Dave Munger is editor of ResearchBlogging.org. He also blogs at Cognitive Daily. Each week, he writes about emerging trends
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In 1939, scientists discovered that DDT, a chemical that had been synthesized decades earlier, was a remarkably effective insecticide that appeared to be completely harmless to humans and most other organisms. By the early 1950s its use was nearly ubiquitous, and it seemed that mosquito-borne malaria might soon become a historical footnote.
We now know that DDT does damage organisms other than insects, especially predatory birds. Indiscriminate spraying of DDT is now tightly controlled, in deference to that fact. So couldn’t it still be used in a limited way to control mosquitoes in populated areas and reduce disease? It could, and it is. But sadly, DDT isn’t nearly as effective against mosquitoes as it once was, due to an explosion of DDT use not for disease-eradication, but on a much larger scale for pest control in agriculture.
Photo courtesy of Geoffrey Gallaway
A report by Georganne Chapin and Robert Wasserstrom published in Nature in 1981 (excerpted here) shows how despite its ban in many industrialized nations, increasing DDT use in farms in developing countries paralleled a huge rise in malaria infections during the 1970s. Unlike the earlier, relatively modest applications in densely populated areas, agricultural DDT use increased exponentially as its target pests developed immunity. Mosquitoes, which weren’t the targets of the agricultural DDT, nonetheless developed resistance that they wouldn’t have if DDT had been used more sparingly. In less than a decade, DDT-resistant mosquitoes were widespread, and malaria again became a killer of millions.
The response of mosquitoes to DDT is just one example of how quickly organisms can adapt to changes in their environment. But not all organisms respond the same way to rapid change. While mosquitoes have shown remarkable resilience in the face of massive quantities of DDT applied all over the world, birds farther up the food chain weren’t so fortunate. Some species, most notably the Peregrine Falcon, nearly went extinct. But if mosquitoes could adapt to DDT, why couldn’t the Peregrines?
“GrrlScientist” is an ornithologist and evolutionary biologist who blogs pseudonymously at Living the Scientific Life. In a recent post, she notes that birds in many isolated ecosystems are nearing extinction. While Hawaii might seem to be a tropical paradise, invasive species and diseases have caused several extinctions and almost destroyed many other endemic bird species, which don’t seem to be able to adapt quickly enough to meet these new threats—just like Peregrines couldn’t adapt to DDT.
Such devastation would seem to be the rule, but for a study led by Sarah Huber and published last week in PLoS ONE. Huber’s team looked at how two species of Darwin’s finches in the Galapagos responded to two parasites that had been introduced to the islands in the 1960s. The birds have already developed antibodies to the pox virus, a disease which can cause limbs to fall off. Birds on other islands where the virus was not prevalent had fewer antibodies. Similarly, the nest fly, whose larvae feed on the blood and tissues of nestlings, has also provoked immune responses in the island birds.
GrrlScientist says the research is impressive, but she wonders why Darwin’s finches are better able to adapt in comparison to other tropical birds in similarly isolated settings. And we could ask the same question about birds like the Peregrine Falcon, which was nearly devastated by DDT. Even if we can’t yet answer that question, Darwin’s finches show that the size and reproductive cycle of an organism alone cannot fully explain why some organisms adapt and others don’t—which means it may also be possible to find an insecticide that mosquitoes don’t readily adapt to.
“Cheshire,” an undergraduate studying entomology at Iowa State University, points to a fascinating study led by Gregor Devine and published in Proceedings of the National Academy of Sciences last year. Devine’s team dosed adult female mosquitoes with the pesticide Pyriproxyfen. Pyriproxyfen doesn’t kill the bugs outright, but instead interferes with the development of juveniles. When the adults breed in pools of water, the poison stuck to their bodies dissolves, and then is transmitted to the developing larvae.
Since Pyriproxyfen doesn’t affect the adults, they don’t learn to avoid it (as many mosquitoes do with DDT). Up to 80 percent of the juvenile population in the study didn’t mature. So just as some birds adapt to new parasites while others don’t, it’s possible that mosquitoes can adapt more or less readily to some insecticides than others. Of course, it remains to be seen whether the mosquitoes will eventually adapt to Pyriproxyfen, but these early results are encouraging.
“Johnny,” a naturalist who blogs at Ecographia, shares the results of a letter from a group led by Scott Loarie and published in Nature last November. These researchers estimate the speed that ecological niches will move in response to climate change, and the anticipated rates range up to more than a kilometer per year. Clearly, some organisms will be able to adapt to these changes, while others will not. Particularly fragile are alpine environments. As temperatures rise, species will be pushed to mountaintops where it is cooler, but even there many species will perish in a climate turning inexorably too warm for them.
While life is amazingly adaptable, and some organisms can respond quickly to some changes, the speed of evolution varies based on the type of threat presented to a species. In some cases, an accommodation can be readily made, while in others, adaptation is slow or impossible for a multitude of varying reasons. For more on how organisms evolve, visit ResearchBlogging.org.
Originally published January 13, 2010
