Inter-specific variability in demographic processes affects abundance-occupancy relationships

Interspecific abundance-occupancy relations along estuarine gradients

Do interspecific abundance-occupancy (A-O) relationships vary systematically along environmental gradients? A-O relationships of macrobenthic assemblages of seagrass and adjacent bare-sediment were compared at series of sites along two types of estuarine gradient; the longitudinal one of the main axial channel, and a transverse one from that channel into progressively smaller channels and creeks enclosed within fringing saltmarsh. Three general features emerged: A-O regression slopes were remarkably uniform across all the disparate assemblages and sites (1.04 ± 0.08); values of R2 were very high (≈0.96); and where small but significant differences between sites occurred, they were associated with the stronger gradients (i.e. those with the larger differentials in abundance and smaller compositional similarities). In two such cases, slope values were correlated with position along the gradient (although only at the margin of statistical significance). Variation in A-O relationships along estuarine gradients does clearly occur, but their constancy leaves the stronger impression.