Threatened species & ecological communities

Pseudemydura umbrina (Western Swamp Tortoise)

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.

Pseudemydura umbrina (Western Swamp Tortoise)

2. Description

The Western Swamp Tortoise is a short-necked, wholly carnivorous freshwater tortoise. It is the smallest of the Australian chelids, with adult males growing to a maximum carapace (shell) length of 155 mm and a weight of 550 grams. Females are smaller, not growing beyond 135 mm carapace length and a weight of 410 grams.

The species inhabits shallow, winter-wet swamps on clay or sand over clay soils with nearby refuges for aestivation (summer dormancy). In most swamps Melaleuca lateritia shrubs form a canopy 1-2 metres above the water over open tall sedges (Leptocarpus canus). Refuges include leaf litter and fallen branches, naturally occurring holes in the clay and at tree bases, or holes left by other animals.

The Western Swamp Tortoise feeds and breeds only in water. It also digs its nest with its forelegs while all other tortoises and turtles dig their nest chamber with their hind legs. In addition to this they only produce one clutch of eggs per year, whereas most other Australian freshwater tortoises produce several clutches.

3. National Context

The Western Swamp Tortoise has been recorded only from scattered localities in a narrow strip (3 km to 5 km wide) of the Swan Coastal Plain IBRA region (defined in Environment Australia 2000) in Western Australia. These localities run roughly parallel with the Darling Scarp, from Perth Airport at Guildford to near Pearce Royal Australian Air Force Base at Bullsbrook. Anecdotal information suggests that the species' past stronghold was the clay soils of the Swan River Valley, the first part of Western Australia developed for agriculture. Almost all this land is now cleared and either urbanised, used for intensive agriculture or mined for clay used in brick and tile manufacture.

While the species was formally identified in 1901 it was not recorded again until 1953. Little is known of its pre-European geographic range.

From the 1960s to the 1980s there were three known populations, one monitored at Ellen Brook Nature Reserve, one at Twin Swamps Nature Reserve, both northeast of Perth, and a third known only from the collection of a single juvenile in 1970 at Five Mile Swamp in the southern part of Perth Airport. Currently, only one population (Ellen Brook) contains naturally occurring mature individuals, while the Twin Swamps population has been supplemented since 1994 with captive-bred individuals after the species was considered to have become practically extinct at the site during the 1980's. No further evidence has been found indicating the presence of a population at Perth Airport, despite extensive surveying. An additional population was established at Mogumber Nature Reserve in August 2000 using captive-bred juvenile stock. This population contains no mature individuals. There are no confirmed indications that the species had a range in the past significantly different from its recent known range.

All populations of the Western Swamp Tortoise are currently being managed under a recovery plan, which has been in place since 1994. The recovery actions being implemented include managing predators, rehabilitating remaining habitat, captive breeding and supplementing existing populations with captive-bred stock.

The Western Swamp Tortoise has been declared as fauna 'likely to become extinct or is rare' under the Western Australian Wildlife Conservation Act 1950. It was listed as a nationally threatened species in 1993 (Endangered Species Protection Act 1993, Environment Protection and Biodiversity Conservation Act 1999). In addition, it is listed on Appendix I of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES).

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

TSSC judges the species 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.

There has been an observed reduction in population size of the Western Swamp Tortoise of more than 80% over the past three generations (generation time is estimated to be about 40 to 60 years).

During the 1960s to 1980s the two significant populations were Twin Swamps Nature Reserve and Ellen Brook Nature Reserve, with both reserves created to protect the species' habitat in 1962.

Currently Ellen Brook is the only viable, naturally occurring population in the wild. At this site, the numbers of breeding adults dropped from about 15 in the 1960s to only 8 in 1979-1982 and then increased slowly to about 20 in the 1990s. There are now an estimated 10 - 15 mature individuals amongst an estimated total of 30 individuals, including juveniles, at the site. Recent increases in the total number of individuals largely reflect an increased number of hatchlings and small juveniles in the population.

