Category Archives: Fragmentation

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

Source:

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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Nature’s burning library

Ghosts of ecology

Urban forests just aren’t the same

Nature’s burning library

Mangroves

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.

Sources:

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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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.

Source:

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

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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.

Source:

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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America's suburban future

Aerial view of Carrollton, Texas

If you think American cities are sprawling now, just wait until 2025. In that time, the U.S. population will grow by 18 percent but the amount of developed land will increase 57 percent. Up to 9.2 percent of the lower 48 could be urbanized by then. And while that number includes cities and the infrastructure to support them—roads, rail, power lines, and so on—that number does not include land impacted by farming, logging, mining, or mineral extraction.

That 10 percent of the lower 48 could be crawling with people is a stark reminder that our nation—while immense—is not immune to the pressures of development. It’s also acknowledgement that despite years of hearing about the resurgence of American cities, sprawl is still king.

Today, it feels like much of what drove the suburbanization of America since World War II has changed. Incomes aren’t rising nearly as fast as they did in the 1950s and 1960s. In fact, when adjusted for inflation, incomes have stagnated or dropped in recent decades. Soaring gas prices and congested freeways have stolen some of the automobile’s glamor, too.

Yet two studies show that while the outlook in the U.S. may have changed, our desire for suburban living has not. The study’s results differ slightly—the 2004 paper says we’ll add 25.8 million hectares (64 million acres) by 2025, the 2009 manuscript says 22.4 million hectares (55 million acres) by 2030—but their conclusions are the same. American cities will continue to sprawl, adding more land per person than in the past.

In recent decades, the locus of suburbanization has shifted from the Northeast and Midwest to the South. With its warmer weather and lower costs of living, the South has grown faster than any other region in the U.S. since 1980. Development has been fueled by flat, cheap land and abundant freeways, which has pushed land demands well above the national average in some states.

That boom also meant the South was hit hard by the housing bust in 2008. But that doesn’t mean the market for suburban housing has disappeared. Living the burbs is still cheaper than the city, and since real incomes for most Americans have suffered in recent years, development will continue to chase lower land prices. The recession and housing slump may have put a damper on suburban development, but I’m guessing it’s just a temporary blip.

Another factor that should conspire against suburban development—higher gas prices—also doesn’t seem to have much of an influence. The 2009 study suggests development rates won’t take much of a hit from high fuel costs. To simulate rising gas prices, the study’s authors reduced the forecasted development rate in states where it was highest—primarily the car-centric South. Only 5 percent less land was converted from rural to urban uses.

It’s possible things could change—perhaps fuel costs will rise even higher, or maybe the home downsizing trend that’s in its infancy will mature. But I think we should prepare for a future filled with suburbs. In the South, where most of the development is happening, land continues to be cheap and easy to access. The same warm weather that drew many people there will also keep them in their cars. Nobody likes walking in the South’s sweltering summers, even if it’s just from the steamy parking lot to the over-air conditioned mall.

The question then is, how can we make the suburbs more environmentally friendly? Encouraging compactness would be a good start, even just at the subdivision level. Hopscotch development inflicts ecological damage well beyond its immediate footprint—there are many plants and animals that cannot survive surrounded by a sea of humanity. Dispersing job and commercial centers is another option, helping to reduce the number of miles people have to drive on a day-to-day basis.

In the end, though, we’ll have to push for more ecologically integrated development. We’ve seen small steps in that direction already—most new subdivisions must deal with run-off from rainstorms on-site rather than shunting it to an overburdened creek. It’s a start, but not enough to offset America’s suburban future.

Sources:

Alig, R., Kline, J., & Lichtenstein, M. (2004). Urbanization on the US landscape: looking ahead in the 21st century Landscape and Urban Planning, 69 (2-3), 219-234 DOI: 10.1016/j.landurbplan.2003.07.004

White, E., Morzillo, A., & Alig, R. (2009). Past and projected rural land conversion in the US at state, regional, and national levels Landscape and Urban Planning, 89 (1-2), 37-48 DOI: 10.1016/j.landurbplan.2008.09.004

Photo by La Citta Vita.

