds. For my purposes, I used only terrestrial sub-footprints. I’ll let the results speak for themselves.



Global Footprint Network. 2011. National Footprint Accounts, 2011 Edition.

Wackernagel, M., Kitzes, J., Moran, D., Goldfinger, S. & Thomas, M. (2006). The Ecological Footprint of cities and regions: comparing resource availability with resource demand, Environment and Urbanization, 18 (1) 112. DOI: 10.1177/0956247806063978

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7 billion

Spare or share? Farm practices and the future of biodiversity

Nature’s burning library


Let’s imagine it’s 48 B.C.E., and the Library of Alexandria is burning.¹ Bits of ash are floating down from the superheated updrafts, remnants of what was the world’s greatest collection of written knowledge to date. You’re standing just outside the door, and you have five minutes to dash in and grab whatever you can carry. Do you focus on one section to save, say, Aristotle’s works on biology and anatomy? Or do you run from stack to stack, hoping to rescue a cross-section of classical scholarship?

Fun choice, huh? Yet those are the sorts of decisions that conservation biologists make all the time. They’re constantly trying to answer a question that has no good answer: “Which remaining bits of nature should we try to protect?” They know we can’t save them all. There’s only so much money and land to go around. They also know that how is an equally important question. Do you set aside a single large reserve or several small ones? Get the answer wrong, and poof, nature as we know it is gone.

With stakes like those, it’s no wonder conservation biologists have been arguing over that question for several decades. David Quammen ably covered the debate in The Song of the Dodo—a highly recommend read—but I’ll briefly summarize it here.

In the 1960s, young scientists Robert MacArthur and Edward O. Wilson were restless. MacArthur was a quantitative ecologist looking to shake up the sleepy field of biogeography—what species occur where and why—and Wilson was an entomologist cum field ecologist tired of merely cataloging new facts about ants. MacArthur was searching for a way to describe mathematically what he saw in biogeographic data. Wilson had such data from his research on Pacific islands and their ant species. Together, they hammered out the theory of island biogeography, which says that larger islands hold more species. Specifically, an island that is 10-times smaller will have two-times fewer species. When MacArthur and Wilson published their ideas, they swept through ecology like wildfire, quickly modernizing the once descriptive and theoretically-challenged science.

Fast-forward to 1975. Jared Diamond—yes, that Jared Diamond—had been conducting his own biogeographic research. Now, he was proposing that to best protect biodiversity, a single large reserve would be preferable to several small ones that totaled the same area. It was, he argued, a logical extension of MacArthur and Wilson’s theory of island biogeography. After all, protected areas are themselves islands marooned in a sea of cities and farms. Just as a larger island tends to hold more species, so too could a larger park protect more biodiversity. And, he added, larger parks are better habitat for big, charismatic megafauna like elephants, lions, and bears.

Needless to say, not everyone agreed. Dan Simberloff—who had been Wilson’s grad student in the 1960s—and Lawrence Abele published a rebuttal to Diamond’s paper the following year. They argued that, given the realities of conservation, putting all our eggs into large parks would be both untenable—purchasing large tracts of land is costly and difficult—and unwise—problems in a single park could doom an entire species. But small parks would provide some insurance and be easier to establish. Plus, there are many species for which reserve size isn’t everything, but geography is. Smaller, more targeted reserves would be a better fit for them.

Not long after Simberloff and Abele published their paper, ecologists began picking sides. It was a contentious debate then, and it still pops up at conferences and in scientific journals. If you talk to participants today, as I did in 2005, each side will say they won, that the debate has been settled. Clearly, that’s not the case.

One of the more recent flare-ups came from a quartet of Australian scientists who asked the same question for the umpteen-thousandth time—single large or several small? But this time, they added a twist. They admitted—in mathematical terms—that we don’t know the answer to a particularly germane question: Which species will go extinct and when?

Typically when conservation biologists design protected areas, they attempt to minimize extinction risk. To do that, they first need to determine what the expected extinction risks are. Those are difficult numbers to pin down accurately. So the authors of the new paper suggest a different approach. Rather than attempting to minimize risks, we should be striving for acceptably small risks. It’s a subtle distinction that could change everything.

Acceptable risk rather than absolute minimum risk is a more realistic target. Reserves designed for minimum extinction risk are setting themselves up for failure, in a way. There’s no way they can reduce extinctions to an absolute minimum. For one, it’s impossible to exclude people from an area entirely, and, as we’ve come to realize, many landscapes wouldn’t exist without human intervention. Besides, no species are really beyond human reach any more—climate change has made sure of that. Aiming for acceptably small risks acknowledges both the limits of our knowledge and the extent of human impact.

But the paper’s authors don’t stop there. They can’t help but toss their two cents into the single large or several small debate. Their answer? Seven reserves. If that number seems too precise to fit all scenarios, remember that this was a modeling experiment—hypotheticals have a way of being oddly exact. What’s more important is that “seven” represents a middle path, of sorts. Seven is neither a single reserve nor is it many. It does lean more toward the “single large” camp—the authors admit as much—but it also recognizes that one monolithic reserve is a risky bet. With seven, individual reserves can be large enough to buffer park interiors, while the overall network provides redundancy.

That’s not to say this paper settles the debate. Quite the opposite, I would bet. But I think it does make an important contribution, that some level of extinction risk is acceptable. For too long, we’ve viewed conservation in black and white, that if we don’t do everything to save a species, we might as well do nothing. In reality, there are a million shades of gray. By liberating ourselves from this binding dichotomy, we can devote more energy and resources to slowing extinction rates.

Ultimately, though, setting aside land for protection will only get us so far. It’s an approach we’ve been using for years, and it hasn’t done much to slow extinction rates. Nature’s library is still burning. If we really want to protect biodiversity, we’ll have to do more. We’ll have to put out the fire. For that, we’ll need more than a few people running in to save what they can. Like an old fashioned bucket brigade, we’ll all have to chip in. That will require real change on our part. All of us.

  1. Historians debate whether the library actually burned then, during Julius Caesar’s siege of the city. But for the sake of the analogy, let’s assume it did.


Michael A. McCarthy, Colin J. Thompson, Alana L. Moore, & Hugh P. Possingham (2011). Designing nature reserves in the face of uncertainty. Ecology Letters, 14 (5), 470-5 PMID: 21371231

Quammen, David. 1996. The Song of the Dodo: Island Biogeography in an Age of Extinctions. Scribner, New York. 702 pp.

Photo by ken2754@Yokohama.

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Interview: Eli Kintisch on art and climate change

A Sam Jury film

Last week I attended an art exhibition with a unique theme—exploring climate extremes—and curator—Eli Kintisch, an MIT Knight science journalism fellow on leave from the journal Science. Kintisch has spent the last year preparing for the exhibition, called “To Extremes: Public Art in a Changing World”. Part of that preparation involved hosting what he called “Climate/Art Pizza”. The small gatherings can best be described as salons: Artists, scientists, and journalists met in his apartment here in Cambridge overlooking the Charles River to discuss how art and science intersect under the rubric of climate change. Entries to “To Extremes” followed similar guidelines.

