Global drifters: the ecological role of non-native macroalgae as beach wrack subsidies

Abstract

Wrack macroalgae play a key ecological role in beach ecosystems, recycling nutrients, reinforcing trophic linkages between sea and land, and strongly shaping consumer populations and food web dynamics. However, the consequences of changing wrack resources for macroinvertebrate communities remain uncertain. We manipulated in situ the availability of eight macroalgae species of different origin (natives vs non-native) and morphological structures (simple vs complex) to test hypotheses about how changes in the type and number of macroalgae contributing to wrack detrital resources might affect beach macroinvertebrate assemblages. Wrack-associated macrofauna assemblages and total abundance were not affected by the presence of non-native wrack. We highlight that the ecological relevance of non-native wrack in the beach ecosystem functioning is potentially equivalent to their native counterparts. The structural complexity of the wrack patches was the main factor driving differences between the wrack-associated macrofauna assemblages. Our study suggests that the aggregation of terrestrial invertebrates in structurally simple wrack represents a general ecological pattern in sandy beach ecosystems. The typical wrack consumer Talitrus saltator showed specific responses to particular macroalgae species, avoiding the less palatable macroalgae. The diversity response of the macrofauna community is more linked to the biochemical composition of the specific macroalgae, such as phenol concentrations than to the morphological structure or the origin of the macroalgae. By providing more diverse habitat conditions and food sources, the mixture of different wrack species showed a synergistic effect enhancing macrofauna abundance and the number of taxa compared to wrack monocultures. Understanding the ecological role of the different types of beach wrack, especially the non-native inputs, is important to develop clear and ecologically sustainable strategies for management responses to beach wrack accumulations.