Anke S. K. Frank

Experimental evidence that feral cats cause local extirpation of small mammals in Australia's tropical savannas

Small mammal species are declining across northern Australia. Predation by feral cats Felis sylvestris catus is one hypothesised cause. Most evidence of cat impacts on native prey comes from islands, where cat densities are often high, but cats typically occur at low densities on mainland Australia. We conducted a field experiment to measure the effect of predation by low-density cat populations on the demography of a native small mammal. We established two 12·5-ha enclosures in tropical savanna in the Northern Territory. Each enclosure was divided in half, with cats allowed access to one half but not the other. We introduced about 20 individuals of a native rodent, Rattus villosissimus, into each of the four compartments (two enclosures × two predator-access treatments). We monitored rat demography by mark-recapture analysis and radiotracking, and predator incursions by camera surveillance and track and scat searches. Rat populations persisted over the duration of the study (18 months) in the predator-proof treatment, where we detected no predator incursions, but declined to extinction in both predator-accessible compartments. In one case, cat incursions were frequently detected and the rat population was rapidly extirpated (<3 months); in the other, cat incursions were infrequent, and the population declined more gradually (c. 16 months) due to low recruitment. We detected no incursions by dingoes Canis dingo, the other mammalian predator in the area. Synthesis and applications. This is the first study to provide direct evidence that cats are capable of extirpating small mammals in a continental setting, in spite of their low population densities. This finding supports the hypothesis that predation by feral cats is contributing to declines of small mammals in northern Australia. The conservation management of native small mammals in northern Australia may require intensive control of cat populations, including large cat-free enclosures. This is the first study to provide direct evidence that cats are capable of extirpating small mammals in a continental setting, in spite of their low population densities. This finding supports the hypothesis that predation by feral cats is contributing to declines of small mammals in northern Australia. The conservation management of native small mammals in northern Australia may require intensive control of cat populations, including large cat-free enclosures.

Correlates of recent declines of rodents in northern and southern Australia : habitat structure is critical

Australia has experienced dramatic declines and extinctions of its native rodent species over the last 200 years, particularly in southern Australia. In the tropical savanna of northern Australia significant declines have occurred only in recent decades. The later onset of these declines suggests that the causes may differ from earlier declines in the south. We examine potential regional effects (northern versus southern Australia) on biological and ecological correlates of range decline in Australian rodents. We demonstrate that rodent declines have been greater in the south than in the tropical north, are strongly influenced by phylogeny, and are consistently greater for species inhabiting relatively open or sparsely vegetated habitat. Unlike in marsupials, where some species have much larger body size than rodents, body mass was not an important predictor of decline in rodents. All Australian rodent species are within the prey-size range of cats (throughout the continent) and red foxes (in the south). Contrary to the hypothesis that mammal declines are related directly to ecosystem productivity (annual rainfall), our results are consistent with the hypothesis that disturbances such as fire and grazing, which occur in non-rainforest habitats and remove cover used by rodents for shelter, nesting and foraging, increase predation risk. We agree with calls to introduce conservation management that limits the size and intensity of fires, increases fire patchiness and reduces grazing impacts at ecological scales appropriate for rodents. Controlling feral predators, even creating predator-free reserves in relatively sparsely-vegetated habitats, is urgently required to ensure the survival of rodent species, particularly in northern Australia where declines are not yet as severe as those in the south.

Interactions of grazing history, cattle removal and time since rain drive divergent short-term responses by desert biota.

Arid grasslands are used worldwide for grazing by domestic livestock, generating debate about how this pastoral enterprise may influence native desert biota. One approach to resolving this question is to experimentally reduce livestock numbers and measure the effects. However, a key challenge in doing this is that historical grazing impacts are likely to be cumulative and may therefore confound comparisons of the short-term responses of desert biota to changes in stocking levels. Arid areas are also subject to infrequent flooding rainfalls that drive productivity and dramatically alter abundances of flora and fauna. We took advantage of an opportunity to study the recent effects of a property-scale cattle removal on two properties with similarly varied grazing histories in central Australia. Following the removal of cattle in 2006 and before and after a significant rainfall event at the beginning of 2007, we sampled vegetation and small vertebrates on eight occasions until October 2008. Our results revealed significant interactions of time of survey with both grazing history and grazing removal for vascular plants, small mammals and reptiles. The mammals exhibited a three-way interaction of time, grazing history and grazing removal, thus highlighting the importance of careful sampling designs and timing for future monitoring. The strongest response to the cessation of grazing after two years was depressed reproductive output of plants in areas where cattle continued to graze. Our results confirm that neither vegetation nor small vertebrates necessarily respond immediately to the removal of livestock, but that rainfall events and cumulative grazing history are key determinants of floral and faunal performance in grassland landscapes with low and variable rainfall. We suggest that improved assessments could be made of the health of arid grazing environments if long-term monitoring were implemented to track the complex interactions that influence how native biota respond to grazing.

