Reducing big predators encourages little ones

The removal of feral cats and foxes in an effort to protect native bird populations may be counterproductive, placing more pressure on them by fuelling a boom in rat and mouse numbers.

That’s the dispiriting finding arising from an Australian review of existing studies, conducted by Graham Fulton from the University of Queensland’s Centre for Biodiversity and Conservation Science and published in the journal Pacific Conservation Biology.

The review took in some 300 papers on the predation of nestling Australian birds and uncovered some complex interactions between medium-sized predators, nests and habitat – prompting a call for more detailed research.

“In Australia, the introduction of foxes and cats, along with the reduction in dingo numbers, is thought to have adversely impacted a range of smaller animals,” Fulton says.

A widely used conservation strategy is therefore to selectively remove introduced medium-sized predators in order to reduce the pressure on prey such as nestling birds. Unfortunately, culling exotics such as feral cats (Felis catus) and the red fox (Vulpes vulpes) can kick-start a phenomenon known as “mesopredator release” – a phenomenon in which smaller predators increase in numbers following the removal of larger ones.

Fulton’ s review details a number of examples in which the removal of both cats and foxes results in increased predation on nestlings by the black rat (Rattus rattus) and even the innocuous looking house mouse (Mus musculus).

Fulton concludes that the net effect of the removal of introduced predators may be negligible in terms of reducing nest predation. The research examines other ways excessive nest predation could be managed in a conservation context, including habitat, and the identity and roles of nest predators.

“It’s not only predators which are to blame, humans are too,” Fulton says.

“Birds such as currawongs,  butcherbirds, the Australian magpie and corvids have increased their range and numbers as a result of the clearing of forest and woodlands.  These same birds are known predators of nestlings and eggs.”

In effect, this is also mesopredator release, but driven by habitat modification rather than the removal of a higher predator.

And it’s not only these “usual suspects” getting in on the act. Unexpected nest predators  captured on camera, or by direct observation, include popular  garden birds such as the grey shrike-thrush (Colluricincla harmonica) and even a tiny honeyeater called the eastern spinebill (Acanthorhynchus tenurostris).

Fulton also points out that most studies to date focus on one predator, and nestling birds are subject to predation by a range of predators, known and novel, and future studies could take this net effect into account.