Our findings suggest that devils influence feral cat behaviour, but contrary to our prediction, we did not find any evidence that devils suppress cat abundance (Fig. 4a) and there was no evidence of increased cat abundance in areas where devils had declined the longest (Fig. 5b). As we predicted, observed cat and devil activity separated temporally, with separation less evident in areas where devils had declined the longest (Fig. 6a). Cat activity was more nocturnal in areas where devils had declined the longest (Fig. 6b). This apparent shift presents an emerging threat to nocturnal competitors and potential prey species that may have infrequently encountered cats prior to DFTD.
Contrary to our predictions, we did not find evidence to support a negative relationship between cat and quoll abundance (Fig. 4b). The overlap in cat and quoll activity was greater in areas with higher quoll abundance (Fig. 7). Overlap was also greater over summer than in winter (Fig. 8), implying a high risk of predation for juvenile quolls. We suggest that while cats do not appear to have caused the recent quoll decline, predation of juvenile quolls by cats could be inhibiting low density quoll populations from recovering their former abundance through a ‘predator pit’ effect [58, 59]. Predation intensity could increase further should cats become increasingly nocturnal in response to devil declines.
Devil and cat interactions
Devil and cat abundance did not differ among DFTD regions (Fig. 5b) and we did not find any evidence that devils suppress the abundance of cats (Fig. 4a). Devil abundance did vary among sites within each DFTD region (S1 Table), but the similarity in mean devil abundance among regions could indicate that, below a certain density, DFTD transmission rates are reduced. This accords with findings of the Save the Tasmanian Devil Program (Sam Fox, STTDP, pers. comm.): relatively consistent, very low devil numbers with reduced disease prevalence, have been trapped in areas where DFTD has long been present. At the time of our surveys, DFTD had been present in the study region for between 5 and 16 years.
The similarity in cat abundance among regions was unexpected. There are two likely explanations. First, if devils were suppressing cat abundance prior to DFTD, the high reproductive capacity of feral cats  might have allowed rapid increase in cat abundance following the decline of devils, so that current abundance could reflect the ‘post-release’ abundance across regions, and the similarity in cat abundance could reflect the similarity in devil abundance among regions. If this is the case, cat abundance appears to have plateaued at new equilibrium levels across DFTD regions, with no apparent effect of time since devil decline at our survey sites (Fig. 5b). While we did not find any evidence for devils suppressing or limiting cat abundance (Fig. 4a), it is possible that devil densities could now be too low to be affecting cats across our survey sites, although Saunders  did not find evidence of suppression at DFTD-free sites supporting high devil densities in north-west Tasmania. However, in the absence of reliable cat abundance data prior to DFTD arrival in these regions, we are unable to ascertain if current cat abundance differs from pre-DFTD abundance. An alternative explanation is that devils do not suppress cat abundance, but rather other factors, possibly bottom-up processes, could be more important in determining cat abundance, as shown by Hollings, et al.  for some regions. Different conditions promote or inhibit the transmission of predatory effects, including predator diversity, strength of interactions, ecosystem productivity, presence of refuges and the potential for compensation [1, 61–64]. For example, top-down processes might be more pronounced where there are strong productivity gradients such as in the high arctic or in arid environments, where food is limiting and competition for scarce resources is high [62, 65], while predator removal in highly productive environments can result in weak effects that do not cascade through trophic levels . Accordingly, Tasmania’s overall higher productivity  might promote only weak competitive interactions between devils and cats, thereby dampening any potential mesopredator release following decline of devils. Weak competitive interactions have been observed between large predators and mesopredators in other systems, such as coyotes (Canis latrans) and racoons (Procyon lotor), although the conditions necessary for these species’ coexistence are not understood . Furthermore, the prey size range and feeding ecology of devils and cats is also quite different, with devils (carnivore/scavengers)  unlikely to reduce or limit the availability of smaller live prey species typically hunted by opportunistic predatory cats [69, 70].
The temporal partitioning of observed cat and devil activity suggests that cats could be avoiding devils. With the exception of the early DFTD region, cats were typically crepuscular or diurnal and their activity was largely separated from the nocturnally active devils (Fig. 6a). In the early DFTD region where devil populations had declined the longest, cats were more nocturnal, exhibiting an increased overlap with devil activity (Fig. 6a). In the absence of temporal activity data for cats and devils prior to DFTD arrival in these regions, we are unable to determine if regional differences in temporal activity are a response to declining devils, or if these differences already existed prior to DFTD arrival. However, the differences in observed cat activity between the early and mid DFTD regions (Fig. 6b) are similar to the differences in observed cat activity at sites where devils were present compared with sites where devils were absent (Fig. 6c). This supports the suggestion that observed differences between regions could be a response to declining devils. Further studies are needed in disease-free areas to investigate activity profiles of devils and cats prior to DFTD arrival, and to monitor if and how carnivore activity changes as DFTD spreads through the region.