Habitat use and behaviour of cattle in a heterogeneous desert environment in central Australia

The activities of livestock in arid environments typically centre on watering points, with grazing impacts often predicted to decrease uniformly, as radial piospheres, with distance from water. In patchy desert environments, however, the spatial distribution of grazing impacts is more difficult to predict. In this study sightings and dung transects are used to identify preferred cattle habitats in the heterogeneous dune system of the Simpson Desert, central Australia. The importance of watering points as foci for cattle activity was confirmed and it was shown that patchily distributed gidgee woodland, which comprises only 16% of the desert environment, is the most heavily used habitat for cattle away from water and provides critical forage and shade resources. By contrast, dune swales and sides, which are dominated by shade- and forage-deficient spinifex grassland and comprise >70% of the available habitat, were less utilised. These results suggest that habitat use by cattle is influenced jointly by water point location and by the dispersion of woodland patches in a resource-poor matrix. The findings were used to build a modified conceptual model of cattle habitat use which was compared with an original piosphere model, and the consequences for wildlife in environments where the model applies are discussed.

Habitat‐ and rainfall‐dependent biodiversity responses to cattle removal in an arid woodland–grassland environment

Biodiversity conservation in rangeland environments is often addressed by removing livestock, but inconsistent responses by biota mean that the efficacy of this form of management is hotly debated. Reasons for this inconsistency include the usually short duration and small spatial scale of manipulations compared to the area of grazing properties, as well as divergent responses amongst biota. In low-productivity arid environments, the pulse-reserve dynamic also complicates the outcome of manipulations. Here, we tested and extended these ideas in a heterogeneous desert environment in central Australia that consists of small patches of open woodland (gidgee) in a grassland (spinifex) matrix. Taking advantage of a controlled property-scale removal of cattle, and a rain event that stimulated productivity, we first quantified differences in the vegetation and small vertebrates of these two habitats, and then tracked the diversity, composition, and abundance of these biota for 6–19 months post-rain. We predicted that the two habitats would differ in the structure, composition, and reproductive output of their constituent plant species. We predicted also that the effects of cattle removal would interact with these habitat differences, with the abundance, richness, and diversity of small mammals and reptiles differing across habitats and grazing treatments. As anticipated, plant species composition in woodland was distinct from that in grassland and varied over time. The effects of cattle removal were habitat specific: Plant composition responded to de-stocking in woodland, but not in grassland; flowers were more abundant, and palatable plant cover also was greater following cessation of grazing pressure. The responses of small mammals but not reptiles showed some accord with our predictions, varying over time but inconsistently with treatment, and perhaps reflected high variability in capture success. We conclude that the timing and length of sampling are important when evaluating the responses of biota to livestock removal, as is the inclusion of all key habitats in the sampling regime.

A Nose for Death: Integrating Trophic and Informational Networks for Conservation and Management

Non-lethal management of wildlife, both ‘problem wildlife’ and pest species, to protect crops and threatened species is becoming increasingly important as non-human animals and humans come into closer proximity. A particularly promising approach is to apply predator scents to manipulate the cost/benefit ratio that influences the behavioral decisions made by prey and other predators about where to forage or rest. However, such olfactory manipulations are not always successful. Using insights from size-structured food webs, we develop a novel integrative model of the information that animals acquire from eavesdropping on predator and conspecific scents. We show how animals can use the information content in predator scents to derive knowledge of other predators and competitors and thus influence their decision to stay in or leave an area. This model framework clarifies how predator scents can influence all trophic levels, from interference competition directed at smaller predators, to predation and herbivory, and exploits direct and indirect pathways to promote landscapes of fear that influence spatial and temporal patch use in target animals. We illustrate how the application of this conceptual model can focus future research to enhance the use of predator scent-based deterrents in conservation and management. This integrated model shows great promise for addressing wildlife management concerns and for eventually improving the success and efficacy of traditional management techniques.

Multiple cameras required to reliably detect feral cats in northern Australian tropical savanna: an evaluation of sampling design when using camera traps.

