Investigating biotic and abiotic factors affecting the recruitment of an intertidal clam on an exposed sandy beach using a generalized additive model

The existence of biotic interactions among macroinfauna resident on exposed sandy beaches and the potential roles that such interactions might play with respect to structuring these assemblages is currently a controversial topic. To elucidate the relative importance of biotic versus abiotic factors in determining the spatial distribution of beach fauna, we use a generalized additive model, which can account for nonlinear relationships between beach clam recruitment and four explanatory variables: two abiotic (beach gradient and distance from a major river mouth); and two biotic (the abundance of adult clams and the abundance of their juveniles). The study organism, Donax serra Röding, is a fairly typical intertidal filter feeder that dominates the macrofaunal communities on many southern African sandy beaches. Results indicate that a mixture of biotic and abiotic factors mediates recruit abundance, with beach gradient being the most influential, followed by adult and juvenile abundances. Of particular importance was strong evidence of intraspecific interactions between D. serra adults and recruits. Furthermore, despite the existence of a published hypothesis stating that the highest recruitment of this species is found beside river mouths and that the subsequent alongshore distribution of older age classes is determined primarily by distance from a major river mouth, we found the opposite. Lowest recruit abundance was in the vicinity of a river mouth and maximum recruit abundance was near the center of the beach. Distance from the river mouth was also less important in structuring the distribution of recruits than the biotic factors. This emphasizes the complex ecology of organisms inhabiting apparently simple intertidal landscapes, and illustrates how the generalized additive modelling approach can be fruitfully applied to problems in beach ecology