Counting the bodies: Estimating the numbers and spatial variation of Australian reptiles, birds and mammals killed by two invasive mesopredators

Abstract

Aim Introduced predators negatively impact biodiversity globally, with insular fauna often most severely affected. Here, we assess spatial variation in the number of terrestrial vertebrates (excluding amphibians) killed by two mammalian mesopredators introduced to Australia, the red fox (Vulpes vulpes) and feral cat (Felis catus). We aim to identify prey groups that suffer especially high rates of predation, and regions where losses to foxes and/or cats are most substantial. Location Australia. Methods We draw information on the spatial variation in tallies of reptiles, birds and mammals killed by cats in Australia from published studies. We derive tallies for fox predation by (i) modelling continental‐scale spatial variation in fox density, (ii) modelling spatial variation in the frequency of occurrence of prey groups in fox diet, (iii) analysing the number of prey individuals within dietary samples and (iv) discounting animals taken as carrion. We derive point estimates of the numbers of individuals killed annually by foxes and by cats and map spatial variation in these tallies. Results Foxes kill more reptiles, birds and mammals (peaking at 1071 km−2 year−1) than cats (55 km−2 year−1) across most of the unmodified temperate and forested areas of mainland Australia, reflecting the generally higher density of foxes than cats in these environments. However, across most of the continent – mainly the arid central and tropical northern regions (and on most Australian islands) – cats kill more animals than foxes. We estimate that foxes and cats together kill 697 million reptiles annually in Australia, 510 million birds and 1435 million mammals. Main conclusions This continental‐scale analysis demonstrates that predation by two introduced species takes a substantial and ongoing toll on Australian reptiles, birds and mammals. Continuing population declines and potential extinctions of some of these species threatens to further compound Australia's poor contemporary conservation record.

Distinctive diets of eutherian predators in Australia

“…The fox was imported into southeastern Australia from Great Britain for sport hunting, becoming established from about 1874 [ 67 ] and subsequently spreading across about 80% of the continent [ 10 , 16 ] and onto ca 50 islands [ 16 ]. It is absent from northern arid regions and monsoonal tropics north of 18°S, and from Tasmania and Kangaroo Island [ 68 , 69 ].…”

“…Symbol size is proportional to the number of samples. Shaded areas represent absences/scarcity for foxes [ 16 ] and dingoes [ 83 ]. Studies of cat diet were somewhat uniformly distributed across the mainland and islands.…”

“…Feral cats are found across the entire continent (except some predator free reserves). Foxes are largely absent from the northern tropics of the continent (green shaded area) and occur at greater densities in southwestern and southeastern forest areas as well as around cities [ 16 ]; as a natural consequence of their geographical range, there was a greater density of fox diet studies carried out in these general areas. The dingo has been removed from around sheep production landscapes (green shaded area) and accordingly there were few dingo diet studies in intensive agricultural zones southwest of the State Barrier Fence in Western Australia and south of the Dingo Barrier Fence (dark lines) where dingoes are either absent or scarce as a consequence of widespread intensive control/exclusion [ 83 ].…”

Integrating terrestrial scavenging ecology into contemporary wildlife conservation and management

“…The Department of Environment and Department of Agriculture intend to trap and humanely euthanize all feral cats (Department of Environment, 2021 ); however, if the feral cat population requires additional measures for eradication, a scavenging study to assess bait type, bait flavor, the impact on non‐target species, and other criteria as outlined above is recommended prior to implementation of a plan. Additionally, along with red foxes, it is estimated that feral cats kill over 2 billion animals per year in Australia (Stobo‐Wilson et al, 2022 ). Invasive vertebrates tend to be highly efficient facultative scavengers (Abernethy et al, 2016 ), so once a species is established, management is often focused around eradication and control (Genovesi, 2005 ), and scavenging studies can be used to guide decision‐making (Figure 2a ).…”

Livestock guardian dog protection of free-range poultry from the red fox

“…The impacts of foxes on livestock, particularly poultry, were recognised within only a few years of their introduction (Saunders et al 2010). Today, it is predicted that there is a population of approximately 1.7 million foxes in Australia (Stobo-Wilson et al 2022), imposing substantial costs on producers and the environment (Bradshaw et al 2021). It is estimated that foxes cause A$28 million loss due to lamb predation, and they represent a significant proportion of the estimated A$46 million spent by broad acre farmers and livestock producers on vertebrate pest control (McLeod 2016).…”