Abstract
Old-growth forests have become rare in North America but provide habitat for unique assemblages of species that often are rare in younger forests. Insects and related arthropods reach their highest diversity in old-growth forests because of their stable moderate temperature and relative humidity and the rich variety of resources represented by high plant species richness and structural complexity. Old-growth arthropod assemblages typically are distinct from those in younger, managed forests. Major subcommunities include the arboreal community that is composed of a rich assemblage of herbivores, fungivores, and their associated predators and parasitoids that function to regulate primary production and nutrient fluxes, the stem zone community that includes bark- and wood-boring species and their associated predators and parasitoids that initiate the decomposition of coarse woody debris, and the forest floor community composed of a variety of detritivores, fungivores, burrowers, and their associated predators and parasitoids that are instrumental in litter decomposition. Insect outbreaks are relatively rare in old-growth forests, where the diversity of resources and predators limit population growth. In turn, insects contribute to plant diversity and limit primary production of host plant species, thereby promoting development of old-growth forest characteristics. Arthropods also provide important functions in decomposition and nutrient cycling that may be lost in younger, managed forests with limited provision of coarse woody debris and accumulated litter. Protection of remnant old-growth forests within the forest matrix may be particularly valuable for maintaining the diversity of plant and arthropod predators that can minimize outbreaks, thereby contributing to resilience to changing environmental conditions.
Highlights
Old-growth forests have been a center of controversy because of their conflicting ecological, political, and social values [1,2,3]
Forests 2017, 8, 97 the western U.S and Canada form relatively pure open-canopied stands maintained historically by frequent fire. These characteristics do not all appear simultaneously, and different forest types acquire these characteristics at different forest ages [1,3]
Arthropod communities in old-growth forest canopies can be distinguished from those in younger forests by their greater diversity and by their relatively higher proportions of folivores, arboreal detritivores and fungivores (Collembola and oribatid mites), and predators (e.g., predaceous beetles, true bugs (Hemiptera), and spiders), compared to sap-suckers (e.g., scale insects, aphids, aleyrodids, leafhoppers, and treehoppers (Hemiptera)) and ants, which are more abundant in younger forests [7,19]
Summary
Old-growth forests have been a center of controversy because of their conflicting ecological, political, and social values (e.g., timber harvest vs. biodiversity conservation) [1,2,3]. Old-growth forests harbor many associated species that are favored by relatively stable environmental conditions and by the rich diversity of habitats and tree species that do not occur in younger forests. Many of these species may be unable to survive and reproduce in younger or managed forests [3]. The diversity of plant species, predaceous arthropods, and insectivorous vertebrates in old-growth forests help maintain lower abundances of herbivores than often occur in younger, managed, forests [3,7]. This paper reviews data on arthropod diversity and ecological functions in old-growth forests of North America and differences between old-growth and younger, managed forests
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