Projected impacts of climate and land-use change on the global diversity of birds

Abstract

Over the past few decades, land-use and climate change have led to substantial range contractions and species extinctions. Even more dramatic changes to global land cover are projected for this century. We used the Millennium Ecosystem Assessment scenarios to evaluate the exposure of all 8,750 land bird species to projected land-cover changes due to climate and land-use change. For this first baseline assessment, we assumed stationary geographic ranges that may overestimate actual losses in geographic range. Even under environmentally benign scenarios, at least 400 species are projected to suffer >50% range reductions by the year 2050 (over 900 by the year 2100). Although expected climate change effects at high latitudes are significant, species most at risk are predominantly narrow-ranged and endemic to the tropics, where projected range contractions are driven by anthropogenic land conversions. Most of these species are currently not recognized as imperiled. The causes, magnitude and geographic patterns of potential range loss vary across socioeconomic scenarios, but all scenarios (even the most environmentally benign ones) result in large declines of many species. Whereas climate change will severely affect biodiversity, in the near future, land-use change in tropical countries may lead to yet greater species loss. A vastly expanded reserve network in the tropics, coupled with more ambitious goals to reduce climate change, will be needed to minimize global extinctions.

Figure 1. Projected Range Transformations

Frequency distribution of projected range transformations for 8,750 bird species by 2100 across the four MA socioeconomic scenarios (histograms, left axis). Within each bar, the height of the shading refers to proportion of projected transformation due to habitat conversion (dark gray) or climate change (light gray). The count of species with exactly zero range transformations is represented as open bars and separated by a gap from the bars representing >0 to ≤100% range transformation. Points with error bars give the average (± SE) current geographic range size (in km²) for species in each range transformation category. Although none of the scenarios is likely to predict the actual pattern of land-cover change, they provide “broad confidence limits” that are likely to span the range of possible futures. The four scenarios are briefly described in Materials and Methods.

Figure 2. Geographic Patterns and Projected Impact of Environmental Change

(A, B) Patterns of change in land cover due to land-use and climate change by 2100. (C, D) The resulting potential impact for birds: the pattern of richness of species with projected range declines ≥50%. This represents the summed, current-day occurrence of qualifying species across a 0.5° grid. Patterns are given for the environmentally proactive “Adapting Mosaic” scenario (A, C), and the environmentally reactive “Order from Strength” scenario (B, D). Maps are in equal-area cylindrical projection. Colors in C and D vary from dark blue to dark red, and the legends provide labels for select colors along this continuous scale (minimum, ~1/3, ~2/3, maximum of data).

Figure 3. Environmental Change, Avian Biogeography, and Loss in Range Size

Projected latitudinal pattern in type of global environmental change, geographic range size, species richness, and the resulting loss in geographic range size (8,750 bird species, 1° bands of latitude). Climate (cyan, on top and semitransparent) and land-use (red) changes between now and 2100 are evaluated for two scenarios: on the left, “Adapting Mosaic” (A, C, E), and on the right, “Order from Strength” (B, D, F). Top (A, B): Total area transformed (area plot, lighter color indicates overlap) and average (± SE) current geographic range size of species per latitudinal band (point and line plot); Middle (C, D): Average proportional loss of range size (area plot, lighter color indicating overlap) and total number of bird species whose range currently overlaps at each latitudinal band (point and line plot). Bottom (E, F): Count of species with ≥50% of range transformed jointly by climate change or land-use change (stacked area plot, coloration indicates the proportion of range area that is transformed by each land change type). Whereas climate change leads to a significant net change of habitat in the polar and temperate regions, the small numbers of bird species that live there on average have very large geographic ranges. Thus, proportional contractions in range size there are much smaller than for the vast majority of bird species that live in the tropics and experience significant reductions in their smaller range sizes due to land-use change. The outcome are many species with significant range reduction in the tropics and subtropics, because of the coincidence of habitat conversion with areas of high species richness. This is particularly the case in the environmentally reactive “Order from Strength” scenario, where large areas of land are converted to agriculture.