Heirisson Prong Research Articles

Reintroduction of the greater stick-nest rat (Leporillus conditor) to Heirisson Prong, Shark Bay: an unsuccessful attempt to establish a mainland population

Greater stick-nest rats were reintroduced to Heirisson Prong from Salutation Island at Shark Bay to establish the first mainland population in Western Australia in over 60 years. Forty-eight animals were transferred over two years from August 1999 to a 17-ha enclosure of natural vegetation that excluded foxes and feral cats. This refuge from introduced predators was located within a larger 1200-ha area where these predators were controlled. Stick-nest rats were able to disperse from the refuge to the wider area. The reintroduction was unsuccessful, with the last record in August 2007. Rats were reproducing in most years, yet only 28 recruits were detected over the reintroduction. Mean condition of rats was better at the reintroduction site relative to the source site. Survivorship of successive translocation cohorts was poorer than that of their predecessors, and survivorship of recruits was poorer than that of translocated animals. The most likely explanations for the decline are predation from monitors and small birds of prey within the refuge, and from monitors, small birds of prey and feral cats outside the refuge. An irruption of other rodents immediately before and coinciding with the reintroduction and building rabbit numbers likely contributed to elevated levels of predation from predators.

Irruption and collapse of a population of pale field-rat (Rattus tunneyi) at Heirisson Prong, Shark Bay, Western Australia

Pale field-rats have long disappeared from Australia’s arid and semiarid zones, other than for some Pilbara islands and a single mainland population of indeterminate status and extent identified at Shark Bay in 1968. Hence, it was noteworthy when a field-rat was first caught at Heirisson Prong in 1994, 40 km north-east of the previous location at Shark Bay. Further individuals were caught regularly from late 1995. The population peaked in July–October 2000 (with captures of ~190 individuals per month) and had collapsed by July 2001 (with only the occasional animal caught thereafter). None were caught beyond 2006, despite regular trapping to 2013. This irruption and collapse was beyond the established range of the species and was in atypical habitat. Widespread trapping after the collapse suggested that the population inhabited few localised ‘source’ areas and a broad area of ‘sink’ habitat, with the latter occupied only after extraordinarily high rainfall events leading to higher grass cover. A return to dry years and the consequent loss of cover (aided by an abundant rabbit population) and strong growth in predator numbers (feral cats and small birds of prey) in response to the high number of field-rats appears to have facilitated the collapse.

Predation by feral cats key to the failure of a long-term reintroduction of the western barred bandicoot (Perameles bougainville)

Context Reintroduction of endangered species potentially places them back in contact with putative factors of historical decline, inadvertently providing the opportunity to evaluate their impact. Aims To monitor the long-term progress of a population of western barred bandicoot reintroduced to mainland Australia and to assess factors involved in its eventual local extinction. Methods Bandicoots were reintroduced from offshore Dorre Island to the nearby mainland peninsula of Heirisson Prong in 1995. The narrow neck of the peninsula was fenced to exclude foxes and feral cats from a 1200 ha area, but the area was subject to periodic incursions. There was parallel management of a confined but unsupported population in an in situ 17-ha predator refuge. Bandicoots were assessed for abundance, body condition and reproduction two to four times annually between 1995 and 2010. In addition, perceived threatening processes (drought, disease and the abundance of cats, foxes and rabbits) were monitored. Key results Bandicoots became well established at the site, spreading to all available habitat. Numbers fluctuated strongly, peaking at ~250 in 1999 and then declining to apparent local extinction (with subsequent re-establishment from the refuge), and at ~470 animals in 2006, followed again by extinction. Conclusions Predation by feral cats was implicated as the primary cause of both free-range extinctions and the eventual elimination of all bandicoots from the predator refuge. Other contributing factors in one or more of the declines were a reduction in reproduction and recruitment in bandicoots during a one-in-100-year drought, the impact of overabundant European rabbits on vegetation used by bandicoots for nesting shelter and brief fox incursions at key times. Implications Existing methods of control of feral cats are rendered ineffective in the presence of abundant and diverse native fauna and abundant exotic species (particularly European rabbits). In addition, episodic drought in arid Australia intensifies the impact of predation by restricting reproduction of prey species. These factors hamper the attempts of conservation managers to re-establish vulnerable species at sites other than those with the infrastructure and/or management intensity to largely exclude exotic predators (and preferably European rabbits) over the long-term.

