The effects of predation risk on prey stoichiometry: a meta-analysis

A decade ago, the general stress paradigm (GSP) aimed to develop a predictive framework linking predator effects to ecosystem function. The GSP was based on the notion that animals, across taxa, exhibit similar physiological responses to predation risk that divert resources from growth and reproduction (which require N-rich biomolecules [i.e., protein]) to emergency functions (which require C-rich biomolecules [e.g., carbohydrates]). The GSP predicts that stressed prey should have a greater dietary demand for C-rich resources, a higher body C:N ratio, and elevated N excretion. Now, 10 yr later, we aim to revisit the GSP-using quantitative meta-analysis to test the original predictions of the GSP and how (1) predator hunting mode, (2) multiple stressors, and (3) prey dietary shifts affect prey stoichiometric responses to predation risk. Our data set was consistent with previous work showing that predation risk increases prey glucocorticoid levels and metabolic rates and decreases prey growth rates. We found that predation risk tended to decrease the fat, carbohydrate, and protein content of prey bodies; increased the C:P and N:P of prey bodies; but had no effect on the C, N, P, or C:N content of prey bodies. Additionally, we found no effect of predation risk on the N content of prey excretions. Prey responses to predation risk were unaffected by multiple stressors or the prey's ability to shift their diet, but predator hunting mode did affect the nature of prey stoichiometric responses. Specifically, ambush predators decreased prey macronutrient content and suppressed prey growth, and active predators had no effect on prey macronutrient content and a smaller effect on prey growth than ambush predators. The significant effects of predation risk were supported by robust fail-safe numbers, despite the high between-comparison heterogeneity that was found in all analyses. Our findings highlight the need (1) to test the underlying mechanisms and emerging patterns of the GSP in diverse taxa, (2) to explore the mismatch between prey macronutrient content and elemental stoichiometry, and (3) to expand the conceptual framework to include more inducible defenses (e.g., behavioral and morphological) and predator traits.