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Despite contributing to the ongoing collapse of native mammal populations across northern Australian savannas, we have limited understanding of the ecological constraints of feral cat population density in this system. Addressing such knowledge gaps is a crucial step towards mitigating the impacts of feral cats, and is particularly important for the large islands off northern Australia that remain as strongholds for numerous species vulnerable to cat predation. Here, we investigated cat density across Melville and Bathurst Island, two large islands in Australia’s monsoon tropics. We deployed large grids (~13 km2) of 70 camera-traps at four locations to investigate how feral cat density varies under different combinations of fire frequency, and feral herbivore presence. Using spatially-explicit capture-recapture models, we estimated feral cat density on Melville Island to be 0.15 cats km−2. We did not record any cat detections on Bathurst Island. Using simulations, we predicted that if cat density on Bathurst Island was equal to that on Melville Island, we would have expected to record 27.9 detections of 9.9 individual cats. Our results, coupled with other recent surveys, suggest that the density of cats is much lower on Bathurst Island than the adjacent Melville Island. The absence of feral herbivores on Bathurst Island may have contributed to this variation in cat density. Management that enhances understorey vegetation density, through feral herbivore control, as well as fire management, could help mitigate the impact of feral cats on northern Australian savanna biodiversity.

Predation by cats (Felis catus) is implicated in the decline and extinction of many Australian mammal species. We estimate the number of mammals killed by cats across Australia through meta-analysis of data on the frequency of mammals in cat diet samples from 107 studies. For feral cats in largely natural landscapes, the spatially-weighted mean frequency of mammals in diet samples was 70% (44% for native species, 34% for introduced species). Frequency was significantly higher on the mainland, and in areas of low temperature and topographic ruggedness. Geographic patterns varied markedly between native and introduced mammals, with native mammals most frequent in northern Australia. We estimate that: (i) 815 million individuals yr-1 are killed by feral cats in natural landscapes, 56% of which are native species; (ii) 149 million individuals yr-1 are killed by unowned cats in highly modified landscapes; and (iii) 180 million individuals yr-1 are killed by pet cats. For the latter two components, mainly introduced species are killed. Collectively, across the three components of the cat population, 1,144 million individuals yr-1 are killed by cats, of which, at least 40% (459 million individuals yr-1) are native species. It remains challenging to interpret this tally in terms of its impact on population viability for Australian mammals, because demographic information is not available for most species. However, our estimate of annual mammal mortality due to cat predation is substantially higher than that due to another key threatening process, land clearing.

Feral cats have been responsible, in part, for the extinction of many species of mammal, bird and reptile globally, especially on islands. Whilst there is extensive evidence of the predatory impacts of cats on mammals and birds, far less is known about their ecological impacts on reptiles, especially in continental situations. We conducted a field experiment to evaluate the impact of feral cats on terrestrial vertebrates in tropical savanna environments of northern Australia. Three experimental treatments were applied to six 64 ha plots to compare and contrast responses of reptile abundance and species richness to predator exclusion and the additive effects of frequent fire. Replicated pitfall-trapping was undertaken in each plot on seven sampling occasions between November 2013 and November 2015. We analysed relative abundance and species richness data using generalized linear mixed models. There was a significant increase in the abundance of reptiles over a two year period in cat-excluded plots with reptile abundance increasing at twice the rate in cat-exclusion plots compared with cat-accessible plots and there was an additive effect of time-since-fire. Cat exclusion had a positive effect on reptile species richness over time, however the evidence for this pattern was weak when seasonal variation was taken into account. Predation by cats, in synergy with other disturbance processes, could adversely impact reptile species and communities elsewhere in the world where feral cats have been established and warrants further investigation.

