While bigger animals may tend to have better survival rates in the wild, that is not necessarily the case for Helms's stag beetles on Stewart Island/Rakiura.
A new study, led by the University of Otago, Ōtākou Whakaihu Waka, found rats preferentially preyed on larger stag beetles, rather than smaller ones.
Lead author Dr Luna Grey, who completed her PhD in the Department of Zoology, says since larger beetles are usually male, this could impact future generations of these beetles and have serious implications for at-risk beetles in the genus.
The Geodorcus helmsi.
"In populations with novel predators, the endemic inhabitants need to adapt quickly or face extinction," Dr Grey says.
"In this case, we see that larger males are being affected, which means they cannot pass on their genes to future generations, which may cause the size of the males and their mandibles to decrease over time."
When a population decreases in size, it also becomes more vulnerable to other threats, such as disease, habitat destruction, and poaching.
"This could lead to the extinction of the Geodorcus helmsi (Helms's stag beetle), removing an iconic feature of New Zealand's southern forests."
Published in the Journal of Insect Conservation, researchers aimed to determine the impact of rats on the beetle, after discovering decapitated heads in a hollow tree and under tree roots while on Stewart Island.
"We visited three sites the next summer: Ulva Island, a predator-free ecosanctuary, Mamaku Point, a fenced peninsula, and some of the forests around Oban.
"At each of these locations we measured the beetles' head width and mandible length. We then compared the sizes of these beetles inside the ecosanctuaries to those outside the ecosanctuaries."
Dr Grey says there are a few possible explanations as to why rats prey on the larger beetles.
"The first reason is that perhaps the larger males are more noticeable than the smaller males and females, which bring attention to the rats more easily than it would for smaller individuals.
"The second potential reason is that the larger size and big mandibles on the male beetles may make it difficult to get into good hiding places when a predator appears, while the smaller beetles can find refuge more easily."
This study is the first to show the effect invasive species can have on animals with exaggerated traits, so the results can also be applied to similar insect species and the more threatened beetles in the genus.
Dr Grey says New Zealand is on the right track for conserving the stag beetle with its Predator Free 2050 protocols, which aim to eradicate rats, stoats and possums.
"However, there is a fear that mice will fill the gap created by the eradication of rats.
"Mice are much more complicated to remove from an ecosystem, but if we want to safeguard our endemic species from predation, we need to consider how eradicating one invasive species will affect the others."
