Wetlands and islands

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This week’s Friday afternoon reads looks at predator research relating to two key habitats in New Zealand – wetlands and off-shore islands.

Man in a canoe with a dog, kayak in backgroun
Sinclair Wetlands coordinator Glen Riley and his conservation dog Pipi check out the waterways. Image credit: Supplied

Colin O’Donnell (Department of Conservation, Christchurch), Kay Capperton (Havelock North) and Joanne Monks (DOC, Dunedin), review research and statistics on the impacts of introduced mammalian predators on the viability of wetland birds, particularly specialist bird species restricted to wetlands. The authors point out that predator control in wetlands has been little studied compared to other New Zealand ecosystems and currently scientific knowledge of predator behaviour and impact in wetlands is limited. There is also little knowledge of how important predator impacts are, compared to other significant wetland environmental threats such as drainage and degradation. Over 90% of New Zealand’s wetlands have been lost and degradation of wetlands is continuing to happen.

Predator control is carried out extensively in many New Zealand wetlands in order to benefit wildlife populations. The authors point out that the benefits of predator control in forests have been well-documented, but little is known, scientifically, about what is happening in New Zealand’s wetlands, even though “over 80 New Zealand bird species use freshwater wetlands in some way”. Eighteen of those 80 species (60%) species are classified as threatened or at risk. In this study, the authors placed particular emphasis on six specialist wetland species: Australasian bittern, banded rail, brown teal, fernbird, marsh crake, and spotless crake.

The objectives of the review, as listed by the authors, were: (1) examine the anecdotal and published evidence for predation by introduced mammalian predators on bird species characteristic of freshwater wetlands; (2) make preliminary predictions about vulnerability of species to predation; (3) summarise information on the occurrence and behaviour of mammalian predators in wetlands; and (4) indicate future directions for research and conservation management.

The study found that “the most frequent predators were cats, but dogs, stoats, rats, and ferrets were also common predators. Most reported predation attributed to dogs was a result of using them for duck shooting.” The authors also reported that “all predators known to impact on indigenous birds are widespread and abundant in wetlands, just as they are in other habitats” and conclude that:

“Monitoring methods for cryptic swamp birds have yet to be developed in New Zealand and are needed urgently if we are to begin assessing population trends. Although techniques are available for monitoring predators, these have yet to be tested in wetland situations, where behaviour of predators might differ from other habitats, or where terrestrial monitoring methods are simply not suitable.”

This review, published in the New Zealand Journal of Ecology can be downloaded in pdf format: Impacts of introduced mammalian predators on indigenous birds of freshwater wetlands in New Zealand (2015)

Another study, also published in the New Zealand Journal of Ecology, looks at the diet flexibility of stoats on Fiordland islands – comparing stoats on rodent-free islands (Chalky Island and Secretary Island), with stoats on an island where mice were present (Resolution Island) and with stoats on the mainland where they had access to their normal range of prey.

The authors, Elaine Murphy, Craig Gillies, Fraser Maddigan, Peter McMurtrie, Kerri-Anne Edge, Maheswaran Rohan and Kay Clapperton examined the stomach contents of stoats which had been removed from the various islands as part of eradication programmes. They found that most of the stoats on Chalky Island had consumed birds, while those on Secretary Island (rodent-free) and Resolution Island (mice present) had eaten invertebrates – particularly weta. Mice were consumed by only 12% of the stoats on Resolution Island, possibly because mice were only present at a relatively low density. On the mainland, where stoats had a wider choice of prey, the average consumption of birds and invertebrates was less.

The authors conclude that their research “demonstrates the adaptability of stoats, and their ability to survive without mammalian prey in different ways. It supports the hypothesis that differences in body weights of stoats are at least partly driven by variation in prey size and/or availability.”
Flexibility of diet of stoats on Fiordland islands, New Zealand (2016)

Richard Griffiths , Fin Buchanan, Keith Broome, John Neilsen, Derek Brown and Michelle Weakley report on the successful eradication of invasive vertebrates on Rangitoto and Motutapu Islands. This project was substantial and is of significant interest in that it occurred close to New Zealand’s largest city, Auckland and involved simultaneous eradication of multiple vertebrate invaders in one operation. Stoats, cats, hedgehogs, rabbits, mice and three species of rats were all removed from an area of 3,842 ha.

In order to successfully eradicate multiple species, “target species were prioritized by likelihood of eradication success with resources allocated preferentially to species posing the greatest risk of failure and methods applied in a sequence that allowed each technique to capitalize on its predecessor”. The approach is reported to have led to benefits of increased operational efficiency, a shorter operation period and reduced costs. Overall, the authors estimate that the project cost 50% less than if each of the target species was eradicated individually.

The abstract to this research article is freely available and the full research report available for purchase from the publishers. Successful eradication of invasive vertebrates on Rangitoto and Motutapu Islands, New Zealand (2015)