Abstract
Individuals can deploy a variety of ecological and behavioural strategies to obtain resources, often using only a subset of the total resource diversity used by their population. This phenomenon of individual specialisation (IS) is nearly ubiquitous across taxa and has the potential to affect population dynamics and ecosystem processes. Pairwise antagonistic interactions such as competition and predation can influence the degree of IS in a population, but little is known about the combined effects of multiple simultaneous interaction types between species, including intraguild predation (competition and predation from a single antagonist). We address this gap by asking how the combination of competition and predation risk from an invasive intraguild predator-Eurasian perch Perca fluviatilis-impacts the degree of dietary and habitat IS in a native New Zealand fish, the common bully Gobiomorphus cotidianus. Bullies exhibit a generalised diet at the population level and compete for benthic and pelagic prey with juvenile perch, while also being subject to predation by larger perch. We used a mesocosm experiment to explore how competition from young-of-year perch and perceived predation risk from sub-adult perch influence IS within bully populations. Over a 3-month period, we monitored individual habitat use and used serial gastric lavage to sample time-integrated individual diets. We found that the presence of juvenile perch was associated with a decrease in dietary IS associated with a shift to more benthic feeding, while habitat IS was affected by an antagonistic interaction between competition and predation whereby presence of small perch negated a negative effect of large perch on IS. This study demonstrates the importance of considering multiple interaction types when evaluating how interspecific interactions influence individual variation within populations.