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Apart from three bat species (one of which lives around braided rivers), New Zealand had no land-based mammals until settled by Māori and Europeans. Instead, birds and insects filled the ecological niche inhabited by mammals elsewhere in the world. Specialists in their habitats, these birds and insects had no reason to evolve defence mechanisms against comparatively more intelligent mammals. Introduced mammals now take an alarming toll on native wildlife through predation, competition and habitat destruction.
Because several pest mammals prey on one another, experience has shown that actions to reduce one species, such as rats or rabbits, leads their natural predators, cats and mustelids (stoats, weasels, and ferrets), to turn their undivided (rather than partial) attention to our native animals. So management strategies must be based on how an entire suite or guild of pest mammals operate.
The video above from Wildlife Management International Ltd. shows how one of the largest colonies of black-fronted tern colonies in the Acheron and Clarence River are under attack. Here, the stoat is killing an adult tern on the nest. The stoat didn’t even eat the bird, just killed it and moved on to another nest. When the footage was shot in December 2015, the colony had gone from 50 nests, down to 7. The video below shows a feral cat wiping out the last of the black-fronted tern nests in the Acheron 2 weeks later. With funding from DOC and ECan, a trapping programme around black-fronted tern colonies elsewhere on the Clarence is in place.
The same cannot be said for predator guilds on braided rivers. The very ‘open’ and dynamic nature of braided rivers differs markedly to relatively permanent and enclosed forest environments. Moreover, because braided river environments are so dynamic, birds may nest in different locations – even different rivers – from year to year. Sometimes these sites are near forests or alpine meadows, other times they are beside open fields or towns, or on beaches along the coast. Consequently it’s not easy to determine let alone implement the best management strategy to control pest mammals in braided river environments. Considerably more research and social education is needed, and needed soon, otherwise rare and endangered braided river birds will soon slip into extinction.
While the greatest threat to braided river birds is from pest mammals, predatory birds, both native and introduced species, are also a problem. Three predatory bird species are known to predate on braided river birds: black-backed gull, magpie, and along the Wairau River near Blenheim harriers are a major threat. Elsewhere harriers present no real problems. This highlights the complexity of predator guilds on braided rivers, and why no single pest management regime can be applied to all rivers.