Impacts of Diseases and Other Disturbances on Non-Timber Forest Resources: A Case Study involving Small Mammals

P art of the diversity of a forest is the variety of agents that can kill trees. These agents differ in the nature, magnitude, and patterns of their impacts on forest resources. Diseases, insect pests, and other small-scale disturbances are commonly assessed on the basis of their impacts on timber production. Tree mortality usually means reduced volume of living stems. Consequently, forest pathologists and entomologists have traditionallymaintained a negative view of these disturbances. Dead trees have traditionally invoked a different perspective for wildlife biologists. To them, dying trees either embody important habitat structures or lead to conditions that support animal existence (Maser et al., 1979; Thomas et al., 1979; Raphael and White, 1984; Ramotnik, 1997). For example, fallen trees create openings in forest canopies that stimulate development of plant understory: tree seedlings, shrubs, forbs, and grasses that in turn provide food and cover for small mammals (Carey et al., 1999). Fallen trees also provide small mammals with refuge from predators, nesting sites, and nursery conditions for production of other food sources, such as arthropods and fungi (Goodwin and Hungerford, 1979; Hayes and Cross, 1987; Carey et al., 1999). Several forest pathology and entomology studies have speculated on the ecological significance of diseases and insects pests, but few have actually addressed their impacts on wildlife habitat (Hart 1993; Bennetts et al. 1996). In addition to diseases and insects, the most frequently cited disturbances in forests are fire, logging, and livestock grazing (Shaw et al., 1993; Swetnam and Baisan, 1994; Tkacz et al., 1994; Geils et al., 1995; Kaufmann et al., 1998). The most commonly recognized expression of a disturbance is probably tree mortality. Not all disturbances kill trees in the same way. Because they often act selectively, the canopy gaps they cause can have distinctive characteristics depending on which trees are killed, what woody structures remain, and how succession proceeds. We hypothesize that particular tree killing pathogens, insects and other disturbances can have specific effects on small mammal habitat, and that the relative importance of different tree-killing agents depends on their nature, magnitude, and spatial patterns. Here we evaluate this hypothesis using a test case involving five species of small mammals common to the forests of the southwestern United States. These small mammals are common prey of a threatened raptor, the Mexican spotted owl (Strix occidentalis lucida) (Ward and Block, 1995). Information detailing influence of biotic factors and linkages with the abundance and distribution of these species will be useful for defining restoration prescriptions that may help conserve these wildlife species.