Predator Functional Responses: Discriminating Between Handling and Digesting Prey

Abstract

We present a handy mechanistic functional response model that realistically incorporates handling (i.e., attacking and eating) and digesting prey. We briefly review current functional response theory and thereby demonstrate that such a model has been lacking so far. In our model, we treat digestion as a background process that does not prevent further foraging activities (i.e., searching and handling). Instead, we let the hunger level determine the probability that the predator searches for new prey. Additionally, our model takes into account time wasted through unsuccessful attacks. Since a main assumption of our model is that the predator's hunger is in a steady state, we term it the steady state satiation (SSS) equation. The SSS equation yields a new formula for the asymptotic maximum predation rate (i.e., asymptotic maximum number of prey eaten per unit time, for prey density approaching infinity). According to this formula, maximum predation rate is determined not by the sum of the time spent for handling and digesting prey, but solely by the larger of these two terms. As a consequence, predators can be categorized into two types: handling‐limited predators (where maximum predation rate is limited by handling time) and digestion‐limited predators (where maximum predation rate is limited by digestion time). We give examples of both predator types. Based on available data, we suggest that most predators are digestion limited. The SSS equation is a conceptual mechanistic model. Two possible applications of this model are that (1) it can be used to calculate the effects of changing predator or prey characteristics (e.g., defenses) on predation rate and (2) optimal foraging models based on the SSS equation are testable alternatives to other approaches. This may improve optimal foraging theory, since one of its major problems has been the lack of alternative models.

Using functional response modeling to investigate the effect of temperature on predator feeding rate and energetic efficiency

“…Moreover, the estimated energy activation for handling time (0.61 ± 0.04 eV) was not different to the one predicted by metabolic theory (0.65 eV). For many predators, prey handling is driven by digestion (Jeschke et al 2002), which depends on metabolism. This is especially true for the 24-h period of our experiment, where handling processes such as killing and ingesting prey are negligible when compared to digestion (Jeschke et al 2002).…”

Evolutionary and plastic rescue in multitrophic model communities

“…First, we implement defence by multiplying attack rate by (1 2 b) and dividing handling time by (1 2 b) (note: the (1 2 b) terms in the denominator cancel each other). We believe this to be the most correct way to implement defence because in our system, defences in the herbivore centre around the production of long posterolateral spines; these spines decrease the likelihood of successful attack (decrease attack efficiency) and also increase the handling time by both wasting time on unsuccessful attacks [33] and by requiring more time to manipulate the herbivore into a position where it can be consumed. Vos et al [15] implemented defence through increasing handling time of predators on defended prey; however, handling time is present only in the denominator of the functional response equation, which is identical for both prey types; thus, defended and undefended prey obtain the same benefit under Vos et al's formulation (C. Kovach-Orr, M. Voss & G. Fussmann 2012, unpublished data).…”

Understanding spatial distributions: negative density-dependence in prey causes predators to trade-off prey quantity with quality

“…Holling's type II functional response has long been thought to be the most widespread among predators [10,11]. In this study, we have shown that negative density-dependence among prey results in a type IV functional response.…”

“…Intake rates may also decline at high prey densities, which results in a hump-shaped functional response (a so-called type IV functional response [12]). As reviewed in [10], the decline in intake rate at high prey densities has been attributed to a decrease in predator searching efficiency (e.g. owing to increased predator detection, confusion, mobbing), and an increase in associated foraging costs (e.g.…”