Urbanisation is causing rapid land-use change worldwide. Populations of freshwater turtles are vulnerable to impacts of urbanisation such as habitat loss, fragmentation and degradation, because many species require interconnected aquatic and terrestrial habitats. Understanding the processes that underpin survival in urban areas is critical in managing species that may vary in their responses to urbanisation. Here, we conducted a mark-recapture study of a common freshwater turtle (Chelodina longicollis) at 20 wetlands over five years across a broad geographical gradient in a large and expanding Australian city. Our aim was to examine relationships between survival and a broad suite of local and landscape environmental variables, and body condition. Using capture-recapture models, we found a positive relationship between the probability of survival of C. longicollis and the proportion of green open space in a 1-km radius around a wetland. There was a positive relationship between survival of female C. longicollis and body condition. Survival probabilities generally did not differ substantially among males, females or juveniles, or seasons. We found evidence of adult turtle mortality resulting from recreational fishing. Our results demonstrate the importance of terrestrial habitat surrounding wetlands for freshwater turtle survival in an urban environment. Our results suggest that management actions for C. longicollis in urban areas need to protect green spaces surrounding wetlands (e.g. parks and remnant vegetation) and discourage human actions that threaten turtle survival. Our study adds to mounting evidence that conserving freshwater turtle populations in urban areas requires managers to consider life cycle requirements over broad spatiotemporal scales.

Feral cats (Felis catus) have devastated wildlife globally. In Australia, feral cats are implicated in most recent mammal extinctions and continue to threaten native species. Cat control is a high-profile priority for Australian policy, research and management. To develop the evidence-base to support this priority, we first review information on cat presence/absence on Australian islands and mainland cat-proof exclosures, finding that cats occur across >99.8% of Australia's land area. Next, we collate 91 site-based feral cat density estimates in Australia and examine the influence of environmental and geographic influences on density. We extrapolate from this analysis to estimate that the feral cat population in natural environments fluctuates between 1.4 million (95% confidence interval: 1.0–2.3 million) after continent-wide droughts, to 5.6 million (95% CI: 2.5–11 million) after extensive wet periods. We estimate another 0.7 million feral cats occur in Australia's highly modified environments (urban areas, rubbish dumps, intensive farms). Feral cat densities are higher on small islands than the mainland, but similar inside and outside conservation land. Mainland cats reach highest densities in arid/semi-arid areas after wet periods. Regional variation in cat densities corresponds closely with attrition rates for native mammal fauna. The overall population estimate for Australia's feral cats (in natural and highly modified environments), fluctuating between 2.1 and 6.3 million, is lower than previous estimates, and Australian feral cat densities are lower than reported for North America and Europe. Nevertheless, cats inflict severe impacts on Australian fauna, reflecting the sensitivity of Australia's native species to cats and reinforcing that policy, research and management to reduce their impacts is critical.

In recent decades severe mammal declines have occurred in the vast and uncleared tropical savannas of northern Australia. Mounting evidence suggests that feral cats (Felis catus), large feral herbivores and increased frequency of high-severity fires, are all contributing to declines; however, the respective influence of each threat remains unclear. There is an urgent need to quantify the relative impacts of both ‘bottom-up’ (i.e. the depletion of resources and habitat simplification from contemporary disturbance regimes) and ‘top-down’ (i.e. increased predation pressure) factors on small mammal populations to inform where, and how, remedial conservation efforts should be targeted in northern Australia. We conducted an extensive survey of mammals across ca. 370,000 km2 of monsoonal northern Australia using both camera-trapping and live-trapping methods. From multispecies occupancy models, we found that feral herbivore abundance, dingo abundance and feral cat occupancy were the best predictors of species richness of small (

Roads are detrimental to wildlife populations that require contiguous networks of terrestrial and aquatic habitats. Many species of freshwater turtles are sensitive to habitat fragmentation caused by roads, and are susceptible to road mortality during overland migrations. The common long-necked turtle (Chelodina longicollis) is an Australian freshwater turtle that frequently moves between wetlands, and so populations may incur negative impacts from road effects. Here, we assessed the relationship between C. longicollis and road density and landscape variables within populations inhabiting 20 wetlands distributed throughout greater Melbourne, Australia. The size frequency distribution of C. longicollis at sites surrounded by high road densities was skewed towards larger individuals, but there was no difference in the frequency of juveniles between high and low road density sites. Regression modelling revealed a clear positive relationship between road density and carapace length (CL) of C. longicollis; the mean CL at a site with the highest road density was predicted to be 23% greater than mean CL at a site surrounded by no roads. Female CL was also positively related to road density. There was a clear positive relationship between wetland age and CL, although this relationship was not as strong. While there was no relationship evident between road density and the proportion of female C. longicollis at a site, more females were captured at smaller ephemeral sites surrounded by a high proportion of green open space and located near drainage lines. We did not find evidence of sex-related differences in road effects. These results suggest that roads may be affecting C. longicollis in the study area, but the direct cause of any effects is difficult to identify.

