Complex Relationships Between Introduced Mammals in New Zealand
We have heard all too many stories about the devastating effects of introduced species on native wildlife. The brown tree snake, for example, has wiped out nine of Guam’s eleven native birds. The toxic cane toad, introduced as a biological control agent in agricultural areas all over the world, has wreaked havoc on local vertebrate communities. The zebra mussel threatens to take over North America’s Great Lakes and the Mississippi River. In 2010, the Global Invasive Species Programme reported that their survey of 57 countries had identified 542 invasive species that they had judged to be threatening to native wildlife. Clearly, this issue is of critical concern to both scientists and local people that value their native flora and fauna. But one lesser-known story is that of the interactions between multiple invasive species in the same locality. How much jostling goes on between newcomers, and what are the effects on the overall invasion dynamics.
New Zealand bears claim the dubious distinction of being an ideal place to study interactions between multiple introduced species. First colonized by Europeans in the late 18th century, the island’s native flora and fauna were given a huge shock with the introduction of many of our associate species, such as sheep, cats, foxes, stoats, and various rodents. New Zealand’s roster of native mammals is limited to two seals and one bat (the other bat species that existed there in historical times, the New Zealand greater short-tailed bat, Mystacina robusta), has not been seen since the 1960’s and is thought to have gone extinct). Native birds, having lived in a relatively serene community with low predation risk for millennia, were not at all equipped to deal with wiley predators such as foxes and cats.
Many native species responded by promptly dying out. Penguins, wrens, snipes, quails, rails, piopios, a bellbird, the exotically named takahe and kokako, an owl, a bittern, the huia, a fernbird, and the aforementioned bat—they all fell off of the extinction cliff after Europeans arrived. The late Stephens Island wren (Xenicus lyalli), one of only three modern flightless songbirds, was wiped out almost entirely by the action of a single cat named Tibbles (although he was not responsible for every single wren killing, as claimed by legend).
The Europeans introduced the vast majority of the new predators, but a few hundred years earlier Polynesian travelers had introduced a rat species that set off an even earlier wave of extinctions, in addition to hunting out the moa and one of the native seal species.
Interactions Between Newcomer Species Studied
The upshot of that series of unfortunate events is that New Zealand now has far fewer birds and far more mammals than it did less than one thousand years ago. This begs an obvious question: how do all of these newcomer species sort out their niches, roles, and territories in the new “neighborhood”? It would seem that dining on native wrens would be plenty to keep them busy, but it’s both interesting and important to learn about the details of the interactions between the invasives themselves. Consider a kind of Kiwi “Alien versus Predator” in which all of the predators are also aliens. Gruesome, yet intriguing.
In light of this, a group of researchers based in New Zealand and Australia designed a study (Ruscoe et al. 2011) to determine how the removal of a top introduced predator, the stoat (Mustela erminea) would impact other invasive species, namely a rat (Rattus rattus), mouse (Mus musculus), and an herbivorous Australian possum (Trichosurus vulpecula). Both the rat and the mouse actually count as predators in this system, due to their broadly omnivorous tastes.
Their study involved setting up several different treatment plots in four different areas of the island. The treatments were: 1) control (no predator removal), 2) stoat removal via trapping, 3) possum removal via poisoning, and 4) rat + possum removal using both poison and trapping. Although these methods might sound harsh, keep in mind that these are all introduced animals that do vigorous harm to native/endemic species.
The researchers had a distinct set of expectations. They anticipated that removing the stoats would have a “mesopredator release” effect on the rats, relieving top-down pressures from a dominant predator and thereby allowing rat populations to increase. They also predicted that possum removal would increase rat populations, due to a relief from competition for food. Finally, they expected that mice populations would increase in the treatments where rats were removed.
Study Yields Both Predicted and Surprising Results
Some of the results were as predicted: the mice did in fact increase in areas where possums and rats were removed. This is likely due to the large overlap in diet between these rat and mouse species: with the rats gone, the mice suddenly had a relative surplus of resources on their paws. They responded in a very mouselike way: by churning out many more mice to take advantage of the boon. This is evidence that rats and mice in New Zealand are jostling for many of the same food resources, possibly having not yet partitioned their niches in their new environment. Competition for resources is known as “exploitation competition,” but it’s also possible that the rats had an “interference competition” effect on mice, altering their foraging behavior through a fear effect, direct antagonistic interactions, or even sometimes consuming their offspring (although the evidence for this is disputed).
The results of the study yielded one surprise, though. It turned out that removing stoats actually did not significantly increase rat populations, contradicting the expectation of a mesopredator release effect. And yet removal of the humble herbivore, the possum, did actually appear to have a releasing effect on rat populations. Despite the reputation of the stoat as a fearsome predator, it was the humble herbivore that seemed to be somehow constraining the number of rats in these habitats. It seems likely that there is some exploitation competition going on between the rats and possums, and perhaps the possums, being more local in origin, have a leg up on resource utilization in these habitats. Further studies will be needed to tease out more details about the relationship between these two species.
This is a valuable insight into the complex dynamics that determine the relative abundances of species within an assemblage. The rat technically ranks “below” the stoat and “above” the possum in a traditional food web schematic, and it yet it was the possum removal that allowed rat populations to increase. This indicates that in this system, rats are more constrained by basic food availability than they are by their interactions with the top predator. It is also a relief to those concerned that stoat removal will cause an epidemic of rats on the beleaguered island.
This study also demonstrates that trophic interactions (usually depicted by a “food chain” or “food web”) are much messier and less linear that we often would like to believe. Many species serve as both predators and prey, or prey on species at a similar trophic “level,” or affect populations of species at other levels without directly consuming them. Or, in the case of New Zealand, an herbivore can even constrain populations of an animal like the rat, which functions as a part-time predator. These complexities are yet another reminder of the intricate, intertwined relationships underlying the communities that we see all around us.
Ruscoe, W. A., D. S. L. Ramsey, R. P. Pech, P. J. Sweetapple, I. Yockney, M. C. Barron, et al. 2011. Unexpected consequences of control: competitive vs. predator release in a four-species assemblage of invasive mammals. Ecology Letters 14:1035–1042.
Anne-Marie Hodge, EGN’s Species Science Advisor, is currently a PhD student at the University of Wyoming in Ecology, and holds a Bachelor of Science in Zoology from Auburn University and a Master of Science in Biology from the University of North Carolina-Wilmington. While at Auburn, she established and served as president of Alabama’s first chapter of the Society for Conservation Biology. Anne-Marie’s research interests include community ecology, predator-prey dynamics, and conservation, with a focus on tropical carnivores. She has research experience in Mexico, Belize, Ecuador, and Kenya.