Breeding Success At The Range Margin Of A Desert Species: Implications For A Climate-Induced Elevational Shift

If the breeding range of a species is limited by biotic or abiotic environmental factors that depress breeding success at the range margin, then range expansion is expected when those limiting factors are alleviated. Over a three-year period, we measured breeding success of a desert species, black-throated sparrow Amphispiza bilineata, along a steep elevation gradient between the Peninsular Mountains and Colorado Desert (San Diego County, California) that is undergoing a warming trend. We compared breeding success at geographically marginal locations (higher-elevation chaparral sites) to more central locations (lower-elevation desert scrub sites) only a short distance apart. Breeding success was measured at the nest level, territory level, and population level. At each level measured, breeding success tended to be greater at higher-elevation chaparral sites at the distribution margin compared to lower-elevation sites where the bird was more common. Black-throated sparrows had 100% reproductive failure at lower-elevation sites during the two driest years of our study (2006-2007), but did relatively well at higher-elevation sites. Only in a wetter year (2008) was breeding success improved at lower-elevation sites. Surprisingly, there was no evidence of an upward elevational shift in distribution over a 26-year period despite a clear warming trend and drier conditions. Greater territory density at lower-elevation sites with reproductive failure during dry years suggests the possibility of an ecological trap in this system, which could prevent or delay climate-induced range shifts. A common presumption has been that desert species will undergo relatively mild negative impacts due to a warming climate, but it is possible that some desert species are already at or near their temperature and aridity tolerance limits within their current range and shifts may not always be possible.