Haack and Byler (1993) have summarized the roles of native pathogens and insects in forest ecosystems. One role is a part of basic ecosystem process, that is, recycling carbon and other nutrients through decay. Others involve beneficial relationships with plants through mycorrhyzal symbiosis and pollination. Still others involve more complex interactions with wildlife and habitat. Pathogens and insects can serve as food sources for vertebrates, invertebrates, and microorganisms and create specific habitat components, such as dead standing and down trees, that provide decayed wood for nesting cavities or forage. They also regulate populations of woody and herbaceous vegetation directly or indirectly through forest succession.
The action of pathogens and insects in successional change is through growth loss and mortality of host trees. These agents can directly regulate host population size and genetic composition and restrict host distribution at various spatial scales. Community diversity may be diminished or enhanced, relative competitive advantage altered, and the availability of food and cover for animals changed by the creation of canopy gaps ranging in size from single trees to large groups or even landscapes (Gilbert and Hubbell 1996).
Changes in community structure and composition during epidemics or outbreaks are a function of the distribution of susceptible individuals on the landscape. For this discussion, community structure refers to the physical arrangement of canopy gaps, down trees, snags, live trees, and other associated vegetation. Composition refers to the distribution of plant species (emphasizing trees) and genetic diversity within species. Both elements can be considered at various spatial scales. There is a common perception that native pathogens and insects prey on the weaker members of the population, culling the less fit and thereby increasing the overall vigor of the population. if the population is normally distributed with respect to vigor, and the low-vigor individuals are randomly distributed on the landscape, then the impacts are minimal and may be barely noticeable. However, these conditions do not always prevail. Large segments of the landscape may be occupied by a single susceptible host species or species group, and/or outbreak conditions may overwhelm even the most vigorous individuals. interactions are complex and changes in composition and structure highly variable, based on existing community structures, host susceptibility, and the virulence or aggressiveness of the agent. Ultimately, wildlife habitat is altered, for better or worse, as a result of changes in forest composition and structure triggered by mortality or growth loss in host trees.
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