A skewed literature: Few studies evaluate the contribution of predation-risk effects to natural field patterns

Abstract

A narrative in ecology is that prey modify traits to reduce predation risk, and the trait modification has costs large enough to cause ensuing demographic, trophic and ecosystem consequences, with implications for conservation, management and agriculture. But ecology has a long history of emphasising that quantifying the importance of an ecological process ultimately requires evidence linking a process to unmanipulated field patterns. We suspected that such process-linked-to-pattern (PLP) studies were poorly represented in the predation risk literature, which conflicts with the confidence often given to the importance of risk effects. We reviewed 29 years of the ecological literature which revealed that there are well over 4000 articles on risk effects. Of those, 349 studies examined risk effects on prey fitness measures or abundance (i.e., non-consumptive effects) of which only 26 were PLP studies, while 275 studies examined effects on other interacting species (i.e., trait-mediated indirect effects) of which only 35 were PLP studies. PLP studies were narrowly focused taxonomically and included only three that examined unmanipulated patterns of prey abundance. Before concluding a widespread and influential role of predation-risk effects, more attention must be given to linking the process of risk effects to unmanipulated patterns observed across diverse ecosystems.

Keywords: fear; natural experiment; non-consumptive; non-lethal; observational; plasticity; predation risk; process from pattern; trait mediated; trait response.

FIGURE 1

Our survey was conducted as a series of decision points which distinguished different experiments as they relate to field contribution (rows), and a sequence of risk effects (columns). In regards to the risk effects (columns), an increase in risk leads to a risk‐induced trait‐response of the prey, which can affect a fitness component, then population growth rate, and then abundance of prey (NCE) or indirectly that of another species such as the prey's resource (TMIE). Nested groups (a–d) correspond to the categorical divisions described in the text. Group S is shown to highlight the difference between our review and that of Sheriff et al. (, see text).

FIGURE 2

(a) Three‐year running average of papers that examined NCEs, TMIEs and all ecology papers shown as a proportion of the total number of papers in each of the three categories over the 29‐year period (inset provides natural logarithm of the data to facilitate visualisation of relative changes through time). (b) Composition of risk effect papers from 1990 to 2018 using all three sub‐searches (n = 3945 total papers). Each square represents ten papers. NCE (c) and TMIE (d) papers categorised by field component metrics. Each square represents one paper (except for one paper that is represented twice having a PLP study of abundance and growth rate). Though PLP studies include a field component, the field component category in the figure does not include PLP studies as to not double count.

FIGURE 3

Proportion (y‐axis) and number (indicated) of responses in PLP studies categorised by the fitness component of the prey measured in NCE studies, and the fitness component of the resource measured in TMIE studies. Multiple rates were included for studies that measured 2 (six studies) or 3 (1 study) different fitness components (fitness component categorisation described in methods).

FIGURE 4

Proportion (width of a bar) of a given prey taxon for NCE and TMIE studies, with the sum of the widths being equal across rows. Four taxa of invertebrates were combined into one group (echinoderms, rotifers, annelids and arachnids). The ‘all’ category includes PLP studies; e.g., of 29 mammal NCE studies, 12 were PLP studies. The total number of studies for each category is given.