In October 2011, the world population reached 7 billion, and the population is expected to pass 10 billion by the end of the 21st century (USA Today). Between 50 percent and 90 percent of these humans live in cities, with the variation depending on geographical area. (Figure 1). In addition, the rate of increase of city population is increasing in the vast majority of countries. This generally leads to the expansion of city boundaries, which in turn has a direct impact on biodiversity. Expanding city boundaries can lead to habitat loss and interfere with the natural life cycles of animals living near cities. One study, by Pillsbury and Miller, on frogs in the state of Iowa in the United States showed that the number of frogs was directly related to the distance of their habitat to an urban area (Figure 2). They found that as the urban density increased, the number of frogs in that area decreased proportionally (Pillsbury & Miller, 2008). Other studies have shown declines in bird populations linked to urbanization (Paucharda, Aguayob, Pena, & Urrutia, 2006).
Humans impact biodiversity in many ways, often negatively. A study done by Czech, Krausman, and Devers analyzed the different ways in which species are endangered by human activities (Figure 3). From this data it can be seen that of the 18 areas of human activity that were examined, the activities that most endanger species are interactions with non-native species (invasive species), urbanization, and agriculture (Czech et al., 2000). According to the Federal Register, urbanization alone has caused 340 species to become endangered. While, in the grand scheme of things, this may not seem to be a very large number, it must be remembered that this is only the number endangered by urbanization within the United States; the total number of species affected by urbanization is proportional to the growing number of cities.Figure 4 analyzes the affects of different human impacts on species. The analysis indicates that the most damaging forms of human activity involve roads, urbanization, industry, and livestock (Czech et al., 2000).
From these and similar studies, it is clear that urbanization has a broad impact on the biodiversity within an area (Pillsbury & Miller, 2008). A study by Michael McKinney breaks down the species affected by urbanization into three categories: urban avoiders, urban adapters, and urban exploiters (McKinney, 2002). Urban avoiders are species that are sensitive to human activity (McKinney, 2002). These are animals, such as large predators, that cannot survive once humans have displaced their food sources. Urban adapters are species that are quick to utilize changes to the environment due to humans. For example, some urban adapters using increased trash for food sources or quickly inhabiting cleared land (McKinney, 2002). Finally, those species that are totally dependent on the presence of humans are classified as urban exploiters (McKinney, 2002). Examples of urban exploiters are species such as sparrows, mice, and rats. These species tend to jump from city to city, thus totally relying on human populations for their survival (McKinney, 2002). It is interesting to note that urban exploiters are only present in urban areas. Thus urban areas actually increase the amount of some species. However, the number of species that this category represents is a very small proportion of total species in the world. Thus existence does not greatly impact overall world biodiversity levels (McKinney, 2002).