The exhibition ended last week, but for Kintisch real challenge lies ahead. He is now working to make the winning entry—an ambitious public art installation—a reality. I caught up with him this week over email.

Tim De Chant: What inspired you to explore climate change through art?

Eli Kintisch: Data suggest that large swaths of the American public know little about climate change and have little interest in the topic. By contrast, the readers of most of my stories I strongly suspect have well-formed views on the topic, so I seek now to reach new audiences who don’t know what Science magazine is, who James Hansen is, or even that the planet has warmed 0.8 ˚C since preindustrial times. 

In an age where everyone pre-filters their news with handheld computers or google news, I figure building public art will:

a) reach broad groups who don’t read science or environmental stories (and can’t run into them by chance reading a paper newspaper) and 

b) use a new visual language that appeals to those who might be turned off or bored by such articles.

De Chant: Has the intersection of art and science been a longstanding interest of yours?

Kintisch: Somewhat—I first got into it in an active way when I designed this Earth Emergency Procedures Safety Card as schwag for my 2010 book Hack the Planet—on real glossy card stock. Artist Benjamin Marra, a friend of mine, helped me layout and did all the drawings, and the give and take with him, plus the challenge of making visual what I had previously described with words.

De Chant: How were the entrants judged? What was the balance between art and science?

Kintisch: To Extremes is what’s known in the art world as a juried exhibition—a fancy name for an art contest whose winners appear in an exhibition. I contacted various curators for suggestions of artists to invite, and invited 50 of them, nearly all American. More than half submitted work in the form of proposed public art works.

Those proposals were to make up the exhibition, and the jury, which included science and art experts, had to choose which pieces to go in the final exhibition. They tried to balance artistic merit with potential for impact with integrity to the science of extremes and climate. They selected nine proposals and the runner up and winner. I was truly inspired by the discussions about each piece—they really embraced the spirit of the exhibition I had created, applying their taste and expertise to choose edgy, creative but solid pieces for inclusion.

(I wasn’t on the jury, though I was present during their deliberations.)

De Chant: Why an exhibition climate extremes and not another aspect of climate change like CO2 concentration or sea level rise?

Kintisch: Mostly I pegged this exhibition with the release of the IPCC special report on extreme events, released in November ’11. I’d consider creating art competitions/juried exhibitions in the future around other aspects of climate.

De Chant: Tell me, what drew you and the other jurists to the winning entry?

Kintisch: Sam Jury’s piece is a one-hour long video loop that explores climate adaptation in her inimitable, eerie/beautiful style. But an algorithm we are designing that responds to real-time climate data will trigger the playing of a bank of videos that explore specific extremes—i.e. extreme rain in the location of the video installation will trigger a video that visually explores the psychological and physical aspects of excess water.

The judges didn’t release a statement, but they seemed to believe Sam Jury’s piece’s a) visual beauty b) direct relevance to climate extremes and c) clever embodiment of the To Extremes theme made it the winner.

De Chant: What are the plans for the winning entry?

Kintisch: Sam Jury (who lives in Cambridge) and I spent three days last week meeting with Boston area foundations, curators, art experts and city officials to figure out how to bring her piece, which right now is just a proposal, to life. It’s audacious, no doubt: to make it work it would have to run continuously over a year or so, Sam thinks. We believe we can make it happen by tapping all the creative interest that Boston has, and our piece might even appear in other cities too.

It won’t be cheap to pull off, but a video installation would cost a lot less than other public art projects, which can run into the hundreds of thousands of dollars. 

Stay tuned!

More than just flora and fauna

Landscape near Paris, Paul Cézanne

Peter Sigrist, writing at Polis about what he sees as ecology’s shortcomings:

Ecology “proper” is currently limited in addressing human habitation. It doesn’t usually incorporate the theory or methods of fields like economics, anthropology, political science, sociology and history. Many subfields have emerged in answer to the need for more detailed study of human-environment interaction, including human ecology, cultural ecology, political ecology, environmental sociology, historical ecology, ecological anthropology, ecological economics and ecological urbanism. But most are not closely integrated with mainstream ecology and its methods, which are primarily focused on nonhuman nature. Many ecologists portray human environmental impact as a kind of alien intervention into the natural world, and don’t attempt to understand the political, social, economic and cultural processes through which it takes place.

Sigrist isn’t entirely wrong—our understanding of the natural world must consider human impacts. The thing is, many ecologist already do that. If Sigrist had written this decades ago, I would concede his point. But times have changed. No serious ecologist draws a firm boundary between natural and anthropogenic spheres. For years, ecologists have widely acknowledged that no part of the Earth is untouched by human influences. Some may still cling to the old distinction between human and wild, but they are increasingly few and far between. Climate change has thoroughly disabused most of that notion.

Sigrist further laid out his argument in a second post, stating:

Incorporating useful elements of cultural landscape, urban political ecology and ecological urbanism can make ecology more attuned to the ways humans experience and influence cities. This is more than a shared analytical framework or conceptual lexicon (Gandy 2008: 567); it means actual integration so that ecologists are equipped to address the full complexity of human environmental relations and help make cities more just, healthy and beneficial to the planet as a whole.

To say too few ecologists study human-environment interactions isn’t just unfair, it’s incorrect. Sigrist seems to misunderstand ecology and its relation to the myriad subfields he listed above. The ecologists he describes—the ones that focus on strictly “natural” ecosystems—aren’t members of an umbrella field but a subfield. They’re just one type of ecologist. The people who study other subfields of ecology? They’re ecologists, too.

What Sigrist is proposing for ecology already exists. Perhaps he wants ecologists to avoid over-specialization.¹ Perhaps what he means to say is that there needs to be more collaboration and cross-pollination between ecologists of different subfields. He’s not wrong—there could always be more. But he’s ignoring what’s already out there. I know ecologists of all stripes—field, physiological, sociological, and so on—who collaborate with environmental historians, economists, even electrical engineers. I know ecologists who write papers about the value of ecosystem services, how to use Wall Street’s data processing techniques to understand the water cycle, or how spirituality can affect the conservation of biodiversity. Hell, I’ve been to conferences where ecologists have wrung their hands about how ecology needs more collaboration. If anyone is conscious of the need for interdisciplinary collaboration, it’s ecologists. After all, ecology is the original interdisciplinary science.

Again, that’s not to say we shouldn’t work harder to identify how humans are affecting the natural world. I’m the last to argue against that. But we need to identify our shortcomings where they actually exist, not where we imagine them to be.

  1. Not a bad idea, but good luck getting it to happen. Despite calls for interdisciplinary research, the trend for individuals is toward increasing specialization. Nipples on the surface of human knowledge and all that.

Landscape near Paris, Paul Cézanne, National Gallery of Art.

Big parks or big lots?

Bubbler in a city park

The United States is not run by godless Communists. Neither is most of the rest of the world. In fact, the godless Communists that do remain are not all that Communist anymore. I bring that up because command and control economies can dictate what development happens where. Land conservation under such a system is technically easier, even if the actual results in Communist nations like the Soviet Union weren’t that inspiring. Land conservation in the free world is a trickier game, one played with carrots and sticks as opposed to edicts and directives. Here, money is your best friend.

