Old mosquito theory holds no water
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The previous year's drought is the cause of high mosquito populations in wetlands the following year because drought drastically reduces mosquito predators.
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The ponds that were usually full but dried out after a drought had lots of mosquito larvae and very few predators and competitors.
SO YOU think you know mosquitoes? Consider the venerable law that rainy weather is cause of increased mosquito populations. Jon Chase, assistant professor of biology in Arts & Sciences at Washington University, has found that the previous year's drought is the cause of high mosquito populations in wetlands the following year because drought drastically reduces mosquito predators from fish to water beetles and other competitor species.
This conclusion stands the ecological world on its head and, eventually could have implications in prediction and control of diseases like West Nile virus, St. Louis encephalitis and malaria. Chase and his group are analysing data from locales across North America to see if their results found in wetlands translate to other landscapes, such as urban and suburban areas. The study appears in the journal Ecology Letters.
"We're dealing with the pure ecology of mosquito larvae and wetlands and trying to see if this translates to the landscape level," said Chase. "This is important because mosquitoes live in all sorts of habitats besides ponds and lakes. With the data we have we're trying to run correlations between mosquito density over 20-year and 30-year spans to see how well it predicts mosquito abundance. We have found that the current year explains almost nothing but the past year explains a lot. We're also looking into disease implications. This is much harder to prove because there is so much epidemiology to take into consideration, for instance the role of intermediate hosts and other complexities. Very few people are looking at these mosquito-borne diseases in a modern ecological context. Most of the work comes from the epidemiological, genomic and/or molecular biology perspectives."
Chase and his team came to their conclusion by accident. A pond they had been studying in Pennsylvania dried up during a drought and the next year, after the pond was replenished, mosquito larvae came back like gangbusters. They then surveyed ponds, some permanent but others semi-permanent and temporary. They found few mosquito larvae in permanent ponds but lots of mosquito predators; in ponds that dried up annually mosquito larvae were scarce because of lots of competitor species, zooplankton, snails and tadpoles. Those ponds that were usually full but dried out after a drought, however, had lots of mosquito larvae and very few predators and competitors because, Chase reasons, those species aren't adapted to dry spells in these ponds.
The team was able to recreate this study by filling tanks with soil and water and stocking them with mosquito larvae and other species found in ponds. They gave these artificial ponds three years to allow the ecosystem to form and stabilise, and then slowly drained some tanks to simulate a drought. Next year, the mosquito larvae numbers were very large compared with tanks that were drained each year or remained full. Analysing data provided by the city of Winnipeg, they found that drought the previous year is a better predictor of high mosquito populations than the current year's rainfall.
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