Competition and metabolism should be linked. Intraspecific variation in metabolic rates and, hence, resource demands covary with competitive ability. The effects of metabolism on conspecific interactions, however, have mostly been studied under laboratory conditions. We used a trait-specific response-surface design to test for the effects of metabolism on pairwise interactions of the marine colonial invertebrate, Bugula neritina in the field. Specifically, we compared the performance (survival, growth, and reproduction) of focal individuals, both in the presence and absence of a neighbour colony, both of which had their metabolic phenotype characterised. Survival of focal colonies depended on the metabolic phenotype of the neighbouring individual, and on the combination of both the focal and neighbour colony metabolic phenotypes that were present. Surprisingly, we found pervasive effects of neighbour metabolic phenotypes on focal colony growth and reproduction, though the sign and strength of these effects showed strong microenvironmental variability. Overall, we find that the metabolic phenotype changes the strength of competitive interactions, but these effects are highly contingent on local conditions. We suggest future studies explore how variation in metabolic rate affects organisms beyond the focal organism alone, particularly under field conditions.

We characterised the metabolic phenotype of individuals of the marine colonial invertebrate, Bugula neritina and, using a trait-specific response-surface design, created pairs of individuals that differed in their metabolic rates. We then followed the performance of all individuals throughout their lives to determine the effects of metabolism on competitive interactions in the field (note that we treated both colonies as the focal and neighbour colony to test for reciprocal interactions). Specifically, we followed the survival, growth, and fecundity (reproductive output) of each individual by checking colony absence/presence and counting the number of bifurcations (colony size, later converted to zooid number) and the number of ovicells (external offspring bearing brood chambers; every ovicell contains a single larva) every two weeks over a period of six months.