Conservation organizations have focused on preserving big tracts of land, and rightfully so. Big buys are often more cost effective and easier to manage. But they’re also becoming trickier to execute in a world dominated by curving cul-du-sacs and one acre lots. If we want functioning ecosystems in these places, we need to focus on land conservation within the subdivision, not along its borders.

Luckily, the carrot seems to be working in those places. A study of subdivisions in Maryland between Washington, D.C., and Baltimore shows that developers have been incorporating more open space into their subdivisions. That’s not because they’re interested in land conservation. Part of it is a bit of command and control—Maryland’s Forest Conservation Act forces developers to conserve a modicum of forested land—but it’s also simple economics. Developers can sell lots and houses at higher prices if open space is nearby. Because proximity matters, that open space typically needs to be within the subdivision.

To developers, though, that open space is fungible. It can exist either as public parks or larger private lots—both raise prices. The Maryland study also found that minimum lot sizes—which governments typically use to preserve open space—can push developers away from shared open space toward larger lot sizes.

This poses a problem for maintaining healthy ecosystems. Like many laws, the way the Maryland Forest Conservation Act is interpreted matters. People can uphold the letter of the law—maintaining forest cover—without changing their usual habits—mowing their entire lot. The result is something that looks like a forest from above but doesn’t function like one.

In a perfect world, everyone would happily tend a few thousand square feet around their house and leave the rest to nature. But that’s not always the case. People will spend all Saturday mowing acres of grass and grumble about it afterwards. That’s because for many people owning a country manor is more alluring than owning a chunk of the great outdoors. You can fight that mentality by increasing minimum lot sizes to the point where mowing it all becomes completely unreasonable,¹ but the closer you get to a metro area, the less tenable that becomes.

There’s also no guarantee that laws dictating minimum lot sizes will remain in place. As the city creeps closer, pressure to further subdivide will mount. Open space preserved in private lots could easily disappear.

Parks, on the other hand, tend to stick around. Unlike large lots, they’re seldom subdivided. Instead, they tend to become institutions. People like their parks and are loathe to lose them—no one wants to see their neighborhood park disappear. So let’s put that to use. Instead of—or in addition to—minimum forest cover and minimum lot sizes, let’s institute minimum park sizes. Everyone will benefit. Developers will be able to sell lots at higher prices. Kids will have playgrounds. Adults will have walking paths. And because big parks often have big natural areas, ecosystems will have a better chance at surviving. It’s a solution that’s a bit more command and control than current vague regulations, but everyone will benefit. It’s also more carrot than stick. Even if you don’t particularly like carrots, it’s better than getting hit with a stick.

  1. Though there will always be exceptions—near where I grew up, one guy mowed 18 acres. He had to buy a bonafide farm tractor so it wouldn’t take him all week.

Photo by JD Hancock.


Lichtenberg, E., Tra, C., & Hardie, I. (2007). Land use regulation and the provision of open space in suburban residential subdivisions Journal of Environmental Economics and Management, 54 (2), 199-213 DOI: 10.1016/j.jeem.2007.02.001

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Flyways and greenways

Red-eyed Vireo

Earlier this week I pointed out that urban areas can actually increase tree cover over time, albeit with a caveat. The two studies I cited measured tree cover and only tree cover—they made no claims about ecological function. Luckily, other studies have done just that, including one that looked at migratory bird use of greenways in urban areas.

Migratory routes are important, though most research into migratory bird decline has focused on habitat loss in their breeding and wintering grounds. That has left a large piece of the puzzle unsolved—the habitat between point A and point B. Think of it this way: If snowbirds—you know, northern (human) retirees who flock to warmer climes in the winter—started disappearing and our best solution was to look for them at their apartment in New York or their rental in Boca Raton—ignoring rest stops and motels along I-95—we’d be doing a great disservice to our older generations. Ignoring flyways is similarly foolish.

There have been studies in more recent years that aim to fill this gap, and one published in 2009 by Salina Kohut, George Hess, and Christopher Moorman picks up the trail along, well, trails. They surveyed bird species abundance and richness—how many and how varied the itinerants were—in 47 greenways in and around Raleigh, North Carolina.

Greenways are a common and convenient way for cities to conserve natural habitat. Their linear form is well suited to urban areas, and they easily double as parks or recreational trails. They also can serve as stop-over habitat for migratory birds. Kohut, Hess, and Moorman were hoping to find the right type of corridor for migrating birds, where our feathered friends can take a load off and fatten up.

It turns out that most birds were not picky and would stop at just about any greenway, regardless of vegetation, adjacent land use, or corridor width. That’s not to say all greenways were entirely equal. Overall, birds favored corridors with taller trees and lots of native shrubs teeming with fruit. And among birds that live in forest interiors far away from human development and even open fields, greenways wider than 150 meters (about 500 feet) surrounded by low-intensity development were the most popular.

None of the greenways Kohut and her colleagues studied were as good as a regular forest, though. Still, with some tweaks—including widening corridors, siting them near low-intensity development, and planting with natives—greenways can make decent stand-ins for the real thing, at least as far as migratory birds are concerned. Residential neighborhoods can even make themselves into agreeable stopover habitat by mimicking vegetation found at popular stops along the flyway.

So greenways make for good bird habitat, but let’s not forget that they’re good neighbors, too. In addition to helping migrating fauna, they boost property values, add recreational opportunities, and work well as commuting corridors for cyclists. Five benefits from one land use. Not too shabby.

Photo by qmnonic.


Kohut, S., Hess, G., & Moorman, C. (2009). Avian use of suburban greenways as stopover habitat Urban Ecosystems, 12 (4), 487-502 DOI: 10.1007/s11252-009-0099-6

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Tree City

City tree silhouette

Cities aren’t called “concrete jungles” for their leafy greenness. But perhaps it’s an inappropriate nickname. Several cities actually have more—not less—tree cover than what came before them. By way of example, take this from historian William Cronon: “There are more trees in southern Wisconsin now than at any point in the last 7,000 years.” That’s in part due to more than a century of fire suppression, but also the intense pace of urban development.

There’s ample scientific evidence to back up Cronon’s assertion. In the early 1990s, David Nowak, an urban forester with the U.S. Forest Service, found that tree cover in Oakland, California, between 1850 and 1989 rose sharply from 2 percent to 19 percent. Now, a new study by Adam Berland, a PhD student at the University of Minnesota, found a similar pattern in and around Minneapolis, Minnesota.

Oakland and Minneapolis—and many other metro areas, I suspect—were sparsely forested before urban development. As far back as 1500 BCE, what would become Oakland was regularly burned by the Coastanoan Indians to clear out the underbrush to simplify acorn gathering. What trees remained in the 1700s were logged for lumber and firewood by the missions. Then in 1848, what was left nearly vanished when gold was discovered in California. By the time Oakland incorporated in 1852, its namesake was nearly gone.

Fire likewise held forests in southern Minnesota at bay for thousands of years. Yet unlike in central California, a part of central Minnesota quickly afforested during a brief climate cooling 400 years ago. It wasn’t long lived, though—shortly after their arrival, European settlers swiftly knocked down most of the Big Woods for farming. The remaining flecks large enough to be called forests cover only 2 percent of the original area. In other words, forests near Oakland and Minneapolis had nowhere to go but up.