The apparent response of cat activity to reduced devil abundance involves a delay, which we did not predict. A delayed response by cats could reflect the persistence of innate anti-predator responses to devils, even after selective pressures have been relaxed. For example, black-tailed deer (Odocoilus hemionus sitkensis) retained innate anti-predator responses to wolves (Canis lupus) during a ca. 100 year period of predator absence . Such behaviours could persist in the absence of a predator due to the low fitness costs associated with the behaviour . Given the high availability of alternative abundant prey sources in Tasmania, avoidance of nocturnally active devils is unlikely to result in reduced fitness for cats. However, selective triggers, such as the drought endured in Tasmania during the three years to 2008 [73, 74], could have been sufficient to increase that cost due to reduced food availability, and therefore might have forced cats to extend their hunting activities nocturnally in an effort to find limited food resources. With reduced devil abundance and reduced interference competition, nocturnal activity would now impose minimal costs to cats, enabling them (and subsequently their kittens) to specialise on nocturnal prey , resulting in the gradual shift in cat activity over a few generations. Even in the absence of increasing cat abundance, temporal shifts in cat activity would present an increased predation risk for nocturnally active species such as eastern quolls that may have rarely encountered cats prior to devil decline.
Higher spotlight sightings of cats identified by Hollings, et al.  in the early DFTD region could reflect an increase in detectability rather than an increase in abundance. We did not find any evidence of higher abundance (Fig. 5b), but the increased nocturnal activity of cats observed in the early DFTD region (Figs. 6a and 6b) would likely make the cats more detectable during spotlighting surveys, which take place at night. Furthermore, while we did not find evidence of cats avoiding devils spatially in the current study, our statewide camera surveys were not performed along roads where spatial avoidance might be more evident. If devils suppress cat behaviour through interference competition, cats may have historically avoided roads where devils forage for road kills , resulting in devils being detected, but cats less likely to be detected in vehicle-based spotlight surveys conducted along roads . Following devil decline, cats might now be more active along roads and therefore more detectable in road-based spotlight surveys . Indeed, Lazenby and Dickman  found that devils can alter the detectability of cats along vehicular trails and roads, with the probability of detecting a cat often more than double at sites where devils were not detected than at sites where devils were detected. Future studies analysing GPS-movement data from sympatric devils and cats are needed to better understand the spatial interactions between these species at finer spatio-temporal scales than can be assessed using either camera or spotlight surveys.
The differing interpretations between Hollings, et al.  and this study will, in part, reflect the different collection methods and data analyses adopted. The analysis by Hollings, et al.  of statewide spotlighting data was the first study to investigate broader ecosystem effects of devil decline as they relate to a range of trophic levels, using the best available data at that time. However, spotlight surveys are known to be an unreliable method for monitoring abundance of cryptic species such as feral cats [78, 79]. An inherent weakness of spotlight survey data is that a brief snapshot on a single night each year is likely to miss or underestimate activity that will more easily be detected by remote cameras left in situ for three continuous weeks. While the use of longitudinal spotlight sightings as an index of abundance does allow comparisons to be made before and after DFTD arrival, such data ignores the importance of detectability . Accordingly, such analyses assume that the non-detection of a species means that the species was absent, whereas a non-detection could simply reflect a behaviour that makes that species less detectable in different places or different times. While longitudinal trends from spotlight surveys have been corroborated with alternative methods such as trapping surveys for devils  and eastern quolls , a similar comparison has not been performed for cats in Tasmania. Accordingly, it might be premature to presume an increase in cat sightings reflects an increase in cat abundance.
While cats appeared to avoid devils temporally, we did not find any evidence that this apparent shift in activity led to a reduction in cat abundance (Figs. 4 and 6). Mammalian and avian mesopredators that avoid larger predators through temporal separation of activity can suffer reduced fitness consequences from hunting at sub-optimal times of day, with reduced resource availability and increased energy demands often leading to reduced breeding success and survival [6, 80, 81]. Such costs of avoidance could be predicted to translate into reduced abundance over time. However, the similarity in cat abundance between regions with different cat activity profiles suggests that temporal shifts are not detrimental to cat fitness and abundance (Fig. 5). Accordingly, the apparent temporal avoidance strategy adopted by cats might simply reduce their likelihood of antagonistic encounters with devils, as has been suggested with subordinate predators avoiding dominant lions (Panthera leo) in Africa’s large predator guild , but otherwise provides no net benefit or loss to cat abundance.
