Abstract
The functional response is a key component of trophic interactions since it quantifies the per capita rate of prey consumption. Determining whether this rate depends on the prey density only (which is the standard assumption), on both prey and predator densities, or simply on their ratio is essential to understand interacting populations. Several experiments have convincingly demonstrated ratio dependence but, with very few exceptions, they were conducted in laboratory conditions. The difficulty of collecting the required data (initial prey density, prey consumption, predator density) probably explains the lack of evidence from functional responses observed in natural systems. A field experiment was previously conducted with a paper wasp and its prey, shield beetle larvae. Both densities were manipulated and the prey consumption was measured. A first analysis led to the conclusion that the functional response of the wasps depended on both prey and predator densities but could not be considered being ratio dependent.Here, we perform an improved analysis of these data, making better justified assumptions and using more appropriate statistical methods. We fit several functional response models to the data and select the best one with information‐theoretic criteria. We also estimate the model parameters and their confidence intervals. This more reliable analysis significantly modifies the original conclusion. Both model selection and parameter estimation indicate that ratio dependence governs the functional response of paper wasps preying on shield beetles in the field. Therefore, ratio dependence is not a laboratory artefact and should be more systematically considered as a potential model for describing functional responses.
Highlights
The functional response g is a key factor to understand the functioning of natural predator-prey systems
The functional response is a key component of trophic interactions since it quantifies the per capita rate of prey consumption
The difficulty of collecting the required data probably explains the lack of evidence from functional responses observed in natural systems
Summary
The functional response g (the instantaneous rate of prey consumption by an average predator) is a key factor to understand the functioning of natural predator-prey systems. The functional response had originally been defined as a function of the prey density only [g 1⁄4 g(N )] but many studies have shown that the predator density influences the individual predation rate [g 1⁄4 g(N, P)] This is known as ‘‘predator dependence’’ (Arditi and Ginzburg 1989). While the existence of predator dependence is widely accepted (e.g., Begon et al 2006, Gotelli 2008, Krebs 2009), a number of authors consider ratio dependence as unlikely to take place in natural systems or complain for a lack of mechanistic explanations In their view, both prey density and predator density affect the functional response independently [g 1⁄4 g(N, P)]. Many mechanisms have been identified as leading to the emergence of ratio dependence (see Chapter 4 of Arditi and Ginzburg 2012)
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