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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.

Sources:

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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Urban forests just aren't the same

Farms giving way to subdivisions in Southeastern Wisconsin

If you were a squirrel living in Southeastern Wisconsin, you’d be pleasantly surprised by the state of things. In many places, there are as many—if not more—trees than there were 200 years ago. But that rosy image doesn’t tell the entire story. Comparing the forests that cover the cities and suburbs around Milwaukee—and likely in many places around the world—is like comparing Rome before and after the fall. It’s still Rome, but it’s not quite the same as it used to be.

Southern Wisconsin is a case study of the changes that were affecting much of the country in the 20th century. Most of the forests had been cleared in the 1800s by farmers, resulting in a landscape that little resembled what came before. The woodlots that remained were small and scattered. In one famous study, only 4.8 percent of the original forests remained by 1935. Milwaukee and its surrounding cities grew steadily in the run-up to World War II, but positively boomed thereafter. They needed room to grow, and since cleared land is easy to build on, farm after farm was subdivided.

The path from forest to front yard seems clear cut. A woods is cleared to make way for farmland, which is later subdivided into lots and sold off to make way for homes. But the reality is much more complex than that. Though a neighborhood may maintain its wooded appearance, it’s original character is gone.

In Wisconsin, subdivisions are invariably preceded by farms. Farming is a tough life. There’s not much money to be made with a small family farm, and an farmer’s property often doubles as his retirement fund. To maximize the investment, he’ll usually subdivide it for housing. It usually works out well for him, because land that’s good for growing crops is also good for building houses—it’s not too steep and most of it doesn’t need to be cleared.

That’s not to say farms are entirely devoid of trees. Most contain small woodlots and extensive fencerows that separated fields of corn, wheat, and soybeans. They’re relics of bygone forests, and in many places that’s all that’s left. Though the relationship is a bit one-sided, relic trees and farms have existed side-by-side for decades.

Maintaining that landscape during subdivision isn’t difficult. Building houses around trees is easy if you don’t take a cookie cutter approach, and houses with big trees in their yards tend to sell for more. But conservation rarely happens. That’s the conclusion of one study of Southeastern Wisconsin. It looked at the fate of extant vegetation as farms gave way to subdivisions between 1937 and 1975. Though the sum total of forested land didn’t drop as much as anticipated, very little of the original vegetation that made it through the transition. By 1975, the trees that dotted subdivisions and roadsides were almost entirely new.

That study reminds us that sum totals seldom tell an entire story. The relationship between forests, farms, and yards is complex and multidirectional. Forests are often cleared for farms, but abandoned farms can return to their forested state over time—much of New England underwent this process. However, urbanization can intervene along the way, removing the little remaining vegetation and replacing it with landscaped yards. But that’s not all the forest loss development is responsible for. Though many subdivisions are carved from land cleared previously for farms, they can be indirectly responsible for the loss of even more forests. Street and yard trees can’t offset this entirely. Similar patterns are well documented in developing nations. In Brazil, for example, expanding soy production has pushed cattle ranchers to clear land further into the frontier. It’s easy to forget these same processes are at work here in the United States.

Even when subdivisions spring fully formed from forested land—skipping the intermediate farm stage—their lots are often cleared of existing vegetation. Some of my research in graduate school documented the stark changes forest edges undergo when houses move in. In old black-and-white aerial photographs, the bare earth of cleared building sites stood out in stark contrast to the dark gray of the surrounding woodlands. Straight, sharp lines separated the two. In time, the edge bled back into the yards, but it wasn’t quite the same.

Suburban development isn’t going away anytime soon, but some of the structure and function of the old woodlands they replaced can be recovered. Homeowners can plant native trees. People can lobby their cities to plant native trees as well, rather than the whatever low-maintenance tree is in fashion among city foresters this year. The result won’t be the same as an intact woodland, but at least it will be similar.

Source:

Sharpe, D., Stearns, F., Leitner, L., & Dorney, J. (1986). Fate of natural vegetation during urban development of rural landscapes in Southeastern Wisconsin Urban Ecology, 9 (3-4), 267-287 DOI: 10.1016/0304-4009(86)90004-5

Photo by sierraromeo.

