Feral cats are more abundant under severe disturbance regimes in an Australian tropical savanna

Abstract

ContextThere is an increasing awareness that feral cats play a key role in driving the ongoing decline of small mammals across northern Australia; yet, the factors that control the distribution, abundance and behaviour of feral cats are poorly understood. These key knowledge gaps make it near-impossible for managers to mitigate the impacts of cats on small mammals. AimsWe investigated the environmental correlates of feral cat activity and abundance across the savanna woodlands of Melville Island, the larger of the two main Tiwi Islands, northern Australia. MethodsWe conducted camera-trap surveys at 88 sites, and related cat activity and abundance to a range of biophysical variables, either measured in the field or derived from remotely sensed data. Key resultsWe found that feral cat activity and abundance tended to be highest in areas characterised by severe disturbance regimes, namely high frequencies of severe fires and high feral herbivore activity. ConclusionsOur results have contributed to the growing body of research demonstrating that in northern Australian savanna landscapes, disturbance regimes characterised by frequent high-severity fires and grazing by feral herbivores may benefit feral cats. This is most likely to be a result of high-severity fire and grazing removing understorey biomass, which increases the time that the habitat remains in an open state in which cats can hunt more efficiently. This is due to both the frequent and extensive removal, and longer-term thinning of ground layer vegetation by severe fires, as well as the suppressed post-fire recovery of ground layer vegetation due to grazing by feral herbivores. ImplicationsManagement that reduces the frequency of severe fires and the density of feral herbivores could disadvantage feral cat populations on Melville Island. A firm understanding of how threatening processes interact, and how they vary across landscapes with different environmental conditions, is critical for ensuring management success.

Cat ecology, impacts and management in Australia

“…The synergistic interaction between invasive predators and major disturbances has been similarly documented in Victorian forests (Hradsky et al 2017). In this special issue, Davies et al (2020) extend the generality of these observations yet further, by showing that the abundance of feral cats on Melville Island, Northern Territory, is positively related to increased disturbance from high frequencies of intense fire and heavy grazing by feral stock. This corroborates another recent study from northern Australia that found that feral cats were less likely to be present in productive habitats unless those habitats experienced high fire activity (Stobo-Wilson et al 2020a).…”

Patterns of niche contraction identify vital refuge areas for declining mammals

“…High fire frequency can lower vegetation cover and reduce mid‐storey vegetation complexity (Russell‐Smith, Price, & Murphy, 2010; Woinarski, Risler, & Kean, 2004), factors that are thought to be important components of habitat quality for C. penicillatus and T. vulpecula (Davies et al., 2017; Geyle et al., 2019; Stobo‐Wilson et al., 2019). Similarly, research has also demonstrated that simplification of understorey vegetation by large exotic herbivores and frequent high‐intensity fire increases the density of feral cats (Davies et al, 2020), while at the same time reducing the ability of small mammals to avoid predators (Legge et al., 2019). Applying a niche lens does not specifically identify threatening processes, but can illustrate how the breadth of conditions (e.g.…”

Variation in feral cat density between two large adjacent islands in Australia’s monsoon tropics

“…comm.). By comparison, there have been 65 recent detections of feral cats on Melville Island from 24 010 camera-trap nights (0.27 cats per 100 camera-trap nights) (Davies et al 2020). As such, the detection rate of feral cats has been more than 30 times higher on Melville Island compared with the adjacent Bathurst Island (i.e.…”

“…The Tiwi Islands, located in the Australian monsoon tropics, remain a stronghold for numerous species that are vulnerable to predation by cats, and are one of the last remaining areas in Australia to support an intact native mammal assemblage. However, there are worrying initial signs of decline for the northern brown bandicoot (Isoodon macrourus), black-footed tree-rat (Mesembriomys gouldii), and brush-tailed rabbit-rat (Conilurus penicillatus) (Davies et al 2018), with evidence that cats have contributed to the decline of at least one species (brush-tailed rabbit-rat, Davies et al 2017), as well as evidence that feral cat activity and abundance are higher in areas that have experienced frequent severe fires and heavy grazing by feral herbivores (Davies et al 2020). Importantly, fire activity is spatially variable across both Melville and Bathurst Island (the two largest of the Tiwi Islands), and feral herbivores are only present on Melville Island.…”