Integrating the costs of plant toxins and predation risk in foraging decisions of a mammalian herbivore

Abstract

Foraging herbivores must satisfy their nutrient requirements in a world of toxic plants while also avoiding predators. Plant toxins and perceived predation risk at food patches should both reduce patch residency time, but the relative strengths of these factors on feeding decisions has rarely been quantified. Using an arboreal generalist herbivore, the common brushtail possum Trichosurus vulpecula, we tested the effects on food intake of the plant toxin, cineole, and regurgitated pellets from one of its predators, the powerful owl Ninox strenua at the small spatial scale of the food patch. We used the giving-up density (GUD) framework, with animals harvesting food items (sultanas) in an inedible matrix (small pebbles). We ran two consecutive field experiments in a eucalypt woodland in eastern Australia, 1 month apart in the same location. In experiment 1, there was a significant interaction between cineole [at 17% of dry matter (DM)] and owl pellets. The GUD was lowest in the absence of both cineole and owl pellet, intermediate in the presence of owl pellet; and highest with cineole ± owl pellet. The effect of owl pellet diminished over time. In experiment 2, only cineole (at 10% DM) increased the GUD significantly. The difference in effect of owl pellet was probably due to both habituation and freshness of the cue. Our study demonstrates the importance of synthesising predator-prey and plant-herbivore ecology to better understand the complex set of constraints influencing foraging herbivores. The greater effect of toxin than fear on possums is likely to be due to its high, but ecologically relevant concentration. This highlights the need to explore the relative and net impacts of a range of concentrations of plant toxins and predation risks.

Survival and causes of mortality for pygmy rabbits (Brachylagus idahoensis) in Oregon and Nevada

“…(Sorensen et al 2005), pygmy rabbits (Shipley et al 2012), cottontails Sylvilagus nuttalli (Shipley et al 2012), and snowshoe hares Lepus americanus (Reichardt et al 1984) decreases with increasing concentrations of PSMs and that PSMs are avoided when higher quality food options are available. Similarly, free-ranging herbivores give up patches of artificial food with higher concentrations of PSMs sooner than patches with lower concentrations of PSMs (Kirmani et al 2010, McArthur et al 2012, Bedoya-Pérez et al 2014a. Selection of artificial diets lower in concentrations of PSMs is consistent with recent field data showing the odds of browsing by pygmy rabbits and sage-grouse on a sagebrush plant increased with decreasing concentrations of monoterpenes (Frye et al 2013, Ulappa et al 2014.…”

“…As a consequence, herbivores must make habitat tradeoffs based on these simultaneous risks (Lima and Dill 1990, Brown and Kotler 2004, McArthur et al 2014. For example, PSMs had a greater effect on foraging patterns of brushtail possums Trichosurus vulpecula (Kirmani et al 2010) and fox squirrels Sciurius niger (Schmidt 2000) than perceived risk of predation. Moreover, brushtail possums left patches of food containing both risks of PSMs and predation sooner (i.e.…”

“…Moreover, brushtail possums left patches of food containing both risks of PSMs and predation sooner (i.e. higher giving up density) than patches with predation risk alone (Kirmani et al 2010). Tradeoffs between PSMs and predation risk have yet to be investigated in a dietary specialist, where higher tolerance to PSMs may influence tradeoffs differently than in generalists.…”

Negotiating multiple cues of predation risk in a landscape of fear: what scares free‐ranging brushtail possums?

“…Thus, although possums have the capacity to detect and perceive risk through olfactory cues (e.g. Kirmani et al ., ), in the current trial they did not perceive the risk to be high enough, or reliable enough, to outweigh the benefits of accessing high‐quality food.…”

“…However, in the wild brushtail possum responses appear variable. Direct cues of predation in the form of predator odours produced weak (Parsons & Blumstein 2010), short term (Kirmani, Banks & McArthur, ) or no response (Russell & Banks, ; Mella, Cooper & Davies, ) in free‐ranging possums. Responses to indirect habitat‐related cues also varied: forest type and open/bush microhabitat had a significant effect on foraging behaviour (Pickett et al ., ), but distance from the safety of trees did not affect foraging (McDonald‐Madden et al ., ).…”