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systems facilitate mapping of forests according to their vulnerability to bark beetle infestation. These hazard maps define a
landscape mosaic comprised of forest stands graded by defense capacity (Fig. 5). Typically, the high hazard stands are also
the preferred habitat of the insect, i.e., populations of bark beetles are clustered in high hazard stands. As long as population
density remains sufficient to overcome host defenses, infestations grow in size through colonization of neighboring hosts.
When population density falls below the level necessary for successful host colonization, the insects must disperse to find
suitable refuges. Two strategies are likely. The insects can disperse to supplement populations in existing infestations at
other locations or they can colonize lightning-struck hosts, which have greatly reduced defense capacity. These two elements,
existing infestations and lightning-struck hosts, add to the picture of host defense for the forest landscape (Fig. 7-1eft
panel). However, forest landscapes in the southeastern US are often highly fragmented, and the vulnerable stands, existing
infestations, and lightning-struck hosts are dispersed. Behavioral strategies associated with dispersal and host selection link
populations of the insect to suitable habitat centers (Fig. 7-1eft panel) (Southwood 11977). Finally, the indices of functional
heterogeneity serve to integrate the information on the distribution of host defenses with knowledge on how the insect
perceives and responds to its environment (Fig. 7-right panel). Further insight into mechanisms of epidemiology of D.
frontalis can be gained through analysis of changes in functional heterogeneity of sequential landscape scenes.
SUMMARY
This paper focused on functional heterogeneity of forest landscapes and epidemiology of D. frontalis. Specific conclusions
from the study include the following:
1. The general premise of this study was that epidemiology of D. frontalis involves a network of high hazard stands,
lightning-struck hosts, and existing population centers. This network is connected at the landscape scale through dispersal
behavior of the insect.
2. The concept of functional heterogeneity (how an organism perceives and responds to its environment) provides a
useful way to organize knowledge about landscape structure and insect behavior. Although we have focused on functional
heterogeneity of forest landscapes and epidemiology of D. frontalis, the approach can be used for other insects and taxa.
3. The :indices of functional heterogeneity were found to be useful in integrating the landscape variables known to
influence epidem iology of D. frontalis.
4. Measurements of functional heterogeneity of forest landscapes incorporate two types of knowledge about host defenses,
The first deals with knowledge embedded in the various types of hazard rating systems used to grade forest stands by
vulnerability to D. J?oHtalis. The second deals with knowledge on the interaction of lighting-struck hosts and D. frontalis. The
functional heterogeneity map provides a visualization of the distribution of host defenses within a meso-scale forest landscape.
5. The functional heterogeneity indices are a tool that can be used to investigate several fundamental issues in epidemiology.
For example we hypothesize that (i) functional heterogeneity of forest landscapes influences the rate and extent of
herbivory by bark beetles and that (ii) initiation, growth, and decline phases of forest insect outbreaks can be predicted by
definition of the change points in functional heterogeneity of landscapes.
LITERATURE CITED
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56. New Orleans, LA: U.S. Department of Agriculture, Forest Service.
BELl.,, S,S._ McCOY, ED., and RUSHINSKY, H.R., eds. 1991. Habitat Structure. Chapman and Hall, NY.
BROWN, V.K. 1991. The effects of changes in habit structure during seccessin in terresterial communities. In Bell, S.S.,
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