Extinction is more likely on the edge

The world is losing its biodiversity, but it’s not losing it in a particularly uniform way. Some places are losing it faster, and some places are losing it slower. North America, for example, has had only one native mammal go extinct since European colonization: the sea mink. Certain parts of the tropics are losing their wildlife a lot faster. It turns out that one of the patterns that drives the speed of extinction is geometry.

That’s according to a new study published yesterday in the journal Nature Communications. A group of American and Czech researchers led by Yale University biologist Petr Keil describe the two primary ways by which conservation biologists estimate species loss. The “species-area relationship,” or SAR, is calculated by comparing the size of a given landscape and the number of species found within it, while the “endemics-area relationship,” or EAR, compares the size of a given landscape and the number of species confined to it. When asking how many species are lost when a certain area is lost, both methods can yield inaccurate estimates.

That’s because areas lost can either be functionally identical to, or different from, those neighboring areas that remain. Perhaps a certain kind of monkey prefers to nest in one area but forage in another, but is fairly flexible in terms of its diet. If it lost the part of the forest where it feeds, the monkey might just survive. If it lost the parts of the forest where it nests, then it might not. In addition, it isn’t entirely clear what ecological role small-scale plots play in coming together to comprise larger landscapes.

Keil and his team noted that habitats can be lost in different spatial or geometrical patterns. Is extinction more likely if habitats are lost from the edge of a landscape toward the center or if they’re lost starting from the center? What if habitat loss is not quite so orderly, but more random? To find out, the researchers created four theoretical models that combined the geometry of habitat loss with species extinction, then compared them to bird, mammal, and amphibian distributions in nine real world regions to see which was most useful at predicting the impacts of habitat loss. In particular, they wanted to challenge an argument made by researchers Fangliang He and Stephen P. Hubbell in Nature that “species–area relationships always overestimate extinction rates from habitat loss.”

What Keil’s group found was that the SAR method sometimes gave higher estimates of biodiversity loss than the EAR method, but also sometimes gave lower estimates. And that depended on the arrangement of species ranges within the landscape in question.

Of greater importance to those who aren’t just biogeography theory wonks, the researchers also discovered that extinctions are more likely to occur when landscapes are destroyed from the outside edge towards the center (such as in sea level rise) than if habitats are lost in the reverse direction (such as in urban sprawl). And the geometrical direction of habitat loss actually played a stronger role in determining the loss of species richness than the overall area lost. In other words, a little habitat loss on the edge of a region was worse for biodiversity than more habitat loss towards the center.

Of course in the real world, habitat loss never proceeds in such an orderly manner, and at this point habitat loss is a reality we have to contend with and plan for rather than a trend that can be reversed. Still, understanding the nuanced ways in which habitat loss leads to species loss is critical for land managers, conservationists, and urban planners, as they assess the potential impacts of human development against a background of climate change. – Jason G. Goldman | 18 November 2015

Source: Petr Keil, David Storch & Walter Jetz. (2015). On the decline of biodiversity due to area loss. Nature Communications, 6, 8837. DOI: 10.1038/ncomms9837.

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