Interactions of cats and eastern quolls
The observed activity profiles of eastern quolls differed between sites with high and low quoll densities, but this was not related to cat activity or abundance (Fig. 7). There was greater temporal overlap between cats and quolls at the high-density quoll sites than at the low density sites, but this was a function of differing quoll activity, with no observed difference in cat activity. Given that the increased overlap was observed at higher quoll density sites, there is no indication that it has resulted in an increased predation risk to quolls. This is further supported by our finding that cat and quoll abundance were not related (Fig. 4b).
The difference in quoll activity between high and low-density quoll sites could reflect differences in intraspecific competition for food. A temporal profile similar to the high density quoll sites was observed in the July 2012 camera survey on BI which supports the only confirmed stable, high density population of eastern quolls in Tasmania. The absence of devils and very low abundance of cats at this island site suggest that quoll activity is unlikely to reflect avoidance strategies in response to perceived threats from larger mammalian predators, although avian predators might still influence quoll activity. Accordingly, the similarity in the profiles between BI and the high quoll density sites on mainland Tasmania suggests that top-down processes are not a primary driver of quoll activity and that bottom-up processes are likely to be important. The delayed peak in activity around midnight at the low density sites likely reflects the reduced quoll activity in response to reduced competition for food at these sites, further supporting this hypothesis. However, to understand the influence of bottom-up processes on quoll activity, further information on the spatial and temporal variation in eastern quoll diet and activity of key prey species would be required.
The consistently low number of quolls trapped and detected at the three declined quoll sites confirms that these populations have shown no sign of recovery (Fig. 2). Further declines were observed in both trapping and camera surveys at the JU site during the course of the study (Fig. 3). This decline in quolls coincided with a rapid and complete decline in detections of the Tasmanian bettong (Bettongia gaimardi) at this site, with declines of both species coinciding with the first appearance of cats at the site . A combination of trapping and spotlight surveys failed to detect any cats in bimonthly surveys performed at the site between May 2011 and March 2012 or in a camera survey performed in February 2012. However, once cats were first detected in May 2012, they continued to be frequently detected on camera and regularly trapped (and removed) up to and including the final trapping survey in July 2013 and the final camera survey in October 2013 . It is possible that cats could have been present at the site but undetected prior to May 2012, however this seems unlikely given the consistent results from a range of complementary survey techniques. While the number of quolls detected and trapped dropped rapidly, low numbers of quolls continued to be detected at the site until the end of the study. It might be that quolls at this site were initially naïve to the presence of cats, and were therefore vulnerable to predation when cats first arrived, with surviving quolls learning to avoid cats and enabling a low number of quolls to persist at this site. While these observations suggest that cats could have contributed to both quoll and bettong declines at this site, this evidence is entirely correlative and does not demonstrate causation. The decline in quolls could alternatively reflect bottom-up processes rather than top-down suppression by feral cats. However, as we did not survey prey abundance as part of the current study, we are unable to discern the mechanism(s) responsible for the quoll decline.
While we did not find any association between cats and quolls generally (Fig. 4b), individual cats could have a disproportionate impact. Our statistical assessment assumes that all individuals are ecologically equivalent . Many populations of generalist species, such as feral cats, comprise specialised individuals whose niches are a subset of the population niche [83, 84]. Cats are known to specialise on the type of prey with which they have had prior experience  and thus individual cats can exhibit preferences in the types of prey they hunt . For example, Gibson, et al.  found that predation by two individual feral cats was catastrophic to vulnerable rufous hare-wallaby (Lagorchestes hirsutus) populations released into the Tanami Desert. Once these two individual cats were removed, no further killings occurred during the next 2–3 years, despite the ongoing presence of other cats in the area. Methods such as camera surveys are not appropriate to establish if and how this individual specialisation of cats might influence cat and quoll dynamics, however specialisation by individual cats provides a possible explanation for the observed rapid decline in quolls at JU following cat incursion at this site (Fig. 3). While predation by individual specialist cats remains one candidate agent of local decline, spatial shifts out of the local study area could also have contributed to the observed reduction in quolls at this site. Indeed, two quolls that were frequently captured prior to cat incursion were subsequently recaptured after a 12 month period of no captures following cat arrival. However, as areas surrounding the immediate study site were not monitored in the current study, we are unable to assess the extent to which this might have occurred.
