All species are limited to a particular geographic area, whether of great or small extent, by historical contingency and interactions between extrinsic biotic or abiotic environmental factors, and intrinsic dispersive and adaptive traits. Distribution margins often occur on ecological gradients, along which the species' range may expand or contract over time. If the distribution of a species is reflective of its adaptive niche, then marginal areas will act as population "sinks" with reduced habitat quality. However, if environmental conditions change at distribution margins so that they are no longer limiting, fitness should improve and result in a range expansion. The general objective of this dissertation research was to elucidate the nature of distribution limits by studying their causes and consequences in marginal populations. Distribution limits of breeding birds were investigated along an arid shrubland gradient in southern California undergoing rapid climate warming. Through the use of multi-scale surveys and historical data, the comparative structure and dynamics of marginal distributions among avian species were related to environmental factors. Elevational distributions were remarkably consistent across three years (2005-2007) and three elevational transects (200-1800 m), with strongly individualistic environmental associations among species. However, over a 26-year period, five species (out of 28 tested) showed statistically significant distribution shifts, all upward in elevation. The average elevational shift for all 28 species was an increase of 116 m, and low-elevation desert species were most likely to show upward shifts. Among species, individual productivity tended to increase with elevation, regardless of whether the species' lower or upper distribution limit occurred along the study gradient. For a focal species, the Black-throated Sparrow (Amphispiza bilineata), breeding success tended to be greater at higher-elevation chaparral sites at the distribution margin compared to lower-elevation desert scrub sites where the bird was more common, but this species did not show an upward elevational shift over a 26-year period. Desert species may be most likely to be at or near their temperature and aridity tolerance limits within their current range, and range shifts can be delayed or prevented by decoupled environmental factors.
