Why ratio dependence is (still) a bad model of predation

Abstract

The history of the idea that predation rates are functions of the ratio of prey density to predator density, known as ratio dependence, is reviewed and updated. When the term was introduced in 1989, it was already known that higher predator abundance often reduced an individual predator's average intake rate of prey. However, the idea that this effect was a universally applicable inverse proportionality was new. That idea was widely criticized in many articles in the early 1990s, and many of these criticisms have never been addressed. Nevertheless, ratio dependence seems to be gaining in popularity and is the subject of a recent monograph by the originators. This article revisits the most important objections to this theory, and assesses to what extent they have been answered by the theory's proponents. In this process, several new objections are raised. The counterarguments begin with the lack of a plausible, generally applicable mechanism that could produce ratio dependence. They include the fact that ratio dependence is a special case of predator‐density effects, which, in turn, are only one of many non‐prey species effects that influence the consumption rate of a particular prey. The proclaimed simplicity advantage of ratio dependence is at best small and is outweighed by its disadvantages; it predicts biologically implausible phenomena, and cannot easily be extended to describe multi‐species systems, trait‐mediated interactions, coevolution, and a number of other important ecological phenomena. Any potential small simplicity advantage disappears with corrections to remove unrealistic low‐density dynamics caused by ratio dependence. The frequent occurrence of strong predator dependence does not make ratio dependence a better ‘default’ model of predation than prey dependence, and empirical studies of the full range of non‐prey species effects on the consumption rates of predators are needed.

Improving the assessment of predator functional responses by considering alternate prey and predator interactions

“…We recognize, however, that ratio-dependent functional responses remain an area of contention in ecology, with critics cautioning that use of relatively simple models necessarily ignore the influence of factors such as movement, landscape heterogeneity, and prey behavior, on predation rates. Such real-world complexities may lead to indirect interference (i.e., pseudo-interference) even in the absence of direct interactions between predators (Free et al 1977, Abrams and Walters 1996, Abrams 2015. It is unlikely that these functional responses are wholly prey or ratio dependent, but exhibit some degree of intermediate interference (Arditi and Akc ßakaya 1990).…”

“…Interestingly, the extent to which predator‐ or ratio‐dependent functional responses outperform prey‐dependent models appears to vary across systems (Arditi and Ginzburg , Abrams ). Of the few empirical tests assessing the variety of functional responses, it seems that, depending on the system, prey‐dependent (Arditi and Saïah ), ratio‐dependent (Vucetich et al.…”

Moving on with foraging theory: incorporating movement decisions into the functional response of a gregarious shorebird

“…As stressed by van der Meer & Ens (), most of these models are phenomenological because they lack a mechanistic underpinning of the processes of prey and competitor encounter, rendering it difficult to use them as firm building blocks in follow‐up studies. Unfortunately, ratio‐dependent predation models, which have been claimed to offer an altered perspective on trophic ecology (Arditi & Ginzburg ), are of phenomenological nature too (Abrams ). But, note that even the few generalized functional response models that do mechanistically include competition have significant drawbacks.…”

Quantifying predator dependence in the functional response of generalist predators

“…An alternative explanation for the weak support for predator dependence in the caging experiment is that the cages, or their placement within an early successional age patch that was dominated by a single barnacle species, altered whelk foraging behaviour from that exhibited across the sets of surveyed patches more generally. This interpretation may seem to challenge the concern that traditional functional response experiments involving isolated predator–prey pairs could be favoring the detection of predator dependence by selecting for strong predator–prey interactions (Abrams ). However, the results of our analysis are also consistent with this concern in that N. ostrina 's estimated interference rate (Fig.…”

“…; Perretti et al . ; Abrams ). Indeed, all methods for quantifying the strengths and hence importance of species interactions make assumptions regarding their functional form (Wootton & Emmerson ; Vázquez et al .…”