The absence of a summer spike in quoll captures at the three declined sites suggests low or no juvenile recruitment at these sites (Fig. 2). The eastern quoll has a short, highly synchronised mating season each year, resulting in a large influx of newly independent juvenile quolls into the population between November and February each year . Numbers typically start to decline around March and usually reach pre-juvenile abundance by July each year . This characteristic annual cycle was observed at the stable site, but was notably absent at the three declined sites (Fig. 2). Individual female quolls trapped at the declined sites had, on average, more pouch young in July (or September at CM) than quolls at the stable site , indicating that individual reproductive output was not reduced. However we are unable to assess if mortality occurred while young were in dens (between August and November) or when they first emerged as independent juveniles. Demographic modelling will be required to assess whether juvenile recruitment is reduced or absent at declined sites, and whether this reflects reduced reproductive success, or mortality of newly independent or emigrating juveniles.
The last mainland Eastern quoll
In June 2008 I had the privilege of examining the last mainland Eastern quoll. Follow the link to see the photo (and specific terms regarding the usage of this photo - it may not be reproduced).
Mainland Eastern quoll sightings
The last confirmed Eastern quoll on mainland Australia (they still persist on the island state of Tasmania but are now under threat by the recent introduction of foxes there) was discovered as roadkill in the Sydney suburb of Vaucluse in January 1963.
Although Eastern quolls were numerous at the time of European colonisation, their numbers dropped rapidly as they became avid poultry theives and were hence hunted.
Since this time it has been argued that Eastern quolls are in fact beneficial to farmers because they predate on insect species which ravage crops.
At any rate, Eastern quoll numbers seemed to decline dramatically almost uniformly across their range in the late 1950s and early 1960s and some researchers conclude that a disease may have contributed to the species demise, much the same way that devil facial tumour disease (DFTD) is currently affecting Tasmanian devils in Tasmania.
The Vaucluse population persisted, it is believed, because of its geographic isolation from disease-affected populations.
East Kurrajong, 2006
In 2006 a news report was released indicating that a resident of East Kurrajong observed two Eastern quolls on her way to work. Eastern quolls are very similar in appearance to spotted-tailed quolls (see last photo on this page) which are known to persist on the mainland. However in this case the witness researched the differences and then made the effort to contact the National Parks and Wildlife service to insist the animals she saw were Eastern quolls, not spotted-tailed quolls.
This sighting is interesting because it occurred during the time of year when young quolls disperse from their parents. The witness saw two animals - one large and one small - fitting the description of adult and young.
Again, the difficulty with Eastern quoll sightings is that spotted-tailed quolls are so similar. However, spotted-tailed quolls grow much larger and heavier than Eastern quolls, so the fact there were two differently sized animals - with the largest corresponding to that of an adult Eastern quoll (and not spotted-tailed quoll) also strengthens the case for this being a genuine EQ sighting. (A juvenile spotted-tailed quoll at the age corresponding to that time of year would have been about the size of the larger animal; likewise a male-female pair of spotted-tailed quolls would have seen the smaller female still at the size of the larger animal observed.)
Near East Kurrajong, 2000 or 2001
My enquiries into the East Kurrajong sighting revealed another sighting made approximately 5 years prior. In this instance a quoll had been trapped in a chook pen. The property-owner contacted authorities who advised it was probably a spotted-tailed quoll. They released the animal and quoll-proofed their pen. Having read the East Kurrajong sighting in the newspaper 5 years later they again contacted authorities to clarify that the animal they had trapped in their pen was in fact an Eastern quoll, not a spotted-tailed quoll.
Due to agreement to maintain confidentiality I am unable to disclose the name of the witness, source of information or exact location.
North of Sydney, 2006
A second sighting occurred in the same month of the East Kurrajong sighting some tens of kilometres distant. In this case the witness was a wildlife worker and his friend. They were spotlighting at night in search of reptiles when they found an Eastern quoll beside the road. They turned their vehicle around and shone their headlights on the animal for a period of 5 to 10 minutes while they watched it move around only a few metres away. The wildlife worker immediately recognised it as an Eastern quoll and said they were familiar with seeing Eastern quolls decades ago. (This is an interesting point to note because they felt they had seen them in the wild in the 1970s). Being in the habit of observing wildlife he made mental notes about specific features of the animal and then researched it in a textbook some days later. Comparing spotted-tailed quolls with Eastern quolls on facing pages of the book, he had no doubt it was an Eastern qoull. He showed the same pages to his friend and asked him which one they saw. The friend agreed it was an Eastern quoll.
Blue Mountains, date unknown
Once again, whilst pursuing sightings data I conversed with a university researcher who had a colleague who is a veterinary surgeon. The vet observed an Eastern quoll run across their backyard in the Blue Mountains. Although I don't have a specific date I got the impression this was relatively recently; perhaps some time from 2000 onwards. I have no way of knowing whether the vet understood the difference between spotted-tailed quolls and Eastern quolls except that the researcher understood this clearly and related the sighting to me as an Eastern quoll sighting.
