Changes in biodiversity and species associations along a latitudinal gradient

Abstract

Biodiversity is currently threatened at local, regional, and global scales, and identifying the species that are vulnerable to these changes is essential for conservation efforts. For example, the breadth of species’ ranges may offer important clues to their susceptibility to loss, as widespread species may be more resistant to loss than species with a narrower range breadth. To determine the potential for shifts in community interactions along the latitudinal geographic ranges of species, we examined pair-wise associations between narrow-range endemic and widespread rocky intertidal species. We surveyed rocky intertidal species composition at eight sites along the California coast from San Diego to Cape Mendocino. Four sites were south of Point Conception, and four sites were north of Point Conception. Point Conception is a major biogeographic feature for coastal marine species, where sea surface temperatures transition from cool temperate waters in the north to warm temperate waters in the south. To determine whether pair-wise species associations were significant, we compared the observed communities’ standardized effect size to a null model to determine which species occurred together more or less often than by chance. Across all sites, widespread species were considerably more abundant than narrow-range endemic species, and the majority of species were widespread. However, total species richness was unrelated to the number of widespread species, and was, instead, determined by the number of narrow-range endemic species present at a site. Our analyses suggest that species are more aggregated than segregated south of Point Conception, but the opposite is true north of Point Conception. Additionally, we found that species associations between narrow-range endemics drove the overall patterns in species associations. One possible explanation for these patterns is that positive interactions, especially those involving narrow-range endemic species, are more important in southern California’s more thermally stressful intertidal habitats.