This summer, biosecurity staff on pest-free Ahuahu-Great Mercury Island (GMI) were alarmed to find a dropping that looked suspiciously like that of a Norway rat. Worryingly, three more were found in the following month.
Yet, further investigation failed to confirm that a rat had invaded. If not a rat, then what could it be?
“It was a puzzle,” says GMI biosecurity Ranger, Tansy Bliss, who found the droppings on her routine checks of the 300-plus surveillance traps on the island. “The droppings were similar in shape and size to those of Norway rats but there was no noticeable smell in the vicinity of the dropping. Rats’ urine has a strong smell, which was absent.”
Rodent tracker dogs doing a routine 3 monthly surveillance check did not detect any rat sign either. Hedgehogs were ruled out, never having been present on the island. Hedgehogs tend to leave their droppings out in the open whereas the scat found was inside trap boxes, on top of a trap box and inside a tracking tunnel.
The staff were stumped and wracked their brains. What else lived on the island that might produce droppings like these?
“That’s when I thought of Southern bell frogs, as there is a sizeable population on Great Mercury,” Tansy recalls. “Claire Wooldridge-way, the Whitianga DOC Biosecurity Senior ranger did some on-line research, and I checked in with the island maintenance staff who had seen scats like these in damp areas around pump and drain housings. Then I found a frog footprint as well as a dropping on a tracking card.”
The evidence was stacking up. But just to be sure, on the advice of frog expert Dr Ben Bell, Tansy caught a Southern bell frog, fed it and waited until – three days later – it produced a dropping. It matched the field droppings in size, shape and rough texture.
The frog was released and the different droppings collected over the investigation were photographed to show fresh, semi-fresh and dry droppings. A dry dropping was crumbled and the contents examined for evidence of the animal’s diet. As expected, there was a lot of insect fragments. Tansy suspects it was insects around the traps rather than the peanut butter bait that had attracted the frogs.
“There were quite large fragments, such as a recognisable beetle casing and weta legs,” says Tansy. “Southern bell frogs don’t chew their food as such. That’s why the scats are rough in texture.”
DOC and GMI staff’s detective work gave reassurance that rats have not reinvaded the privately owned Ahuahu-Great Mercury Island and are not preying on the vulnerable native birds that find sanctuary there. The impacts of the Southern bell frog, which was introduced from Australia and is now widespread in most of New Zealand, have not been measured. However, they are known to have voracious appetites and will prey on anything of a suitable size – mainly insects but have also been known to eat small lizards.