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International audience; Predation risk, the probability that a prey animal will be killed by a predator, is fundamental to theoretical and applied ecology. Predation risk varies with animal behavior and environmental conditions, yet attempts to understand predation risk in natural systems often ignore important ecological and environmental complexities, relying instead on proxies for actual risk such as predator-prey spatial overlap. Here we detail the ecological and environmental complexities driving disconnects between three stages of the predation sequence that are often assumed to be tightly linked: spatial overlap, encounters, and prey capture. Our review highlights several major sources of variability in natural predator-prey systems that lead to the decoupling of spatial overlap estimates from actual encounter rates (e.g., temporal activity patterns, predator and prey movement capacity, resource limitations) and that affect the probability of prey capture given encounter (e.g., predator hunger levels, temporal, topographic, and other environmental influences on capture success). Emerging technologies and statistical methods are facilitating a transition to a more spatiotemporally detailed, mechanistic understanding of predator-prey interactions, allowing for the concurrent examination of multiple stages of the predation sequence in mobile, free-ranging animals. We describe crucial applications of this new understanding to fundamental and applied ecology, highlighting opportunities to better integrate ecological contingencies into dynamic predator-prey models and to harness a mechanistic understanding of predator-prey interactions to improve targeting and effectiveness of conservation interventions.

International audience; Non-consumptive effects of predators result from the cost of responses to perceived risk. Prey modulate risk exposure through flexible habitat selection at multiple scales which, in interaction with landscape constraints, determines their use of risky habitats. Identifying the relative contributions of landscape constraints and habitat selection to risk exposure is a critical first step towards a mechanistic understanding of non-consumptive effects. Here, we provide an integrative multi-scale study of roe deer spatial responses to variable hunting pressure along a landscape gradient of open habitats and dispersed refuges. Between low-risk and high-risk periods, we investigated shifts in 1) home-range location, 2) probability of using risky habitats (between-habitat scale) and 3) distance to the nearest refuge (within-habitat scale). For 2) and 3), we disentangled the contributions of landscape constraints and habitat selection to risky habitat use. We found that when risk was high, roe deer did not shift their home-range, but generally decreased their use of risky habitats, and sometimes reduced their distance to cover (particularly older animals). There was a functional response in between-habitat selection, with animals living in more open landscapes responding more than those living in landscapes with more refuges. However, individuals living in more open landscapes avoided open risky habitat less. Finally, we found that among-individual variation in risk exposure was generally, but not always, minimized by habitat selection across gradients of landscape constraints. To our knowledge, this is the first study simultaneously documenting prey responses to risk at the within-habitat, between-habitat and home-range scales. Our results support the view that between-habitat selection acts at a higher hierarchical level than within-habitat selection, and provide a framework for disentangling the contributions of habitat selection and landscape constraints to risk exposure. Selection cannot always compensate for landscape constraints, indicating a need for further investigation of the processes underlying habitat selection.

Models and experimental studies of the impact of mammalian browsing on plant populations typically use biomass to quantify resource pools and herbivore impacts. For many woody plant populations, however, plant height is a better measure of fitness than biomass. We describe a function that links browsing to shoot and height growth by considering both the direct impact of biomass consumption and the effect of bud-loss. The relative importance of direct consumption and bud-loss for height and shoot growth is tested through experimental clipping. Experimental results are used to parameterize and compare the performance of our function. We show that the impacts of bud-loss are indeed significant for both shoot and height growth and may cause the responses of biomass and height to diverge. We also show that our function results in an improved fit to experimental results when compared with a linear function only describing the direct impact of consumption. These results suggest that the demographic impacts of browsing may diverge from the responses expected for biomass based on herbivore consumption alone. Particularly, height growth can be drastically reduced at low and intermediate levels of consumption if the impacts of bud-loss on shoot growth are high.

The current dramatic increase in atmospheric CO 2 concentration favours C 3 versus C 4 photosynthesis, and although other aspects of environmental conditions come into play, it implies an uncertain future for C 4 grasses. If it has been suggested that the poor quality of C 4 grasses contributed to large mammalian herbivores declines as C 4 grasslands spread from the late Miocene, these investigations of the past have not been matched by a similar attention focused on the future implications of C 4 to C 3 shifts. Here we discuss how these may affect grazing systems, also considering other aspects of C 3 /C 4 differences (productivity, phenology) which might affect herbivore performance. Current knowledge suggests that important changes in herbivore performance could be observed, but is too fragmentary to allow general quantitative conclusions. We urge plant and herbivore ecologists to collectively address these limitations, as the future of grazing systems has important implications for biodiversity and human livelihoods.