At least 80 introduced animal species have established populations on the Australian continent. Assessing invasive animals in 200853 stated that invasive animal species are one of the top three greatest threats to threatened species and ecosystems—mainly by competing for, or destroying, habitat and food resources—and that they continue to colonise new areas. Some of these species were initially established in past centuries and continue to expand their ranges (e.g. the cane toad spreading into Western Australia in February 2009; Figure 4.18), but new threats have also emerged, such as red-eared slider turtles and tilapia. Carp were found to occur in 11.5% of Australian rivers. In addition to carp and cane toads, feral pigs were identified as nationally significant invasive animals by the Australian Vertebrate Pests Committee, due to their impact on inland river systems (especially wetlands).
Southern Australia and New Zealand, considered together, represent one of six major invasion ‘hot spots’, where non-native freshwater fish species represent more than one-quarter of the total number of fish species in river systems, and where the proportion of native fish species that have a high risk of extinction in the wild is the highest.54 In New South Wales, three alien species—common carp, gambusia and goldfish—are present in all inland rivers. Redfin perch, brown trout and rainbow trout are also widespread. Carp are overwhelmingly dominant (Figure 4.19), making up 87% of alien fish biomass and 58% of total fish biomass. Carp and gambusia were the dominant species in all lowland rivers in the Murray–Darling Basin.