Food predictability changes the structure of animal populations

Posted on October 15, 2015

Although it’s known that animal behaviour is affected by the predictability of food resources, the population-level implications of food predictability haven’t received much attention. Researchers at Oregon State University recorded the behaviour of 3-cm long juvenile steelhead trout (Oncorhynchus mykiss) in artificial streams fitted with programmable feeders. Each stream contained three feeding patches made of nylon mesh that retained floating flake food, and food densities in the three patches were different (low, medium and high). There were three experimental treatments: in one, the spatial predictability of food was low (the configuration of high, medium and low density patches was changed each day); in another, spatial predictability was intermediate (the food density was changed every three days); and in a third, high predictability treatment the food density stayed the same throughout the experiment. Although the three patches varied in food density, the total amount of food per stream was always the same. Each stream was stocked with 60 fish having a mean length of 3 cm, and the experiment ran for 21 days. After four days, the streams were opened to emigration into a one-way fish trap at the downstream end of each channel. In streams with highly predictable food, the emigration rate was high but almost all the remaining fish took up territories and had relatively high rates of foraging and growth. These fish also had large otoliths (earbones) at the start of the experiment – an indication of fast metabolism and behavioural dominance. The results showed that the spatial predictability of food induced fish to modify their foraging tactics, with a predictable supply leading to the separation of physiologically-distinct cohorts. When the supply was unpredictable and the benefits of territorial defence less certain, more individuals were willing to be unattached “floaters”.

Reference: Sloat, M.R. & Reeves, G.H. 2014. Demographic and phenotypic responses of juvenile steelhead trout
to spatial predictability of food resources
Ecology 95(9), 2423–2433.