September 17, 2012
The endangered tomato frog in Madagascar. Photo by Rhett A. Butler.
City populations are expected to grow by five billion people and expand by 1.2 million square kilometers by 2030. Much of this expansion is forecast to occur in the tropics, which contain the bulk of the world's species. The new study attempts to quantify the impact of urbanization on the world's so-called "hotspots" — nearly three dozen areas with exceptionally high levels of species found no where else.
Using data from a variety of sources, researchers at Yale University, Texas A&M University, and Boston University developed a probabilistic model for estimating the impacts of urbanization on vegetation, carbon stocks, and threatened species. They found that by 2030, nearly three percent of hotspot areas will be urbanized, up from one percent in 2000. While the extent seems small, paving over marshes, forests, and grasslands could generate 1.38 billion tons of carbon emissions (5 billion tons of CO2) from direct land use change. Some 214 species currently listed as endangered and critically endangered and considered focal species by the Alliance for Zero Extinction (AZE) would be affected by urban expansion, including 20 — 15 of which are amphibians — that would experience complete urbanization of their habitat.
The biggest increase in hotspot urbanization is forecast in Africa and Asia, specifically the Eastern Afromontane, the Guinean Forests of West Africa, the Western Ghats and Sri Lanka hotspots, according to the study. The biggest biodiversity impacts would occur in Africa and Europe.
"Africa and Europe are expected to have the highest percentages of AZE species to be affected by urban expansion, 30% and 33%, respectively," the authors write. "However, the Americas will have the largest number of species affected by urban expansion, 134, representing one quarter of all AZE species in the region."
The study however does not capture the full impact of urban expansion on biodiversity or carbon emissions. Urban expansion is being fueled by rural abandonment, which in some places is enabling natural vegetation to regenerate, sequestering carbon and potentially restoring habitat for wildlife. But on the flip side, city-dwellers generally have a larger carbon footprint than poor farmers, consuming more commodities that are often sourced from conversion of lowland habitats, which store more carbon than the upland habitats where natural vegetation is typically regenerating, according to other studies. Therefore the overall impact of urbanization is difficult to estimate, as noted by the authors.
"The analysis in this article only examines the direct spatial 'imprint' of urban expansion on biodiversity hotspots, AZE species, and carbon biomass, and not the indirect land-change processes that both drive and respond to urbanization. Urban expansion can also affect land uses in distal places, which in turn can alter carbon stocks, especially in the tropics. This 'indirect' urbanization affect is difficult to fully quantify," they write.
"In some cases, it will amplify and in other cases attenuate carbon losses. We know that cities have always relied on their hinter- lands and other distal places for resources from food and fuel to waste assimilation. For example, a typical household in Sydney or Melbourne is responsible for greenhouse gas emissions, water withdrawals, and land use distributed across all of Australia. The magnitude of the virtual carbon, water, and land embodied in urban areas means that the bulk of environmental impacts from future urban expansion is also likely to occur outside of the areas forecasted to become urban."
The authors conclude by arguing that while "urbanization is often considered a local issue", their study shows "shows that the direct impacts of future urban expansion on global biodiversity hotspots and carbon pools are significant."
"At the same time, the full environmental impacts will not be confined to urban boundaries and will largely be felt elsewhere."
CITATION: Karen C. Seto, Burak Buneralp, and Lucy R. Hutyra. Global forecasts of urban expansion to 2030 and impacts on biodiversity and carbon pools. PNAS Online Early Edition for the week of September 17-21, 2012.
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