Individual variability in trophic position and diet of a marine omnivore is linked to kelp bed habitat

Abstract

Omnivores play an important role in the routing and distribution of organic matter across food webs. We demonstrate a novel approach to quantifying the coincidence of landscape- scale nutritional gradients with niche breadth in terms of variability in trophic level and use of basal organic matter sources. We provide an example of the links between individual variability in resource use and habitat of a broad-spectrum omnivore, the red rock lobster Jasus edwardsii. Information on the co-occurrence of J. edwardsii with kelp bed habitats Ecklonia radiata and with their preferred prey Mytilus edulis galloprovinciallis were collected at 60 sites across Fiordland, southwest New Zealand. Analysis of distance-based linear models (DISTLM) indicated that the presence of mussels was the best predictor of lobster occurrence in the model set. At a subset of sites, we collected lobster muscle for stable isotope analysis and measured 3 demographic para- meters from the lobsters: relative abundance, sex and carapace lengths. We characterised habitats with surveys of common kelp and mussels. Using stable isotope signatures (δ 13 C and δ 15 N), we calculated individual-based estimates of trophic level and the mixture of organic matter sources, i.e. phytoplankton and macroalgae. Using DISTLM, lobster and mussel densities best explained variability in lobster diet. Variability in resource use was distinct inside and outside of kelp bed habitat. In kelp beds, lobsters fed at a higher average trophic level, with low variability among individuals in trophic level and use of organic matter sources. Outside kelp beds, individual vari- ability indicated broad trophic diversification. These patterns indicate a strong influence of the nutritional landscape at the scale of the metapopulation, which has important implications for understanding dietary influences on population structure.