Why sparrow distributions do not match model predictions

A companion paper in this issue describes the mapping of Cape Sable seaside sparrow (Ammodramus maritimus mirabilis) habitat using satellite imagery. In general, those maps are correct. However, testing against an independent survey of the sparrow population does identify errors. Those errors fall into two categories, model errors and bird errors. The model errors result from genuine problems with the model. More interesting for conservation are the bird errors, which are more numerous. They are of two types. (1) Commission errors: Some suitable habitat does not contain birds. These are areas where prior events have depleted sparrow numbers, but because the sparrows do not disperse long distances, they cannot occupy it quickly. (2) Omission errors: Some birds rema in in unsuitable habitat that was formerly suitable. This results from the sparrow's very high site fidelity. Often, they will not leave an area even when it is no longer suitable. The consequences of these bird errors are that some habitat and some birds are not contributing to the species' survival. Thus, an estimate of only the amount of habitat or only the sparrow population may present an overly optimistic view of the sparrow's plight. © 2003 The Zoological Society of London.

Duke Scholars

Author Stuart L. Pimm Environmental Sciences and Policy