Decomposing risk: Landscape structure and wolf behavior generate different predation patterns in two sympatric ungulates

Abstract

Abstract. Recolonizing carnivores can have a large impact on the status of wild ungulates, which have often modified their behavior in the absence of predation. Therefore, understanding the dynamics of reestablished predator-prey systems is crucial to predict their potential ecosystem effects. We decomposed the spatial structure of predation by recolonizing wolves (Canis lupus) on two sympatric ungulates, moose (Alces alces) and roe deer (Capreolus capreolus), in Scandinavia during a 10-year study. We monitored 18 wolves with GPS collars, distributed over 12 territories, and collected records from predation events. By using conditional logistic regression, we assessed the contributions of three main factors, the utilization patterns of each wolf territory, the spatial distribution of both prey species, and fine-scale landscape structure, in determining the spatial structure of moose and roe deer predation risk. The reestablished predator-prey system showed a remarkable spatial variation in kill occurrence at the intra-territorial level, with kill probabilities varying by several orders of magnitude inside the same territory. Variation in predation risk was evident also when a spatially homogeneous probability for a wolf to encounter a prey was simulated. Even inside the same territory, with the same landscape structure, and when exposed to predation by the same wolves, the two prey species experienced an opposite spatial distribution of predation risk. In particular, increased predation risk for moose was associated with open areas, especially clearcuts and young forest stands, whereas risk was lowered for roe deer in the same habitat types. Thus, fine-scale landscape structure can generate contrasting predation risk patterns in sympatric ungulates, so that they can experience large differences in the spatial distribution of risk and refuge areas when exposed to predation by a recolonizing predator. Territories with an earlier recolonization were not associated with a lower hunting success for wolves. Such constant efficiency in wolf predation during the recolonization process is in line with previous findings about the naı¨ve nature of Scandinavian moose to wolf predation. This, together with the human-dominated nature of the Scandinavian ecosystem, seems to limit the possibility for wolves to have large ecosystem effects and to establish a behaviorally mediated trophic cascade in Scandinavia.

Prey Selection of Scandinavian Wolves: Single Large or Several Small?

“…Due to their small size (relative to moose) roe deer may not constitute a profitable prey type for wolves if they must actively be searched for at lower densities. In this scenario wolves should only prey on roe deer opportunistically when encountered [59]. An alternative hypothesis is that there are indirect effects of roe deer population density on their vulnerability through intra-specific competition and limitation of resources [60, 61].…”

“…Theoretically, predation on moose would then be further limited as wolves may experience lowered encounter rates with moose in roe deer habitat [64, 65]. However, this hypothesis was not supported in an investigation of wolf predation and space use in Scandinavia [59]. That study used a subset of the data available for the current study in order to investigate the effect of prey density, wolf distribution, and landscape structure on the probability of occurrence of a wolf kill for moose and roe deer.…”

“…In this sense, these two studies may seem to yield contradictory results. However, whereas Gervasi et al [59] modelled the probability of a site being a wolf kill site (as compared to randomly distributed locations within the wolf territory), the current study tested the probability for a known wolf kill being either roe deer or moose and how this was related to the density of each prey species. These studies are therefore complementary and show that 1) wolves do not spend their time in proportion to the spatial variation in roe deer density [59] and, 2) when killing an ungulate prey, this is far more likely to be a roe deer than moose in areas of high roe deer density (this study).…”

“…These results were surprising because other studies have found a strong relationship between snow conditions and the selection of different prey species [8, 23, 78] and our study area seems to have included enough variation in snow depth assumed to be important for the vulnerability of small-sized deer to predation [66, 79, 80]. Previous studies in Scandinavia have shown that increased snow depth resulted in both a higher proportion of a vulnerable segment of the moose population (calves) in wolf kill [59] as well as reduced chase distances of wolves on both moose and roe deer [72]. Obviously, we have currently an insufficient understanding of how winter conditions may affect prey vulnerability and the selection pattern of wolves in Scandinavia.…”

Long‐ and short‐term temporal variability in habitat selection of a top predator

“…Wolves did select habitat on the basis of forest cover, elevation, and slope. These are certainly important features to wolf predation (Mao et al 2005, Bergman et al 2006, Kauffman et al 2007, Gervasi et al 2013 and it is plausible that wolves' preferences with respect to hunting grounds are captured by those features. We suggest that, with respect to the predator-prey relationship, predator habitat selection at small spatial scale, like the one used in our study, is driven by features of the landscape that enhance hunting success, rather than overall prey distribution.…”