Abstract
Emerging evidence increasingly illustrates the importance of a holistic, rather than taxon-specific, approach to the study of ecological communities. Considerable resources are expended to manage both introduced and native mammalian carnivores to improve conservation outcomes; however, management can result in unforeseen and sometimes catastrophic outcomes. Varanid lizards are likely to be apex- or mesopredators, but being reptiles are rarely considered by managers and researchers when investigating the impacts of mammalian carnivore management. Instances of mesopredator release have been described for Varanus gouldii as a result of fox and cat management in Australia, with cascading effects on faunal community structure. A meta-analysis showing extensive dietary niche overlap between varanids, foxes and cats plus a review of experimental and circumstantial evidence suggests mesopredator release of V. gouldii and about five other medium to large species of varanid lizard is likely in other regions. This highlights the need for managers to adopt a whole-of-community approach when attempting to manage predators for sustained fauna conservation, and that additional research is required to elucidate whether mesopredator release of varanids is a widespread consequence of carnivore management, altering the intended faunal responses.
Highlights
The predatory impact of introduced carnivores on native prey has been widely documented; only recently has much attention been given to the indirect effects that result from competitive interactions between predators [1,2]
Some instances of mesopredator release have yielded counterintuitive outcomes; for example, the removal of cats from an island resulted in a decline rather than an increase in the resident seabird colony because rats were released from regulation by cats, resulting in an increase in rat numbers and egg predation rates [4]
Conspicuous examples involving nonmammalian predators include the competitive release of ghost crabs (Ocypode quadrata) after raccoon removal resulting in higher rather than lower sea turtle egg predation [11], mesopredator release of marine fish, elasmobranchs and squid as a consequence of humans over-exploiting top-order predators such as sharks and cases of intraguild predation among raptors leading to population level effects when the dominant predator declined in abundance
Summary
The predatory impact of introduced carnivores on native prey has been widely documented; only recently has much attention been given to the indirect effects that result from competitive interactions between predators [1,2]. Australia has suffered the greatest rate of mammal extinction and range contraction of any continent since European arrival [19] and the introduction and recent establishment of two eutherian carnivores, the red fox (Vulpes vulpes) and feral cat (Felis catus), on the Australian mainland has been implicated in many of these catastrophic declines [2,19 – 24]. Both foxes and cats are actively managed to reduce their impacts on agricultural and/or conservation values in many parts of Australia [21,25].
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