Does temperature, light or inter-individual variation explain feeding motivation in a tropical freshwater predator?

Abstract

Variability in environmental conditions in freshwater ecosystems are increasingly driven by human activity. Increased temperature and light intensity are among the anthropogenic stressors dramatically altering these ecosystems, for example through deforestation that reduces canopy cover of riparian vegetation. Simultaneous exposure to multiple stressors complicates predictions of responses to environmental stressors due to potential interactions, yet the interaction between temperature and light intensity on feeding motivation remains poorly understood. Here, a fully factorial design was employed to investigate the combined effect of increased temperature and light intensity on the feeding motivation of a freshwater predator, the pike cichlidSaxatilia proteus. Strikes toward food items were used to quantify the subjects' motivation to feed. We found no effect of temperature or light intensity on feeding motivation, either individually or as an interaction. Our repeated measures design allowed us to test whether the predatory fish showed personality variation, i.e. consistent inter-individual differences, in their motivation to feed. While the time taken to make the first strike was not consistently different between individuals, the number of strikes in 1 minute, 3 minutes and the time taken for the 10th strike (which were strongly correlated to one another), was consistently different between individuals. This variation could not be explained by variation in body length, which had no effect as a main effect. Variation between individuals is likely to be magnified in wild predators that are likely to vary more in experience and genetic background than the captive subjects in our study. Thus, we suggest that anthropogenic effects that alter the composition of individuals in a population of predators, for example selective harvesting, will have a greater ecological effect than direct short-term effects of variability in environmental factors.