The arrival of dense settlement was something of a godsend for trees. Young neighborhoods and cities are often depauperate—it’s easier to build without big trees in your way—but they tend to accumulate tree cover as they age. And relative to the denuded landscape that came before Oakland and Minneapolis, those urban forests are more akin to a real jungle than a concrete one.

Urban forests are certainly an improvement from a tree’s perspective, but they’re not a panacea for habitat loss. Neither of these studies examined how those forests function ecologically. Just like 11 random people do not make a soccer team, a bunch of trees is not the ecological equivalent of a real forest. Not only is the understory substantially different in cities—houses are terrible forage for most insects and animals—but the types of trees are often radically different.

Still, these two studies should make abundantly clear that cities do function as ecosystems, albeit limited ones. And in some cases, they are more diverse and productive than what came before. This is especially true for metropolitan Minneapolis, where monocultures of wheat and corn were less diverse than the Big Woods they replaced and maybe less ecologically complex than the cities that replaced them. These two cases also underline the need for an urban ecology that doesn’t just study what systems cities create, but strives to shape those systems for greater ecological complexity and diversity.


Berland, A. (2012). Long-term urbanization effects on tree canopy cover along an urban–rural gradient Urban Ecosystems DOI: 10.1007/s11252-012-0224-9

Nowak, David J. (1993). Historical vegetation change in Oakland and its implications for urban forest management Journal of Arboriculture, 19 (5), 313-319

Photo by frozenchipmunk.

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Southern regions nurtured languages

La Florida, by Abraham Ortelius

In the last few years, I’ve had the good fortune of befriending a pair of Italians. Before meeting them, I admit I knew relatively little about Italian culture apart from the typical American stereotypes. I grew up in an area with strong German roots, and the college I attended maintains close ties with Norway. Needless to say, I was not well acquainted with southern European cultures.

But thanks to my friends, that’s been changing. Among other things, I’ve been picking up bits of Italian, both the standard tongue and the Veneto dialect. Italy, I’ve learned, is a country defined by a common language which many Italians don’t speak at home. There doesn’t seem to be much agreement on the exact number of dialects, but estimates range from around a dozen to over 50.

That Italy has so many dialects shouldn’t surprise an astute student of history. The region was heavily balkanized prior to unification in the mid-1800s. But Italy’s dialectal diversity may also be the product of another quirk of geography. A study done in the mid-1990s by two British professors—an evolutionary anthropologist and an evolutionary biologist—revealed a distinct trend in the languages of North American native peoples at the time of European contact. More languages were spoken in southern latitudes and the range over which those languages were spoken was smaller. In other words, language density increased closer to the equator.

The scientists discovered this trend when analyzing the first comprehensive map of the world’s languages, Atlas of the World’s Languages, which was initially published in 1993.¹ Focusing on languages spoken by native peoples when Europeans first arrived, they counted the number of tongues that a line of latitude crossed as it ran east-west across the continent. Their survey spanned 8 ˚N and ended at 70 ˚N, the furthest north an entire latitudinal span was inhabited by humans.

Upon tallying their results, a few things stood out. First, the number of languages peaked at 40 ˚N—the parallel that runs approximately through Philadelphia, Denver, and Reno.² Perhaps coincidentally—or perhaps not—this northing is also where the number of mammal species peaks in North America.³ They also discovered the number of languages per square kilometer rises exponentially as you head south. Further, the number of parallels each language intersected increased as they moved north, a function of both language density and the non-overlapping nature of native peoples’ languages at the time. Finally, the number of languages increased with habitat diversity.

The authors speculate that greater habitat diversity at southern latitudes was responsible in part for the greater density of languages. More habitat diversity tends to increase resource abundance, which would allow smaller groups of people to survive in those areas. After groups divided or a new group formed, cultural or geographic barriers may have fostered linguistic diversification.

With the advent of global communications networks, many languages and dialects are slowly dying out. That’s partially driven by the the need to communicate with ever more people in ever more places. But what’s pushing in that direction? One answer could be the world’s population. Earth is a planet of finite resources, and perhaps efficient use requires more interaction. People learned long ago that we need to cooperate to survive. Language is an amazingly efficient vehicle for that. Today, the need to cooperate—and communicate—is greater than ever.

¹ I’d love to get my hands on it, but it sells for over $700. Time to hit the library.

² The top of Italy’s boot heal is at about 40 ˚N. That’s not to imply any correlation, just to provide a frame of reference.

³ Expect more on the species-latitude relationship in a later post.


Mace, R., & Pagel, M. (1995). A Latitudinal Gradient in the Density of Human Languages in North America Proceedings of the Royal Society B: Biological Sciences, 261 (1360), 117-121 DOI: 10.1098/rspb.1995.0125

Map scanned by Norman B. Leventhal Map Center at the BPL.

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Ghosts of ecology

Roman mosaic

If you want a glimpse of our ecological future, take a look at present-day Europe. Continuous and intensive human habitation for millennia have crafted ecosystems that not only thrive on human disturbance, they’re dependent on it. But even in places where pastoral uses have fallen by the wayside, the ghosts of past practices linger. If you have any doubt that the changes we’re making to the earth right now will be felt thousands of years from now, these two studies should wipe those away.

This post was chosen as an Editor's Selection for ResearchBlogging.orgThe first takes place in a post-apocalyptic landscape masquerading as a charming woods, the Tronçais forest. Smack in the middle of France, Tronçais is the site of a recent discovery of 106 Roman settlements. Photographs of the settlements call to mind Mayan ruins in Yucatan jungles, with trees overtaking helpless stone walls. Tronçais was not unique in this way—following the fall of the Roman Empire, many settlements reverted to forest after the 3rd and 4th centuries CE.

Ecologists studying plant diversity in the area noticed two distinct trends. First, the soil became markedly different as they sampled further from the center of the settlements. Nearly every measure of soil nutrients declined—nitrogen, phosphorous, and charcoal were all lower at further distances. Soil acidity declined, too. Second, plant diversity dropped off as sample sites moved further into the Roman hinterland, and likely a result of changes in the soil.

The researchers suspect the direct impacts of the settlement and Roman farming practices are behind the trends. High phosphorous and nitrogen levels were probably due to manuring. The abundance of charcoal is clearly from cooking fires, while soil pH was affected by two uses of lime common in the Roman empire—mortar used in building and marling, the spreading of lime and clay as a fertilizer. The combined effects of these practices fostered plant diversity after the settlements fell into ruin, the effects of which can be seen to this day.

The second study was undertaken by another group of ecologists who canvased grasslands in northern and western Estonia. While threatened today by the usual suspects—intensive agriculture and urbanization—the calcareous grasslands of Estonia have a long history of human stewardship which helped a wide variety of grasses and herbs to flourish. They were greatly expanded by the Vikings, who settled the area between 800 and 1100 CE. Knowing this history, the researchers suspected population density may have boosted floral diversity. They sampled exhaustively, recording plant species and communities in 15 quadrats at 45 sites for a total of 675 sample plots. They also drew 20 soil samples at each site. To estimate population density during the Viking Period, they used an established model that estimated settlement size and extent based on known ruins.

