Kakadu still battling South American invader

In a paper recently published in Nature, an international group of scientists led by CSIRO Ecosystem Sciences’ Dr Shon Schooler explained how this unpredictability could lead to a strategy for ongoing control of the weed.

A native of South America, the aquatic plant Salvinia molesta is one of the most widespread and environmentally and economically destructive invasive plants because it can form huge mats of floating plant material that clog waterways and crowd out native species.

Biological control using a weevil about 2mm long has successfully cleared massive infestations of salvinia in 13 countries including parts of Australia, but Kakadu proves a hurdle for it.

“Salvinia in Kakadu’s billabongs exists in two stable states,” Dr Schooler said.

“In one it is suppressed by the weevil while, in the other, it isn’t and, in some years, seasonal flooding allows the system to flip between the two states.”

After a flood event, the system is delicately balanced and could tip either way – into a state where salvinia is uncontrolled and forms thick mats, or where it is controlled by the weevils and there is mostly open water.

Dr Schooler said the research has implications not just for the control of salvinia but for restoration ecology in general.

“Systems with two stable states are generally viewed as potential environmental disasters because, once the system falls into a degraded state, it is very hard to flip it back to the desired state,” he said.

“However, our research suggests that in Kakadu’s billabongs – after flooding when the system teeters between states – opportunities could arise to tip the salvinia system towards successful control by weevils.”

The research team is now investigating how to introduce weevils when salvinia is especially vulnerable, thus ensuring years of control.

“Understanding more about how complex ecological systems work produces opportunities to use their complexities to help manage or repair them,” a co-author of the paper Dr Anthony Ives from the University of Wisconsin said.