Carnivores usually occur over large areas in low numbers, which sometimes makes them difficult to find. In their recently published article, Hayley Geyle and colleagues assessed the effectiveness of different camera trap survey designs for detecting feral cats and red foxes, and looked at how this influenced their ability to determine whether populations had changed in response to control through time.
Introduced carnivores in Australia
As with other invasive carnivores, feral cats and red foxes have contributed to the decline and extinction of many native species. In Australia, these impacts are most severe for small- to medium-sized, ground-dwelling mammals (~35-5500g), with predation by cats and foxes being identified as a major contributing factor in almost all of the extinctions that have occurred since the arrival of Europeans.
In response, land managers have developed many tools for protecting vulnerable wildlife, including the establishment of predator-free enclosures, translocation of native species to predator-free islands, guardian animals, and lethal control techniques. These approaches can be very expensive to implement, and in some contexts may not work as effectively as desired.
Regardless of what approach is used, it is vital that we monitor the response of introduced carnivores to management interventions to ensure that our approaches are working properly, that we are investing resources wisely, and that we are able to achieve our intended goals.
Monitoring comes with challenges
Cats and foxes can be cryptic, they often have relatively large home-ranges, and their territorial nature means that non-urban populations often occur in low densities. Each of these factors makes them difficult to detect during ecological surveys. To complicate things further, co-occurring carnivores often interact in complex ways, either through competition (and sometimes predation) or through changes to behaviour.
One way to improve our ability to detect these carnivores may be to target our surveys on roads. Several studies have shown that carnivores frequently use roads and tracks, possibly to facilitate movement between open and fragmented habitats such as forest edges, or recently burnt areas where they are able to hunt more effectively.
Case study: Grampians National Park, SE Australia
As part of a collaboration with the Victorian State Government agency Parks Victoria, we set up an experiment to test whether cats and foxes were more easily detected on roads compared with off road locations, and to see if there was any evidence to suggest that these species avoid one another, either spatially (by using different areas of habitat) or temporally (by being active at different times).
The Grampians National Park was an ideal site to carry out this experiment. It supports several threatened species, many of which are likely to be at risk of predation by cats and foxes. Lethal fox control has been occurring in the park for several years, and a cat control program is being trialled later this year. Hence, we wanted to assess whether our survey methods would be useful for detecting population changes in response to these control techniques.
We found that cats and foxes were far more likely to be detected on road-side cameras than cameras set up in adjacent woodland and forest, and that there was no real evidence of avoidance or suppression between the two species. This means that a single, broad survey approach is likely to be useful for monitoring both species within this landscape.
Our comparison of different survey approaches revealed that declines were more reliably detected for both species when monitoring efforts were targeted on roads, and that for cats especially, monitoring of off-road habitats was likely to be ineffective. We also found that we could detect population declines after surveying fewer sites when efforts were targeted on-road. This means that the overall costs of monitoring could be reduced using this approach, as savings can be made on equipment and personnel costs (time in the field), or more sites could potentially be surveyed with the same level of investment.
Implications for monitoring
Our study suggests that targeting roads is likely to be an effective approach for determining whether introduced carnivore populations are declining in response to management interventions, particularly when resources for monitoring are limited, which is a widespread problem in pest and wildlife management.
However, there are limitations with this approach. For example, it would provide little information about the types of habitats used by cats and foxes, and limit the opportunity for concurrent monitoring of predators and prey that typically avoid roads. We urge practitioners to think carefully about the types of questions they are interested in, and to consider whether their monitoring program is sufficient to answer those questions.
Nevertheless, our study provides land managers with an approach to ecological survey design that balances monitoring effort with resource constraints, potentially aiding management questions to be more easily and efficiently answered. Our results may also be applicable to other contexts where land managers are interested in monitoring declines of other species which are known to readily use roads and tracks within more extensive habitat.
Read the full research: “Evaluation of camera placement for detection of free‐ranging carnivores; implications for assessing population changes” in Issue 1:1 of Ecological Solutions and Evidence.