Advice to the Minister for the Environment, Heritage and the Arts from the Threatened Species Scientific Committee (the Committee) on Amendments to the List
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Summary of assessment: Escaped garden plants are very likely to be a cause of decline in the population size and distribution of Pimelea spicata, with local extinctions likely in the absence of effective weed control measures. In particular, the invasion of Kikuyu and Bridal Creeper is likely to have had an adverse impact on the species through direct competition.The Committee judges that this threatening process is adversely affecting Pimelea spicata. Sandhill Greenhood Orchid The Sandhill Greenhood Orchid is endemic to South Australia and is currently listed as vulnerable under the EPBC Act. This seasonal terrestrial orchid emerges from subterranean tubers in June, flowers in September and October, and dies back during November-December (Sorensen and Jusaitis, 1995; SA DEH, 2005). In the early 1990s, the species’ distribution was restricted to three well known small populations, totalling an estimated 330 plants, and a fourth newly discovered population (Groves et al., 2005; DEWHA, 2009e). Two of these populations occur within a State Reserve at Tailem Bend and the third and fourth populations are located on a golf course in the Adelaide Metropolitan area and in the Coorong National Park, respectively (Groves et al., 2005; DEWHA, 2009e). The invasion of escaped garden plants has been identified as a threat at each of these populations. Bridal Creeper and Avena spp. are the main weed species at Tailem Bend and Coorong National Park and Soursob (Oxalis pes-caprae) is the prominent weed species at the golf course (DEWHA, 2009e). These weed species negatively impact on Sandhill Greenhood Orchid by directly competing for light, water, nutrients and space. Sorensen and Jusaitis (1995) quantified the impact of Bridal Creeper on Sandhill Greenhood Orchid numbers through analysis of quadrats at Tailem Bend. As depicted in Figure 3, in July 1994, an average of more than 40 orchids per m2 were recorded in the absence of Bridal Creeper, and this plant density reduced to just over ten orchids per m2 when Bridal Creeper was present. The adverse impact of Bridal Creeper was also apparent temporally. For instance, in Bridal Creeper-infested areas, the weed’s coverage almost doubled during the September flowering period, at the expense of the Sandhill Greenhood Orchid which declined in density to about 2.5 orchids per m2. Sorensen and Jusaitis (1995) attribute the strong adverse impact on the Sandhill Greenhood Orchid to the close congruence in phenology of weed and orchid – both species grow from a tuberous root stock over autumn and winter and both also fruit and die back during spring and summer. Summary of assessment: Escaped garden plants are very likely to be a cause of decline in the population size and distribution of the Sandhill Greenhood Orchid. The invasion of Bridal Creeper, in particular, has had an adverse impact on the species through direct competition. The Committee judges that this threatening process is adversely affecting the Sandhill Greenhood Orchid. Figure 3: Numbers of Sandhill Greenhood Orchid (Pterostylis arenicola) with and without Bridal Creeper (Asparagus asparagoides) at Tailem Bend, SA (Sorensen and Jusaitis, 1995).
Wing-fruited Lasiopetalum The Wing-fruited Lasiopetalum is a shrub endemic to the Serpentine National Park of Western Australia near Perth. The species is currently listed as endangered under the EPBC Act. The species is known from one natural location and two translocated locations and is estimated to comprise 555 mature individuals (excluding 102 translocated plants that have not yet been proved viable) (DEWHA, 2009c). Invasive garden plants including Blackberry, Gladioli (Gladiolus undulatus) and Watsonia (Watsonia sp.) pose the most immediate threat to the species and its habitat by suppressing both the growth of adult plants and recruitment through competition for soil moisture, nutrients and light (Stack and English, 2003; DEWHA, 2009c). The presence of these weed species also potentially threatens the survival of the Wing-fruited Lasiopetalum by contributing to fuel loadings and thereby increasing the risk of fire (Stack and English, 2003). Despite weed control that has been undertaken in accordance with the species’ recovery plan, invasive garden plants are still present within the species’ distribution and propagules are continually introduced from infestations upstream of the natural location (Stack and English, 2003). Summary of assessment: Escaped garden plants are very likely to be a cause of decline in the population size and distribution of the Wing-fruited Lasiopetalum. The invasion of Blackberry, Gladioli and Watsonia, in particular, has had an adverse impact on the species through direct competition.The Committee judges that this threatening process is adversely affecting the Wing-fruited Lasiopetalum.Blue Gum High Forest of the Sydney Basin Bioregion The Blue Gum High Forest of the Sydney Basin Bioregion (‘Blue Gum High Forest’) is currently listed as a critically endangered ecological community under the EPBC Act. The Blue Gum High Forest is a tall forest that has several vegetation layers in its undisturbed state (DEH, 2005). The common components of the dominant tree layer of this ecological community are Blackbutt (Eucalyptus pilularis), Sydney Blue Gum (Eucalyptus saligna) and Smooth-barked Apple (Angophora costata) (DEWHA, 2009a).The Blue Gum High Forest once covered the Sydney region, however, following European settlement and extensive vegetation clearing, this ecological community is now restricted to small bushland remnants nestled amongst the northern suburbs of Sydney. These remnants constitute less than 5 percent of the original forest area (DEH, 2005). Many of the threats to the Blue Gum High Forest relate to the juxtaposition of the remnants with intensively disturbed urban areas. For instance, remnants with a high perimeter to area ratio are subject to edge effects and are highly susceptible to invasion by weed species, many of which are escaped garden plants (TSSC, 2005; DEWHA, 2009a). It is estimated that at least 50 percent of the remaining areas containing the Blue Gum High Forest are subject to weed invasion (TSSC, 2005). Problematic escaped garden plants that are commonly associated with Blue Gum High Forest and threaten its condition include:
(NSW Scientific Committee, 2008a; DEWHA, 2009a). Camphor Laurel, Lantana, Privet, Ochna and Blackberry are of most concern, as these species are often present even in the least disturbed parts of high quality Blue Gum High Forest remnants (DEWHA, 2009a). A number of remnants have highly modified understories, in which the native woody component has been largely replaced by various garden escapees through direct competition. In addition to altering the native floristic structure and composition of the ecological community, the reduction of understorey complexity that results from the invasion of escaped garden plants also leads to the degradation of habitat for a range of native bird and mammal species. Moreover, the invasion of escaped garden plants contributes to a reduction in the overall natural ecological function of the community (NSW Scientific Committee, 2008a). Although weed infestations have been managed to some extent in some areas, the current level of work is not enough to significantly reduce the continued invasion of escaped garden plants from neighbouring private properties and public land (NSW NPWS, 2004a; DEH, 2005). |