Potorous gilbertii (Gilbert's Potoroo)

Advice to the Minister for the Environment and Heritage from the Threatened Species Scientific Committee (TSSC) on Amendments to the list of Threatened Species under the Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act)

1. Scientific name, common name (where appropriate), major taxon group.

Potorous gilbertii (Gilbert's Potoroo)

2. Description

Gilbert's Potoroo is a small nocturnal marsupial, with a densely furred body, which lives in small groups or colonies. It has long hind feet and long, curved claws on its front feet that it uses to dig for food. Adults range in size from 900 to 1200g.

The diet of Gilbert's Potoroo consists almost entirely of fungi which are considered to make up 90% of the species' food source. Many of these fungi are mycorrhizal (i.e. they grow in association with the roots of plants).

3. National Context

Originally discovered in the 1840s near Albany, Western Australia, Gilbert's Potoroo had been considered to be extinct since the early 1900s until its rediscovery in 1994 at Two Peoples Bay Nature Reserve near Albany. Currently, Gilbert's Potoroo is known from only one population in the wild on Mount Gardner headland in Two Peoples Bay Nature Reserve.

Gilbert's Potoroo's is now found on low, dense long-unburnt heath (approximately 50 years) on valley slopes. Its core range falls within a restricted vegetation type characterised by Melaleuca striata and M. thymoides over dense sedges. The species is considered to avoid areas where dieback disease, caused by the root pathogen Phytophthora cinnamomi, has modified the structure and floristic assemblage of heathlands.

Gilbert's Potoroo is currently listed as 'fauna that is likely to become extinct or is rare', under the Western Australian Wildlife Conservation Act 1950 and was listed as a nationally threatened species in 1993 (Endangered Species Protection Act 1993, Environment Protection and Biodiversity Conservation Act 1999).

A draft recovery plan for the Gilbert's Potoroo is in preparation, a recovery team is in place, and recovery actions have been undertaken since the species' rediscovery in 1994. These include baiting to control foxes, investigating the impact of cats on Potoroos, hygiene protocols to stop the spread of Phytophthora cinnamomi and a captive breeding program. A captive colony was established soon after the Gilbert's Potoroo rediscovery in 1994. While the colony grew from the initial nine founders (including three pouch young and young-at-heel brought in with their mothers) to a maximum of 14 animals in 1998, reproductive rates have been much lower than anticipated.

4. How judged by TSSC in relation to the EPBC Act criteria.

TSSC judges Gilbert's Potoroo to be eligible for listing as critically endangered under the EPBC Act. The justification against the criteria is as follows:

Criterion 1 - It has undergone, is suspected to have undergone or is likely to undergo in the immediate future a very severe, severe or substantial reduction in numbers.

Gilbert's Potoroo was described by John Gilbert as very common around Albany in 1840, and between 1840 and 1870s a small number of specimens were collected. Only one modern specimen has ever been recorded outside the Albany area. This specimen (date and collector unknown) was collected at Birches Cave, near Margaret River and suggests that the species occurred recently on the Leeuwin-Naturaliste Ridge. Sub-fossil specimens have been collected on the Leeuwin-Naturaliste Ridge, the Devil's Lair and Yanchep Caves. There is no other evidence of Gilbert Potoroo's past distribution.

Currently, Gilbert's Potoroo is known to occur at one very small site, Mount Gardner headland in Two Peoples Bay Nature Reserve, near Albany. Its current extent of occurrence is estimated to be 8km2 and the area of occupancy is estimated to be less than 5km2. Recent trapping in suitable habitat at the known major sites on Mount Gardner headland indicates that no more than 30 individuals occur in this population. It is possible, although unlikely, that other populations persist in some undisturbed areas along the south coast of Western Australia.

Research on the species' DNA indicates that Gilbert's Potoroo has undergone a recent population bottleneck (i.e. a sudden decrease in population density with a resulting decrease in genetic variability within a population).

While the degree of decline and reasons for the decline remain uncertain, there are a number of current threats operating, some of which may be related to the past decline:

  • Gilbert's Potoroo, being in the critical weight range for mammals, are vulnerable to predation by introduced taxa (foxes, Vulpes vulpes, and feral cats, Felis catus), both of which occur in the Two People's Bay area. Potoroos are considered to have also suffered such predation in the past.
  • The extent and quality of habitat has been affected by Phytophthora cinnamomi which is considered to impact on, or eliminate, the host plant to the fungi on which the Potoroo feeds. Currently, it is thought that the Potoroo only occurs in areas that are free of Phytophthora cinnamomi. Areas that may have been suitable for the Potoroo in the past may no longer be suitable because of the spread of Phytophthora cinnamomi in the region.
  • Another current threat is clearing of suitable habitat in the region which would restrict the dispersal of the species. It is likely that clearing within the range of the Potoroo also had a similar affect in the past, as well as removing suitable habitat for the species.
  • Gilbert's Potoroo's appears to require dense long-unburnt heath for habitat. Since European settlement, fire regimes in the area are likely to have altered and to have removed suitable habitat for the Potoroo.
  • Currently, recruitment of young to the adult population is low and the reasons for this remain unclear.

