Researchers have spent the last year digging through wasp faeces, proving four species thrive on the same island without being in competition with one another. This could change the way we think about invasive species, writes Matthew Scott.
Great Mercury Island or Ahuahu is an idyll of white sand beaches and thick native bush. Privately-owned and remote, without crowds or infrastructure, it’s like a slice of New Zealand yet to join the modern age.
And like premodern New Zealand, it is free from mammalian pests - at least it has been ever since island owners Michael Fay and David Richwhite matched $750,000 from the Department of Conservation to drive the last rat and weasel back to the mainland in 2016.
But like on many of our offshore islands functioning as nature preserves, if you train your ears you can still hear the buzzing of one of our most common introduced pests.
Four species of wasp inhabit Great Mercury, and a recent study from the University of Auckland has found their foothold on this environment can reveal how introduced species can coexist.
Researchers were surprised to discover the four species of wasp keep to their own parts of the island and only minimally overlap in terms of favoured prey, which may be a worrying result for ecologists.
This is because wasps prey on endangered native bugs and insects such as weta, moths and butterflies.
Previous knowledge dating back to the works of Charles Darwin suggests animals with a similar ecological niche become competitive, reducing the amount of available prey and overall making life more difficult for each species.
The new research means invasive species who occupy similar ecological niches are more capable of success than previously considered.
Researcher Julia Schmack said these findings may amplify the ecological impact of pests around the country.
“If they don’t actually push each other over the edge like we previously thought, they could have a greater effect on their environments,” she said.
This could help us to understand how other closely related introduced species thrive, despite occupying the same broader habitats - for example, the black rat and the brown rat.
The study used a novel approach to find out what the wasps were eating and how much of their diet was shared between them - digging through wasp faeces.
“It’s a bit like a crime scene,” said Schmack. “We extract the DNA and use it to identify the prey.”
This method is also used to uncover which of our endemic species are at risk to these introduced predators.
As generalists, wasps will eat almost anything, and on Great Mercury, they make a meal from a range of New Zealand native species like moths, weta and spiders.
Professor Jacqueline Beggs, the co-author of the study, said only a few studies around the world have looked at more than one invasive species at a time.
“There are real implications for our understanding of how invasive species interact as well as for invasive wasp control in New Zealand,” she said.
“If we understand their combined ecological impact, at least we have a better chance of developing strategies to better protect our valuable native species.”
Julia Schmack first thought to examine the wasp populations of the islands on a trip to Rangitoto. “There were heaps of wasps over there!”
“Nothing was known about the wasps on the islands,” she said.
Successful groups of voracious predators in our natural reserves should put our focus on the islands.
“It’s important that we protect these offshore islands,” said Schmack.
The wasp has a double-edged effect on our ecology. Despite hunting our own sometimes endangered fauna, they are pollinators who also like to snack on other exotic species.
“They have the potential to be natural pest agents."