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City wildflowers

Tucked amidst acres of asphalt jungle are cities’ unsung environmental heroes. Yards, lawns, gardens—call them whatever you please—these bits of unpaved earth play a real role in supporting thriving urban ecosystems. And they could play the part even more eloquently if we thought of them as parts of a larger whole.

Anyone who has spent more than five minutes in a city knows they are not always welcoming to people, let alone plants and animals. It’s common to see thin, scrappy weeds straining against their concrete binders, or birds clinging to wiry utility lines in lieu of more customary branches and brambles. But behind houses, or even hidden out front in plain sight, are postage stamps of possibility. With all the clipping, prodding, and spraying these diced green patches receive, it’s easy to forget that they are part of a real ecosystem.

Cities have long been overlooked by ecologists, mostly because the logistics involved in studying them can be convoluted. Getting permission from dozens, even hundreds of landowners is one of the biggest headaches, so urban ecologists typically resort to the next best thing—parks, forest preserves, greenways, and so on. Parks can be fantastic reservoirs of habitat, but their area pales in comparison to the amount of land scattered throughout the city as yards and gardens. Real urban conservation plans needs to account for everyone’s little patch of nature.

Here’s where landscape ecology can help. Landscape ecology teaches us to look beyond—or within—the various bits and pieces, components and parts that make up an ecosystem. Scale is king in landscape ecology, be it spatial or temporal. In the context of a city, this means that each individual yard and garden—which on its own can seem hopelessly small, only able to support a salamander, some insects, and a few birds at best—is just one piece of a larger patchwork, one connected by birds that fly across town, salamanders that waddle beneath fences, bees that hum between flower beds, and seeds that disperse on the wind. Protecting them all is only possible when conservation plans cover the gamut of spatial and temporal scales.

Unfortunately, much of this potential has yet to be tapped. By and large, cities remain natural wastelands. Habitat fragmentation keeps down the abundance and diversity of species. Inspired landscaping can counter the diversity problem, but it usually does so with exotic species that are poor ecological substitutes for natives. Pets—especially cats—take a toll on native animal populations, while air, light, and sound pollution add further disruptions to an already taxed ecosystem.

Still, most cities have enough material for a solid conservation foundation. Many people are earnestly invested in their yards, carefully curating selections of grasses, trees, and shrubs, attracting musical entertainment through bird feeders, and in doing so supporting a diversity of mammals, amphibians, reptiles, and invertebrates. This has all been accomplished without significant coordination. Programs like the Audubon Society’s “Audubon at Home” or the National Wildlife Federation’s backyard certification scheme have nibbled at the edges, but lots more could be done.

A recent review of the landscape ecology of gardens suggests that to encourage habitat friendly yards and gardens a bottom up approach would be best. Top down programs can help cities meet conservation targets, but they do little to change people’s attitudes. Encouraging a “conservation ethic of the city” would probably be more successful, but also more difficult to engender. Lawn culture is heavily embedded in many Western nations, especially the U.S. and Canada. Lawns will always have their place; besides recreation, they are surprisingly productive ecosystems. Yet most are far larger than they need to be. Substituting appropriate plantings for classic Kentucky bluegrass would save people time and effort, reduce emissions from mowing, and boost habitat diversity and complexity.

Lawns are just one part of the equation. Landscape ecologists can step in to identify the driving forces behind landowner decisions. Where the conservation ethic exists, ecologists can encourage neighbors to coordinate their landscaping, clumping their native plantings so that four quarters can add up to one whole, for instance. Depending on the area’s ecology, landscape ecologists can further define optimal sizes for these native plots—for example, will it take twenty percent of four yards or six to meet the needs of a native bird? Or in other cases, water features like ponds may be more important than contiguity. Each city, even neighborhood, will have its own gestalt, and landscape ecologists can help discover it.

Sources:

Falk, J. (1976). Energetics of a Suburban Lawn Ecosystem Ecology, 57 (1) DOI: 10.2307/1936405

Goddard, M., Dougill, A., & Benton, T. (2010). Scaling up from gardens: biodiversity conservation in urban environments Trends in Ecology & Evolution, 25 (2), 90-98 DOI: 10.1016/j.tree.2009.07.016

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Photo by Per Ola Wiberg.

