Summary of assessment: Invasive invertebrate pests are causes of decline in the population size and distribution of many threatened species and ecological communities, with local extinctions likely in the absence of effective control measures. In particular, yellow crazy ants are having an adverse impact on the species and ecological communities through direct competition and habitat destruction.
The Committee judges that this threatening process is adversely affecting many listed threatened species.
Competition, habitat loss and degradation caused by terrestrial weeds Invasive terrestrial weeds are having major impacts on Australian native species and ecological communities. Invasive weeds change the natural environment by out-competing native species for space, nutrients and sunlight (Adair and Groves, 1998). Weeds can change fire frequency and behaviour (gamba grass), species composition and richness (camphor laurel (Cinnamomumcamphora)), better colonise disturbed environments, smother native species (bridal creeper) and provide shelter for invasive vertebrate pests such as rabbits (blackberries (Rubus spp.)).
All of Australia’s vegetative communities have been or are subject to some level of weed invasion. It has been estimated that on a national scale exotic weed species account for 15% of flora, and of these one-quarter are either serious environmental weeds or have the potential to be serious weeds (Weeds, 2010). It is estimated that 80% of invasive plants were deliberately introduced, 65% as ornamental plants and approximately 15% for pasture or forestry (CRC Weed Management, 2003b). Further to this, a 2006 study of Australian weeds determined that approximately 2800 weed species are now naturalised in Australia, with 71 being recognised as being of national significance (Coutts-Smith and Downey, 2006).
The majority of nationally listed threatened flora species and ecological communities have weed invasion listed as a key threat to their survival. Currently one group of Northern Territory grass species, ‘Invasion of northern Australia by Gamba Grass and other introduced grasses’ is listed as a KTP under EPBC Act.
Intentionally introduced plants can also become weeds, for example murram grass (Ammophila arenaria) and sea spurge (Euphorbia parallias), which were introduced to sand dunes to provide stabilisation. Due to the success of the introduction of these species there is a complete change to the morphology and natural selection process in many of the affected upper beach and dunes area (Parks and Wildlife Service Tasmania, 2003). These changes eliminate many nesting areas for species that utilise the coastal zone, such as Thinornis rubricollis rubricollis (hooded plover) and Haematopus longirostris (pied oyster catcher).
For this nominated group of novel biota that impact on biodiversity, lantana (Lantana camara) is provided as one example to further detail the impact of terrestrial weeds.
Lantana (Lantana camara) Lantana is a profusely branching, scrambling, aromatic shrub 2–4 (up to 15) m tall and wide, with square-sectioned, often prickly cane-like stems (Swarbrick et al., 1995; Swarbrick et al., 1998, p 199). Lantana is an 'aggregate species', or 'species complex' (Day et al., 2003; Sanders 2006) containing diverse varieties that have arisen from natural and horticultural hybridisation (Howard, 1969; Swarbrick et al., 1998, p 120; Parsons & Cuthbertson, 2001b).
Lantana was first recorded in Australia in 1841 from the Botanic Gardens in Adelaide, there have since been many introductions for horticultural purposes and lantana has spread rapidly (Swarbrick et al., 1998, p 121). Lantana is listed as a Weed of National Significance and has spread along the east coast of Australia, from southern New South Wales to Cape York, and from sea-level up to 600 m altitude, or less commonly to 1000 m. It has invaded at least four million hectares, mainly in New South Wales and Queensland (CRC Weed Management, 2003c), and is expected to significantly increase its distribution and density within this range unless properly managed
Lantana is found in New South Wales and Queensland and is also naturalised in the Northern Territory, South Australia and Western Australia, on Lord Howe and Norfolk Islands, on three islands in the Torres Strait and (apparently marginally) in Victoria. Lantana is not yet known to be naturalised in Tasmania, but ornamental plantings have been found in the State.
Lantana has an adverse effect on biodiversity as it suppresses native vegetation by forming dense thickets that shade seedlings, out-competes native species for nutrients, and smothers plants (Biosecurity Queensland, 2010). Lantanais also thought to be allelopathic, i.e. able to inhibit or suppress by chemical means the germination and/or growth of at least some competing plant species (Swarbrick et al., 1995; Gentle and Duggin, 1997; Day et al., 2003). Lantana can flower and set seed all year round and its seeds are widely dispersed by many birds and mammals. Open forests and woodlands on Australia’s east coast can be readily invaded by lantana without disturbance, with lantana becoming distributed throughout these community types. Lantana also invades many other ecological communities especially after disturbance, and can become the dominant understorey species in disturbed native forests (Day et al., 2003). It can inhibit succession following a disturbance by out-competing native colonisers and preventing natural recovery. Declines in plant species richness with increasing levels of lantana infestation have been documented in dry rainforest (Fensham et al., 1994). Lantana is one of the worst invasive weeds in Australia and has been estimated to threaten populations of more than 1 300 native plants and animals. There have been records of some native species such as bandicoots, whipbirds, quail, wrens, birdwing butterflies, and brush turkeys using lantana thickets as substitute habitat where it has replaced the natural understorey vegetation, however this is only as a substitute for native species (Biosecurity Queensland, 2010).
This impact can be demonstrated for the critically endangered ecological community ‘Littoral Rainforest and Coastal Vine Thickets of Eastern Australia’. Within Australia, littoral rainforest occurs along the coast from far eastern Victoria up the east coast through New South Wales and Queensland and across the Northern Territory and Western Australia. This ecological community is listed as critically endangered in its southern occurrences along the eastern coastline of Australia (including offshore islands) from Princess Charlotte Bay, Cape York Peninsula to, and including, eastern Victoria. The ecological community provides a range of benefits to the landscape, it is an important buffer to coastal erosion and wind damage, provides natural refugia, suitable nest sites and food resources for resident and seasonally migratory species (DEWHA, 2008b and references therein).
