Chap. 1: Cities & Nature
Humans altered the biosphere, greatly expanded their population, and now feel the effects of those alterations. A hunter–gatherer lifestyle gave way to an agrarian one, and an agrarian lifestyle gave way to an urban lifestyle. Urban dwellers exist more detached from the biosphere and from the environment that shapes and interacts with it. In this book I review the ecological, environmental, and sociological features of urban life and a world increasingly changed by its human occupants.
The human population exploded in recent centuries, reaching numbers 60 times greater than the population 2,000 years ago and sixfold more than just 150 years ago, bringing with it serious environmental challenges. In this chapter I put human population densities into the context of other creatures and explain several important ecological concepts that show the stark immensity of our present population. I look at people’s use of land and water, and how human densities and resources connect to important general ecological principles. Reasonable estimates show that our global population exceeds “natural” levels by up to 400-fold. If we were just another species, then, given our body size there ought to be about one person per square kilometer. However, suburban and urban areas of the United States have densities of 1,000 to 10,000 people per square kilometer.
All these people, mostly concentrated into cities, put tremendous stresses on our natural resources. Water and fertilizers represent two fundamentally important aspects of sustaining a large human population because both factors relate to important ecological features, including evapotranspiration and net primary productivity. These two features drive many aspects of nature, an important one being biodiversity, meaning the number or richness of species within broad groupings of organisms. For humans, evapotranspiration is important to demands of growing food, but I also demonstrate our immense dependence on producing nitrogen fertilizers using fossil fuel energy. While fertilizer greatly amplified food production and supported more people, along with increasing technologies, it meant an increased agricultural efficiency. That change in efficiency affected the economic viability of small family farms, and as these businesses collapsed, it pushed a tremendous land-use change toward urbanization.
An important feature of increased urbanization is that when precipitation falls on impervious surfaces, the water makes its way through constructed stormwater systems to urban streams. As a result, urban streamwater quality suffers, killing the sensitive organisms living in urban streams. These consequences correlate directly to the amount of impervious surface contained in a watershed. I refer to many examples from my city, Durham, North Carolina, and like most cities, Durham’s stormwater system uses urban streams as above-ground stormwater pipes, and the stormwater flows into drinking water reservoirs. As a result, decreased water quality in urban streams correlates with various measures of increasing urbanization.