Community disassembly and the resulting loss of natural population control mechanisms for such species generally is associated with the conversion of natural landscapes to urban and agriculture landscapes. Broad spectrum pesticide use and habitat simplification, along with habitat fragmentation, are important contributing factors. The reduction in species diversity can contribute to the phenomenon of 'ecological release' in remaining species, whose predators, competitor and parasites are reduced in numbers or eliminated. Some of these may be already serving as zoonotic reservoirs or vectors. If so, ecological release may result in their proliferation. This helps explain why many emerging zoonotic diseases occur in areas where settlement and agricultural expansion recently have encroached into tropical forests. A similar phenomenon, associated with the regrowth of forests in developed regions, can lead to zoonotic disease emergence. The pattern of reforestation in Eastern North America during the past half-century resulted in increased habitat favored by forest edge adapted species such as whitetail deer and white-footed mice. White-tailed deer, the principle host of the adult tick Ixodes scapulari, reinvaded the area and with few predators and competitors the population exploded. This pattern of ecological change arguably has been a major factor in the emergence of Lyme disease in this region.