The Asian tiger mosquito can carry dread diseases like Zika, and yellow and dengue fever. After it vanished from Palmyra Atoll, an island in the tropical Pacific, USGS researchers and partners set out to find out why.
The scientists’ first hint that something had changed on the island was the lack of daytime bites. About 30 researchers from the U.S. Geological Survey (USGS), University of California at Santa Barbara, and others were stationed on Palmyra Atoll, a reef island in the tropical Pacific. They were painfully familiar with the two species of mosquito that plagued them — the larger, aggressive Asian tiger mosquito attacked during the day, and the smaller southern house mosquito preferred to strike at night. So when a species that fed on them went missing, they noticed.
In other parts of the world, the Asian tiger mosquito can carry dread diseases like Zika, and yellow and dengue fever. It likely arrived at Palmyra Atoll with humans, as have most of the insect species on the island. The mosquito has enjoyed a steady diet of non-native rats, birds, and visiting scientists ever since it invaded.
In 2001, the U.S. Department of the Interior designated Palmyra Atoll as a National Wildlife Refuge and placed it under the care of the U.S. Fish and Wildlife Service. 10 years later, the Service and partners removed all the ~40,000 rats infesting the island, to allow the damaged environment a chance to recover.
“Rats were everywhere, and became the dominant consumer on land,” says Kevin Lafferty, an ecologist with the USGS Western Ecological Research Center. He has traveled to Palmyra to lead research projects since 2006 and has seen firsthand how the island ecosystem changes when non-native species make landfall.
After the rats’ removal, native plants and crustaceans flourished again, and the researchers stopped being bitten by mosquitoes during the day. The Asian tiger mosquito prefers mammalian prey, like the rats and people. But they sometimes feed on birds, which are plentiful at Palmyra. Although the researchers suspected the mosquito no longer existed on the Atoll, they wondered if it might be lurking at low densities by surviving off seabirds away from the field station.
Lafferty and researchers from UC Santa Barbara, the University of Maryland, and Stanford University decided to find out whether the Asian tiger mosquito really had vanished from the island. For two years, the team set up traps marked with human scent, and “black light” traps that emitted UV light to attract insects. They also took careful note of when (day or night) they were bitten by mosquitoes.
Hillary Young had been a graduate student at Stanford in 2009, while rats still inhabited Palmyra. For her dissertation, she studied the structure of the island’s food web, including its insects. Using data from Young’s dissertation work, the scientists could compare the food web before and after the eradication of rats.
Prior to the rats’ removal, Asian tiger mosquitoes made up about 6% of the mosquito species found in black light traps. Post-removal, the traps captured many bird-feeding southern house mosquitoes, but no Asian tiger mosquitoes. This met the standards set by the World Health Organization to document the collapse of a mosquito population. Mosquito populations have been driven locally extinct before, but only through applying insecticides and removing breeding habitat. With rats gone from Palmyra, the people on the field station did not have enough blood to support a mosquito population. The island’s Asian tiger mosquitoes starved to death.
“In retrospect it makes perfect sense. But hindsight is always 20:20. I don’t think anyone went into this rat eradication thinking about potential effects on mosquitoes,” says Young, now an associate professor in UC Santa Barbara’s Department of Ecology, Evolution, and Marine Biology.
“This is a rare example of a ‘secondary extinction,’ when one species goes extinct because one that it depends on goes extinct,” says Lafferty. But secondary extinctions are probably common, at least for parasites. In fact, there was at least one more secondary extinction on the atoll. Lafferty and his colleagues had previously found that Palmyra rats had a large parasitic worm living in their stomachs. That worm went extinct along with its rat host.
The study offered Lafferty and colleagues a rare look into how dependent one species can be on another. The Asian tiger mosquitoes on Palmyra could have, presumably, switched to a diet of seabird blood when rats were no longer available — but they didn’t. And that has potential implications for controlling Asian tiger mosquito populations elsewhere.
“In terms of human lives lost, mosquitoes are the most dangerous animal on the planet, so there’s been a lot of effort to control their populations,” says Lafferty. The general assumption has been that mosquitoes are primarily limited by climate and larval habitat (standing water, etc.). This study, Lafferty says, “shows how reducing access to blood meals can also drive mosquitoes extinct.”
The researchers published their results in the journal Biology Letters.
Lafferty’s research program with the USGS Western Ecological Research Center specializes in infectious disease ecology, marine nearshore ecology, and the role of parasites in ecosystems across the world. Today, he and Young lead an NSF-funded project to study how the Palmyra Atoll food web has changed after rat removal. Visit Lafferty’s research profile and Young’s laboratory website to learn more.
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