Risks in Large Populations: The Cape Sable Sparrow as a Case History
Pimm SL and Bass OL. Risks in Large Populations: The Cape Sable Sparrow as a Case History
A draft copy of the report discussing population species extinction rates and risk. Very small populations- those numbering a few to a few dozen breeding pairs-often go extinct quickly. The reasons for their doing so are well-understood and relatively easy to model. Considerable experience teaches that much larger populations can become extinct quickly too. Understanding the fate of these species is the much more difficult challenge that this paper will address. The species of concern is the Cape Sable sparrow. We explore two methods of calculating the sparrow's risk of extinction. The first employs the idea that one can characterize the natural limits of population size fluctuations over time. So armed, one can predict whether the lower limit will encompass such low levels that rapid extinction will be probable. This is a familiar recipe. We show that this method failed spectacularly even when applied to a situation where it would seem entirely appropriate. Our second method identifies the causes of the sparrow's population fluctuations. By understanding the mechanisms underlying population fluctuations we deduce an altogether bleaker picture of the bird's future.