The relative influence of temperature, moisture and their interaction on range limits of mammals over the past century.

Aims I determine the relative importance of temperature and moisture acting alone and in tandem for range contractions and expansions of mammalian species over c. 70 years. Location The contiguous United States west of the eastern border of the Rocky Mountains (103.77° W). Methods Museum records of 67 mammalian species from two time periods (1900-39 and 1970-2009), mean annual temperature and mean annual precipitation are used to model historic and modern distributions. For each era the region outside each species' range in the opposing era is assigned to zones of range limitation: temperature alone is prohibitive, moisture alone is prohibitive, both preclude the species, or both have values that alone are within the species' tolerance range but in combination are outside its climatic niche ('interaction' limitation). The relative importance of each type of limitation is estimated by their intersection with regions of range contraction and expansion. Results Averaged across species, 82 ± 3% (± SE) of the areas of contraction are now occupied by interaction limitation, and 74 ± 5% of areas expanded into were historically occupied by similar zones. In comparison, areas of range shift in which just temperature or moisture acting alone would have excluded the species comprise between only 9 and 17%, and areas in which both factors are limiting are negligible. Expansions into and contractions from regions of interaction limitation occurred more often than expected by chance given the distribution of this type of limitation in the wider study region. Main conclusions Particular combinations of temperature and moisture can interact directly or indirectly to limit species ranges even when these factors alone do not exceed species tolerances. Change in the correlation between of range-limiting factors can have as much effect on species ranges as absolute change in those factors by themselves. [ABSTRACT FROM AUTHOR]