Trade-offs between sight lines and escape habitat determine spatial strategies of risk management by a keystone herbivore

Abstract

Prey individuals possess four basic strategies to manage predation risk while foraging: time allocation, space use, apprehension, and foraging tenacity. But there are no direct tests of theory detailing how spatial strategies change and covary from fine to coarse scales of environmental variability. We address this shortcoming with experiments that estimated space use and vigilance of snowshoe hares while we measured foraging tenacity in artificial resource patches placed in risky open versus safe alder habitat. Hares employed only two of eight a priori options to manage risk. Hares increased vigilance and reduced foraging in open areas as the distance from cover increased. Hares did not differentiate between open and alder habitats, increase vigilance at the coarse-grained scale, or reduce vigilance and foraging tenacity under supplemental cover. Hares were more vigilant in the putatively safe alder than in the purportedly risky open habitat. These apparently paradoxical results appear to reflect a trade-off between the benefit of alder as escape habitat and the cost of obscured sight lines that reduce predator detection. The trade-off also appears to equalize safety between habitats at small scales and suggests that common-sense predictions detailing how prey reduce risk may make no sense at all.

Contingent strategies of risk management by snowshoe hares

“…My design mimics that of Morris and Vijayan (2018) who quantified hare foraging on pine boughs placed in an open field versus thick alder habitats while wildlife cameras recorded the hares' behaviour. The nutritional quality of jack-pine boughs, a favoured food of hares in winter (Bergeron and Tardif 1988), declines from the farthest tip towards the base (Palo et al 1992;Hodson et al 2010).…”

“…Boughs thus represent a depletable resource patch with diminishing returns to browsers such as snowshoe hares. The diameter of the residual stem at the point of browse yields a reliable and repeatable estimate of the hares' GUD (Morris 2005;Hodson et al 2010;Morris and Vijayan 2018). The GUDs, in combination with time-synchronized photos, yield the basic data required to assess risk management and possible trade-offs between time allocation and vigilance.…”

“…I completed my analyses with paired t tests comparing the number of images with feeding hares between full-and new-moon phases and between near and far stations. I repeated the tests with Morris FACETS | 2019 | 4: 407-422 | DOI: 10.1139/facets-2018-0048 412 facetsjournal.com Morris and Vijayan's (2018) data on the number of hares foraging on pine boughs in alder and open habitats. I evaluated whether these differences corresponded with the trade-offs predicted by eq.…”

“…Prey individuals can use a variety of behavioural options to reduce risks of predation while foraging. Prominent among them are (i) time allocation (when and how long to forage), (ii) space use (habitat and patch selection), (iii) apprehension (detecting risk by being alert or vigilant), and (iv) foraging tenacity (capability to maintain feeding rates in the presence of predation risk) (Fraser and Gilliam 1987;Brown and Kotler 2004;Morris and Vijayan 2018). The forager's use of each mechanism will OPEN ACCESS Citation: Morris DW.…”

“…A better option is to merge those experiments with direct observations on foraging animals. Experiments on risk management by snowshoe hares (Lepus americanus) illustrate that, indeed, it is possible to assess space use, time allocation, vigilance, and foraging tenacity directly and simultaneously (Morris and Vijayan 2018). The experiments revealed that snowshoe hares used vigilance and foraging tenacity as their main mechanisms to manage risk.…”