Age estimation of pouch young and growth of the burrowing bettong, Bettongia lesueur, (Marsupialia: Potoroidae) on Heirisson Prong, Shark Bay, Western Australia

Measurements of hind foot length, weight, head length and tail length were collected from pouch young, young at foot, sub-adults and adults of a reintroduced population of the vulnerable burrowing bettong, Bettongia lesueur, on Heirisson Prong, Shark Bay, Western Australia. A key based on qualitative characteristics was used to age individuals and a Richards growth function was fitted to plots of hind foot length versus age. The patterns of growth and morphological characteristics were similar for males and females. Both sexes exhibited a growth pattern typical of a medium-sized macropod with an initial slow phase of growth followed by a rapid phase that slowed as adult size was reached. The growth curves are suitable for broad estimates of the age of wild-caught animals from pouch young to early adulthood.

Control of feral cats for nature conservation. IV. Population dynamics and morphological attributes of feral cats at Shark Bay, Western Australia

The dynamics of feral cats (Felis catus) were assessed at Shark Bay at two adjoining sites subject to differing intensities of predator control. The Heirisson Prong conservation reserve (12 km2) was fenced to exclude predators and was subject to intensive control actions, while a portion of the adjoining Carrarang pastoral lease (60 km2) was subject to a lesser level of control. Foxes (Vulpes vulpes) were largely absent at both sites owing to effective control. Densities of cats were highly variable over time, showing strong annual fluctuations over 14 years of records Three independent estimates of peak density were made, varying between 1.5 and 2.8 km–2. Rate of increase was assessed as 0.98 on the pastoral lease and 0.99 on the conservation reserve (to give an approximate doubling of the population every 8.5 months). A logistic model, with K = 1.5 km–2 and r of 0.98, gave a maximum sustained yield of 0.37 cats km–2 year–1 and a harvest rate of >0.6 cats km–2 year–1 for their elimination in 5 years or less (for K = 2.8 km–2, these values increase to 0.69 and >1.05 km–2 year–1 respectively). Harvest outcomes at both sites were consistent with these models. However, the effort required to maintain a given offtake rate increased 6-fold at low cat densities and offtake by trapping as a function of cat density took the form of a Type 3 functional response. The functional response for cat trapping (the offtake with constant effort per unit time) overlaid against the curve of cat productivity suggested a stable equilibrium point at low cat densities (0.07–0.13 cats km–2). Hence, trapping effort needed to be greatly intensified at low cat densities and/or augmented by other methods of control to eradicate cats from the closed system of the reserve. The strongly male-biased sex ratio of captures at the barrier fence suggested high levels of reinvasion from beyond the harvested area of the pastoral lease and this made effective control in this open system difficult.

Reintroduction and establishment of the western barred bandicoot Perameles bougainville (Marsupialia: Peramelidae) at Shark Bay, Western Australia

The western barred bandicoot was reintroduced to the Australian mainland in 1995 after an absence of at least 60 years. The new population was derived from 14 animals, reintroduced to Heirisson Prong from Dorre Island in Shark Bay, Western Australia. Introduced predators (the European red fox and the feral cat) were controlled at the reintroduction site, but European rabbits were not. A large fenced area of natural vegetation within the reintroduction site was used as a secure refuge from mammalian predators. Bandicoots were released from this predator refuge to the 12 km 2 conservation site. Dispersal from the point of free release was minimal. The reintroduced population has persisted for 4 years and increased, with at least 175 bandicoots recruited to the population in this time. The recapture rate of marked bandicoots was low, suggesting that adult mortality was high. Reproductive output at Heirisson Prong appeared greater than that of the two surviving wild populations on Bernier and Dorre Islands. Litter size was similar, but there was an extended annual breeding season at the reintroduction site. Body condition of reintroduced and wild bandicoots were similar, although there was some indication that reintroduced males may have been in poorer condition than their island counterparts. The litter size of bandicoots increased with a decrease in rabbit abundance, however, bandicoots were able to reproduce, maintain condition, and sustain recruitment to allow the population to increase despite the presence of rabbits. Two fox incursions occurred during the 4-year period of establishment, and feral cats were present on occasion in low numbers. Feral cats may be responsible for a lower rate of population increase than that observed on predator-free Dorre Island. Ongoing predator control is essential for any mainland reintroduction of bandicoots.

