Prey animals stay put as predators increase

A new mathematical model finds that the reasoning used in studies of predator-prey relationships is flawed – and often delivers predictions that run counter to evidence.

Predator-prey relationships are predicated on flight response. When a potential prey animal becomes aware of a predator, it will likely flee. Prior modelling suggests that the tendency to flee – the wariness of the prey animal, in other words – increases as the number of predators in the vicinity increases.

The assumption is a key tenet of the standard model used predator-prey research, known as signal detection theory.

It seems logical enough, but new modelling from a team led by biologist Pete Trimmer at the University of California, Davis, and mathematician John McNamara from the University of Bristol in the UK has found that adding extra variables to the calculations produces the opposite result. As the number of predators increases, the chance that a prey animal will run for its life actually drops.

The researchers call this new approach state-dependent detection theory or SDDT.

The reversed result, Trimmer and his colleagues write in a paper published in the journal Proceedings of the Royal Society B, arises because signal detection theory assumes that prey animals make one decision at a time – and that the choice between staying put or heading for the hills is the only thing on the agenda.

This, says Trimmer and his colleagues, is simplistic.

Any prey animal will have to weigh up multiple factors in making a possible life-or-death choice. If the prey animal is hungry and eating much-needed food, for instance, then the urge to remain in place might be stronger than the urge to run away.

This reasoning led Trimmer and his colleagues to plug a value for “energy reserves” into the calculations. If the potential prey is well stocked with food itself, it can afford to miss a meal and flee. If its reserves are low, it may choose to stay put.

SDDT also predicts that increasing numbers of predators makes prey less likely to run away.

“If predators are rare, when an organism receives a signal of possible danger, it can afford to flee because it should have plenty of opportunity to make up for lost energy at a later time,” the authors write. 

“In contrast, if predators are very common, the organism cannot afford to flee every time it receives a signal of possible danger.”

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