View or print this publication - Northern Research Station - USDA ...

Functional Heterogeneity of Forest Landscapes: How Bark Beetles Perceive and Respond to Their Environment
Studies of epidemiology begin with a consideration of issues associated with landscape structure. The term landscape
structure refers to the spatial relationships between distinctive ecosystems (ecotopes, see Coulson et al. 1993), i.e., the
distribution of"energy, materials, and species in relation to the sizes, shapes, numbers, kinds, and configurations of components
(Turner 1989). Characteristics of landscape structure (porosity of the forest matrix, boundary configurations, patch size
and shape, etc.) influence epidemiology of bark beetles. Structural characteristics of landscapes are often summarized and
represented as heterogeneity. Although defined in several ways, the term heterogeneity is generally taken to mean composition
of'parts of"different kinds. In considering the interaction of D. frontalis with the forest landscape it is necessary to
distinguish between measured and functional heterogeneity. Measured heterogeneity deals with physical features or components
of a landscape. Functional heterogeneity deals with how an organism perceives and responds to its environment
(Kolasa and Rollo 1991). There is obviously a direct link between patches, boundaries, and heterogeneity as boundaries
define patches, and patchiness is what produces heterogeneity (Wiens 1992, Hansen and di Castri 1992).
Tied to concepts associated with landscape heterogeneity is the subject of habitat structure. In general, habitat
structure refers to the physical arrangement of objects in space (Bell et al. 1991). In studies of epidemiology, an essential
task is to define the distribution, abundance, and location of habitat units. In effect, habitat structure is an element of measured
heterogeneity, i.e., habitat can be viewed as a mappable element of a landscape mosaic. Southwood (1977) suggested
that habitat serves as the template for ecological strategies.
Examination of how bark beetles perceive and respond to their environment at the landscape scale, i.e., evaluating
functional heterogeneity, was difficult before the availability of (i) GIS technologies, (it) spatially referenced databases, and
(iii) spatial statistics. Also, until the text by Forman and Godron (1986), there was no standardized nomenclature for describing
landscapes from an ecological perspective (McIntosh 1991). With these problems partially solved, it is now possible to
consider the substantial knowledge base on natural history of bark beetles (and other organisms) in the context of the forest
landscapes where they occur (Dunning et al. 1992).
In reference to D. frontalis, we have indicated that a great deal is known about host selection relative to the initiation
and subsequent growth of infestations. In the case of initiation of infestations, lightning-struck hosts play a prominent role.
For infestation growth, variables associated with forest stand hazard become significant. With knowledge of how these
variables are spatially and temporally distributed, we can consider scenarios, based on the natural history of the insect, that
explain causes for changes in the distribution and abundance of D. frontalis at the landscape scale.
Ways and Means of Measuring Functional Heterogeneity
Ways and means for measuring landscape heterogeneity have been reviewed by Kolasa and Pickett (I991), Turner
(1987, 1989), S. Turner et al. (1991), and M. Turner et al. (1991). Although there are several methodologies available, no
single index is suitable for characterization of functional heterogeneity for a given organism. Each organism perceives and
responds to its environment in unique ways. The elements of landscape structure that are important in defining herbivory by
bark beetles, for example, may be substantially different from those needed to characterize the process for deer populations.
Figure 2 illustrates the relation of functional and measured heterogeneity. In Figure 2 functional heterogeneity is represented
as a gradient ranging from homogeneity on one end to heterogeneity on the other. Interpretation of the significance of index
values in the mid-range of the gradient is difficult. We emphasize, also, that intermediate heterogeneity conditions are most
difficult to interpret from the behavioral perspective of the organism, i.e., it is difficult to predict how an organism will react
to conditions of intermediate heterogeneity. For this reason measurement of functional heterogeneity for a specific organism
will likely require use of several indices, each of which is sensitive to different aspects of landscape structure, e.g., connectivity,
porosity, fragmentation, boundary configuration, etc. To address this issue, we developed (or adapted) three different
procedures to characterize functional heterogeneity of forest landscapes relative to herbivory by D. frontalis. These indices
are described briefly below.
Indices of Heterogeneity
Our initial goal in defining functional heterogeneity for D. frontalis was to combine knowledge of natural history of
the insect with information on landscape structure. We developed (or modified) three indices to characterize functional
heterogeneity. All use a weighted pattern-recognition computational approach, and each is sensitive to a different aspect of
275