The response of macrofauna communities and shorebirds to macrophyte wrack subsidies on exposed sandy beaches of southern California

Abstract

To investigate the influence of marine macrophyte wrack subsidies on community structure, relationships between community attributes, including species richness, abundance, and biomass of macrofauna and abundance of shorebirds, and a variety of factors, including the standing crop of wrack and beach morphodynamics, were examined on 15 exposed sandy beaches on the southern California coast. The beaches sampled were primarily modally intermediate morphodynamic types, and three were groomed regularly. Species richness, abundance, and biomass of the macrofauna were high compared to values reported for similar beach types in other regions and were not predicted by morphodynamics or other physical factors. Overall species richness and abundance, and the species richness, abundance, and biomass of wrack-associated fauna and selected taxa were significantly correlated with the standing crop of macrophyte wrack. Wrack-associated macrofauna, such as amphipods, isopods, and insects, made up an average of >37% of the species on ungroomed beaches and comprised 25% or more of the total abundance on half of those beaches. The abundance of two shorebird species, plovers that forage using visual cues, was positively correlated with the standing crop of wrack and with the abundance of wrack-associated invertebrates. Significant differences in community structure, including depressed species richness, abundance, and biomass of macrofauna, especially for wrack-associated taxa, were associated with beach grooming and provided strong evidence for the bottom-up effects of wrack subsidies. Grooming also reduced the prey available to vertebrate predators, such as shorebirds. Substantial ecological effects of the large-scale disturbance and removal of organic material, food resources, and habitat are associated with beach grooming. These results suggest that macrophyte wrack subsidies strongly influence macrofaunal community structure, higher trophic levels, and ecological processes on exposed sandy beaches. The supply of macrophyte wrack should be considered as a factor in ecological studies of exposed sandy beaches, particularly where macrophyte production is high.