The population at Twin Swamps Nature Reserve was estimated at above 250 individuals in the early 1960s but declined to below 10 individuals by 1984. The population at this site has increased as a result of supplementation with captive-bred individuals since 1994. It currently contains only 6 mature individuals.

The population at Mogumber Nature Reserve was established in August 2000 with the release of 6 captive-bred juvenile tortoises. A further 20 juveniles were released in August 2001. This population currently contains no mature individuals. The threats that have contributed to the decline in numbers of this species have included the clearing of its habitat for agricultural, industrial and urban development, and the draining of swamps for agricultural purposes. In addition, the Western Swamp Tortoise's habitat is under threat due to increasing aridity in the region, which is resulting in early drying of swamps during some years. Monitoring of the species has shown that hatchlings must achieve a body weight of about 25grams in their first six months in order to survive the period of dormancy during their first summer. In years of below average rainfall, the hatchlings cannot achieve this because the swamps retain water for a shorter time than other years, reducing the months available for feeding (which is only done in winter and early spring when swamps are full). Also, evidence also shows that females are not able to produce eggs in low rainfall years. For recruitment to succeed, there must be two successive years of average or above average rainfall. Since the mid-1960s Perth has had many years of below average rainfall, which has undoubtedly affected the numbers of the Western Swamp Tortoise.

Predation of the Western Swamp Tortoise is also a threat. The European Red Fox (Vulpes vulpes) was abundant in both nature reserves before fencing occurred in 1990 at Ellen Brook and 1993 at Twin Swamps. Foxes are known to eat adult and juvenile tortoises as well as destroy eggs. While the fox-proof fences have reduced the number of fox-related deaths at both sites, they have also caused some problems. During years when swamps have dried early some tortoises have been attracted to water in farm dams on adjoining private land but have died from overheating when they have been unable to negotiate the fencing. This problem has been mitigated to some extent by the installation of tortoise-gates in fencing, patrols of fencing and pumping ground water into swamps during low-rainfall years. Despite the greatly reduced number of foxes within the reserves, from time to time some foxes have been able to breach the predator fence and kill tortoises.

Kookaburras (Dacelo novaeguineae) may predate eggs and hatchling and juvenile tortoises, and Australian Ravens (Corvus coronoides) and introduced rats (Rattus rattus and R. novegicus) are also believed to attack juveniles, particularly as the tortoises leave swamps in late spring to begin summer aestivation.

It is also thought that fire may have contributed to the decline of the species at Twin Swamps. The Twin Swamps population uses leaf litter or fallen branches as aestivation refuges, and are therefore susceptible to fire. In past decades the surrounding areas have been increasingly developed for urban and agricultural use, and the risk of frequent fires has increased. At Ellen Brook the species aestivates underground so fire is not thought to be a major threat. The Western Australian Department of Conservation and Land Management currently have fire management policies in place for the area.

The Western Swamp Tortoise has undergone a very severe reduction in numbers. It has declined from an estimated population of around 250 in the 1960's to around 15-25 mature individuals. The species currently occurs in a single viable population in the wild, with a further two populations currently being maintained by supplementation with translocated individuals. A number of threats have been identified. Therefore the species is eligible to be listed as critically endangered under criterion 1.

Criterion 2 -Its geographic distribution is precarious for the survival of the species and is very restricted, restricted or limited

The extent of occurrence of the Western Swamp Tortoise is estimated to be 100 - 150km2. It is currently known from a single viable, naturally occurring population (Ellen Brook Nature Reserve) and two populations maintained through translocation of captive-bred individuals (Twin Swamps Nature Reserve and Mogumber Nature Reserve).

The number of mature individuals has undergone a very severe reduction (from around 250 in the 1960's to around 15-25 mature individuals at present). The species is subject to low fecundity and slow growth rates, with sexual maturity not being reached until 10-15 years of age. Therefore it will be a number of years before juveniles become breeding individuals. Meanwhile, as the number of remaining mature individuals is very low and subject to a number of threats, as outlined in Criterion 1, such as increasing aridity leading to reduced recruitment to the population and the presence of predators such as foxes, rats, ravens, and kookaburras, that particularly feed on hatchlings and juveniles. Despite conservation efforts, these threats are still operating to various degrees and numbers may continue to decline in the near future.