Soil qualities naturally had an affect on present-day plant diversity, but human population density during and shortly after the Viking Period also emerged as a significant predictor. As with the Roman study, changes to soil nutrients because of human activities are likely behind the results. But that’s not all. The researchers point out that seed dispersal 1,000 years ago also influenced present-day diversity. When the Vikings expanded the grasslands, they connected different patches that had previously been isolated, allowing previously isolated species to germinate in new areas.

These are not the first studies to reveal a shadow of human habitation in present day ecosystems—the Amazonian rainforest is littered with evidence of agriculture before European contact, for example. But these studies show the ghosts of ecology persisting for millennia, not centuries. Not only does it bolster the notion that no landscape is pristine—an idea that has been gaining traction with the ecological community—it should underscore the persistence of any human activity.


Dambrine, E., Dupouey, J., Laüt, L., Humbert, L., Thinon, M., Beaufils, T., & Richard, H. (2007). Present forest biodiversity patterns in France related to former Roman agriculture Ecology, 88 (6), 1430-1439 DOI: 10.1890/05-1314

PÄRTEL, M., HELM, A., REITALU, T., LIIRA, J., & ZOBEL, M. (2007). Grassland diversity related to the Late Iron Age human population density Journal of Ecology, 95 (3), 574-582 DOI: 10.1111/j.1365-2745.2007.01230.x

Photo by mharrsch.

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A primer on the metabolic theory

Rhinoceros with tick birds

Lurking the background of many previous articles here at Per Square Mile is a recently formulated framework that describes everything from the heart rate of a mouse to the aorta size in a blue whale. It’s called the metabolic theory.

Metabolic theory’s origins span two states and six decades. Max Kleiber originally formulated the basic relationship between body mass and metabolism back in the 1930s. Kleiber, a professor at the University of California, Davis, produced a famous plot (see below), where body mass is related to metabolism over nine orders of magnitude, from mice to elephants. His analysis of a subset of that data revealed a trend by which metabolism scaled with body mass raised to the ¾ power. He proposed an underlying theory which he felt could explain the results, but it didn’t stick.

Relationship between body mass and metabolism, Kleiber 1947From Kleiber 1947 Physiological Reviews 27(4)

Decades later, Geoffrey West, a theoretical physicist at the Santa Fe Institute in New Mexico, was ruminating on matters of life and death. Specifically, he wondered why he could live to be 100 years old but a mouse would only live a few years, despite vast similarities between himself and a mouse on a cellular scale. In the course contemplating this, West had become interested in the work of Max Kleiber.

Meanwhile, an hour north on Interstate 25, ecologists Jim Brown and Brian Enquist had recently combed through the literature and discovered that Kleiber’s law also applied to plants, but they were struggling with an explanation as to why. Fortunately, West was soon introduced to Brown and Enquist, and the trio embarked on a collaboration that would revolutionize the way scientists think about ecology.

Metabolic theory was one of the main scientific contributions of that partnership. Put simply, it states that a whole host of ecological phenomena are governed by metabolism, which itself is limited by the rate at which its chemical reactions can acquire the necessary compounds. And those reactions are limited by the distribution network which supplies chemical compounds, whether that be an animal’s circulatory system or a plant’s vascular system. Important to this relationship is that the final branch of the network is the same size across species within a group. And they are—capillaries, for example, are the same diameter in everything from mice to elephants. As these distribution networks branch from large to small—aortas to capillaries, trunks to leaves—they follow a fractal pattern, which provides the mathematical basis for the ¾ power in Kleiber’s law.

Klieber’s law turned out to be just one of many such relationships, all of which scaled according to power laws with exponents in increments of ¼. I can imagine West, Brown, and Enquist started feeling about ¼ exponent power laws the same way you and I do when we buy a new car or computer—we start seeing them everywhere. The researchers had stumbled upon what appeared to be a fundamental law of biology which exerted its influence in seemingly disparate systems. Mortality scales to the -¼ exponent of body mass, meaning smaller animals live shorter lives than larger ones. Plant resource use scales to the ¾ power of mass. Plant population density scales with biomass to the -¾ power, a refinement of earlier self-thinning laws that reported a -⅔ exponent relationship. Total standing biomass scales with individual body size raised to the ¼ power. And on and on and on.

The metabolic theory of ecology hasn’t been without it’s detractors, but no good theory should go untested. Still, no matter the outcome of those challenges it has done ecology a world of good, pushing the field to think beyond it’s descriptive roots.


Brown, J., Gillooly, J., Allen, A., Savage, V., & West, G. (2004). Toward a Metabolic Theory of Ecology Ecology, 85 (7), 1771-1789 DOI: 10.1890/03-9000

Enquist, B., Brown, J., & West, G. (1998). Allometric scaling of plant energetics and population density Nature, 395 (6698), 163-165 DOI: 10.1038/25977

Kleiber, Max (1947). Body size and metabolic rate. Physiological reviews, 27 (4), 511-41 PMID: 20267758

Photo by Ferdi’s -World.

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Density solidified early human domination

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Hunter-gatherer populations show humans are hardwired for density

Density solidified early human domination

Sunset over the Kenyan savanna

It’s no surprise that Homo sapiens dominates the Earth. After all, we’re resourceful, social, and smart. No, the surprise is how we did so in just 50,000 years. Such a pace is unprecedented, especially for a long living, slow reproducing species such as ours. Intelligence and opposable thumbs certainly helped, but we aren’t the only ones who can use a tool or solve a puzzle. Rather, a peculiarity of our social nature may be what has set us apart, allowing us to live in nearly every biome on Earth.

The exact mechanics of how sociality fostered our dominance are fuzzy. Myriad archaeologists and anthropologists work hard to resolve those uncertainties, but history is vast and their resources are comparatively small. There is another option, though, one that relies on mathematical machinations and close study of the characteristics of modern day hunter-gatherer groups. Using those methods, a group of anthropologists and biologists think they may have solved part of the migratory riddle. Our predisposition to living densely, they suppose, may have contributed to our stunning success beyond the savannas of Africa.

A sublinear relationship between population size and home range size—meaning that larger groups live at higher densities—imparts special advantages for species that can deal with the twin burdens of density, overshoot and social conflict. Overshoot describes a population that overwhelms its habitat, devouring all available food and otherwise making a mess of the place. Social conflict is as it sounds, where tight proximities provoke fights between individuals. Together, those snags can bring a once booming population to it’s knees.

But social animals are uniquely adapted to cope with those problems. For one, social behavior soothes tensions when they do rise. And when it comes to the necessities of life, density conveys a distinct advantage for social species—resources, chiefly food, become easier to find. Larger, denser populations squeeze more out of a plot of land than an individual could on his or her own.

Density itself wasn’t directly responsible for the first forays out of Africa. Those groups were were too small and dispersed to receive a substantial boost from density. They faced the worst the natural world had to offer, and many probably couldn’t hack it.

Where population density conferred its advantages was when subsequent waves of colonizers followed. Density allowed those people to thrive. They joined the initial groups, growing more populous and drawing more resources from the land. This made groups more stable both physically and socially—full bellies lead to happier and healthier people. As each group’s numbers grew larger, their social bonds grew stronger and their chances of regional extinction plummeted. In other words, once people worked together to establish themselves, they were likely there to stay.

