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Wildlife ecologists are frequently asked to assess the effects of urbanization on wildlife resources. Converting natural open space to urban landscapes invariably results in changes in the habitats that are available to wildlife. But by definition, "habitat" is a species-specific concept and wildlife managers have found it very difficult to assess and compare the impacts of different land use decisions on wildlife in general.
For these reasons, we tend to focus on key species that have special significance because of their biological vulnerability or because of their significance to society. For example, in Tucson recent or ongoing studies are describing urban and suburban populations of coyotes, Harris hawks, Cooper's hawks, bird communities, deer, javelina, burrowing owls, and other species. Each of these studies helps to fill gaps in information needed to evaluate urban land use decisions. But they represent only a small sample of the species that use metropolitan environments and even the most optimistic among us cannot expect that we will have comprehensive studies for all urban vertebrates in the foreseeable future. For this reason, we need to complement these species-oriented studies with an approach to biological assessments that can generalize beyond individual species and provide land use decision-makers with a basis for evaluating alternative landscapes in terms of their effects on wildlife-in-general.
The results presented below are selected from a study of the vegetative characteristics of different urban landscapes in the Tucson Metropolitan area (Shaw et al. 1996). That study, funded by the Arizona Game and Fish Department, Heritage Fund, developed a comprehensive GIS database of land covers in Tucson. Representative samples of 33 landscape types were selected and a variety of vegetative data was gathered about each. This paper focuses on two of those vegetative parameters that have very important implications for wildlife: % vegetative cover and % native plant vegetative cover. top
All life is ultimately dependent on the conversion of solar energy into plant materials that in turn sustain animals and fuel complex ecosystems. Ironically, in spite of the growing interest in urban conservation issues, until recently we have not had answers to fundamental biological questions like: How do different metropolitan landscapes compare in terms of primary productivity? Although empirical measures of primary productivity for different metropolitan landscapes are not feasible, a reasonable index of this parameter is the percentage of ground covered by perennial vegetation. By comparing the percentage ground cover for different landscape types, we can produce a rough index of the biomass of animal life that is might be sustained in each land use category. top
Using this approach, our study revealed that among the 33 land cover categories in Tucson, golf courses and neighborhood parks have the highest vegetative cover (93% and 77 % respectively). Interestingly, even schools (45%) and moderate density housing (1–3 houses/acre, 44%) have a higher % vegetative cover than natural open space in this semi-arid region. Perennial vegetative cover in the natural upland areas in this region averaged only 22%.
Results such as these may initially appear to be a developer's dream because they imply that golf courses are more valuable biological communities than the natural open space they supplant. But the goal of conservation is not simply to maximize biological productivity. Wildlife conservation in metropolitan environments is concerned with providing habitats for the whole spectrum of wildlife species native to an area. Golf course vegetation is dominated by exotic grasses and other non-native plant species which, while they may be utilized by some native wildlife, generally lack the diversity of species that are present in natural communities. Furthermore, some types of native wildlife are very specific in their requirements for native plant communities. These types of animals tend to drop out of communities when exotic plants replace natives. For this reason, if your goal is to promote native wildlife, it is important to consider the % of vegetative cover that is attributable to native plant species. top
When only native plant species are considered, the rankings of land use categories differ markedly from the total vegetative cover rankings. Riparian areas, low density housing, and natural open space areas have the highest percentages of native plant cover. It is interesting to note that low density housing (<1 house/acre) ranks higher (42%) than natural open space (22%). This is probably attributable to the heavily landscaped and irrigated areas near homes and to the enhanced vegetative growth that results from increased runoff created by roofs, roads, and other hard surfaces. top
Obviously, total vegetative cover and native vegetation are only two of many plant community attributes that determine which species of wildlife can utilize a given urban landscape type. Other important factors include structural diversity, plant species diversity, presence of escape cover vegetation, etc. Collectively, knowledge about these attributes provides the foundation for understanding and comparing the values of different urban and suburban landscapes for wildlife in general.
In this study, we developed a simple model of wildlife habitats values built upon the assumption that the objective of urban conservation efforts is to provide habitats for the spectrum of wildlife types native to this region. That model utilized four vegetative variables: total vegetative cover, native vegetative cover, a structural diversity index, and a measure of plants which may provide escape cover for wildlife (i.e. shrubs and trees with branches and foliage near the ground surface). Using this model, riparian areas are clearly the most valuable habitats in Eastern Pima County. Other high ranking landscape types include low density housing (<1 house/acre), natural open spaces, and federal/state parks and forests. Furthermore, when these values are portrayed in a spatial format using GIS maps, land use planners and wildlife managers can examine patterns of habitat distribution and identify gaps and isolated habitats. top
Although in many situations, urban wildlife conservation efforts will continue to focus on individual species, this general approach provides urban land planners and developers some ability to compare the implications of different land use decisions for wildlife-in-general. Based on this model, we offer the following recommendations for incorporating wildlife conservation into planning for Tucson and other Cities in the Sonoran Desert region:
Shaw, William W., Lisa K. Harris, Margaret Livingston, Jean-Paul Charpentier, and Craig Wissler. 1996. Pima County Habitat Inventory: Phase II. Report to Arizona Game and Fish Department. 94pp + ARCINFO database.