Tuesday, June 11, 2019

Controlling rats doubles `elepaio nesting success, and other conservation success stories

Hawai`i `elepaio, Image: Kelly Jaenecke, USGS

A new study, published this year in the journal The Condor, found that removing black rats from a forest environment quickly improves the ability of the native `elepaio to bounce back.
It is part of a growing body of evidence that removing rats from environments where they are not native can significantly improve bird survival and forest recovery. 
And unexpected benefits can happen. As when black rats were removed from Palmyra Atoll. While it was mainly intended to protect seabirds and native crabs, the removal also wiped out the Asian tiger mosquito, Aedes albopictus, which carried disease. It turned out the mosquitoes needed the rats as a blood source.
And with rats gone, suddenly the Palmyra forest floor burgeoned with seedlings of native trees.
The new `elepaio study is entitled, "Increased nesting success of Hawaii Elepaio in response to the removal of invasive black rats." You can find it here. Authors are Paul C. Banko, Kelly A. Jaenecke, Robert W. Peck and Kevin W. Brinck.
It's not news that rats are toxic to the natural environment in the Islands. They eat everything--seeds, seedlings, eggs, adult birds, insects and lots more. What's new here is clear proof of the direct impact on one important deep forest species.
"In Hawaii and other oceanic islands with few native land mammals, black rats (Rattus rattus) are among the most damaging invasive vertebrate species to native forest bird populations and habitats, due to their arboreal behavior and generalist foraging habits and habitat use," the authors wrote.
There are models that suggest that growth rates for native bird species populations should respond well to removing rats, but there hasn’t been a lot of evidence—mainly because that evidence is hard to get. Many of the critical native forest birds are rare, their nests are hidden and hard to observe and they can be high in trees.
One reason black rats are a special problems is that they climb trees, and will take females and eggs right off the nest. There are wildlife video images of it.
"Lower female survival rates have been attributed to nest predation by rats for a number of Hawaiian species," the authors write.
"Hawaiian forest bird nesting studies have indicated that rats are an important cause of nest failure for at least the Oahu Elepaio in lowland mesic forests dominated by invasive fruit-bearing tree species and for the Puaiohi in wet montane ‘ōhi‘a forests." O`ahu `elepaio are known to science as Chasiempis ibidis, and puaiohi or small Kaua`i thrush as Myadestes palmeri.
In the paper's study, researchers used rodenticide to reduce rat populations by 90 percent in two Hawai`i Island forest areas, each 120 acres in size, along the Mauna Loa Strip Road in Hawai`i Volcanoes National Park. They also trapped rats, catching thousands of them.
Their finding: Once the rat populations were reduced, for the Hawai`i puaiohi or Chasiempis sandwichensis, nesting success doubled, from 33 to 62 percent, and female survival also increased dramatically.
"The rapid response of Hawaii Elepaio to rat removal indicates that predator management could be a powerful tool for restoring the entire forest bird community. Hawaii Elepaio are representative of other forest bird species because they nest in a variety of widespread, abundant tree species and they build their nests throughout the forest canopy," they wrote.
And one of the benefits of keeping the bird numbers elevated, they argue, is to give the species time. Time to evolve natural resistance to one of the other critical threats, mosquito-borne avian malaria.
It's good news for conservation. 
Another bit of positive news from a couple of years ago was that native forest birds like the `elepaio quickly inhabit newly established native forest areas.
It's another case of, if you build it, they will come.
© Jan TenBruggencate 2019

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