Wilderness housing boom challenges conservation

Houses in Keystone, CO, located in the Arapaho National Forest

The housing boom may be over in the United States, but things look very different when you take a step back. Since the 1940s, housing has grown at about 20 percent each decade. And while the current recession may have slowed things down, we’ll have to start building more houses eventually if we’re to house the 120 million more people by 2050. The decades-long housing boom magnifies in intensity when you start looking around national parks, wilderness areas, and national forests, as a recent paper in the Proceedings of the National Academy of Science did. While there were just 14.9 million housing units within 50 kilometers of a protected area in 1940, there were 61.9 million units in that same buffer by 2000. And not only have the raw numbers increased, so too has their share of all American housing.

It’s no surprise that people want to live near parks. Clean air, open space, and gorgeous scenery draw people in as tourists and bring them back as prospective property buyers. Retiring Baby Boomers, freed of their work commitments, have been particularly drawn to the borders of national parks, forests, and wilderness areas. Even working-age people—many willing to suffer through a long commute to live near natural beauty—have gravitated towards protected areas.

None of this would be a big problem if we didn’t mind clustering our houses together, limiting our impact on the land around us. But the reality is no one moves to the country to live in an apartment building. People buy a handful of acres, erect a house a good distance from the road, and string in a long driveway.

Though it’s tempting to think the environmental impacts of a house stop when grass gives way to trees, the reality is that houses cast a shadow far larger than their physical footprint. Roads, utility lines, and driveways dice up the landscape. This fragmentation reduces species’ ability to travel out of the now-noosed protected areas, trapping them in reserves that may not fulfill their needs. Houses also bring hordes of exotic plants and animals, including pets, which often prey on native fauna. All of these factors—and more—create disturbances that can affect protected areas, though they may be miles away.

The authors of the study cite a few examples of how wilderness housing has already changed the habitat in and around protected areas. In the east, the Great Smoky Mountains National Park is choked with pollution. In Colorado, the number of visitors to the Mount Evans Wilderness Area has increased so much that the Forest Service now requires permits. Wildfires in Cleveland National Forest near San Diego, most of which are set by humans, have skyrocketed in number, pushing out the native coastal sage scrub and ushering in exotic grasses. In Michigan, houses peppered within the boundaries of Huron-Manistee National Forest have led to the suppression of wildfires, which has depressed reproduction of the fire-dependent Jack Pine, which has hurt populations of the Jack Pine-dependent Kirtland’s warbler.

The surge in housing near protected areas seems poised to continue apace through at least 2030. The authors of the PNAS study forecast that 17 million additional housing units will be erected within 50 kilometers of protected areas. For those of us who love to live near wilderness—myself included—this news is bittersweet. The good part is that there will be plenty of places for me to live, if I choose to move there. The bad is that nature-lovers like myself run the risk of damaging that which we admire. But not all is lost. The paper’s authors offer a few suggestions as to how we can minimize our footprint. (Really, these are suggestions that we should all follow, whether we live in the city or the country.) Push for zoning laws that clump houses closer together, leaving more open space in tact. Site and build your house with your surroundings in mind. Landscape with native plants. Keep your pets from running rampant. Don’t fertilize too much. Turn off your outside lights, and keep the noise down. And one more (this one’s mine)—keep your lawn small.

Source:

Radeloff, V., Stewart, S., Hawbaker, T., Gimmi, U., Pidgeon, A., Flather, C., Hammer, R., & Helmers, D. (2009). Housing growth in and near United States protected areas limits their conservation value Proceedings of the National Academy of Sciences, 107 (2), 940-945 DOI: 10.1073/pnas.0911131107

Photo by Kara Allyson.

