Sprawl follows the growth of cities worldwide. An estimated 54% of us now live in urban areas, which are expected to grow dramatically in the next few decades. Prudential Inc. recognizes urbanization as a global megatrend: “City Limits Becoming Limitless Cities” proclaims an advertisement in Washington’s Reagan National Airport.
My interest in the environment goes back to grade school. I was more of a naturalist then, but collections of rocks, bugs, birds’ nests, and other artifacts filled my bedroom. Now nothing hurts more than to go back to some of my early haunts, only to find that they have succumbed to suburban development.
Sprawl has many causes. It stems from increasing wealth that allows people to escape the city and live in a pastoral setting. It stems from a high rate of population growth and the need for all those folks to live somewhere. Sprawl follows the construction of a convenient highway transportation system through the countryside, with land development springing up at each interchange.
Loss of species has many causes—climate change, loss of predators, invasions of exotics, and purposeful harvest—but land development ranks at the top of the list. Remove the natural habitat for species and many of them will disappear. Only a few animals—squirrels, feral pigeons, rats, and cockroaches—seem to flourish in urban habitats. Many others disappear soon after their native realm is fragmented into small parcels and criss-crossed by roads.
The road network in the U.S. now exceeds 4 million miles, so that you can get no further than 22 miles from a road anywhere in the U.S., outside of Alaska. On average, each km2 of land contains 0.7 km of road length. In the United States, the area of impervious (paved) surface– what is often called the “built environment”—is 112,610 km2—about 1.1% of the total land area and larger than the area of remaining wetlands.
The display of lights in night-time photos of the world taken from space shows the pervasive influence of Homo sapiens nearly everywhere on Earth that is not too dry or too cold for human habitation. There are 62 million miles of roads on the Earth’s surface, enough to go around the Earth about 2500 times. Humans manage 55% of the world’s ice-free surface, largely for agriculture and rangeland. The area of the U.S. that is managed for agriculture, grazing, and rangeland amounts to 35% of the total land.
With only 25% of the world in a natural state and much of it broken up into small disconnected land parcels, it is no wonder that species extinction rates are now much higher than anything we see in the fossil record. Much of our attention has focused on endangered species, but I am particularly concerned with the slow downward spiral in the populations of nearly all species of my childhood. When was the last time you saw a whip-poor-will in the Northeast or a chuckwalla in the Mojave Desert of California? The World Wildlife Fund (WWF) recently reported that animal populations were down 52% worldwide in the last 40 years, largely as a result of habitat destruction.
Many people will send $25 to a conservation organization and expunge their conscience about species extinction. Yet, almost without a thought, the same folks will applaud hearing of a new road being built to shorten their commute to work, health care, or shopping. Few deplore the ongoing loss of species both at home and abroad, spurred by population growth, land development and the loss of natural habitat. Separation of the remaining parcels of natural habitat means that most species will have difficulty migrating to favorable areas as the climate warms. Turtles have a tough time crossing interstate highways!
One only needs to think about the Irish potato famine to witness what happens to a human society that is dependent on a single species for its sustenance. Increasingly we are beginning to see that diverse natural ecosystems lower the risk of certain diseases for humans, such as Lyme disease. These and other lessons should make us recognize that we are part of nature and not above it. A growing body of ecological research shows that diverse systems are more resistant to disturbance and more resilient following perturbation. Remove a significant component of the species and the ecosystem becomes less productive, less tolerant to drought, and less likely to bounce back to its prior level of function when harsh conditions are alleviated.
Human population growth puts our species on a collision course with the maintenance of nature around us. Whether we can manage our population within certain limits or suffer the consequences that will accompany the decline of nature remains to be seen—perhaps not by us, but certainly by the next generation of humans looking out to their sprawling urban habitat.
Barnosky, A.D. et al. 2011. Has the Earth’s sixth mass extinction already arrived? Nature 471: 51-57
Dobson, A.P., J.P. Rodriguez, W.M. Roberts, and D.S. Wilcove. 1997. Geographic distribution of endangered species in the United States. Science 275: 550-555.
Ellis, E.C., K. K. Goldewijk, S. Siebert, D. Lightman, and N. Ramankutty. 2010. Anthropogenic transformation of the biomes. Global Ecology and Biogeography 19: 589-608.
Goldewijk, S., A. Beusen, G. van Drecht, and M. De Vos. 2011. The HYDE 2.1 spatially explicit database of human-induced global land-use change over the past 12,000 years. Global Ecology and Biogeography 20: 73-86
Martinuzzi, S.N., J.C. Withey, A.M. Pidgeon, A.J. Plantinga, A.J. McKerrow, S.G. Williams, D.P. Helmers, and V.C. Radeloff. 2015. Future land-use scenarios and the loss of wildlife habitats in the southeastern United States. Ecological Applications 25: 160-171.
Keesing, K. et al. 2010. Impacts of biodiversity on the emergence and transmission of infectious diseases. Nature 468: 647-652.
Martinuzzi, S.N., J.C. Withey, A.M. Pidgeon, A.J Plantinga, A.J. McKerrow, S.G. Williams, D.P. Helmers, and V.C. Radeloff. 2015. Future land-use scenarios and the loss of wildlife habitats in the southeastern United States. Ecological Applications 25: 160-171.
Milesi, C., C.D. Elvidge, R.R. Namani and S.W. Running. 2003. Assessing the impact of urban land development on net primary productivity in the southeastern United States. Remote Sensing of Environment 86: 401-410.
Newbold, T. et al. 2015. Global effects of land use on local terrestrial biodiversity. Nature 520: 45-50
Watts, R.D. et al. 2007. Roadless space of the conterminous United States. Science 316: 736-37.
Wilcove, D.S. and L.L. Master. 2005. How many endangered species are there in the United States? Frontiers in Ecology and Environment 3: 414-410.
Vogelmann, J.E., S.M. Howard, L. Yang, C.R. Larson, B.K. Wylie and N. Van Driel. 2001. Completion of the 1990s National Land Cover data set for the conterminous United States from LANDSAT Thematic Mapper data and ancillary data sources. Photogrammetric Engineering and Remote Sensing 67: 650-662.