Decades of Predator Studies May Be Wrong Due to Flawed Mathematical Assumptions

We examined how multiple predators affect prey survival has been a central question in ecology, with researchers developing sophisticated methods to detect 'emergent effects' - situations where predators hunting together kill more or fewer prey than predicted from their individual effects. However, new research reveals that the standard mathematical approach used by ecologists contains systematic biases that may have led to incorrect conclusions for decades.

The study examined the Multiplicative Risk Model, the consensus approach that emerged from years of scientific debate for predicting what happens when multiple predators hunt the same prey. This model accounts for the fact that prey cannot be eaten twice, but we discovered it makes a critical assumption that predators maintain constant feeding rates throughout experiments.

Using mathematical simulations of predation experiments, We compared predictions from the standard model against more realistic models that account for how predator feeding rates actually change over time. They found that when prey are depleted during experiments - which happens in most studies - the standard model produces systematic bias.

An examination of 100 multiple predator studies from the literature revealed that prey were depleted by 70% on average over the course of experiments. This depletion violates a key assumption of the Multiplicative Risk Model, which assumes constant per capita mortality rates. In reality, most predators exhibit Type II functional responses, meaning their feeding rates change as prey become scarce.

The bias manifests differently depending on experimental design. Studies using additive designs (where predator densities are combined) consistently overestimated 'risk enhancement,' while substitutive designs (where total predator density remains constant) usually overestimated 'risk reduction,' even when predators were actually having completely independent effects.

These findings have significant implications for understanding ecosystem function. Predator-prey interactions comprise the foundation of food web dynamics, and if researchers have been systematically overestimating how often predators interact, then current understanding of ecosystem function may be flawed. This affects conservation efforts, ecosystem management, and basic understanding of how marine and terrestrial food webs operate.

The research suggests that meta-analyses and syntheses of multiple predator effects may need to be reconsidered in light of these systematic biases. Moving forward, the field needs better methods that account for prey depletion and nonlinear functional responses, recognizing that the dynamic nature of biological processes can significantly affect study outcomes.

Citation

McCoy, Michael W.; Stier, Adrian C.; Osenberg, Craig W. (2012). Emergent effects of multiple predators on prey survival: the importance of depletion and the functional response. Ecology Letters.

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