Context Small- and medium-sized native mammals have suffered severe declines in much of northern Australia, including within protected areas such as Kakadu National Park. Several factors have been implicated in these declines but predation, particularly by feral cats (Felis catus), has been identified as potentially the most direct cause of decline for many species. Aims We evaluated how prey frequency changed in cat and dingo scats in Kakadu from the early 1980s to 2013–15, with this period spanning a severe decline in the small- and medium-sized mammal fauna. Methods Chi-square test of independence and Fisher’s exact test were used to compare prey frequencies between dingoes and cats, and among years to assess significance of temporal change. Key results Small-sized native mammals were the prey item occurring at the highest frequency in scats for both dingoes and cats in the 1980s. Prey content in dingo and cat scats differed in the 2010s with macropods predominating in the scats of dingoes, and medium-sized native mammals predominating in cat scats. The frequency of occurrence of small-sized native mammals declined in both dingo and cat scats between the 1980s and 2010 sampling periods, while the frequency of occurrence of medium-sized native mammals remained constant in dingo scats and increased in cat scats. Conclusions Small mammals were a major component of the diets of both dingoes and cats in Kakadu in the 1980s, when small mammals were much more abundant. Despite marked reduction from the 1980s to the 2010s in the capture rates of both small- and medium-sized native mammals, some species continue to persist in the diets of cats and dingoes at disproportionally high frequencies. Both predators continue to exert predatory pressure on mammal populations that have already experienced substantial declines. Implications Although predation by feral cats is a major threat to small- and medium-sized native mammals, dingoes may also play an important role in limiting their recovery. Disturbance from fire and grazing by introduced herbivores has been shown to augment predatory impacts of feral cats on native mammals. Predation more generally, not just by feral cats, may be exacerbated by these disturbance processes. Management programs that solely focus on mitigating the impact of feral cats to benefit threatened species may be inadequate in landscapes with other significant disturbance regimes and populations of predators.

Context Feral cats (Felis catus) are a threat to biodiversity globally, but their impacts upon continental reptile faunas have been poorly resolved. Aims To estimate the number of reptiles killed annually in Australia by cats and to list Australian reptile species known to be killed by cats. Methods We used (1) data from >80 Australian studies of cat diet (collectively >10 000 samples), and (2) estimates of the feral cat population size, to model and map the number of reptiles killed by feral cats. Key results Feral cats in Australia’s natural environments kill 466 million reptiles yr–1 (95% CI; 271–1006 million). The tally varies substantially among years, depending on changes in the cat population driven by rainfall in inland Australia. The number of reptiles killed by cats is highest in arid regions. On average, feral cats kill 61 reptiles km–2 year–1, and an individual feral cat kills 225 reptiles year–1. The take of reptiles per cat is higher than reported for other continents. Reptiles occur at a higher incidence in cat diet than in the diet of Australia’s other main introduced predator, the European red fox (Vulpes vulpes). Based on a smaller sample size, we estimate 130 million reptiles year–1 are killed by feral cats in highly modified landscapes, and 53 million reptiles year–1 by pet cats, summing to 649 million reptiles year–1 killed by all cats. Predation by cats is reported for 258 Australian reptile species (about one-quarter of described species), including 11 threatened species. Conclusions Cat predation exerts a considerable ongoing toll on Australian reptiles. However, it remains challenging to interpret the impact of this predation in terms of population viability or conservation concern for Australian reptiles, because population size is unknown for most Australian reptile species, mortality rates due to cats will vary across reptile species and because there is likely to be marked variation among reptile species in their capability to sustain any particular predation rate. Implications This study provides a well grounded estimate of the numbers of reptiles killed by cats, but intensive studies of individual reptile species are required to contextualise the conservation consequences of such predation.