Ecological Correlates and Conservation Implications of Overestimating Species Geographic Ranges

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Abstract
Abstract:  Species range maps based on extents of occurrence (EOO maps) have become the basis for many analyses in broad‐scale ecology and conservation. Nevertheless, EOO maps are usually highly interpolated and overestimate small‐scale occurrence, which may bias research outcomes. We evaluated geographical range overestimation and its potential ecological causes for 1158 bird species by quantifying EOO map occurrence across 4040 well‐studied survey locations in Australia, North America, and southern Africa at the scale of 80–742 km2. Most species occurred in only 40–70% of the range indicated by their EOO maps. The observed proportional range overestimation affected the range‐size frequency distribution, indicating that species are more range‐restricted than suggested by EOO maps. The EOO maps most strongly overestimated the distribution of narrow‐ranging species and ecological specialists with narrow diet and habitat breadth. These relationships support basic ecological predictions about the relationship between niche breadth and the fine‐scale occurrence of species. Consequently, at‐risk species were subject to particularly high proportional range overestimation, on average 62% compared with 37% of nonthreatened species. These trends affect broad‐scale ecological analyses and species conservation assessments, which will benefit from a careful consideration of potential biases introduced by range overestimation.