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Bubbler in a city park

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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,¹but 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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Red-eyed Vireo

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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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Tree City

City tree silhouette

Note to WordPress.com followers: Per Square Mile has moved to a private host. Your old WordPress.com follows and email subscriptions won’t work as WordPress will not share that information. Head over to the new Per Square Mile for the latest.

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.

Sources:

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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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.

Source:

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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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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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.

Sources:

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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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.

Source:

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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