Shifting Boundaries

The disastrous consequences of foxes, cane toads and other overseas animals settling in Australia are well known. But there can be problems too when our native species move beyond their natural home ranges.

- Tim Low

Imagine 6.3 billion snails releasing 23 tonnes of faeces a day. That’s the situation today in the Swan River in Perth, where mud snails now reign supreme.

Their numbers have exploded since they were first detected there in 1954 and, in the river’s lower reaches, they now crowd the bed in densities of up to 400 per square metre. This species is native but its natural range lies more than 3000km away in eastern Australia.

Seagrass beds in the Swan are now under siege thanks to the snail’s success. Before it arrived, the river had few hard surfaces on which small seaweeds could grow. Now, seagrass is being uprooted by snails and smothered by seaweed that dislodges from snail shells at the rate of 433 million fragments a day. Snail faeces are thought to be aggravating the problem by fertilising the seaweed.

We often hear of harmful animals from overseas – foxes, cats, toads and the like – but problems also ensue when our own native species spread from one region to another. In two centuries, hundreds of species in Australia have moved in every possible direction, amounting to a major reshuff ling of the ecological pack. Anything and everything is involved, including spiders, cicadas, earthworms, marsupials, f ish, birds, frogs and even diseases, although only some are regarded with concern.