| Shorebirds migrate long distances between breeding and wintering grounds, stopping to feed and rest at wetlands along the way. Half of the 50 shorebird species in North America have declined, in parallel with declines in wetland habitat. Current conservation efforts target wetland sites with spectacularly large numbers of shorebirds, an approach that favors aggregated and abundant species. However, it is unknown which species benefit from this approach, as continental space-use by shorebirds is poorly understood. Further, there has been no attempt to evaluate whether space-use during migration has changed systematically through time, or whether it is correlated with species status.; I used Camargo's Index of Evenness (selected from four candidate indices: Simpson's Index of Evenness, Lloyd's Mean Index of Crowding, Smith-Wilson Index, and Dispersion Index) as a metric for quantifying spatial pattern. Based on Montecarlo simulations, Camargo's out-performed the others in its sensitivity to underlying differences in spatial distribution (ranging from even to patchy), and robustness to incomplete or variable sampling effort/design. Using a 26-year database of 36 shorebird species surveyed at 2000 wetland sites throughout the conterminous United States, shorebirds were generally aggregated among the wetlands surveyed but species varied significantly in extent of aggregation. Species exhibited similar within-species dispersion patterns during fall and spring and among months of a season, suggesting that space-use of wetlands is intrinsic to each species. Grouping species by body size, migration distance or habitat affinity did not reveal spatial assemblages in either season, nor did cluster analyses comparing distributions among all species.; Nineteen (53%) species demonstrated linear trends in space-use through time; most became more dispersed. Species classified as coastal specialists displayed more coherence than either inland specialists or generalists, and correlations between spatial dispersion and established indices of population status revealed that a higher proportion of imperiled species demonstrated directional changes in dispersion than non-imperiled species. The lack of overlap in space-use among species with similar ecology and changes in spatial dispersion through time suggest that current approaches to shorebird conservation should be broadened to account for space-use differences among and within species. |