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Animals seek calm seas in oceans of human influence

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Animals seek calm seas in oceans of human influence

Highfields, looking over the valley

Habitat loss can be like death by a thousand cuts for ecosystems. Each conversion to farmland, housing, or pasture, when taken on its own, may seem a small, inconsequential nick on the surface of a vast planet. But together, and over decades and centuries, these cuts add up, leaving only tiny remnants of the original scattered across the landscape. The result can be devastating for animals and plants which depend on the original ecosystem. Yet unlike the grisly metaphor drawn from an ancient Chinese torture, the wounds of habitat loss are not entirely fatal. Farms and subdivisions may not supplant virgin forest or grasslands, but they need not be inhospitable wastelands, either.

This post was chosen as an Editor's Selection for ResearchBlogging.orgThe field of ecology typically focuses on the leftovers of habitat conversion—the bits and pieces that somehow evaded plows and bulldozers. But just as no island is truly separated from the rest of the world, habitat remnants remain connected by the fields and yards that constitute the interstitial spaces. Known as “the matrix,” such spaces are important to the continued vitality of habitat fragments. In fact, according to new research out of Australia, the qualities of the matrix may matter more to the survival of native animal species than characteristics of the remnants themselves.

The matrix in the study are primarily cattle stations prevalent in the Toowoomba Regional Shire in southern Queensland. The area has been grazed heavily since the early 1900s, and pastoral uses are still the predominant land use, though urban development has been increasing in recent years. To quantify the amount of human influence within the matrix, the researchers measured a host of variables, mostly pertaining to roads and ranging from length to width to driving surface and even road kill statistics. They also closely studied the forest patches within the matrix and surveyed their resident mammal populations.

Many previous studies have focused on remnants’ size, shape, and geographic relationship to one another. This study did that, too, but also investigated the matrix itself. Of all possible landscape characteristics, the intensity of human development and number of tall trees within the matrix exerted more influence than any other.

To see how the matrix plays a role in this case, imagine you’re part of a group of people who are stranded on one of those islands, and your island doesn’t have everything you need to survive. Other islands do, though, so you’d need to set sail from time to time, just as the animals in the forest remnants may have to cross a pasture or pass through a neighborhood. The island metaphor is often used to describe how habitat remnants and their inhabitants interact with each other. Typically, people wax lyrical about the size, shape, and distance separating the islands. In this case, however, its not just the span of open water between the islands that matters, but the qualities of that water.

Neither the sea nor the matrix are absolute barriers, but they aren’t entirely hospitable, either.

Now let’s say you had a choice of islands on which to be stranded. Which would you pick? Quite obviously you’d prefer an island close to the mainland, but let’s say that’s out of the question. In lieu of that, you’d probably pick one that’s surrounded by calm seas so that exploring nearby islands would be infinitely easier. Some shallow waters between islands might be nice, too, to anchor your boat when you needed to take a rest. And if I were you, I’d also go for one with a bit of elevation to avoid high tides or tsunamis.

If those three wishes were granted, you’d probably have a good chance of living to a ripe old age. Conveniently, those wishes mirror what mammals in the Toowoomba study seemed to prefer, too. The researchers found fewer mammals traversing heavily developed areas—analogous to rough seas—and more where tall trees provided cover from predators—calm seas. The animals also favored a matrix with more nooks to roost or hide as they travelled—similar to shallow water for anchorage. Finally, the best remnants were those where human activities didn’t encroach too often—akin to higher elevation islands that guard against tides and tsunamis.

As a stranded soul, you wouldn’t have much influence over the ocean. But as stewards of the matrix, we decide its flora, its structure, and its ease of passage for animals. The biggest difference we can make is in reducing our developed footprint. The changes don’t have to be as drastic as ripping up subdivisions or farms. They could be as simple as carving roads only where they are most needed or avoiding areas where animals frequently tread. And failing that, plant trees.

Sources:

Brady, M., McAlpine, C., Miller, C., Possingham, H., & Baxter, G. (2009). Habitat attributes of landscape mosaics along a gradient of matrix development intensity: matrix management matters Landscape Ecology, 24 (7), 879-891 DOI: 10.1007/s10980-009-9372-6

Brady, M., McAlpine, C., Possingham, H., Miller, C., & Baxter, G. (2011). Matrix is important for mammals in landscapes with small amounts of native forest habitat Landscape Ecology DOI: 10.1007/s10980-011-9602-6

Photo by Shaun Johnston.

