Insects and pathogens are major regulators of forest in many ecosystems, each year there are millions of hectors affected by insects and pathogens throughout North America.
Although the visual effect of insects and pathogens is less dramatic than that of fire or mass land movements their ability to kill either individual trees or small groups of trees in a forest has a major influence in controlling forest succession.
However recent outbreaks of bark beetles and the introduction of invasive pathogens have greatly influenced forest succession on large scales.
There ability to have dramatic influences on forest succession is even more pronounced due to the fact that many insects and pathogens attack only one species.
Insects and diseases like all other disturbance agents tend to work together this is particularly true with respect to fire, although little information is available about this relationship.
James Solomon, USDA Forest Service, www.forestryimages.org
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