Rat poo discovery prompts rethink about destructive invasive species

Selling, giving it away and releasing it into the wild are all illegal. And as you can probably guess from its name, African boxthorn is very spiky. Since it was introduced in the 1800s, it has created impenetrable five-metre-high thickets along roadsides, railways and waterways.

But although humans consider it a destructive weed, new research has prompted a rethink of its work. A scientist has discovered the plant has become critical to the survival of a rare animal that lives across tiny islands off the Australian coast.

Annie Kraehe spent weeks doing something only a scientist would do – collecting and “poking through” the poo pellets of a large native rat, trying to work out what they were scavenging.

Like many nightshades, African boxthorn is poisonous to humans. But it seems the greater stick-nest rat is able to not only eat the plant, but digest massive amounts of it. “This gave us proof of what the stick-nest rats have been eating,” Kraehe said.

Kraehe found that around half of its diet is made up of the plant, even though it only makes up around one tenth of available vegetation. Flinders University, where she's completing her PhD, called her discovery that it prefers to eat the weed over native foods a "twist to the native animal's survival story".

Giant rat finds another use for invasive spiky plant

But it’s not just a food source for the native rodents. Following their footprints through the sands, Kraehe also discovered they use the plant as a building material for their massive nests which are made from interwoven sticks and stones.

Reevesby Island, off the coast of Port Lincoln in South Australia, is an important stronghold for the guinea pig-sized rat. It was once widespread across dry savannahs from Shark Bay in Western Australia to the meeting of the rivers at the Murray–Darling basin.

Because its populations were decimated by foxes and cats, it was extinct on the mainland by the 1930s. Today it survives only on islands free from invasive predators, as well as fenced-off mainland sanctuaries.

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Not the first time African boxthorn has protected Aussie animals

Study co-author Flinders University Associate Professor, Vera Weisbecker, said it was “good news” for the greater stick-nest rat because most weeds, including African boxthorn, are generally bad for biodiversity.

"African boxthorn offers excellent cover from these raptors, so we think that this makes boxthorn thickets a preferred nesting location for the stick-nest rats,” she said.

“A similar effect has also been observed for little penguins in South Australia, where African boxthorn provides better protection against cats and foxes than native vegetation.

“There are a few cases of individual native animals benefiting from invasive weeds around the globe.”

The CISRO has published the paper 'Threatened stick-nest rats preferentially eat invasive boxthorn rather than native vegetation on Australia’s Reevesby Island'. The research was a collaboration between Flinders and Adelaide Universities.

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