The densities of four species of gall-forming sawflies were found to vary significantly among willow host plant clones. Two of the speices varied among host plants at four sites in each of three years. The other two species varied in density among host plants at most of the sites in two of the three years. Total sawfly density also varied significantly among clones. Individual species densities on willow clones were significantly positively correlated between years when all sites were combined and frequently when sites were considered separately. Most pairwise species combinations were independent in density between years, but some negative correlations existed between the stem galler and the leaf galler. Gall-former densities also were largely independent among clones within years with all sites combined and with sites considered separately. The significant correlations were nearly all positive. At all four sites the combination of significant variation in sawfly densities among willow clones in the field and independence of species densities among clones resulted in significantly different communities (relative abundance of species) among willow clones in three years. Although sawfly abundances differed substantially among the four sites, this remained true. It is argued that the pattern of community structure among clones is the result of variation in host plant quality of clones. We propose an hypothesis to account for patterns of herbivore species associations based on intrapopulation host plant variation.

One factor limiting the population size of secondary cavity-nesting birds in ponderosa pine is the number of suitable nesting cavities. Snags in the pine forest provide a large number of species with nesting and roosting sites. To maintain secondary cavity nesters at their natural population level, a density of 5.2 snags per ha is recommended for mature ponderosa pine. # /

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Four species of gall-forming sawflies were each frequently found to have clumped distributions among shoots within their willow host plant across four sites and three years. When all species were considered together by clone, year, and site, species showed independence of distribution among shoots two thirds of the time and showed positive covariance one third of the time. When pairs of species were considered separately, but clones were combined within sites and years, 60% of the chi-square tests of association were significant. All but one of the significant tests showed positive associations between pairs of species. The stem galler was positively associated with the leaf folder at all sites in all years, and the petiole galler was positively associated with the stem galler and leaf folder for most year by site combinations. When species paris co-occurred on shoots they were usually found at the same or higher density as when found alone on shoots. Only 2 of 100 tests showed a depressed density of a species when co-occurring on shoots with heterospecifics.All sawfly species were found on shoots that were significantly larger (mean node number) than on shoots without sawflies, and species responded to shoot size variation similarly. Sizes of shoots occupied by heterospecific species combinations were usually significantly larger than shoots with only conspecifics, for all species. These data supported the hypothesis that similar species' responses to within-plant variation would lead to positive rather than negative or random species associations. The data do not support the hypothesis that interspecific competition was important in determining shoot choice or species density.

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