Global warming affects foraging efficiency of fish by influencing mutual interference

Predator-prey interactions underpin ecological dynamics from population to
ecosystem scales, affecting population growth and influencing community
stability. One of the classic methods to study these relationships is the
functional response (FR) approach, measuring resource use across resource
densities. Global warming is known to strongly mediate consumer-resource
interactions, but the relevance of prey and predator densities remains
largely unknown. Elevated temperature could increase consumer energy
expenditure, which needs to be compensated by greater foraging activity.
However, such greater activity may concurrently result in a higher
encounter rate with other consumers, which potentially affects their total
pressure on resource population because of synergistic or antagonistic
effects among multiple predators. We performed a laboratory experiment
using three densities of a fish predator (pumpkinseed, Lepomis gibbosus)
(one, two, and four specimens), two temperatures (25 and 28 oC), and six
prey densities. Using the FR approach, we investigated the combined
effects of elevated temperature and predator and prey density on
consumer’s foraging efficiency. We observed a reduced maximum feeding rate
at the higher temperature for single predators. The foraging efficiency of
predators in groups was generally associated with antagonistic
interactions, whose strength was greater for higher predator densities,
but was further mediated by the temperature. Specifically, we observed a
general decrease in antagonistic interactions in elevated compared to the
ambient water temperature for multiple predator groupings. Irrespective of
temperature, antagonistic multiple predator effects increased with
predator density and peaked unimodally at intermediate prey densities,
indicating multiple dimensions of density-dependence which interact to
supersede the effects of warming. This study shows that increasing
temperature affects the per capita performance of predators, but that this
effect is dampened with increasing predator densities. Their adaptive
response to temperature consists of limited food intake and further
reduced intraspecific interactions. Including conspecifics as food
competitors may thus offer more realistic outcomes compared to widely-used
experiments with only single predator individuals, which could
overestimate the effect of increasing temperature.