Dying from the lesser of three evils - facilitation and non-consumptive effects emerge in a model with multiple predators.

12 April 2019

Fouzai, Nadia; Opdal, Anders; Jørgensen, Christian; Fiksen, Øyvind

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 cod Gadus morhua larvae in a water column, and find the minimal vulnerability from three common predator groups with different hunting modes; i) ambush predators that sit-and-wait for approaching fish larvae; ii) cruising invertebrates that eat larvae in their path; and iii) 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.

Keywords: Multiple predators, Consumptive effects, Non-consumptive effects, Trait mediated indirect effects, Facilitation, Optimal behaviour, Dynamic programming, Predator-prey interactions.