It’s a heartwarming story the scientific paper tells in the unsentimental language of mathematics. It implies that the essential success of our species can be boiled down to one variable, β, and one value of that variable, ¾. The variable β is an exponent that describes how populations scale numerically and geographically. Its value of ¾ is significant. When β equals one or greater, each additional person requires the same amount of land or more—the group misses out on density’s advantages. But when β is less than one—as it is in our case—then a population becomes denser as it grows larger.

The degree of our sociality has allowed us to bend the curve of population density in our favor. If early humans had been an entirely selfish species—each individual requiring as much or more land than the previous—β would be equal to one or greater. We wouldn’t have lived at higher densities as our populations grew, and early forays beyond the savanna might have petered out. Instead of conquering the globe, we’d have been a footnote of evolution.¹

And here is where we can consider how this affects our modern lives. Population density may have aided our sojourn out of Africa, but it’s clear there are limits. Hunter-gatherer populations appear to be limited to around 1,000 people, depending on the carrying capacity of the ecosystem. Technology has raised carrying capacities beyond that number—as evinced by the last few millennia of human history—but we don’t know it’s limits. A scaling exponent equal to ¾ may have helped our rise to dominance, but it also could hasten our downfall. Technology may be able to smooth the path to beyond 7 billion, but what if it can’t? What if ¾ is an unbreakable rule? What happens if we reach a point where density can no longer save us from ourselves?

¹ I might point out here that β=¾ could tell us something about the viability of libertarianism, but that’s a subject for another post.


Hamilton, M., Burger, O., DeLong, J., Walker, R., Moses, M., & Brown, J. (2009). Population stability, cooperation, and the invasibility of the human species Proceedings of the National Academy of Sciences, 106 (30), 12255-12260 DOI: 10.1073/pnas.0905708106

Photo by lukasz dzierzanowski.

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Floral metabolic densities

Hunter-gatherer populations show humans are hardwired for density

The curious relationship between place names and population density

Floral metabolic densities

Cross-section of a rose pedicel, 10x magnification

Nature has a funny way of not behaving geometrically. When you plot all sorts of variables that describe the natural world—metabolism with body size, population size with home range, place names with population density—they don’t follow a linear relationship, they adhere to a power law. Plants, of course, are no exception. Plant population densities—like human population densities—follow a power law, where larger plants are dispersed ever farther apart.

The phenomenon was first described by Kyoji Yoda in a 1963 paper. For his Master’s thesis, he sampled weeds on vacant lots around Osaka, plotting their dry weight against the density at which they grew. He observed that as plant weight increased, density decreased by -³⁄₂. Competition for resources, he reasoned, caused the populations to thin. As plants grew larger, some died out for lack of resources. They just couldn’t compete against the others. Yoda’s self-thinning law is why the Earth isn’t entirely consumed by weeds—they may produce innumerable seeds, but not all make it to maturity. Yoda published his discovery in the Journal of Biology of the Osaka City University. It languished in obscurity for 15 years before other plant ecologists took notice.

The self-thinning rule is often called Yoda’s law, and it’s been empirically tested in ecosystems around the globe. It’s a pretty rad name for an ecological theory, which is why it’s a pity that it may not be entirely accurate. A more recent study suggests the theory describing plant population densities should hew closer to an exponent of -⁴⁄₃, not -³⁄₂. It’s a subtle distinction, but one that ultimately means plant densities are driven by their metabolisms. This revelation comes courtesy of Brian Enquist and his colleagues Geoffrey West and Jim Brown. West and Brown are no stranger to metabolic relationships—the pair described how metabolism changed with body size (it follows a power law, naturally).

The trio put together a model which describes plant resource use based on the amount of water and nutrients a plant moves through its xylem, or the tube-like tissues found in a plant’s stem. Measuring xylem transport rates is a roundabout way of measuring total photosynthetic rate, which in turn lets you also determine metabolic rate. The logic goes something like this: The more water and nutrients a plant’s xylem can handle, the more it can photosynthesize and metabolize. From these measurements, they estimated that the way plants use resources scales at ¾ power. At first glance this seems way off from the -⁴⁄₃ exponent noted above, but it’s really just flipped. Plant ecologists tend to write the equation one way, while animal ecologists write it the other way. The two exponents are, in fact, equal.

Enquist, West, and Brown’s model may not completely sink Yoda’s law, though. They point out that their model doesn’t necessarily describe self-thinning of plant populations witnessed in the real world. Rather, their -⁴⁄₃ power relationship predicts how much plant biomass an ecosystem will produce based on its resources. A grassy ecosystem can have the same level of productivity as a forested one provided the same level of resources are available.


Enquist, B., Brown, J., & West, G. (1998). Allometric scaling of plant energetics and population density Nature, 395 (6698), 163-165 DOI: 10.1038/25977

Marinus J.A. Werger, Masahiko Ohsawa, Mamuro Kanzaki, & Takuo Yamakura (1997). Obituary: Kyoji Yoda (1931–1996) Plant Ecology, 133 (2) DOI: 10.1023/A:1017190418074

Photo by Tatcher a Hainu.

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Hunter-gatherer populations show humans are hardwired for density

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Creativity—the disturbance that distinguishes urban ecosystems

Spatial contagion in an urban ecosystem

Mimicking nature is nothing new for human beings. Ceremonial dress and dances have long imitated totemic animals. Leonardo da Vinci’s plans for a flying machine were closely modeled on the birds he saw out his window. And more recently, nature has inspired designers of everything from velcro to solar cell installations.

Cities and suburbs would seem to be an exception to that rule. They are arguably anthropogenic down to the last blade of grass. We pile dirt up and grade it down, plant grass and pull it from sidewalk cracks. But like other human endeavors, urban areas unwittingly imitate natural landscapes. Or more accurately, people tend to mimic each other, which in turn makes urban landscapes mimic natural ones.

Ecosystems are clumpy. They aren’t randomly distributed in small bits so much as they are consolidated into large associations—think forests, prairies, and so on.¹  It stands to reason that when people move in, the clumpiness of nature moves out. After all, yards are managed by individuals, each with their own tastes and preferences. Just because Sally plants a shrub doesn’t mean George will. But in reality, we’re not so creative. If Sally does plant a shrub, George is more likely to follow suit and so are the other neighbors. Thusly, neighborhoods take on specific ecological forms—a shrubland in this case.

The spatial contagion of landscaping was first demonstrated by two studies of the Hochelaga-Maisonneuve district in Montreal. Hochelaga-Maisonneuve is an old neighborhood that dates back to the mid-19th century and is complete with residential, commercial, and industrial zones, though most of the current houses were built between the end of World War II and 1970. Researchers drove up and down 17 streets, recording the spatial and floral characteristics of 646 front yards.

They discovered that the distance between two yards was responsible for 20 percent of the variation between them. In other words, next door neighbors are more likely to have similar landscaping than people three blocks apart. Next door neighbors were also more likely to have more similar vegetation than houses across the street from one another.² Lastly, the shape and size of the yard also drove landscaping choices. Together, they created a neighborhood that, when examined spatially, exhibited some of the same clumpy characteristics of natural ecosystems.

