“Prey limitation drives variation in allometric scaling of predator-prey interactions”

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Raul Costa-Pereira, Márcio Silva Araújo, Renan da Silva Olivier, Franco L. Souza, and Volker H. W. Rudolf (Oct 2018)

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Variation in allometric scaling of predator-prey interactions is predicted by gradients of prey limitation

What drives variation in relative size of predators and their prey?

Sometimes, Leptodactylus chaquensis may prey on large-sized food types, such as Physalaemus centralis.
(Credit: Raul Costa-Pereira)

Predators should carefully choose their prey. Small-sized prey may be tricky to find and capture, and usually have low energetic return. In turn, too large prey are difficult to overcome, subdue, and even ingest. Therefore, foraging theories predict an intermediate optimal relative size-difference between predators and their prey. Although this theoretical concept has been widely used to model the dynamics and structure of predator-prey populations and food webs, empirical systems show a tremendous variation in predator-prey size ratios.

To investigate the underlying causes of this unexpected variation, the authors analyzed more than 6,000 trophic interactions between tropical frog species and their prey across a diverse range of communities in the Pantanal wetlands of Brazil. Surprisingly, they demonstrated that variation in the relative size of predators and their prey is not simply “noise” but instead can be predicted by gradients of prey limitation consistent with predictions from Optimal Foraging Theory. Importantly, this study shows for the first time that the variance of size-scaling ratios is neither negligible nor constant as currently assumed, but instead it changes predictably across communities. This variation has important consequences for ecological and evolutionary dynamics of food webs. Together, these findings make clear that prey limitation leads to deviations from the universal size-scaling constant and thus challenges current paradigms of metabolic and food web theories.

Other times, Leptodactylus chaquensis preys on much smaller prey types, such as ants, termites, larvae, and beetles. The photo shows the gut contents of one L. chaquensis individual.
(Credit: Raul Costa-Pereira)


Ecologists have long searched for a universal size-scaling constant that governs trophic interactions. Although this is an appealing theoretical concept, Predator-Prey Size Ratios (PPSR) vary strikingly across and within natural food webs, meaning that predators deviate from their optimal prey size by consuming relatively larger or smaller prey. Here, we suggest that this unexpected variation in allometric scaling of trophic interactions can be predicted by gradients of prey limitation consistent with predictions from Optimal Foraging Theory. We analyzed >6,000 trophic interactions of 52 populations from four tropical frog species along a gradient of prey limitation. The mean of PPSR and its variance differed up to two orders of magnitude across and within food webs. Importantly, as prey availability decreased across food webs, PPSR and its variance became more size-dependent. Thus, trophic interactions did not follow a fixed allometric scaling but changed predictably with the strength of prey limitation. Our results emphasize the importance of ecological contexts in arranging food webs and the need to incorporate ecological drivers of PPSR and its variance in food web and community models.

One of the sampled ponds in the Pantanal wetlands, Central Brazil.
(Credit: Raul Costa-Pereira)

A limitação de presas explica a variação na alometria de interações predador-presa

Ecólogos têm buscado há muito tempo uma constante alométrica universal que governe relações tróficas. Embora esse seja um conceito teórico atraente, razões de tamanho predador-presa (PPSR) variam amplamente entre e dentro de teias tróficas naturais, o que indica que predadores desviam de seus tamanhos ótimos de presa consumindo recursos relativamente maiores ou menores. Aqui, nós sugerimos que essa inesperada variação alométrica em interações tróficas pode ser predita por gradientes de limitação de presas, o que seria consistente com a Teoria do Forrageamento Ótimo. Nós analisamos >6.000 interações tróficas em 52 populações de quatro espécies de rãs tropicais ao longo de um gradiente de limitação de presas. A PPSR média e sua variância diferiram até duas ordens de magnitude entre e dentro das teias tróficas. Particularmente, conforme a disponibilidade de presas diminuiu nas teias tróficas, a PPSR e sua variância se tornaram mais dependentes do tamanho corpóreo dos predadores. Desse modo, interações tróficas não seguem uma constante alométrica fixa, mas variam previsivelmente de acordo com a magnitude da limitação de presas. Nossos resultados enfatizam a importância de diferentes contextos ecológicos no arranjo de teias tróficas e a necessidade de incorporar os mecanismos ecológicos que afetam a PPSR e sua variância em modelos de teias tróficas e comunidades.