Abstract
Knowledge on how functional responses (a measurement of feeding interaction strength) are affected by patch size and habitat complexity (represented by refuge availability) is crucial for understanding food-web stability and subsequently biodiversity. Due to their laborious character, it is almost impossible to carry out systematic empirical experiments on functional responses across wide gradients of patch sizes and refuge availabilities. Here we overcame this issue by using an individual-based model (IBM) to simulate feeding experiments. The model is based on empirically measured traits such as body-mass dependent speed and capture success. We simulated these experiments in patches ranging from sizes of petri dishes to natural patches in the field. Moreover, we varied the refuge availability within the patch independently of patch size, allowing for independent analyses of both variables. The maximum feeding rate (the maximum number of prey a predator can consume in a given time frame) is independent of patch size and refuge availability, as it is the physiological upper limit of feeding rates. Moreover, the results of these simulations revealed that a type III functional response, which is known to have a stabilizing effect on population dynamics, fitted the data best. The half saturation density (the prey density where a predator consumes half of its maximum feeding rate) increased with refuge availability but was only marginally influenced by patch size. Subsequently, we investigated how patch size and refuge availability influenced stability and coexistence of predator-prey systems. Following common practice, we used an allometric scaled Rosenzweig–MacArthur predator-prey model based on results from our in silico IBM experiments. The results suggested that densities of both populations are nearly constant across the range of patch sizes simulated, resulting from the constant interaction strength across the patch sizes. However, constant densities with decreasing patch sizes mean a decrease of absolute number of individuals, consequently leading to extinction of predators in the smallest patches. Moreover, increasing refuge availabilities also allowed predator and prey to coexist by decreased interaction strengths. Our results underline the need for protecting large patches with high habitat complexity to sustain biodiversity.
Highlights
The interplay between stability, complexity and biodiversity of ecological networks is a subject of a long lasting and still ongoing discussion in ecology (e.g., MacArthur, 1955; May, 1972; McCann, 2000)
Interaction strength refers to feeding interactions, which can be studied by investigating the ‘functional response’: how the number of prey eaten by a predator changes with increasing prey densities
Our model comparison of patch size and refuge availability dependent functional response models, based on Bayesian Information Criterion (BIC), included a scaling of half saturation density with patch size and refuge availability, but it did not include any scaling of the Hill exponent with either parameter (Table 2)
Summary
The interplay between stability, complexity and biodiversity of ecological networks (especially food webs) is a subject of a long lasting and still ongoing discussion in ecology (e.g., MacArthur, 1955; May, 1972; McCann, 2000). Interaction strength refers to feeding interactions, which can be studied by investigating the ‘functional response’: how the number of prey eaten by a predator changes with increasing prey densities. In his seminal work, Holling (1959b) described the mechanism of functional responses mathematically and showed that under the simplest assumptions it should follow a hyperbolic curve. This experiment was performed in a simple experimental trial with blindfolded students (the predator) on a plain nine-square foot table as the experimental arena and sandpaper discs as prey. One of the descendants of Holling’s functional response models based on enzyme kinetics (Real, 1977) unifies the type II and the type III functional responses: f (N )
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