Abstract Context. Feral cats are a major cause of mammal declines and extinctions in Australia. However, cats are elusive and obtaining reliable ecological data is challenging. Although camera traps are increasingly being used to study feral cats, their successful use in northern Australia has been limited. Aims. We evaluated the efficacy of camera-trap sampling designs for detecting cats in the tropical savanna of northern Australia. We aimed to develop a camera-trapping method that would yield detection probabilities adequate for precise occupancy estimates. Methods. First, we assessed the influence of two micro-habitat placements and three lure types on camera-trap detection rates of feral cats. Second, using multiple camera traps at each site, we examined the relationship between sampling effort and detection probability by using a multi-method occupancy model. Key results. We found no significant difference in detection rates of feral cats using a variety of lures and micro-habitat placement. The mean probability of detecting a cat on one camera during one week of sampling was very low (p = 0.15) and had high uncertainty. However, the probability of detecting a cat on at least one of five cameras deployed concurrently on a site was 48% higher (p = 0.22) and had a greater precision. Conclusions. The sampling effort required to achieve detection rates adequate to infer occupancy of feral cats by camera trap is considerably higher in northern Australia than has been observed elsewhere in Australia. Adequate detection of feral cats in the tropical savanna of northern Australia will necessitate inclusion of more camera traps and a longer survey duration. Implications. Sampling designs using camera traps need to be rigorously trialled and assessed to optimise detection of the target species for different Australian biomes. A standard approach is suggested for detecting feral cats in northern Australian savannas.

Does Historical Coexistence with Dingoes Explain Current Avoidance of Domestic Dogs? Island Bandicoots Are Naïve to Dogs, unlike Their Mainland Counterparts

Introduced predators have a global reputation for causing declines and extinctions of native species. Native prey naiveté towards novel predators is thought to be a key reason for predator impacts. However, naiveté is not necessarily forever: where coexistence establishes, it is likely that naiveté will be reduced through adaptation, and the once alien predator will eventually become recognised by prey. For example, native marsupial bandicoots in Sydney avoid backyards with domestic dogs (C. lupus familiaris), but not domestic cats (Felis catus), even though cats and dogs were both introduced about 200 years ago (Carthey and Banks 2012). The authors attributed bandicoots’ recognition of dogs to long-term exposure to a close relative of dogs, dingoes that arrived in Australia 4000 years ago. Here, we test a prediction of this hypothesis by taking the study to Tasmania, where dingoes have never been present but where domestic dogs also arrived about 200 years ago. We use a similar survey design to that of Carthey and Banks (2012): asking Hobart residents to report on pet-ownership, bandicoot sightings and scats within their backyards, as well as an array of yard characteristic control variables. We predicted that if long term experience with dingoes enabled mainland bandicoots to recognise domestic dogs, then Tasmanian bandicoots, which are inexperienced with dingoes, would not recognise domestic dogs. Our results indicate that Tasmanian bandicoots are naïve to both dogs and cats after only 200 years of coexistence, supporting our hypothesis and the notion that naiveté in native prey towards alien predators (as observed on the mainland) may eventually be overcome.

Cattle removal in arid Australia benefits kangaroos in high quality habitat but does not affect camels

Removing cattle as a management tool to conserve biodiversity may not necessarily alter grazing impacts on vegetation if other introduced or native herbivores move in and replace the cattle after removal. This study investigated whether there was a difference in the abundance of native red kangaroos (Osphranter (Macropus) rufus) and introduced feral camels (Camelus dromedarius) on arid rangelands where cattle had been recently removed compared with where cattle remained. Activity was measured by clearing and weighing dung, and by counting animal sightings. Kangaroos were encountered more frequently in high quality habitat (gidgee woodland) where cattle had been recently removed. However, kangaroo dung in newly cattle-free areas comprised only ~1.5% of the weight of cattle dung in this habitat where cattle still grazed, indicating no grazing compensation by the native herbivore. Camels showed no clear preference for particular habitat types but used dune tops usually avoided by kangaroos and cattle. There was no indication of camels using habitats differently in areas where cattle were removed. Camel dung collected across all habitats comprised less than a tenth the weight of cattle dung, but almost five times as much as kangaroo dung. As cattle removal had occurred relatively recently, further monitoring is needed to determine its impact over longer periods, especially through low and high rainfall cycles. Methods to improve the monitoring of large herbivores in the presence and absence of livestock and to assess whether anticipated conservation goals are achieved are discussed.