Evidence for male-biased dispersal in a reintroduced population of burrowing bettongs Bettongia lesueur at Heirisson Prong, Western Australia.

The pattern of dispersal of burrowing bettongs (Bettongia lesueur) was studied in a population reintroduced to a peninsula protected from exotic predators at Heirisson Prong in Shark Bay, Western Australia. The reintroduced population was growing strongly in numbers and expanding in area during the study. Young were first marked in the pouch and subsequently monitored by trapping and radio-telemetry after independence to establish their movements relative to those of their mothers. B. lesueur on Heirisson Prong dispersed between the ages of 170 and 250 days, coinciding with the period from weaning to sexual maturity. Dispersal was male-biased. Young males dispersed significantly further than young females with mean dispersal distances of 4600 m and 1100 m respectively. Male B. lesueur also frequented significantly more warrens than females, being located at a mean of 0.37 warrens per daily radio-tracking fix compared with a mean of 0.24 per fix for females. Scarring from intraspecific aggression occurred in male bettongs only. Males may disperse to the periphery of the population to escape aggressive interactions with established adult males and visit more warrens to increase mating opportunities. Dispersal in females may be related to resource quality or inbreeding avoidance.

Do European rabbits (Oryctolagus cuniculus) influence the population ecology of the burrowing bettong (Bettongia lesueur)?

The influence of the European rabbit (Oryctolagus cuniculus) on the survival of medium-sized native mammals remains unclear despite 60 years of speculation. Most medium-sized native species that might have been affected by the presence of rabbits are extinct, rare, or endangered. This limits the opportunity to study their interaction with introduced herbivores. We studied the effect of changes in rabbit density on aspects of the ecology of burrowing bettongs (Bettongia lesueur) reintroduced to mainland Australia on Heirisson Prong, Shark Bay, Western Australia. The rabbit population at the site grew to a 10-year high concomitant with a dry summer and low and declining pasture cover. Rabbit browsing led to widespread defoliation of, and subsequent death of, many palatable shrubs. Despite these adverse conditions, reproduction, recruitment and rate of increase of bettongs did not vary with changes in rabbit density, nor did the survival of adult males and females. This work casts doubt on the idea that competition with an introduced herbivore, such as the rabbit, was an important factor in the decline of the burrowing bettong. It highlights the need to understand the ecologies and life histories of native and introduced species in order to manage for the long-term persistence of native species.

Reintroduction of the burrowing bettong Bettongia lesueur (Marsupialia: Potoroidae) to mainland Australia

The burrowing bettong was successfully reintroduced to the Australian mainland in 1992 after an absence of 50 years. The population, derived from 42 individuals translocated from a remnant population on an offshore island, has persisted for over seven years on the Heirisson Prong peninsula at Shark Bay in Western Australia. It has grown to over 260 individuals that are distributed widely through available habitat. The successful management of exotic European foxes and feral cats proved crucial to the outcome of the reintroduction. Factors contributing to the successful management of predators and to a reduction in their impact included: the choice of a narrow peninsula as the site for reintroduction (permitting cost-effective use of predator-proof fencing); effective baiting (fox and cat) and trapping (cat) strategies; the maintenance of an in situ breeding colony (to provide insurance against major loss of free-range animals to predators); choice of high quality habitat (providing reasonable cover and promoting a high and relatively stable rate of increase for bettongs, even during dry years), and choice of a site that was accessible for regular management visits. An abundance of European rabbits at the reintroduction site appeared not to be a limiting factor. The success of this reintroduction has stimulated a range of other reintroductions of endangered mammals to arid and semi-arid sites across Australia, particularly to peninsulas or other sites where exotic predators can be controlled effectively.

