Mt Arthur Boronia (Boronia hemichiton)

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)
13 April 2005

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

Boronia hemichiton (Mt Arthur Boronia)

A review of the Boronia group in eastern and northern Australia was undertaken in 2003 (Duretto 2003). Prior to this review, six species of Boronia were recognised in Tasmania (i.e. B. anemonifolia, B. citriodora, B. nana, B. parviflora, B. pilosa, and B. rhomboidea). The 2003 revision identified a total of fifteen taxa for Tasmania with nine endemics, including Boronia hemichiton. Following this revision, surveys were commissioned to verify both the distribution and the population parameters of a number of Boronia species including Boronia hemichiton. These surveys were undertaken during August to December 2003. Numbers of mature plants were determined either through direct counts or by estimates of plant density and areas of occupancy.

2. Description

Mt Arthur Boronia is a woody shrub that grows to approximately 1.5m high. The species occurs in poorly drained wet heath or scrub, usually with emergent eucalypts (Eucalyptus gunnii or Eucalyptus ovata).

Mt Arthur Boronia is endemic to northern Tasmania, occurring in two populations in the upper catchment of the Piper River on the western flanks of Mt Arthur. The species was previously only known from one record from the western flanks of Mt Arthur. More recently another population has been located 3km from the first population on the western flanks of Mt Arthur in wet heath and in association with the woody shrubs Leptospermum lanigerum, Callistemon viridiflorus, Melaleuca squamea and Westringia rubiaefolia, and the large tussock-forming sedge Gahnia grandis.

3. National context

Mt Arthur Boronia only occurs in Tasmania. It is known from two populations on the western side of Mt Arthur. All known plants occur on State Forest in areas classified as 'production'. Mt Arthur Boronia has been nominated for listing under Tasmania's Threatened Species Protection Act 1995. The Tasmanian Threatened Species Scientific Advisory Committee has not yet made a decision on its conservation status.

EPBC Act criteria.

TSSC judges the species to be eligible for listing as vulnerable 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.

Until recently the distribution of Mt Arthur Boronia was poorly known. The species was only known from one record in 1983 from the western flanks of Mt Arthur in northern Tasmania. Subsequent surveys have found another population located 3km to the southeast in wet heath. The number of mature individuals is estimated to be between 1000 - 2000, with an area of occupancy of 0.3 - 0.5km². The plants are found at an altitude range of 480-660 m. Targeted searches in similar habitat in the region, at both higher and lower attitudes, have failed to locate other populations.

Potential threats to this species are considered to be possible infection with the plant pathogen Phytophthora cinnamomi, an inappropriate fire regime, and activities that directly disturb the species habitat (e.g. road works) or indirectly affect its hydrology (e.g. upstream or upslope activities in the upper catchments of nearby rivers that increase runoff and sediment loads).

The actual level of susceptibility of Mt Arthur Boronia to Phytophthora cinnamomi is, at this stage, unknown. However, the susceptibility of other Boronia species to Phytophthora cinnamomi has long been recognised. Phytophthora cinnamomi can be spread by dispersing independently through very moist but well aerated soil aided by water, wildlife and human activities such as road building and maintenance. Previous advice from experts suggest that fire, or frequent fires, can create conditions conducive to an increased abundance and spread of Phytophthora cinnamomi. Fire reduces organic matter in the soil and associated soil micro-flora/fauna that can have a suppressive effect on Phytophthora cinnamomi. Fire also reduces the vegetation cover and allows soil temperature and water availability to rise, facilitating an increase in the pathogen. The movement of fire fighters and associated machinery is a secondary mechanism aiding the spread of Phytophthora cinnamomi. It is also believed that too frequent fires in the area would be a problem for this species, while too long a period between fires would run the risk of plants dying off and seed stored in soil eventually losing its viability. Activities in the area such as road side maintenance can cause disturbance to the Mt Arthur Boronia habitat.

The species is only known from two populations. There is insufficient evidence however to indicate a reduction in population size in the past or a likely reduction in the future. There is insufficient quantitative data available against this criterion. Therefore, the species is not eligible for listing under this criterion.

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

It is estimated that the population of Mt Arthur Boronia has an extent of occurrence of between 1.4 to 1.5km², and its area of occupancy between 0.3-0.5km². Two populations of Mt Arthur Boronia are currently known. They occur in the upper catchment of the Piper River on the western flanks of Mt Arthur and 3km from the first population on the western flanks of Mt Arthur in wet heath.

While the species has a restricted geographic distribution, and is known from only two populations, there is insufficient evidence to indicate that this geographic distribution is, in itself, precarious for the survival of the species. The species' habitat may potentially be impacted on by forest activities and upstream activities that increase runoff and sediment loads, however, there are no known impacts from these potential threats at this stage. The actual susceptibility of Mt Arthur Boronia to Phytophthora cinnamomi is, at this stage, unknown. Fire, or frequent fires, can create conditions conducive to an increased abundance and spread of Phytophthora cinnamomi. Fire reduces organic matter in the soil and associated soil micro-flora/fauna that can have a suppressive effect on Phytophthora cinnamomi. Fire also reduces the vegetation cover and allows soil temperature and water availability to rise, facilitating an increase in the pathogen. The movement of fire fighters and associated machinery is a secondary mechanism aiding the spread of Phytophthora cinnamomi. Too frequent fires is considered to be detrimental to the survival of the species. Conversely, should long periods occur between suitable fire events, plants may become senescent with soil-stored seed becoming unviable.

