Relationship between abundance and range size trends in North American breeding birds

Background/Question/Methods

When data on abundance are lacking, trends in the extent of the geographic range are often used as a proxy for trends in population size.  These trends are then used to estimate species extinction risks, for example through the IUCN Red List Criteria. The utility of this approach depends on the relationship between abundance and range size. We tested the relationship between trends in abundance and trends in range size using data from the North American Breeding Bird Survey (BBS) between 1981 and 2005. Range sizes were estimated based on the route locations where each species was recorded, after correcting for spatial and temporal bias in BBS route locations.  Abundances were estimated based on the number of recorded individuals of each species along each route.  For each species we tested the effect of calculating range size as the area of the minimum convex polygon (MCP) of occurrence locations or as alpha-hulls, which use the average distance between all occurrence locations to exclude areas likely to be unoccupied.

Results/Conclusions

Preliminary results indicate that there is no consistent relationship between trends in range size and trends in abundance across species, regardless of the method used to estimate range size. These results suggest that approaches using spatial occurrence data alone to identify species that may be at risk of decline and endangerment may be inadequate and that the relationship between range size and abundance is more complex than commonly assumed.