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However, in pre-colonization forests of the southern US, herbivory by D. frontalis (along with fire) was likely
involved in structuring the landscape (Schowalter et al. 198 l, Rykiel et al. 1988). As D. fror_talis infestations are r_ormally
suppressed in an effort to reduce economic loss by the insect, it has not been possible to evaluate this hypothesis un,it
recently. Outbreaks of D. frontalis in Rare II Wilderness Areas on National Forests in Texas have been allowed to follow a
natural course, with only modest intervention to protect endangered species. The outbreaks, which occurred over a period of
1983 to t994, have radically changed the pattern of the forest landscape mosaic of these wilderness areas (Fig. t). Under this
epizootic condition, individual infestations coalesced and the pattern and extent of tree mortality substantially modified the
structure of the forest landscape.
Figure I .---The impact of herbivory by D. frontalis in pine fores_ landscapes in east Texas. Under this epizootic condition,
individual inl_stations coalesced and the pattern and extent of tree mortality substantially modified the structure of
the tbrest landscape.
The alternate way of viewing insect outbreaks is to consider how landscape pattern affects the process of herbivory.
Again, because D. frontalis infestations are normally suppressed, it was not possible in the past to examine this relation.
Figure I clearly illustrates how the process of herbivory is influenced by the pattern of patches that were created by previous
in|izstations of D. j}ontalis. There are few examples of empirical studies of the eflEcts of landscape pattern on the process of
herbivory (Turner 1989).
By definition, epidemiological studies are dynamic and therefore the process of herbivory and the pattern of the tk_rest
landscape mosaic change through mutual interaction, i.e., the pattern influences the process and the process influences the
pattern. Although not investigated in a rigorous manner, the scenario observed on the National Forest wilderness areas of
Texas sugges{s that bark beetle herbivory drives the process of ecosystem succession (from the release to reorganization
phase in Holling's scheme 1992b) (see also Brown 1991).
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