That’s in spite of our top predators—invasive mammals such as stoats—being much smaller.
Associate Professor Wittmer, from the School of Biological Sciences, says the research in and next to Wyoming’s Grand Teton National Park showed interactions between species on the same “trophic” level of the food chain—determined by their feeding behaviour—could be just as important as those between levels.
“In New Zealand, we don’t have comparably sized apex predators in our terrestrial ecosystems. However, we have a range of invasive mammalian predators, such as stoats and rats, that occupy the apex predator position in a similar way to wolves and pumas.
“We are embarking on a predator-free strategy where we will manipulate the abundances and interactions of a range of top predators. Our research shows that changes within trophic levels and resulting changes in species interactions, such as competition, will likely have important follow-on effects as well, particular if one species is managed and the other isn’t.”
In a recent paper in Proceedings of the Royal Society B, Associate Professor Wittmer and co-authors highlighted how reintroducing wolves in Grand Teton National Park negatively affected pumas.
The Grand Teton area gave researchers a “wonderful opportunity” to study puma population dynamics using a unique 16-year data set, he says.
“In particular, we were able to look at the effects wolves might have on pumas. Wolves were reintroduced into this system north of our study area in Yellowstone National Park and then naturally dispersed into our study area, where they had been absent for at least 80 years or so.
“We found the effect of re-established wolves explained most of the observed decline of the puma population in our study area, and it explained it better than other variables we looked at, such as food availability or hunting.
“That was due to quite complex sideways species interactions between pumas and wolves, which included direct mortality of kittens and increased starvation of juvenile pumas.”
Interactions within trophic levels need to be understood before New Zealand implements conservation and management strategies, particularly in systems where species are either reintroduced or heavily controlled or even removed, says Associate Professor Wittmer.
“That is exactly what we are planning on doing with Predator-Free 2050. We have demonstrated we are quite good at reintroducing our native birds to predator-free offshore or mainland islands, but now we are proposing to completely eradicate seven of the most damaging invasive species we have in our ecosystem.
“We know a lot about how these species affect our endangered bird populations, but we don’t really know what is going to happen when we are removing or controlling only some of those species but not others.
“Take rats and mice. The relationship between them is complex and includes competition for food, and at times rats kill mice.
“What we are doing right now is proposing to remove rats from the ecosystem, but not mice. So will we see a mice eruption afterwards because they are now freed from the competitive interactions with rats? We just don’t know.
“Both rats and mice have negative effects on native species. But will there be benefits from removing rats for our rare invertebrates, such as weevils or wētā, if mice numbers increase because they are no longer competing with rats? It highlights that all these systems are highly complex.”
Associate Professor Wittmer is now investigating how scavengers can affect predators in North America and then plans to look into New Zealand ecosystems and apply lessons from overseas.
He hopes increased long-term research funding of 10 years or more will become available in New Zealand.
“Understanding and teasing apart complex species interactions is difficult and requires long-term data. Until we make an investment into make long-term studies, it will be difficult to address topics that require research over many years.”