Influence of prey-food abundance on predator–prey foraging games: A test with little egrets and goldfish

Background: A predator–prey relationship abounds with variables that might affect the behaviour of either species as it responds to the challenges of the predation game. In a series of experiments, we have studied the behavioural responses of goldfish (prey) and little egrets (predator) when brought together in a large laboratory apparatus – an aviary with three pools of water. To obtain a baseline, we measured their behaviour in an aviary without manipulating any variable (Katz et al., 2010). Because the results of that study suggested that both fish and egrets follow the rules of optimal foraging, we undertook a series of experiments in which we manipulated the number of food patches (Katz et al., 2014a), the number of fish per pool (Katz et al., 2014b; Vijayan et al., 2018a), and the size of goldfish refuges (Vijayan et al., 2018b). In each case, the results supported the hypothesis of optimal behaviour. Mitchell (2018, this issue) makes four predictions about the behaviours of both species as the abundance of fish resources changes in a system such as the aviary. Specifically: (1) Prey should respond very weakly, if at all, to variation in the food ration among patches. (2) The predator should allocate more time to patches with a higher prey-food ration. (3) When moving to another patch, the predator should choose the patch with the higher prey-food ration more often than it would at random. (4) The predator will kill more prey in patches receiving a larger ration of prey-food. Aims: Explore the effect of varying the abundance of fish-food on the behaviours of goldfish and little egrets in the aviary. Determine whether variation in their behaviours agrees with the predictions of Mitchell’s theoretical model. Methods: We used little egrets (the predators) and goldfish (the prey). We conducted experiments in two identical aviaries (7 m diameter). Each aviary contained three separate patches, i.e. three pools (1.52 m diameter, 0.60 m deep, ∼1000 litres), each open to foraging by a single egret in the aviary. A fixed cover (22.75 cm radius) at the centre of each pool provided a refuge for the goldfish. Each pool had 15 fish. We manipulated intraspecific competition by offering three different amounts of goldfish food in an aviary each day: full (1.13 g per day), half (0.56 g per day), and quarter (0.28 g per day) rations. We then observed and recorded the behaviours of both players for 6 hours. Results: Patch-to-patch variation in the food ration had no effect on fish activity. But the egret allocated more time to foraging in the patch where the prey-food ration was highest (1.13 g). When moving to another patch, the egret moved to the patches containing high food rations (1.13 and 0.56 g) rather than the patch with the lowest food ration (0.28 g). And the predator caught significantly fewer goldfish in these poorest patches. Thus all four predictions were validated.