An intriguing aspect of social foraging behaviour is that large groups are often no better at capturing prey than are small groups, a pattern that has been attributed to diminished cooperation (i.e., free riding) in large groups. Although this suggests the formation of large groups is unrelated to prey capture, little is known about cooperation in large groups that hunt hard-to-catch prey. Here, we used direct observations of Yellowstone wolves (Canis lupus) hunting their most formidable prey, bison (Bison bison), to test the hypothesis that large groups are more cooperative when hunting difficult prey. We quantified the relationship between capture success and wolf group size, and compared it to previously reported results for Yellowstone wolves hunting elk (Cervus elaphus), a prey that was, on average, 3 times easier to capture than bison. Whereas improvement in elk capture success levelled off at 2–6 wolves, bison capture success levelled off at 9–13 wolves with evidence that it continued to increase beyond 13 wolves. These results are consistent with the hypothesis that hunters in large groups are more cooperative when hunting more formidable prey. Improved ability to capture formidable prey could therefore promote the formation and maintenance of large predator groups, particularly among predators that specialize on such prey.

Aim Non-native reptiles are often detrimental to native communities and ecosystems and can be extremely difficult to manage once established. Thus, there is considerable interest in predicting the likelihood of establishment of nonnative reptiles. We assessed three hypotheses describing possible factors contributing to the successful establishment of introduced reptiles in an effort to better identify potential invaders.Methods Using a global invasion database (1307 introductions of 398 species) and Bayesian generalized linear mixed models, we tested the relative importance of event-level (e.g. propagule pressure), location-level (e.g. climate matching) and species-level (e.g. parthenogenesis) factors in reptile establishment success. ResultsThe factors that positively influenced establishment success included: (i) Event-level: longer time since initial introduction, greater number of introduction events and intentional introductions; (ii) Location-level: smaller differences in latitude between native and introduced ranges and the presence of native congeners in the introduced range; and (iii) Species-level: smaller body size, herbivores, larger native range size, parthenogenesis and high fecundity.Main conclusions We found that location-level factors were most important in describing reptile establishment success, followed by event-and species-level factors, respectively. This pattern matches closely with what others have found in a variety of vertebrate taxa. However, the importance of species traits may be underestimated considering the insufficient knowledge of reptile life history within introduced ranges. Importantly, individual variables from all three hypotheses contributed to global reptile establishment. Managers should be especially cognizant of small herbivorous and fecund reptiles that are frequently introduced into areas with a strong climate match to their native range. Further, parthenogenesis greatly facilitated establishment, indicating that obligate parthenogenetic species may become ubiquitous through modern globalized trade.

Trophic interactions are a fundamental topic in ecology, but we know little about how competition between apex predators affects predation, the mechanism driving top-down forcing in ecosystems. We used long-term datasets from Scandinavia (Europe) and Yellowstone National Park (North America) to evaluate how grey wolf (Canis lupus) kill rate was affected by a sympatric apex predator, the brown bear (Ursus arctos). We used kill interval (i.e. the number of days between consecutive ungulate kills) as a proxy of kill rate. Although brown bears can monopolize wolf kills, we found no support in either study system for the common assumption that they cause wolves to kill more often. On the contrary, our results showed the opposite effect. In Scandinavia, wolf packs sympatric with brown bears killed less often than allopatric packs during both spring (after bear den emergence) and summer. Similarly, the presence of bears at wolf-killed ungulates was associated with wolves killing less often during summer in Yellowstone. The consistency in results between the two systems suggests that brown bear presence actually reduces wolf kill rate. Our results suggest that the influence of predation on lower trophic levels may depend on the composition of predator communities.

Summary Prey switching occurs when a generalist predator kills disproportionately more of an abundant prey species and correspondingly spares a rarer species. Although this behaviour is a classic stabilizing mechanism in food web models, little is known about its operation in free‐living systems which often include dangerous prey species that resist predation. We used long‐term (1995–2015) data from a large mammal system in northern Yellowstone National Park, USA, to understand how prey preference of a wild, generalist predator (Canis lupus) responds to a shift in prey species evenness involving rising numbers of dangerous prey (Bison bison) and dropping numbers of relatively safer prey (Cervus elaphus). Contrary to the prey switching hypothesis, wolves attacked and killed disproportionately more of the rarer, but safer, species. Wolves maintained a strong preference against bison even when this species was nearly twice as abundant as elk. [Correction added after online publication on 26 April 2017: ‘more than’ changed to ‘nearly’]. There was also evidence that wolves were increasingly averse to hunting bison as relative bison abundance increased. Wolves seldom hunted bison because capture success was limited to a narrow set of conditions: larger packs (>11 wolves) chasing smaller herds (10–20 bison) with calves. Wolves scavenged bison carrion instead and did so more frequently as bison abundance increased. Our study demonstrates the overarching importance of prey vulnerability to understanding the prey preferences of generalist predators in ecological communities with dangerous prey. The formidable defences of such prey diminish the potential for switching and its stabilizing influence on population dynamics. In these communities, shifts from hunting to scavenging are perhaps more likely than shifts in prey preference. The assumption of switching may therefore overestimate the stability of multi‐prey systems that include dangerous prey species. A http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.12866/suppinfo is available for this article.