The extent of occurrence of the Western Swamp Tortoise is restricted, and it only occurs at few locations where threats are still operating to some extent and therefore continued decline may occur. The geographic distribution of the species is precarious for its survival. Therefore, the species is eligible for listing as 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 Western Swamp Tortoise's estimated total number of mature individuals is very low. There are currently an estimated 15-25 mature individuals in the wild, which has declined from around 250 during the 1960's.

The species has a very restricted extent of occurrence about 100 - 150km2. Most of their range has been converted for agricultural, industrial or urban use. Within this restricted range, it is dependent on a specific habitat, which is characterised by the presence of ephemeral (intermittently wet) swamps on clay or sandy-clay soils, with suitable aestivation refuges nearby. It does not occur on the permanent swamps of the Swan Coastal Plain in the same region. It is currently known from a single viable, naturally occurring population (Ellen Brook Nature Reserve) and two populations maintained through translocation of captive-bred individuals (Twin Swamps Nature Reserve and Mogumber Nature Reserve).

The number of mature individuals has undergone a very severe reduction (from around 250 in the 1960's to around 15-25 mature individuals at present). The species is subject to low fecundity and slow growth rates, with sexual maturity not being reached until 10-15 years of age. Therefore it will be a number of years before juveniles become breeding individuals. Meanwhile, as the number of remaining mature individuals is very low and subject to a number of threats, as outlined in Criterion 1, such as increasing aridity leading to reduced recruitment to the population and the presence of predators such as foxes, rats, ravens, and kookaburras, that particularly feed on hatchlings and juveniles. Despite conservation efforts, these threats are still operating to various degrees and numbers may continue to decline in the near future.

The Western Swamp Tortoise has a low estimated number of mature individuals occurring at a low number of sites, and numbers of mature individuals may continue to decline. Therefore, the species is eligible to be listed as critically endangered under criterion 3.

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

The estimated total number of mature individuals of Western Swamp Tortoises is extremely low (between 15 and 25 mature individuals occurring at two very small remaining sites), as noted in previous criteria. Even though tortoises have been successfully translocated to Twin Swamps Nature Reserve and Mogumber Nature Reserve, the long time to reach maturity in this species means that the success of these translocations will not be apparent for some years.

Therefore, the species is eligible for listing as critically endangered under Criterion 4.

Criterion 5 - Probability of extinction in the wild

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

5. Conclusion

Since the 1960s, the Western Swamp Tortoise has undergone a very severe reduction in numbers. During the 1960's numbers were estimated at around 250 and have now declined to less than 50 mature individuals. The species requires a specialised habitat that has been significantly reduced and degraded since European settlement, particularly through the effects of agricultural development. It is also subject to a number of ongoing threats including predation by introduced and native animals and increased aridity. The species is currently being intensively managed under a recovery plan but despite conservation efforts, its long period until maturity, low numbers of breeding individuals and continuing threats mean it is likely to continue to decline in the near future.

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

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:

Pseudemydura umbrina (Western Swamp Tortoise)

Publications used to assess the nomination

Burbidge, A.A. and Kuchling, G. (1994) Western Swamp Tortoise Recovery Plan, 1st edition. Wildlife Management Program, Department of Conservation and Land Management

Burbidge, A.A. and Kuchling, G. (1999) Western Swamp Tortoise Recovery Plan, 2nd edition. Wildlife Management Program, Department of Conservation and Land Management

Burbidge, A.A. and Kuchling, G. (2003) Western Swamp Tortoise Recovery Plan, 3rd edition. Wildlife Management Program, Department of Conservation and Land Management

Cogger, H.G. (1996) Reptiles and Amphibians of Australia. Reed Books Australia, Port Melbourne Victoria

Environment Australia (2000), Revision of the Interim Bioregionalisation of Australia (IBRA) and the Development of Version 5.1 - Summary Report. Department of Environment and Heritage, Canberra.