Moose, trees, and ground-living invertebrates: indirect interactions in Swedish pine forests

Abstract

The role of moose in structuring the boreal forest ecosystem has been studied extensively in recent pears. This research has focused mainly on the effects of moose on vegetation and soil dynamics. However, the extent to which these effects influence animal communities has received little attention. We studied how invertebrate assemblages on the forest floor of two Swedish pine forests were affected by the presence of moose. Our approach was to trap invertebrates using pitfall traps in unbrowsed exclosures and browsed reference plots, estimate the percentage cover of herb-layer plants, measure the height and density of tree species, and measure the accumulation of leaf litter. The effect of moose on tree strata differed between the two areas. In Sunnas, where the stand was a mixture of deciduous trees and pines, the main effect of moose was to eliminate most of the deciduous trees from the canopy. Here the amount of deciduous leaf litter was ten times higher in unbrowsed exclosures compared with browsed plots. The stands on the dry sites in the other area, Furudal, were almost pure pine. Here, moose browsing reduced the density of taller pines, but increased the number of pine saplings. At Sunnas, the herb-layer plant cover did not differ between browsed and unbrowsed plots. However, there was a difference between exclosures and browsed plots in terms of the ground-dwelling invertebrate assemblage. Number of individuals of most invertebrate groups that differed between exclosures and browsed plots were higher in unbrowsed plots and the total number of invertebrates was also higher in unbrowsed plots, but diversity of invertebrate fauna was higher in browsed plots. At Furudal both the vegetation and invertebrate assemblages differed between browsed and unbrowsed plots. Browsed plots had a higher cover of ground lichens; while unbrowsed plots had more mosses. Among the invertebrate taxa that differed between treatments, the number favored by browsing and the number disfavored by browsing were roughly equal, and diversity was not affected by moose. We suggest that by affecting the density and composition of tree species, moose indirectly influenced the structure of invertebrate communities. At Sunnas, the reduction in the amount of deciduous leaf litter was probably the key effect. Vegetation was not affected much, but the differences in litter layer composition had affected the microclimate on the forest floor while also providing an important additional trophic resource and habitat for ground-dwelling invertebrates. Al Furudal the main effect of moose was to reduce the density of the pine canopy: thus increasing the amount of light penetrating to the forest floor. As a consequence, the floral composition and microclimate was altered which, in turn, affected the invertebrate fauna.