Effects of competition and predation risk from a life history intraguild predator on individual specialisation

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.