Invasive species are one of the two most frequently cited pressures for EPBC-listed species (the other being fragmentation of habitat).15 Invasive species and pathogens include plants, animals, fungi and a range of pathogenic microorganisms; their significance and impacts are also discussed in Chapter 5: Land.
Invasive plants (weeds) affect biodiversity in many ways: they may outcompete native plants, they may reduce or alter the resources available to native animals and they may markedly affect fire regimes. In 2007, more than 2800 of the 27 000 alien plant species that had been imported into Australia had become established in the wild, and that number was growing at around 10 species per year.120 The number is now much higher. As part of the National Land & Water Resources Audit phase II, the jurisdictions mapped 98 major weeds nationally.121 Of these 98 species, 20 are Weeds of National Significance (WoNS). The remaining 78 are either candidate WoNS, on the national environmental alert list, targets for biocontrol (control of an invasive species by other species, usually by the managed introduction of predators or pathogens), or a combination of these three categories.
As with many threatening processes, the mechanisms by which weeds have impacts are poorly defined, and processes for determining which species they affect and their relative importance compared with other pressures are inconsistent and often unclear.88,122 Scott and Grice122 recently reviewed the state of research on weeds in Australia. Since 1995, the number of quantitative studies of the impact of weeds on the Australian environment has more than doubled to more than 70 studies, covering 30 species. Most studies show that weeds reduce plant biodiversity by reducing native species richness and changing community composition and structure. Weed invasion is considered to be a threatening process for one-third of rare species in Australia. Scott and Grice conclude that the measures currently adopted to understand the invasion of weeds in Australia are not at the level required to plan strategies to mitigate the problems they create.
Many introduced fungi have become established in Australian ecosystems, especially in soils. Several are known to have become invasive species, and the list is likely to grow in the future as a result of more research and as climate changes. Three invasive fungi are of particular concern nationally: chytrid amphibian fungus (Batrachochytrium dendrobatidis), myrtle rust (Uredo rangelii) and Phytophthora cinnamomi.123
The chytrid fungus124 has been responsible for mass deaths of frogs worldwide and is widespread in Australia. A significant association between amphibian declines in upland rainforests of northern Queensland and three consecutive years of warm weather125 suggests future warming could increase the vulnerability of frogs to the fungus.
Myrtle rust is of particular concern, because it affects a wide range of plant species in the Myrtaceae family of plants, including Australian natives like bottlebrush (Callistemon spp.), tea tree (Melaleuca spp.) and eucalypts (Eucalyptus spp.).126 There are fears that this emerging invasive species could transform the Australian environment in major ways, especially if its impacts are magnified by climate change.
Phytophthora cinnamomi is a rootrot fungus that attacks a range of native plants as well as crops. It is a particular problem in Tasmania and Western Australia, but is more widespread nationally and is likely to become an even bigger problem as soil warms with climate change in the future.127
The most significant invasive vertebrate animal species are the European fox (Vulpes vulpes), domestic cat (Felis catus), European rabbit (Oryctolagus cuniculus), feral goat (Capri hircus), feral pig (Sus scrofa) and cane toad (Bufo marinus). Significant invertebrate invasive species include the red fire ant (Solenopsis invicta), the yellow crazy ant (Anoplolepis gracilipes) and a range of tramp ant species.128 European wasps, bumblebees and European honeybees are also widely cited species of concern. Asian honeybees are a recent threat that appears to have potential for major impacts (Box 8.5).