The impact of cats and foxes on the small vertebrate fauna of Heirisson Prong, Western Australia. II. A field experiment

The hypothesis that predation by feral cats and introduced foxes reduces population sizes of small, native vertebrates was supported by results of a predator-removal experiment at Heirisson Prong, a semi-arid site in Western Australia. The methods of control used against cats and foxes to protect native mammals reintroduced to Heirisson Prong produced three broad ‘predator zones’: a low-cat and low-fox zone, where foxes were eradicated and spotlight counts of cats declined after intensive cat control; a high-cat and low-fox zone where spotlight counts of cats increased three-fold after foxes were controlled; and a zone where numbers of cats and foxes were not manipulated. Small mammals and reptiles were monitored for one year before and three years after predator control began. Captures of small mammals increased in the low-cat and low-fox zone, but where only foxes were controlled captures of small mammals declined by 80%. In the absence of cat and fox control, captures of small mammals were variable over the sampling period, lower than where both cats and foxes were controlled, yet higher than where only foxes were controlled. The capture success of reptiles did not appear to be related to changes in predator counts. This study presents the first experimental evidence from mainland Australia that feral cats can have a negative impact on populations of small mammals.

The impact of cats and foxes on the small vertebrate fauna of Heirisson Prong, Western Australia. I. Exploring potential impact using diet analysis

The diets of cats (Felis catus) and foxes (Vulpes vulpes) killed during predator control at a semi-arid site in Western Australia were studied to see which prey species may be affected by predation from these introduced predators. The number of items, biomass and frequency of occurrence of each food type in the gut contents from 109 feral cats, 62 semi-feral cats and 47 foxes were used to calculate an Index of Relative Importance for each food category for each predator. Mammals were the most important prey group for all three predators, with rabbit being the most highly ranked prey species. The diets of feral and semi-feral cats were similar in dietary diversity but differed in the frequency of occurrence of some food categories. Native rodents, birds and reptiles occurred more frequently and were ranked higher in the diet of feral cats, and food scraps occurred more frequently in the diet of semi-feral cats. The diet of foxes was less diverse than that of either group of cats. Invertebrates and sheep carrion were more important prey categories for foxes than for cats. In the summer–autumn period, foxes ate more sheep carrion and invertebrates than they did in winter–spring. The diet of feral cats was more diverse in summer–autumn, including a greater range of invertebrates and more rodents, birds and reptiles than in the winter–spring period. We predict that cats are more likely to have an impact on small vertebrates at this site and that the control of cats could lead to recoveries in the populations of native rodents, birds and reptiles. By contrast, the control of foxes alone may lead to a rise in cat numbers and a consequent detrimental impact on small vertebrate populations.

Wedge-tailed Eagle Aquila audax Predation on Endangered Mammals and Rabbits at Shark Bay, Western Australia

SummaryThe diet of Wedge-tailed Eagles Aquila audax was assessed from a single nest on Bernier Island and two nests on Heirisson Prong at Shark Bay, Western Australia. The nests and their immediate surrounds were searched for mammalian skulls and the numbers of each species recorded. The nest at Bernier Island was examined annually between July 1992 and October 1995 and the two nests on Heirisson Prong in October 1995. Banded Hare-wallabies were the most abundant prey item found in the Bernier Island nest over the four year period comprising 53.0% of all skulls, followed by Rufous Hare-wallabies at 35.9%, Burrowing Bettongs at 8.5% and fish at 2.6%. On Heirisson Prong, one bettong skull and 91 rabbit skulls were collected. Wedge-tailed Eagles displayed a preference for larger prey items on both Bernier Island and Heirisson Prong. They are diurnal predators but on Bernier Island their prey species are largely nocturnal. Possible hunting techniques are discussed.

Control of Feral Cats for Nature Conservation. I. Field Tests of Four Baiting Methods

Four methods of baiting were evaluated on a radio-collared population of feral cats on Heirisson Prong, Shark Bay, Western Australia. Dried-meat baits, baiting rabbits to kill cats through secondary poisoning, a fishmeal-based bait and a bait coated in the flavour enhancer Digest were tested. All proved to be ineffective for controlling feral cats. Future research should explore baits more ‘natural’ in appearance and the effect of visual lures, and possibly bait over a larger area to increase the number of cats exposed to baits.