Spatial patterns of sandy beach habitat use by mobile invertebrates vary with wrack type and tide phase

Abstract

Habitat distributions of many coastal species are expected to shift with climate change and sea level rise, affecting local resource use and biotic interactions. Intertidal zones of sandy beaches support a diversity of highly mobile invertebrates that move frequently to adjust to changing tides as well as habitat availability and condition. Many of these species rely on allochthonous inputs of marine macrophytes, or wrack, for food and shelter. We evaluated the effects of habitat availability and resources on spatial niche partitioning by macroinvertebrate detritivores and predators using field surveys and a field experiment on wrack colonization and consumption. Semi-lunar tide phase greatly affected the amount of suitable habitat (54 % more on spring tide) and the mean position of wrack subsidies on the beach (34 % more landward). Predator and detritivore distributions overlapped, but predators were concentrated higher on the beach. From neap to spring tide, the mean intertidal positions of wrack detritivores expanded more (biomass 49.8 %; abundance 45.4 %) than predator positions. In the experiment, significantly more kelp wrack, Egregia menziesii and Macrocystis pyrifera, was consumed than seagrass wrack, Phyllospadix torreyi, at all intertidal levels. Wrack-associated invertebrate community composition differed among wrack types and intertidal levels. Our findings demonstrate how highly mobile intertidal macroinvertebrates partition beach habitat and food resources, with notable spatial separation between detritivores and predators, and how this behavior can be mediated by tide phase and wrack type and position. Linking spatial dynamics of mobile intertidal animals to future projections of habitat availability and resource supply can inform management of sandy beaches with relevance for their conservation as ecosystems in the face of global change.