The reason, the researchers think, is because most of us tend to be pretty unoriginal when it comes to aesthetic decisions. They trot out the works of 19th century American philosopher Charles Sanders Pierce to support their case.³ Pierce proposed that there is no truth, only knowledge filtered by interpretation. In other words, one person’s knowledge is merely an interpretation of another person’s knowledge. Building on that, Pierce surmised that there were three types of human experience, which he cleverly called firstness, secondness, and thirdness. Firstness deals with sensory perceptions—smell, taste, touch, and so on. Secondness connects those sensory perceptions to another bit of information, say associating a smell with the type of flower, for example. Thirdness goes further, establishing symbolic links between two otherwise unrelated pieces of information, like the crunch of leaves underfoot and the beginning of the school year. Loosely, Pierce’s three levels of experience correspond with feeling, knowing, and understanding.

Most people in a neighborhood experience front yards on a primary or secondary level, the researchers suggest. People may take a neighbor’s landscaping and reproduce it wholesale—a primary interpretation—or add a little twist like planting a different variety—a secondary interpretation. Rare are the people who come up with entirely new ideas, the people who produce tertiary interpretations. Not all tertiary interpretations catch on—if a design is too daring, neighbors won’t mimic it—but those that do change the neighborhood. These people are the germs of spatial contagion in cities. Their creativity can begin a cascade of ecological change within cities.

Tertiary interpreters make urban ecosystems fundamentally different from other ecosystems—they have agency, unlike many other processes—yet strikingly similar—they are disturbances, like fires, hurricanes, and floods. They can drive widespread change like a giant forest fire or be confined to a yard like a small tree-fall gap in a forest. But no matter their scale, creative people are a distinct type of disturbance and one unique to urban ecosystems.

  1. While it’s true that our scale of perception is partly responsible—after all, we don’t consider a small patch of grass a prairie, though both contain grasses—there’s functional support for the distinctions between ecosystems.
  2. Across-the-street neighbors’ landscaping is more “in your face”, and so people strive to differentiate themselves more, the authors suspect.
  3. This is where things get heavy, man.


Julien, M., & Zmyslony, J. (2001). Why Do Landscape Clusters Emerge in an Organized Fashion in Anthropogenic Environments? Landscape Research, 26 (4), 337-350 DOI: 10.1080/01426390120090139

Jean Zmyslony, & Daniel Gagnon (1998). Residential management of urban front-yard landscape: A random process? Landscape and Urban Planning, 40 (4), 295-307 DOI: 10.1016/S0169-2046(97)00090-X

Jean Zmyslony, & Daniel Gagnon (2000). Path analysis of spatial predictors of front-yard landscape in an anthropogenic environment Landscape Ecology, 15, 357-371

Photo from Google Earth.

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Salvaging disturbed forests may not save biodiversity

Never buy a car with a salvage title. Anyone who has ever driven a car after a major accident can tell you why—it’s just not the same as before the crash. Though all the parts might be in the right place and the paint just as shiny as before, there’s invariably some new rattle, shake, or whistle that you can’t fix. The magic that is gone, and nothing will bring it back. Cars are a lot like primary tropical forests in that way.

Biodiversity thrives in undisturbed tropical forests. But once they have been selectively logged, burned, or leveled, what grows back in their place just isn’t as rich, vibrant, or diverse as the original, according to a new paper released online today in Nature. The meta-analysis—written by a number of authors including Bill Laurence and Tom Lovejoy, two deans of tropical conservation—synthesized 2,220 pairwise comparisons of primary and disturbed tropical forests from 138 different studies on four different continents to arrive at that one conclusion.

The dominant image of deforestation—at least from an American perspective—is the Amazon. Photographs and satellite images of logging and agricultural conversion show in graphic detail splintered tree stumps, smoking ashes, and herringbone tentacles of human influence. But while the authors found South American forests are greatly threatened by human disturbance, Asian forests are even more imperiled.

To compare results from numerous studies, the study’s authors the measured effect size of human disturbance on biodiversity. It’s a statistical technique which describes the magnitude of differences between populations. The effect size of land-use changes in Asia was more than twice that of second place South America and even larger still than those of Africa and Central America.

To give you an idea of the severity of Asia’s biodiversity threats, let’s review the guidelines on interpreting effect sizes. Generally, a small effect size is 0.2, medium is 0.5, and large is 0.8 and above. In the study, Central America checks in at 0.11, Africa at 0.34, and South America at 0.44. (A quick caveat before we continue: The African result may not be representative. The continent’s tropical forests are understudied because of continued conflict, and future disturbance rates could accelerate in the face of population growth.) Asia is far ahead of the rest of the pack, blowing them all away with an effect size of 0.95.

Asian tropical forests are more threatened by every type of human impact than tropical forests on other continents. Agricultural conversion is responsible for a large portion of biodiversity loss in the region, with plantations and selective logging operations following not far behind. Plantations are of particular concern because the crops they yield—primarily palm oil and exotic woods—are lucrative. Their profit potential draws interest not only from multinational corporations, but governments as well. These organizations have large amounts of capital and can convert vast tracts of primary forest into ecologically sterile plantations that practically print money.

Plantations also have the advantage—for governments and corporations, at least—of looking deceptively like natural forests to many people. Asia Pulp & Paper, a company with large plantation holdings throughout Southeast Asia, has been exploiting this confusion through a series of recent TV ads. The Indonesian government has been in on the ruse, too, suggesting that it may push for their plantations—many of which were carved from primary forests—to count as forest land under REDD schemes, or reduction of emissions through deforestation and forest degradation. That means the government would not only profit from the plantations’ crops, but also from international payments to purportedly offset or reduce carbon emissions.

If we have to use forest land at all, the best bet to preserve biodiversity seems to be selective logging. Though the practice still harms overall biodiversity, it does so less than other land uses. Still, the paper’s authors caution that selective logging’s ill effects may be masked by proximity to less disturbed primary forests, which may export species to depauperate tracts. If this is the case, then selectively logged areas may be running the ecological equivalent of a trade deficit with primary forests. Without some reciprocation, the two will eventually go bankrupt.

This new meta-analysis confirms what many ecologists have long suspected—that minimally disturbed primary forests are some of the best bastions of biodiversity. It puts another hole in the idea that agroforestry projects, plantations, and even selective logging can extract resources without adversely affecting ecosystems. Like a car that’s been in an accident, primary can never be the same as before. But unlike cars, we can’t go out and buy new ones.


Gibson, L., Lee, T., Koh, L., Brook, B., Gardner, T., Barlow, J., Peres, C., Bradshaw, C., Laurance, W., Lovejoy, T., & Sodhi, N. (2011). Primary forests are irreplaceable for sustaining tropical biodiversity Nature DOI: 10.1038/nature10425

Photo by WWF Deutschland.

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The importance of sentimental landscapes

Looking out from Melrose Rock

When I was packing for the move from Chicago to Cambridge, I figured the transition would be easy for two reasons, both of which are related. First, the two cities share a temperate climate. I grew up in Wisconsin and love—absolutely love—the changing seasons. For example, I’m not merely unfazed by below zero weather, I revel in it. The second reason is partially a consequence of the first—the Midwest and New England share a similar flora. Deciduous forests were the playground of my youth, where I went to escape the heat of the summer or romp through the snowy winter.