There is insufficient evidence to indicate that the existing population of Mt Arthur Boronia is subject to a continuing decline or extreme fluctuations in population numbers. The geographic distribution, while restricted, is not considered precarious for the survival of the species and there is insufficient evidence to indicate ongoing decline. Therefore, the species is not eligible for listing under this criterion.

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

The estimated total number of mature individuals is low, totalling between 1000 - 2000. However, there appears to be no information available indicating a decline in numbers in the past or that the population is currently declining in numbers or that it is likely to in the foreseeable future.

Potential threats to this species are considered to be possible infection with the plant pathogen Phytophthora cinnamomi, an inappropriate fire regime, and activities that directly disturb its habitat (e.g. road works) or indirectly alter its hydrology (e.g. upstream or upslope activities in the upper catchments of nearby rivers that increase runoff and sediment loads). There is limited evidence however to indicate that these threats are currently having a detrimental affect on the population of Mt Arthur Boronia or that the species is continuing or likely to decline in numbers. Geographic distribution is addressed under Criterion 2.

There is insufficient quantitative data available against this criterion. Therefore, the species is not eligible for listing under this criterion.

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

The total number of Mt Arthur Boronia is between 1000 - 2000 mature individuals. The population is restricted to two locations on the western flanks of Mt Arthur in northern Tasmania. The population may be prone to a number of potential threats including possible infection with the plant pathogen Phytophthora cinnamomi, inappropriate fire management, and activities that either directly disturb its habitat (e.g. road works) or indirectly impact on its hydrology (e.g. upstream or upslope activities in the upper catchments of nearby rivers that increase runoff and sediment loads).

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

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

Mt Arthur Boronia is endemic to northern Tasmania, occurring in two populations on the western flanks of Mt Arthur. Potential threats to this species are considered to be the possible infection with the plant pathogen Phytophthora cinnamomi, an inappropriate fire regime, and activities that either directly affect its habitat (e.g. road works) or indirectly alter its hydrology (e.g. increased runoff and sediment loads). However there is little evidence to indicate that these threats are currently having a detrimental affect on the population of Mt Arthur Boronia.

Mt Arthur Boronia has a low number of mature individuals between 1000 - 2000 and a very restricted area of occupancy of 0.3 - 0.05km². This species is eligible for listing as vulnerable under criterion 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 vulnerable category:

  • Boronia hemichiton (Mt Arthur Boronia)

Conservation Advice

Mt Arthur Boronia (Boronia hemichiton) is a woody shrub that grows to around 1.5m high and occurs only in northern Tasmania on the western flanks of Mt Arthur (North Tasmania NRM region). The species is limited to approximately 1000 - 2000 plants at two locations.

Activities that directly affect its habitat (e.g. road works) or that alter the hydrology of these sites (e.g. increased runoff and sediment loads) and infection with the plant pathogen Phytophthora cinnamomi are possible threats to the survival of this species. Inappropriate fire management may also have a detrimental impact.

The priority recovery and threat abatement actions required for this species are:

  • restricting any activities that may have a direct impact on the species (e.g. clearing, spraying with chemicals, invasion of weeds, inappropriate grazing; changes to the hydrology of the habitat);
  • incorporation of the marshes at Mt Arthur that support the two Mt Arthur Boronia populations within a new Phytophthora Management Area and establishment of appropriate hygiene protocols to minimise infection of these two sites; and
  • areas of heath on State Forest land that are known to support Mt Arthur Boronia should be identified for inclusion within the relevant authority's Tactical Fire Management Project with the objective of determining and implementing an appropriate fire management strategy for the species. This list does not encompass all actions that may be of benefit to this species, but highlights those that are considered to be of the highest priority at the time of listing.

A Recovery Plan is not yet in place for this species. A national Threat Abatement Plan exists for the plant pathogen Phytophthora cinnamomi: 'Threatened Abatement Plan for Dieback Caused by the Root-rot Fungus Phytophthora cinnamomi, Commonwealth of Australia 2001."

Priority for the development of recovery plan: Medium.

Publications used to assess the nomination

Duretto, M.F. (2003) Notes on Boronia (Rutaceae) in eastern and northern Australia. Muelleria 17, 19-135.

Schahinger, R. (2004) Distribution and Conservation Status of the Tasmanian Endemic Shrubs Boronia gunnii, Boronia hemichiton and Boronia hippopala. Report to the Threatened Species Unit, Department of Primary Industries, Water and Environment, Hobart, Tasmania.