In summary, since the mid-1800s, the species has apparently decreased in distribution and abundance from around Albany and possibly from other areas of the south west of Western Australia to the extent that it now occurs in one very small population in one location. It is also considered that a recent population bottleneck has occurred in this species. From this evidence, it may be suspected that a very severe decline in numbers has occurred since the mid-1800s.

Therefore, the species is eligible for listing as critically endangered under this criterion.

Criterion 2 -Its geographic distribution is precarious for the survival of the species and is very restricted, restricted or limited Gilbert's Potoroo is known to exist at only a single location, in one population, on Mount Gardner headland, at Two Peoples Bay Nature Reserve near Albany. It occurs in at least four separate patches of unburnt dense shrubland on valley slopes. The extent of occurrence for Gilbert's Potoroo is estimated to be 8km2 and its area of occupancy less than 5km2.

Gilbert's Potoroo's core range falls within a restricted vegetation type characterised by Melaleuca striata and M. thymoides over dense sedges and the species is known from low, dense unburnt heath (at least 50 years unburnt). Gilbert's Potoroo is also considered to avoid areas where dieback disease caused by the root pathogen Phytophthora cinnamomi has modified the structure and floristic assemblage of heathlands.

Field surveys since 1994 have found no evidence of other wild populations.

The species and its habitat are subject to a number of ongoing threats including: predation by foxes and feral cats (both of which are known from the Two People's Bay area); the loss or degradation of host plants for the fungi that the Potoroo feeds on (as a result of the impacts of Phytophthora cinnamomi); and the lack of recruitment of young to the adult population. The population is also potentially threatened by a catastrophic fire (fuel loads within Gilbert Potoroo habitat are very high due to the species apparent preference for long-unburnt vegetation). The clearing of suitable habitat that may contain undiscovered populations or in which Gilbert's Potoroo could, in future, be reintroduced or disperse is also considered to be a threat.

While recovery measures are in place to combat the impact of foxes and feral cats, as well as the degradation or loss of the species habitat from Phytophthora cinnamomi or fire, it is not considered that these threats are sufficiently abated as yet. There is still uncertainty surrounding the causes of the past declines of Gilbert's Potoroo, and there is concern that the reasons for the low recruitment of young to the adult population are not known. Therefore, it is reasonable to consider that the species may continue to decline in the future.

The geographic distribution of Gilbert's Potoroo is precarious for the survival of the species and is very restricted. Therefore, the species is eligible for listing as critically endangered under this criterion.

Criterion 3 - The estimated total number of mature individuals is limited to some degree and: (a) evidence suggests that the number will continue to decline at a particular rate; or (b) the number is likely to continue to decline and its geographic distribution is precarious for its survival.

The total number of mature individuals is very low. The population size of Gilbert's Potoroo is estimated to number no more that 30 individuals which occur in one population at one location (the recovery plan does not indicate whether the estimate was for mature individuals).

The species and its habitat are subject to a number of ongoing threats including: predation by foxes and feral cats (both of which are known from the Two People's Bay area); the loss or degradation of host plants for the fungi that the Potoroo feeds on (as a result of the impacts of Phytophthora cinnamomi); and the lack of recruitment of young to the adult population. The population is also potentially threatened by a catastrophic fire (fuel loads within Gilbert Potoroo habitat are very high due to the species apparent preference for long-unburnt vegetation). The clearing of suitable habitat that may contain undiscovered populations or in which Gilbert's Potoroo could, in future, be reintroduced or disperse is also considered to be a threat.

While recovery measures are already in place for the species and the site is being appropriately managed for the survival of the species, there are still a number of threats operating and the species may decline in the future.

Therefore, the species is eligible for listing as critically endangered under this criterion.

Criterion 4 - The estimated total number of mature individuals is extremely low, very low or low.

The estimated total number of mature individuals of Gilbert's Potoroo is extremely low, occurring in one population at one location on Mount Gardner headland, at Two Peoples Bay Nature Reserve near Albany. Trapping, conducted in all the major sites of suitable habitat at Mount Gardner indicates that no more than 30 individuals occur in the population.

Therefore, the species is eligible for listing as critically endangered under this criterion.

Criterion 5 - Probability of extinction in the wild

There are no quantitative data available against this criterion. Therefore, the species is not eligible for listing under this criterion.

5. Conclusion

Gilbert's Potoroo is known from one population in the wild of less than 30 individuals. Its geographic distribution is precarious for the survival of the species. The species and its habitat are subject to a number of ongoing and potential threats including a catastrophic or uncontrolled fire, predation from foxes and feral cats, low recruitment of young to the adult population, the impact of the dieback disease caused by the root pathogen Phytophthora cinnamomi on fungi host plants and the clearing of areas of suitable habitat within the species extent of occurrence.

The species is eligible for listing as critically endangered under criterion 1, 2, 3 and 4.

6. Recommendation

TSSC recommends that the list referred to in section 178 of the EPBC Act be amended by including in the list in the critically endangered category:

Potorous gilbertii (Gilbert's Potoroo).

Publications used to assess the nomination

Courtenay, J. and Friend, A. (2003). Draft Gilbert's Potoroo Recovery Plan 2003-2008. Wildlife Management Program, Department of Conservation and Land Management.

Sinclair, E.A. Darks, A. and Wayne, A.F. (1996) Rediscovery of Gilbert's potoroo, Potorous tridactylus, in Western Australia. Australian Mammalogy 19(1), 69-72.