Having been a Cantabrigian for just just over two months, I can’t speak to the winters yet. But I can say something about the plants. A jaunt to Middlesex Fells over the Labor Day weekend affirmed my fondness for temperate deciduous forests. Still, I wasn’t quite at home. The Fells has a marvelous mix of deciduous oaks and evergreen pines perched on rolling hills and rocky outcrops. The whole landscape is reminiscent of the Calvin and Hobbes cartoons I devoured as a kid, but there was something missing. That something is my history with the place, or lack thereof. Research confirms it.

I wasn’t a part of the study in question—it took place almost a decade ago—but its findings confirm why I am both predisposed to liking New England’s woods and why they aren’t quite home yet. The study’s authors surveyed 328 park users in Ann Arbor, Michigan, to see whether they were attached to a particular park or just a particular setting. The study’s authors classified participants as park neighbors, visitors, volunteers, or staff, reasoning that these backgrounds would tint the lenses through which people viewed the parks.

The researchers found that neighbors who frequented a particular park were smitten by that place in particular. Perhaps the bond was formed during solitary reflective walks, or maybe weekend picnics with the family. Regardless, they liked those place in particular and didn’t find substitutes as appealing. Park volunteers and staff, however, were more inclined to treasure a park’s ecological contributions rather than sentimental ones. When shown photographs of a particular ecosystem, say a prairie, volunteers and staff were more likely to rate those shots highly regardless of their location. Volunteers and staff, who the researchers reasoned to be more ecologically knowledgeable, were also more open to restoration projects that supplanted invasive species with natives. Park neighbors and visitors tended to be happy with the landscape the way it was and generally opposed changes.

The differing perspectives of sentimental park users and ecologically principled individuals may help explain my hesitant fondness for the Massachusetts wilderness. The study seems to confirm that I straddle the line between two types of people. I have a feeling that many people are like me, especially those who recently moved. Our sentimental side aches for a favorite tree or preferred vista, but the rational ecologist in us appreciates native plant assemblages and landscapes.

People develop not just an affinity for nature, but the nature outside their window. That suggests not only that we should get outside, but also bring the outdoors closer to home, whether that be in the form of a city park or wild backyard. First-hand experiences with nature can be powerful ways to inspire people to adopt their own environmental ethic. I’m not the first to posit this theory—David Gessner does just that in his book My Green Manifesto, which I’m currently reading, as have others before him. Indeed, I can trace part of my own environmental ethic to a childhood spent in the park down the street or at the seven acres of scrubby, overgrazed woods just outside of town that my dad was rehabilitating. They are the type of landscapes I love and am fighting to preserve. Indeed, part of the reason I’m fascinated with higher density living is the potential it has to keep the wild places wild, the semi-wild places semi-wild. Calvin and Hobbes’s zany woodland adventures captured my childhood imagination because I saw in them a bit of my own al fresco self. I want future generations to have that chance, too.


Ryan, R. (2005). Exploring the Effects of Environmental Experience on Attachment to Urban Natural Areas Environment and Behavior, 37 (1), 3-42 DOI: 10.1177/0013916504264147

Photo by Paul-W.

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Spare or share? Farm practices and the future of biodiversity

Forest-farm edge in the Bolivian Amazon

Farming giveth and farming taketh away. Let’s parse that statement: Farming provides food—that much is obvious. But farming also gobbles up land that would otherwise accommodate endless biodiversity and beneficial ecosystem services. To counter the ecological harm done by farms, we have two options. One is to make farming more ecosystem friendly. Known as land sharing, this choice differs from garden variety organic farming by enmeshing cultivation with conservation rather than just minimizing detrimental impacts. The other option, land sparing, intensifies current cultivation while leaving other land as wild as possible. If you’re looking to feed people and maximize biodiversity conservation, you have to pick one.

The correct answer, at least according to a study published today in Science, is land sparing. The study’s authors examined farms and forests in southwest Ghana and northern India. They found more overall diversity of bird and tree species per square kilometer in land sparing setups—where farming is intense and reserves off limits—than in land sharing schemes—where farming and conservation occur on the same plot of land.

The ecologists involved in the study mapped out 25 one square kilometer plots in Ghana and 20 in India. The Ghanaian plots were divided almost equally among forest (8), large-scale oil palm plantations (8), and forest-farm mosaic (9). In India, they were split among five forest and 15 farm plots, five of which were low yield and ten of which were high yield. In each plot, the researchers measured average population densities of bird and tree species and binned each species into two broad categories—those that would thrive under a particular farming regime and those that would suffer. They then compared biodiversity statistics for land sparing regions (which contained both farmed and forested plots) with land sharing ones.

Unsurprisingly, all species fared worse when land was farmed. But the disheartening part—at least for those of us who dream of harmonious, ecotopian farms—was that more species were worse off on a region-wide basis under land sharing than land sparing. So although land shared between farm and forest is better for biodiversity on a single plot scale, the overall region is better off when some plots are intensively farmed and others are left alone.

In other words, sparing appears to be the least worst option. While some generalists thrive under land sharing, less mobile species with higher habitat constraints need special protection. Habitat reserves provide that, and land sparing schemes can support larger reserves. The only way land sharing excels at protecting biodiversity is when farm yields are impossibly low.

Land sharing, then, is the futon of biodiversity conservation. Just as a futon is both a middling bed and mediocre couch, land sharing is merely passable at producing food and so-so at protecting biodiversity. Neither futons nor land sharing systems excel at their dual tasks. As The Dude in The Big Lebowski would say, “This is a bummer, man.”

One drawback of land sparing is that it requires an immense amount of self-control on the part of individuals and society as a whole. Time and again we’ve challenged the inviolability of protected areas when we are—or think we are—short on resources. Conservation is hard, and plowing more land will always be the easier option. To prevent ourselves from doing that, we need to raise yields, which takes resources, training, and discipline. None of this will be easy.

Furthermore, raising yields sustainably, which the authors endorse, is going to be difficult. There are certainly some easy places to start—yields in much of Africa are dishearteningly low. But the world has embraced fossil fuel-driven, industrial agriculture for a reason—it’s the easiest way to squeeze more food from the land. If non-fossil fuel farming were the easiest option, we would have done that by now. Land sharing, on the other hand, trades low yields for closeness to nature. Locally this may be more sustainable, but is there enough land to feed 10 billion people that way? Probably not.

The choice between land sparing and land sharing is just one of many we will face as the planet’s resources stretch thin. While I’m quietly rooting for integrated, ecologically friendly approaches, there seems to be growing evidence that intensively exploiting a limited footprint may be the better option. If that’s true, the Romantic in me hopes we don’t lose our connection with nature in the process.

Ben Phalan, Malvika Onial, Andrew Balmford, & Rhys E. Green (2011). Reconciling Food Production and Biodiversity Conservation: Land Sharing and Land Sparing Compared Science, 333 (6047), 1289-1291 : 10.1126/science.1208742

Photo by Sam Beebe / Ecotrust.

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