The U.S. Census bureau designates two classifications for urban settlements: urbanized areas with 50,000 or more people; and urban clusters (outside of urbanized areas) with 2,500 to 50,000 people. Outside of these areas, land is classified as rural. While needed for Census purposes, this definition does not provide clarity regarding what “urban” actually means. A host of social and technological characteristics are often associated with the term, such as activity, commerce, innovation, detachment from nature, pollution, and more. Yet, these notions are often simplistic. For example, many urban areas throughout the world have fantastically productive ecological systems within their borders, including naturalized areas and more altered ones. Also, many city residents have significant affinity for the natural world. Lastly, urban areas may actually offer residents more energy efficient lifestyles than rural areas for a given number of people. Rather than discuss common conceptions, it is more instructive to consider urban areas as areas exhibiting increased human population density and managed land, usually characterized by built infrastructure such as roads and buildings.
Urban areas display unique levels of landscape heterogeneity, which has been influenced by human and biogeophysical factors over layers of history. Human approaches divide up land within a city based on political designations, such as neighborhoods, boroughs, counties, zip codes, and voting districts. Human needs such as water and electricity have designated service maps that are a function of the built infrastructure (pipes and transmission lines), which may or may not follow political boundaries.
This human-driven heterogeneity is complemented by variations that result from biotic and geologic characteristics. While many urban residents may be aware of environmental concerns or enjoy activities in wilderness, the rituals of daily urban life can obscure the dynamic biotic processes taking place in a city. Even in highly altered systems, examining ecosystem processes provides useful and unique insights into system function. Understanding how organisms interact with other organisms as well as the surrounding environment lies at the heart of urban ecology. Studying landscape heterogeneity provides a useful framework for understanding city function and structure, including the interactions of humans, other biotic organisms, and built infrastructure.
As noted above, heterogeneity can come from many sources. The photographs below show views East and West along P St. in Midtown Sacramento. Looking east towards downtown, a landscape of medium-density residential and significant tree cover (in perspective) is visible. The mix of trees will be highly dependent upon human decisions, but also influenced by ecological processes. Looking west, one sees a change in landscape from the raised Capitol City Freeway. This division in landscape is driven by human decisions, creating a microclimate underneath the bridge.
Traveling west underneath the overpass, one moves into areas designated as East Sacramento. The Capitol City Freeway serves as a physical representation of the boundary for many residents, even though the designation does not align other markers such as zip codes. Nevertheless, while the composition of organisms would not necessarily change in the few blocks of P Street due to biogeophysical factors, the highway creates a noticeable difference in landscape composition.
Urban systems are also characterized by layers of history. The picture below in East Sacramento shows a recently re-developed area, characterized by mixed-use commercial and residential buildings. As a newer strip, the block has younger vegetation than other surrounding areas since vegetation takes years or decades to mature. This lack of established tree cover can affect the function of buildings and the perspective of the residents. Trees in cities often have significant positive psychological effects on residents. Many city managers seek to increase urban tree cover for benefits such as increased land value. Additionally, vegetation can serve to mitigate urban environmental impacts through uptake of carbon dioxide and cooling through shading and transpiration. Urban ecology attempts to understand the mechanisms of these processes in the context of an urbanized environment.
While trees are often viewed as beneficial for an urban landscape, they are not necessarily natural. Examining the composition of organisms within a city is one way to understand how city managers view urban ecology, both today and in the past. Walking through Sacramento, one sees many towering palm trees along the streets, an exemplary non-native species. Humans should be aware of the ramifications of introducing non-natives to urban areas, as well as the mechanisms through which such introductions occur. So while landscape heterogeneity is a fundamental characteristic urban and biotic landscapes, urban areas contribute to rapid introduction of non-natives, with potentially homogenizing effects on species composition. Reduced biodiversity in an area can impact ecosystem health, such as increasing vulnerability to disease. Thus, urban systems show both heterogeneity and homogeneity in landscape and resident organisms, which urban ecology must continue to research. Recent urban planning approaches, rooted in a revisiting of planning based on thinkers such as Jane Jacobs and others, have begun to re-emphasize the role of heterogeneity in the built environment for economic health and social outcomes. As the planet urbanizes, understanding these dynamics in the context of the biotic component of cities will be a challenging task with potential benefits for ecosystems overall.