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The health of ecosystems is intimately connected with human health. A line of research from the Cary Institute of Ecosystem Studies in Millbrook, New York, has shown that wildlife-rich regions can lower the risk of Lyme disease to humans. This is because environments with higher numbers of animal species harbor fewer ticks infected with Borrelia burgdorferi, the bacteria that cause Lyme disease. Therefore, humans in these environments are less likely to be bitten by infected ticks.

In the northeastern United States, some of the most common Borrelia hosts, such as opossums and squirrels, infect only a small percentage of the ticks that feed off them. Opossums and squirrels also kill nearly all the ticks that feed on them by consuming or destroying them during grooming. Thus, they do not spread Borrelia easily. Computer models by the Cary team indicate that the loss of these less efficient hosts from tick-prone environments mean the ticks end up feeding on more efficient Borrelia hosts such as mice. The Lyme risk to humans is compounded by the fact that mice are highly adaptable generalist species: they are able to flourish in environments with a human presence, such as residential, agricultural, or industrial areas.