December 21, 2005

Habitat area and species extinction

Larger areas harbor more species. For some groups of species and types of habitats species-area curves are so well documented that they allow us to predict the number of species in an unstudied area only by knowing its size.

Species-area curves also allow us to guess how many species disappear from a given region when human activities reduce the size of suitable habitat.

For example, using this method E. O. Wilson estimated that 50% of tropical rainforest species will disappear with each 90% loss of forest area.


If most populations were originally globally rare but locally abundant, then depending on how the fragmentation process proceeds, many populations would remain abundant if local patches were protected. If enough of these patches were protected, then global species richness would not decline as much as predicted.
This is from a study by Brian Wilsey, Leanne Martin and Wayne Polley published in the current issue of Conservation Biology. They studied the effect of habitat heterogeneity on species richness in a set of prairie fragments in Iowa, where humans have converted 99.9% of the original prairie area to other uses. They found that there are more native plant species (491) than models of uniformly distributed species predict (27–207). "Even tiny remnants continued to support a large number of native species."

Second, species-area curves tell us nothing about the time dimension. Some populations may dwindle to extinction for a long time. Aveliina Helm, Ilkka Hanski and Meelis Pärtel found that the current number of habitat specialist plant species in 35 calcareous grasslands (alvars) in Estonia reflected not the current sizes of their habitats but those of 70 years ago, before 70% of alvar area disappeared due to changes in land use (published in the current issue of Ecology Letters).
We estimated the magnitude of extinction debt at around 40% in individual alvars, corresponding to predicted loss of around 20 vascular plant species per alvar in the future. With current landscape structure, many of these species will be lost from the entire region, although this will be an even slower process than extinction from individual alvars.
If the causes of habitat destruction cease and the ecosystem returns to its original condition populations may recover before going extinct.

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