Effects of interspecific interactions on individual specialisation

In many species, some individuals are specialist users of a subset of the total resources available to their population, a phenomenon known as individual specialisation. This intraspecific variation has long been suspected of shaping population dynamics and has potentially large effects at the ecosystem level. Variation in the degree of individual specialisation can be driven by ecological interactions. The common bully Gobiomorphus cotidianus, a New Zealand native fish, displays a generalised diet at the population level, with moderate levels of individual specialisation. Juvenile perch Perca fluviatilis compete for the same food resources, while larger perch are potential predators. These antagonistic biological interactions may have an impact on the trophic niche and the degree of individual specialisation in bullies. I used surveys of fish communities and cross-sectional diet data to assess individual specialisation in wetland bully populations in relation to ecological factors. Then, in two series of controlled manipulative experiments, I asked how individual specialisation in bullies responded to intraspecific competition, interspecific competition and predation risk from perch. I hypothesised that ecological niches at the population and individual levels would vary according to the composition of the fish community. Finally, I led a collaborative comparative phylogenetic analysis of the literature which compiled indices of individual specialisation in diet across ecosystems and taxa and tested whether individual specialisation was predicted by cumulative measures of human impacts. I anticipated that due to global resource and habitat disruptions, human impacts on individual specialisation would generally be negative. Consistent with the niche variation hypothesis, I found that intraspecific competition led to overall higher intraspecific diet diversity, while interspecific competition led to lower degrees of individual specialisation. Predation risk had a positive effect on dietary, and a negative effect on habitat individual specialisation. However, the direction of these effects changed when interspecific competition occurred along predation risk. At the population level, I recorded niche shifts due to interspecific competition both in habitat use and in diet. Finally, I found negative effects of human activities on individual specialisation in terrestrial and freshwater ecosystems. There was however no evidence of a similar negative relationship in marine ecosystems. This discrepancy in results might be linked to differences in the types of drivers that impact individual specialisation between each ecosystem type. This work underscores the importance of integrating individual variation when evaluating a population’s response to antagonistic biological interactions