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Plants rockin’ the suburbs, animals not so much

Plants rockin’ the suburbs, animals not so much

tufted hair grass (Deschampsia caespitosa) in a yard

Where there are more people, there’s less nature. It’s a fairly well established fact. Manhattan may have the odd hawk or falcon, but the paved island’s diversity of plants and animals just can’t compare to that of 23 square miles of pristine wilderness. What’s less known is how well biodiversity fares in human landscapes that are somewhere between the Empire State Building and Daniel Boone’s back forty.

Many scientific papers have been written about how specific types of plants and animals fare in the countryside, the city, and places in between, but few summarize the big picture. One review paper did take a wider view and surveyed 105 studies. It found that though most types of animals avoid the city, plant life seems perfectly happy living the suburban dream.

Animals, specifically mammals, reptiles, and amphibians,¹ dropped precipitously in most studies as researchers moved from the countryside to the city. In the first transition—from the countryside to the suburbs—only three studies found the same or greater numbers of mammal, reptile, and amphibian species, while 14 studies reported fewer. Invertebrate diversity rose in 14 studies, but fell in 30 others and remained the same in only three reports. These negative trends were magnified in the transition between the suburbs and the city proper. All but one study reported the same or fewer numbers of species of invertebrates in the big city as in the suburbs. As the researchers moved into the city and human population density increased, they found fewer mammal, reptile, and amphibian species.

Loss of habitat is probably behind this steady decline. Larger animals like mammals, reptiles, and amphibians need relatively large plots of land to survive. Invertebrates like insects are better off in human-dominated areas because of their smaller size—even a single tree can support dozens of different insects.

Amidst the gloom, plants were the one bright spot. In the suburb-city transition, plant diversity advanced in seven studies and retreated in seven others. It thrived when moving from the countryside to the suburbs. Plants’ success is probably due to their negligible requirements. Many only require a bit of soil, some water, and moderately clean air. On top of that, people often lend plants a helping hand by planting, watering, and fertilizing them. And while a home may have a cat and a dog, many sport dozens of different flowers, trees, and shrubs in their garden. Suburban lots are both large enough to encourage gardens yet small enough for people to support more diversity than on sprawling country lots. Even different landscaping preferences between different households fosters higher diversity.

Suburban plant diversity, though, probably comes at the expense of native flora. Most yards are beautified with species exotic to the area. Many are chosen simply based on their appearance or low maintenance. Native gardens are becoming more popular, but their numbers still pale in comparison to more traditional yards. It’s my suspicion that non-native landscaping holds down the diversity of mammals, reptiles, amphibians, and invertebrates. Native plantings would probably aid native animals, helping to offset some of the land taken by development.


  1. This review did not include birds—there are so many studies of birds in cities that it would be another paper in and of itself.

McKinney, M. (2008). Effects of urbanization on species richness: A review of plants and animals Urban Ecosystems, 11 (2), 161-176 DOI: 10.1007/s11252-007-0045-4

Photo by pluckytree.

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"What if" development ran rampant in Silicon Valley?

Silicon Valley

Silicon Valley exploded in every way possible following World War II, well before it was known as Silicon Valley. The population boomed, houses were thrown up, and roads slithered out from the peninsula’s main drag, the El Camino Real. But as the strip malls started to take over, locals set about preserving much of the open space that remained.

Throughout the Bay Area, over 1,500 square miles have been protected from development. Some sprawl advocates such as Robert Bruegmann have suggested the sheer volume of protected land within the Bay Area has forced suburbs far into California’s Central Valley, and that the land closer to the city centers would be better used for housing. Silicon Valley itself holds a not-insignificant 181 square miles of the Bay Area’s protected land. Detractors claim the open space has hemmed in development too much, forcing additional housing elsewhere, increasing commute times, and reducing the amount of affordable housing in the Valley.

About the Finder easter egg

Even Apple engineers were enamored with the golden hills that surround Silicon Valley.

