Aim Over 250 species of obligate terrestrial cave-dwelling animals (troglobionts) are known from single caves in the eastern United States. We investigate their geographical distribution, especially in relation to other troglobionts. We relate these patterns to taxonomic group, opportunities for dispersal and geographical location.
Location Caves of the United States east of the Mississippi River.
Methods We associated over 3000 records of more than 450 troglobiotic species and subspecies with hexagons of 1000, 5000 and 10,000 km2 in size. We calculated Moran's I, black–white joins and cubic regression of endemics on non-endemics at all three spatial scales. For 5000 km2 hexagons, we modelled the spatial autocorrelation of the residuals of the cubic regression of endemics on non-endemics.
Results Differences among orders in percentage single-cave endemism were not significant, except for Pseudoscorpionida, which was higher (69%) than any other order. At all three scales, Moran's I and black–white joins were significant, indicating a clumped distribution of both single-cave endemics and other troglobionts. Spatial patterns were similar at all three scales and Moran's I was highest at 5000 km2. The cubic fit of endemics to non-endemics was consistently better, with less systematic error or residuals, than were linear or quadratic models. Residuals showed a significant geographical pattern with excess endemics in more southerly locations.
Main conclusions There was both a non-spatial and spatial component to the pattern of single-cave endemism. The non-spatial component was the association of high levels of single-cave endemism with areas of high diversity of non-endemics. It may be that both are high because of high secondary productivity. Spatially, single-cave endemism is high in central rather than peripheral areas and in the southern part of the range. It is not higher in areas of more dissected limestone, which would reduce migration rates; if anything endemism is lower. Regional spatial effects are important, indicating that cave communities cannot be understood (or protected) in isolation.