Mass‐dependence in the predation risk of unequal competitors; some models

Abstract

Foragers can monitor their survival through the size of body reserves in a starvation/predation risk trade‐off. Energy reserves reduce the risk of energetic shortfall, while survival will be maximised at intermediate reserve levels when there is a cost of carrying mass loads. The size of reserves that will maximise survival may not be identical for unequal competitors, when unequal access to resources will affect the costs and benefits of energy reserves. Here, I evaluate the effect of competitive ability (dominance) for the mass‐dependence in predation risk and how it is affected by (1) attack rate (attack rate effect), (2) distance to the emergence of an unconcealed predator attack (attack distance effect) and (3) distance to cover (cover distance effect). This general model is illustrated by empirical data for parameters specific for birds. The effect of competitive ability for the mass‐dependence in predation risk is ambiguous and depends on how rank is mediated into mass‐dependent predation risk. Dominants pay a lower cost in predation risk for mass loads than sub‐ordinates when competitive ability entails that they feed closer to cover (cover distance effect) and when the exposure to attacks and attack rate is lower than for sub‐ordinates (attack rate effect). In contrast, a shorter distance to the emergence of an unconcealed attack (attack distance effect) implies a lower increase in predation risk with mass for sub‐ordinates. As a consequence of how the cost of mass load varies with conditions there is no unambiguous relationship for how predation risk can be traded off for starvation risk for individuals with different competitive ability.