Such criticism can be difficult to deflect without ripping up woodlands and grasslands, so a group of geographers set about asking “what if” the protected land in Silicon Valley were open to development. They created a series of maps to predict where and how much development would take place by analyzing six characteristics—slope of the terrain, presence of wetlands, distance to streams, distance to railroads, and distance to historical urban centers—that would help them determine the number of additional houses each piece of land could support.

In total, they found that 51,000 additional housing units could be added in Silicon Valley if all the parks, protected watersheds, and protected wetlands were be converted to housing. For reference, the region has 790,000 units currently. Crucially, the study’s authors estimate only around 3,400 units would be on lots small enough to be considered affordable by Bay Area standards.

Topography is the main reason few affordable units would be added to the area’s housing stock. Much of the protected area in Silicon Valley is high in the hills and on steep slopes. Due to concerns over wildfires and mudslides, housing density in these areas is restricted. Existing houses built on unprotected wetlands have also been spaced far apart. Since the Valley’s large protected tracts are either up in the hills or down in the wetlands, there is little room for additional high density (and affordable) development. Over 20,000 of the additional units, they estimate, would be single family homes on large lots, around 1.6 acres each. Real estate of that size in the Bay Area is not cheap, even by Bay Area standards. Single family homes on one acre or larger lots in San Jose list for $1.5 million.

Six and a half percent more housing units might make a bit of difference in cramped Silicon Valley, but it would also do away with the open space that makes the area both livable and attractive to many. Plus, much of the gains in affordable housing would come at the expense of parks within city limits, many of which were created give residents of the surrounding high density housing a bit of fresh air and greenery. Sounds like it was a pretty good trade-off to me. Having worked with many high school students from such neighborhoods, I know many of them never made it to the Bay Area’s large regional parks. But they did spend many hours at their local neighborhood parks.

Source:

Denning, C., Mcdonald, R., & Christensen, J. (2010). Did land protection in Silicon Valley reduce the housing stock? Biological Conservation, 143 (5), 1087-1093 DOI: 10.1016/j.biocon.2010.01.025

Photo by calwhiz.

The map that started it all

Blue Mounds, WI

Buried in a dusty tome grandly titled Man’s Role in Changing the Face of the Earth lies a map that changed my life. Granted, my life was already headed in a direction amenable to this map’s wiles, but that lone figure’s influence cannot be understated. 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. In that one figure, John T. Curtis posthumously changed my life.

Cadiz Township, Wisconsin

Growing up in southeastern Wisconsin, I was always peripherally aware that the forest fragments I frequented had not always been mere fragments. But Curtis’s map drove the point home. His fragments were dead ringers for my fragments. His maps were my environmental awakening, but in black-and-white.

Curtis grew up in Waukesha, Wisconsin, the same place my grandfather lives. He attended Carroll College—also in Waukesha—for his AB and then moved 65 miles west to Madison for his PhD. Curtis more or less remained in Madison till his death in 1961, proving that you don’t have to go far to accomplish big things. Curtis is best known within the ecological community for his work with Roger Bray on ordination, a statistical technique that enables botanists to make sense of the distribution, frequency, and abundance of plant species on a plot of land. His Cadiz Township maps are almost an afterthought, a minor footnote in an otherwise sweeping treatise on Wisconsin vegetation.

Unlike Bray-Curtis ordination, the Cadiz Township maps are astonishingly simple. The figure’s earliest frame depicts a wild Wisconsin, untamed by the plow and dominated by a grand deciduous forest the likes of which I sought as a kid. The scene rendered fifty-one years later in the second frame is entirely different. The smooth curve of the prairie-forest border is gone. Inky shards replace the previously continuous forest. In the third map, those bits grow smaller still. In the 1950 frame, the remaining woodlots are barely visible, like the last specks of glass from a broken platter waiting to be swept into a dustpan.

What makes Curtis’s map all the more remarkable was that he recreated the scenes from survey data and his own observations, painstakingly piecing together handwritten records of the six-by-six mile township. To my knowledge, it is one of the first visual reconstructions of fragmentation-as-it-happened, and one of the most influential—at least to me.

Photo by Ron Wiecki.

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