Global warming affects foraging efficiency of fish by influencing mutual interference

1. 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.

2. 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 populations because of synergistic or antagonistic effects among multiple predators.

3. 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°C) and six prey densities. Using the FR approach, we investigated the combined effects of elevated temperature and predator and prey density on the consumer's foraging efficiency.

4. We observed a reduced maximum feeding rate at the higher temperature for single predators. However, the foraging efficiency of predators in groups was negatively affected by antagonistic interactions between individuals and further mediated by the temperature. Specifically, we observed a general decrease in antagonistic interactions at 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 that interact to supersede the effects of warming.

5. This study shows that conspecific presence negatively affects the per capita performance of predators, but that this effect is dampened with increasing temperature. Their adaptive response to temperature consists of limited food intake and further reduced intraspecific interactions. Including intraspecific competition in study design may thus offer more realistic outcomes compared to widely used experiments with only single predator individuals, which could overestimate the effect of increasing temperature.