Loss Of Avian Diversity Climate Change Versus Habitat Loss

We have used the geographic distribution database for birds described above to evaluate potential impacts of projected environmental change on each of the major continents (Jetz et al., 2007). The Millennium Ecosystem Assessment (MEA) used four quantitative scenarios to examine how land cover would change across the land surface of the Earth over the next 50 and 100 years (Alcamo et al., 2005; Carpenter et al., 2005). The scenarios were driven by quantitative climate models derived from the Intergovernmental Panel on Climate Change (IPCC) and projections of human population growth, wealth, and other socioeconomic parameters across regions (Image_Team, 2001). In these projections, rates of land conversion would be driven either by climate change or by the need for new agriculture land. The four MEA scenarios were defined by whether or not governments take a proactive or reactive response to environmental management, and by whether the world's nations become more unified and interactive or they become more protectionist and isolated (Cork et al.,

2005). Jetz et al. (2007) used the output from the scenarios to examine the

potential impact on the world's land-bird species under the simplifying assumption of stationary geographic ranges.

Projections of land-use change based on the different MEA scenarios have revealed consistent geographical patterns of impact. The projections differ mainly in the magnitude of their impacts, with the reactive and isolationist scenarios experiencing about twice the rate of habitat conversion as the scenarios for proactive and connected worlds (Carpenter et al., 2005). In all cases, the impacts of climate change in the next 50 to 100 years are largest in polar regions. Although climate change also has effects in the temperate and tropical zones, these are almost completely masked by human agricultural expansion, particularly in the tropics. This pattern of land-use change will interact directly with the geographical variation in the range sizes of bird species. In particular, bird species with small ranges are at a much greater risk of extinction than those with large geographical ranges (Jetz et al., 2007). Unfortunately, most avian species living in the tropics have small ranges and a significant number will experience large declines in range size due to agricultural habitat conversion. In contrast, the minority of species that live in the polar zones are projected to experience large potential loss of range due to climate change, but they usually have sufficiently large geographical ranges that some of their environment remains habitable (Fig. 4.5) (Jetz et al., 2007).

FIGURE 4.5 The relationship between geographic range size and percentage range transformations for all of the world's 8,750 land birds under two MEA scenarios of future land-use change. (Left) "Adaptive mosaic'' (which assumes a world with open political dialogue that deals proactively with environmental problems). (Right:) ''Order from strength'' scenario (which assumes a more insular political world that only deals retroactively with environmental problems). Jetz et al. (2007) provide complete detail for how the analyses were developed. The dots illustrate number of avian species, lightest shading denotes range change due to climate change, and black illustrates land-use change due to agricultural expansion.

FIGURE 4.5 The relationship between geographic range size and percentage range transformations for all of the world's 8,750 land birds under two MEA scenarios of future land-use change. (Left) "Adaptive mosaic'' (which assumes a world with open political dialogue that deals proactively with environmental problems). (Right:) ''Order from strength'' scenario (which assumes a more insular political world that only deals retroactively with environmental problems). Jetz et al. (2007) provide complete detail for how the analyses were developed. The dots illustrate number of avian species, lightest shading denotes range change due to climate change, and black illustrates land-use change due to agricultural expansion.

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