Paws without claws? Ecological effects of large carnivores in anthropogenic landscapes

Abstract

Large carnivores are frequently presented as saviours of biodiversity and ecosystem functioning through their creation of trophic cascades, an idea largely based on studies coming primarily out of relatively natural landscapes. However, in large parts of the world, particularly in Europe, large carnivores live in and are returning to strongly human-modified ecosystems. At present, we lack a coherent framework to predict the effects of large carnivores in these anthropogenic landscapes. We review how human actions influence the ecological roles of large carnivores by affecting their density or behaviour or those of mesopredators or prey species. We argue that the potential for density-mediated trophic cascades in anthropogenic landscapes is limited to unproductive areas where even low carnivore numbers may impact prey densities or to the limited parts of the landscape where carnivores are allowed to reach ecologically functional densities. The potential for behaviourally mediated trophic cascades may be larger and more widespread, because even low carnivore densities affect prey behaviour. We conclude that predator-prey interactions in anthropogenic landscapes will be highly context-dependent and human actions will often attenuate the ecological effects of large carnivores. We highlight the knowledge gaps and outline a new research avenue to study the role of carnivores in anthropogenic landscapes.

Keywords: behaviourally mediated effects; density-mediated effects; human-dominated landscapes; recolonizing carnivores; trophic cascades.

Figure 1.

Evidence for large-carnivore-induced trophic cascades is biased towards a few study systems, as indicated by a search on the string ‘trophic cascade’ on Web of Science (4 May 2015). The search generated 1102 papers, of which 55 concerned terrestrial large carnivores (>15 kg) that either trigger trophic cascades or significantly affect the behaviour or abundance of their prey or mesopredators. (Online version in colour.)

Figure 2.

Apex predators can cause tritrophic cascades (symbolized by wolf–deer–tree) and mesopredator cascades (symbolized by wolf–fox–hare–tree). Bottom-up factors (upward arrows) can influence these trophic interactions in the opposite direction. Humans influence all components of this food web directly by altering animal densities or behaviour, and indirectly by influencing food resources.

Figure 3.

Large carnivores increasingly recolonize European landscapes, illustrated here for wolf based on Chapron et al. [4] and updated with recent confirmed sightings. Two main gradients can seriously alter carnivore impact in European landscapes: (i) anthropogenic effects as indicated by a human footprint index [83] and (ii) net primary production (NPP) [84]. The functional role of carnivores in high-human-footprint and high-NPP areas is largely unknown.