CHOPPING down trees to build houses and plant crops isn鈥檛 just damaging the environment 鈥� it鈥檚 threatening human health, too. Small patches of forest such as those left by housing developers turn out to be breeding grounds for disease.
Researchers have long speculated that interfering with the environment makes certain diseases more prevalent, but proof has been hard to come by till now.
Breaking up forests into small patches is known to alter populations of animals living there. In the eastern US, animals such as weasels, foxes and coyotes avoid smaller patches of woodland as there is not enough food food there to sustain them. But mice thrive, since most of their predators and competitors leave. 鈥淵ou get hundreds of mice per hectare,鈥� says Felicia Keesing, a biologist at Bard College in Annandale, New York.
Advertisement
Keesing thought this would affect Lyme disease, an infection that is on the rise in the US. The disease, which causes flu-like symptoms and sometimes leads to arthritis and neurological problems, is caused by bacteria (Borrelia burgdorferi) that live predominantly in white-footed mice. Ticks feeding on the mice pick up the disease and pass it on to people.
Keesing鈥檚 team studied 14 patches of forest in a hot spot for Lyme disease in New York state. They found that the number of disease-carrying ticks increased dramatically as the patches shrank. Plots of less than 2 hectares had on average 3 times as many ticks and 7 times as many infected ticks per square metre as larger patches. In one 1-hectare plot, a staggering 80 per cent of ticks were infected (Conservation Biology, vol 17, p 1).
The glut of infected ticks is almost certainly due to the altered animal population, says Keesing. If more species were around, some ticks would feed off animals that do not carry the Lyme bacteria.
The effect of deforestation on Lyme disease also seems to hold for other diseases. An as-yet unpublished study by Jonathan Patz, director of the Health Effects of Global Environmental Change programme at Johns Hopkins University in Baltimore, Maryland, shows that deforestation in the Amazon has benefited one species of mosquito (Anopheles darlingi) more than others. A. darlingi is the main carrier of the malaria parasite in the region.
Keesing hopes her results will help persuade local officials to bring in rules that will prevent developers from leaving patches of wood smaller than 2 hectares. 鈥淭he results are so clear, planners would be foolish to ignore them,鈥� she says.