Abstract
In eastern North American forests, eastern hemlock (Tsuga canadensis) is a foundation species. As hemlock is lost from forests due to the invasive hemlock woolly adelgid (Adelges tsugae) and pre-emptive salvage logging, the structure of assemblages of species associated with hemlock is expected to change. We manipulated hemlock canopy structure at hectare scales to investigate the effects of hemlock death on assemblages of ants, beetles, and spiders in a New England forest. Relative to reference hemlock stands, both in situ death of hemlock and logging and removal of hemlock altered composition and diversity of beetles and spiders, and logging increased the species richness and evenness of ant assemblages. Species composition of ant assemblages in disturbed habitats was non-random relative to the regional species pool, but we found no evidence that interspecific competition shaped the structure of ant, beetle, or spider assemblages, in either manipulated or intact forest stands. Environmental filtering by hemlock appears to maintain low levels of species richness and evenness in forest stands, suggesting that the loss of hemlock due to the hemlock woolly adelgid or human activities will not likely lead to extirpations of ant, beetle, or spider species at local scales.
Highlights
Many ecosystems rely on foundation species that have architectural and functional features that shape communities and modulate ecosystem functions (Dayton 1972, Ellison et al 2005a)
Temporal changes in ant assemblages In seven years of collecting we accumulated a total of 26 ant species in 1295 species occurrences (Fig. 1; raw data available in Ellison 2009)
This is greater than 50% of all known species collected from Harvard Forest, which covers a much broader range of habitats than Harvard Forest Hemlock Removal Experiment (HF-HeRE)
Summary
Many ecosystems rely on foundation species that have architectural and functional features that shape communities and modulate ecosystem functions (Dayton 1972, Ellison et al 2005a). Forest landowners are logging hemlock and realizing economic gain before the adelgid arrives (Orwig et al 2002, Foster and Orwig 2006). These different types of structural changes to hemlock forests—slow disintegration of boles, coarse woody debris, twigs, and leaves after the trees have died in place versus essentially instantaneous removal of boles and large limbs leaving only small branches, twigs, and leaves atop soil compacted by heavy machinery— should have profound consequences for species dependent on hemlock’s different life stages and the environment that it creates
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