Mudgee, circa 1985
In April 2008 I met with an associate professor who has been acquiring mystery animal sightings since 1950. He recalled seeing a news article in about 1985 of an Eastern quoll being captured in the Mudgee area. Whilst he is currently tracking down the article for me, much of his work was destroyed by flooding of his house in 2007.
(The Eastern quoll shown here is the less common black form)
New England area, circa 1990s
Another amatuer researcher reported to me that he recalled an Eastern quoll being trapped in the New England area in the 1990s as part of a wildlife monitoring program. It was his impression that the NSW Parks department was aware of this capture. I have not obtained any further information on this claim.
Northern NSW, May 1996
In a NSW parliamentary hearing, the Honorable I. Cohen asked the Honorable J. W. Shaw about an Eastern quoll sighting reported in mainstream media on 11 May 1996. The news article, in the Daily Telegraph was written by Simon Benson and described biologists employed by State Forests sighting an Eastern quoll in a forest in northern NSW.
The Honorable J. W. Shaw's reply was made in November 1996 and cites the Minister for Land and Water Conservation saying that biologists employed by State Forests have not recorded any sighting of this species in this area. Further, the National Parks and Wildlife Service has not advised State Forests of this species in this area.
Carrai Forest, NSW, circa before 1991
In answering the Honorable I. Cohen's question in NSW parliament (see "Northern NSW, May 1996" above), the Honorable J. W. Shaw cites the Minister for Land and Water Conservation as advising that State Forests is aware of a CSIRO sighting of this species from a vehicle in Carrai State Forest more than 5 years prior to May 1996. Extensive surveys were undertaken by a number of organisations including CSIRO without any further evidence of the species.
Nowra, NSW 1986
One reader of this website reported seeing an Eastern quoll near a picnic area in Minnamurra Falls National Park near Nowra in 1986 in full view of hundreds of visitors plus park rangers. After commenting on this page that "I have no way of knowing whether this witness might have confused a spotted-tailed quoll which have been documented numerous times to become bold around picnic grounds.", the witness contacted me again to clarify: "You are right, at the time I didn't know the difference between the Eastern and Spotted Tail Quoll so I couldn't guarantee that it wasn't the latter although the size (half the size of a cat) and the lighter brown colour makes me still think it was an Eastern Quoll."
Casino, NSW 1980
I have been informed of an account that comes from Casino in about 1980. Allegedly an Eastern quoll was found to have killed 32 fowl on a property over the course of 2 nights. Traps were laid and the animal was caught. The farm owners were sympathetic and released the quoll at a more remote location, but not before taking photographs.
At time of writing I have not yet seen the alleged photographs, but the witness describes the quoll as their favourite animal - perhaps indicating a good chance at a correct identification?
Little River, Victoria, 2005
I have reports of two deceased Eastern quolls being recovered from Victoria - one a roadkill, the other a drowning. In both cases the body is no longer available for examination.
Little River, Victoria, 2008
Two more quolls were discovered as roadkill at Little River in 2008. These were collected and forwarded to Museum Victoria. As with the 2005 specimens they likely originated with the nearby free-range feral-proof sanctuary.
(This is a spotted-tailed quoll, shown here for comparison)
Despite the possibility of a misidentification of spotted-tailed quolls, I believe there is merit in investigating these sightings further. Certainly in a number of cases the witness either explicitly compared spotted-tailed quolls with Eastern quolls, or is of an employment status to suggest they understand this difference and would not intentionally confuse the two.
The two most recent sightings are reasonably near to Sydney, thus making it possible for me to deploy trail cameras in search of Eastern quolls. In March 2008 I purchased my first commercial trail camera unit and began testing with cats in the backyard. The result is this review of the Moultrie GameSpy I-40 infra-red trail camera. It was first deployed in search of Eastern quolls, in the vicinity of the more recent sightings, in April 2008.
Updates on the results of this camera's deployment will be periodically posted on this page. The camera's name is Charlie :)
- All images of fawn Eastern quolls in this article are copyright Andrea Little / Mt Rothwell Sanctuaries. Used with permission.
- The images of the black Eastern quoll and spotted-tailed quoll are copyright Where Light Meets Dark
The black Eastern quoll is part of Secret Creek Sanctuary's breeding program in Lithgow, NSW. The spotted-tailed quoll is a female from Healesville Sanctuary, Victoria - see more of this beautiful animal here. The fawn quolls were all part of Mt Rothwell Sanctuary's breeding program in Victoria.