‘Littoral Rainforest and Coastal Vine Thickets of Eastern Australia’ once formed an archipelago of patches along the eastern coast of Australia however this has now been reduced and fragmented primarily by coastal development, sandmining and agriculture (DEWHA, 2008b and references therein). The resulting fragmentation and reduction in patch size render the ecological community more vulnerable to other threats including weed invasion, edge effects and fire. The ecological community is subject to invasion by transformer weeds, these being highly invasive taxa with the potential to seriously alter the structure and function of the ecological community. If left unchecked, such weeds will eventually take over and destroy the affected patch. Transformer weeds include lantana, bitou bush (Chrysanthemoides monilfera subsp. rotundata) and rubber vine (Cryptostegia grandiflora), which are all recognised as Weeds of National Significance i.e. high impact, highly invasive species (DEWHA, 2008b and references therein).
Table 3 provides a list of species or ecological communities, listed as threatened under the EPBC Act, that are considered to be adversely affected by lantana.
Table 3: EPBC Act listed species and ecological communities that list lantana as a threat.
Eastern Suburbs Banksia Scrub of the Sydney Region
Littoral Rainforest and Coastal Vine Thickets of Eastern Australia
Blue Gum High Forest of the Sydney Basin Bioregion
Semi-evergreen vine thickets of the Brigalow Belt (North and South) and Nandewar Bioregions
Swamp Tea-tree (Melaleuca irbyana) Forest of South-east Queensland
Turpentine-Ironbark Forest in the Sydney Basin Bioregion
Summary of assessment: Invasive terrestrial weeds are the cause of decline in the population size and distribution of many threatened species and ecological communities, with local extinctions likely in the absence of effective control measures. In particular, lantana is having an adverse impact on the species and ecological communities through direct competition and habitat destruction.
The Committee judges that this threatening process is adversely affecting many listed threatened species.
Competition, habitat loss and degradation caused by aquatic weeds and algae Invasive aquatic weeds of inshore marine and fresh water ecosystems constitute a significant component of Australia’s weed flora and make up a third of Australia’s 20 Weeds of National Significance (WoNS, 2009). Another invasive aquatic weed species that is not listed as a Weed of National Significance that has been introduced to Australia, water hyacinth (Eichhornia crassipes) was nominated by the IUCN as among 100 of the world’s most invasive species (Lowe et al., 2000). A number of semi-aquatic invasive exotic grass species, including gamba grass are listed as KTPs under the EPBC Act.
Many of Australia’s aquatic weeds were introduced and spread by the aquarium trade including water hyacinth, cabomba (Cabomba caroliniana), hydrocotyl (Hydrocotyle ranunculoides), parrot’s feather (Myriophyllum aquaticum), Caulerpa taxifolia and salvinia (Salvinia molesta). Others including olive hymenachne (Hymenachne amplexicaulis) were introduced as feed for livestock and have spread through the wetlands and some, such as arum lily (Zantedeschia aethiopica), were escaped garden plants. Aquatic weeds fall into four groups: emergent; floating; floating leaf; and submerged weeds (Blanchard et al., 2009).
The arum lily, a native of South Africa, is an emergent weed that has stem and leaves growing above the water line and is often found growing along the edges of creeks and ponds. The arum lily is toxic to wildlife and can form dense clumps along water courses inhibiting native animals from access as well as changing the bank structure and out-competing native plant species (Blanchard et al., 2009).
Floating aquatic weeds such as salvinia and water hyacinth form thick mats on the surface of the water blocking out light and deoxygenating the water, preventing germination and smothering native flora and fauna. Other weeds, such as cabomba and leafy elodea or dense waterweed (Egeria densa), are submerged aquatic weeds and many have the potential to spread through Australia’s aquatic habitats (Schooler et al., 2006; Blanchard et al., 2009). These species also smother native species, restrict water flow, increase siltation and alter the environment for fauna, blocking pathways and making feeding difficult.
Aquatic plants intentionally introduced for beneficial environmental purposes can also become invasive and result in more problems than they remediate. The semi-aquatic upper tidal weed common cordgrass or rice grass (Spartina anglica) was introduced to Tasmania and Victoria to reclaim salt marshes and mudflats, but has successfully colonised and now clogs many Tasmanian estuaries. Cordgrass has changed the morphology of the upper tidal area that is important for spawning for many aquatic species and birds (Kriwoken and Hedge, 2000).
The diatom didymo (Didymosphenia geminata) is not currently present in Australia, but is a major pest in New Zealand (Whitton, 2009). This species has the potential to affect fish such as galaxiids through abrasion of skin and gills.
The endangered ecological community 'Assemblages of plants and invertebrate animals of tumulus (organic mound) springs of the Swan Coastal Plain' is threatened by two major wetland weed species Isolepis prolifera and Pennisetumclandestinum (DEWHA, 2007). The endangered Epthianura crocea tunneyi (yellow chat (Alligator Rivers)) is threatened by invasion of exotic weeds such as para grass, gamba grass, and olive hymenachne, which modify the species’ habitat by increasing the density of the grass and shrub layer, smothering native grasses and increasing fuel loads which promote intense, late dry-season fires. These changes in the habitat make it unsuitable for the yellow chat (Alligator Rivers) (DEWHA, 2009a).
Summary of assessment: Invasive aquatic weeds are the cause of decline in the population size and distribution of many threatened species and ecological communities, with local extinctions likely in the absence of effective control measures. The Committee judges that this threatening process is adversely affecting many listed threatened species.