Ecologists do most of their work accounting for humans as a “disturbance” and something out of the natural order of an ecosystem. However, one of the greatest effects humans have had upon the planet is through the building of cities which are becoming greater, bigger, dirtier. Over half of the world’s population lives in cities and it is likely a greater proportion will continue to do so. Urbanization changes landscape structure in a way that results in habitat loss and fragmentation, which further affects biodiversity and ecosystem processes. Landscapes are comprised of mosaics of patch types that have varying values to various taxa but in urban regions, patches are often defined by human land use. Organisms, such as birds, respond to this and utilize these patches in a variety of ways. Some birds are very adept at utilizing urban resources, some avoid urban areas entirely, and some at least commonly pass through them. It is important to understand how “people centric” patches affect biodiversity, specifically how patches of various urban land use types act as supplemental habitats or barriers for birds. While this has been studied in woodland or forest type environments, it has not been studied in arid environments. Jennifer Litteral and Jianguo “Jingle” Wu at Arizona State University (Litteral at the School of Life Sciences and Wu at the School of Life Sciences and School of Sustainability) studied passerine birds at fifteen reserves in the Phoenix metropolitan region (in the Sonoran Desert) and explored the relationships between species diversity and reserve characteristics and urban land uses surrounding the reserves. They found that, while reserve characteristics were important, the urban environment outside the reserves also made an impact on bird density within the reserves.
Birds were categorized into four types based on life history characteristics: synanthropic, non-synanthropic, feeding generalist, and insectivore. Synanthropic organisms benefit from humans or from human activity, but are not domesticated animals (e.g. rats). The authors hypothesized that a negative relationship between species density and land use types surrounding the reserves would indicate that those land uses act as hostile edges that birds are less likely to cross when adjacent to desert habitat fragments and further isolate the reserves or reduce habitat quality within the reserve by increasing disturbance or edge effects. They hypothesized that positive relationships, in turn, would provide some kind of supplemental habitat that the bird guilds could use for extra resources or move through towards other habitat patches.
Multiple spatial scales are important for birds’ choices of habitat and the authors wanted to examine that factor, but because the area was relatively homogenous, they chose larger-scale (200, 1000, 2500 m) boundaries surrounding the reserves.
All the reserves in the study were surrounded by flat, developed areas that were dominated by irrigated, often mesic vegetation. The populated parts of the city have such different water regimes and are dominated by so many non-native species, that the boundaries of these fragments often represented legal and ecological boundaries. Areas outside the reserves were more similar to woodland ecosystems than native desert.
The study showed a negative relationship between increase in urban area and synanthropy and a positive relationship between area and non-synanthropy. Further, urban area increases were positively associated with generalist feeding species but negatively correlated with insectivore species.
The results showed that bird guilds were consistently influenced by reserve characteristics than by surrounding land use, but land use did prove to be influential to certain guilds. Specifically, not only did the degree of urbanization matter, by the type.
The authors used a random sampling approach in this study. They assigned one random sampling point for each vegetation type per reserve. In areas that lack dense vegetation and are environments with minimal noise (as in urban and desert bird communitites) species are assumed to have equal detection probabilities. Each random sampling point received four bird counts: one during two breeding season per year, for two years. The reserves with quantified by five characteristics: area, shape, distance to outlying desert, distance to next fragment, and time since isolation. They assigned 14 labels to various fragments of the areas surrounding the reserves based on land use (e.g. tourist and visitor accommodations, low density residential, commercial, office, transportation, passive open space, industrial, agriculture). The authors also accounted for unequal sizes of different fragments by using rarefaction process common. They also evaluated birds by the quantity bird density rather than raw species richness. Because area available for conservation is generally limited, species density for such studies is considered more useful.
With a population of over four million people, as the fourteenth largest metropolitan population in the United States, and with the second fasted population growth rate in the country, it is important to understand how reserves in Phoenix are faring. This study helped show that reserves close to urban areas are useful for increasing the species density of synanthropic and insectivore species. However, if the goal is to conserve non-synanthropic and generalist species, reserves farther away from urban development and surrounded by environmental conditions closer to what is “natural” to the region may be more helpful.
This study found that area surrounding reserves up to 2500 m are important for determining target species. Urbanization has negative effects upon non-synanthropic and insectivorous species, however, and if urban planners and conservationists intend to use reserves to protect them, they should be as large as possible and as close to land use types that enhance their diversity. This study adds to the understanding that one cannot form a reserve and expect it to function as an isolated, ideal ecosystem. Rather, it functions as an island that is affected to some degree by its surroundings.
Litteral, J. and J. Wu. 2012. Urban landscape matrix affects avian diversity in remnant vegetation reserves: Evidence from the Phoenix metropolitan region, USA. Urban Ecosystems DOI 10.1007/s11252-012-0245-4.