The distribution and abundance of a species are often associated with environmental gradients. If the breeding distribution of a species is limited by biotic or abiotic environmental factors along a gradient, then reduced breeding success is expected at distribution margins resulting in reduced abundance. However, if environmental conditions change at distribution margins so that they are no longer limiting, breeding success should improve and result in a range expansion. We measured breeding success of Black-throated Sparrows (Amphispiza bilineata) along an elevation gradient between the Peninsular Mountains and Colorado Desert (San Diego County, California) in a region with a gradual warming trend. We compared breeding success at marginal locations (higher-elevation chaparral sites) to those at central locations with more typical habitat (lower-elevation desert sites) over a three-year period under varying climatic conditions. Breeding success was measured at the nest-level, territory-level, and population-level.

Results/Conclusions

At each level measured, breeding success tended to be greater at higher-elevation chaparral sites at the distribution margin, and was more strongly dependent on rainfall at lower-elevation desert sites. At lower-elevation sites, Black-throated Sparrows had 100% reproductive failure during the two dry years of our study (2006-2007), but did relatively well at higher-elevation sites. Only in a wetter year (2008) was breeding success slightly greater at lower-elevation sites. If the climate continues to become warmer and drier in this system, then the distribution boundary of this species is expected to track upward in elevation as a result of greater breeding success at higher-elevation sites; however, there has been little evidence of an upward shift. Lags in climate-induced boundary shifts can be due to several potential causes, including decoupling of local environmental factors and range-wide population dynamics.