The effect of competitor presence on the foraging decisions of small mammals

Competitive interactions between species can have marked effects on the diets and foraging behaviours of the interactants. Dominant competitors can constrain the foraging decisions of subordinate competitors, reducing the individual fitness of subordinates and potentially driving their populations to low levels. Following a sustained population decline of the bush rat, Rattus fuscipes, in the presence of the competitively dominant common brushtail possum, Trichosurus vulpecula, at Booderee National Park in south-eastern Australia, we investigated whether possums affected the foraging decisions of bush rats. Using a manipulative feeding experiment, we predicted that bush rats would (1) increase visits to baited sites where possums had restricted access to the bait and (2) restrict visits to baited sites where possums had free access. We used camera traps to investigate visit patterns and time spent foraging at 40 baited sites with two treatments, one that allowed full access by both species (full access) and the other that attempted to prevent possum access (restricted access). We also measured additional covariate factors that may influence visits. Bush rats visited both treatments less when there were more possum visits. We also found that bush rats spent less time eating bait at sites regularly visited by possums, regardless of possums’ access level. Our results indicate that negative interactions, such as competition, can restrict the ability of subordinate species to successfully forage, contributing to species declines.