Abstract
Prey modify their behaviour to avoid predation, but dilemmas arise when predators vary in hunting style. Behaviours that successfully evade one predator sometimes facilitate exposure to another predator, forcing the prey to choose the lesser of two evils. In such cases, we need to quantify behavioural strategies in a mix of predators. We model optimal behaviour of Atlantic codGadus morhualarvae in a water column, and find the minimal vulnerability from three common predator groups with different hunting modes; 1) ambush predators that sit‐and‐wait for approaching fish larvae; 2) cruising invertebrates that eat larvae in their path; and 3) fish which are visually hunting predators. We use a state‐dependent model to find optimal behaviours (vertical position and swimming speed over a diel light cycle) under any given exposure to the three distinct modes of predation. We then vary abundance of each predator and quantify direct and indirect effects of predation. The nature and strength of direct and indirect effects varied with predator type and abundance. Larvae escaped about half the mortality from fish by swimming deeper to avoid light, but their activity level and cumulative predation from ambush predators increased. When ambush invertebrates dominated, it was optimal to be less active but in more lit habitats, and predation from fish increased. Against cruising predators, there was no remedy. In all cases, the shift in behaviour allowed growth to remain almost the same, while total predation were cut by one third. In early life stages with high and size‐dependent mortality rates, growth rate can be a poor measure of the importance of behavioural strategies.
Highlights
Predator–prey interactions structure ecological communities, and understanding the nature and strength is a central challenge for ecologists
These changes in prey traits can influence the interaction of the prey with other species, prey resources or predators of the prey, causing trait-mediated indirect effects (TMIEs) on these other species (Abrams 2007, Peacor and Werner 2008)
Schmitz and Suttle (2001) have shown that sitand-wait spiders cause grasshoppers to move from nutritious grass to less nutritious but safer herbs, and that this predatorinduced habitat change in prey led to a positive TMIE on grass, and a negative TMIE on herbs
Summary
Predator–prey interactions structure ecological communities, and understanding the nature and strength is a central challenge for ecologists. We model optimal prey responses to multiple predators in the pelagic, upper layers of the open ocean up to 100 m depth, which is not a homogenous environment with nowhere to hide, as one might initially believe, because of vertical and diurnal variation in light In this system, tradeoffs are strong, and we model an organism with a limited behavioural repertoire: fish larvae about one centimeter long that can move up and down in the water column and influence their encounter rate with food by swimming slowly or not at all. We develop a general model of optimal behaviour, growth and survival of larval fish exposed to ambush and cruising invertebrates and visually hunting fish We use it to assess if larvae can effectively adjust behaviour to avoid specific predatory modes; to quantify the relative importance of consumptive, non-consumptive and traitmediated indirect effects; to map the scope for behaviour to reduce overall predation; And to investigate how variable composition of predator communities affect overall predation rates
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