Effects of wrack composition, age, and cover on the spatial distribution of beach arthropods

Background/Question/Methods

Talitrid amphipods (Talitridae) and kelp flies (Diptera) are major processors of stranded wrack on sandy beaches and are extremely important in making carbon and nutrients available for consumers higher up on the trophic web, such as shorebirds, but the factors that drive their distributions are not fully understood. To test for resource partitioning in wrack-associated sandy beach macrofauna, I examined the effects of wrack pile age and composition on fly and talitrid amphipod distributions using a factorial experiment on a Northern California sandy beach. Observational surveys were used to look for correlations of animals in naturally occurring wrack piles with wrack composition, wrack cover, and distance from the swash zone. 

Results/Conclusions

Ephydrid flies were correlated with local wrack pile size while Fucellia rufitibia flies appeared to be correlated with broader algal cover on the beach. While both presumably feed on algae, algal cover on different spatial scales determined the distribution of these flies. The effect of experimental wrack pile age on animal abundances depended on the type of wrack, with the older piles of Cryptyopleura ruprechtiana and Zostera marina harboring more large flies (Fucellia rufitibia). Amphipods were found closer to the swash zone, where fresher, moister wrack tends to be. These differences in animal distribution patterns imply that talitrid amphipods and beach flies exhibit resource partitioning as a result of how their natural histories and traits interact with tidal and diurnal cycles. Spatial distributions of sandy beach macrofauna appear to be determined by many various abiotic and biotic factors.