Invertebrate diversity in old-growth versus regenerating forest canopies

Debate over old-growth forests has focused on the fate of threat-ened plant and vertebrate species. Invertebrates (which comprise atleast half of all forest species) have been largely ignored. Manyinvertebrates also depend on old-growth resources. Reduced diver-sity in younger managed forests promotes pests and deprives us ofvaluable biological resources.
Old-growth (450-yr-old) and regenerating (10-yr-old) canopieswere sampled at six replicate sites at the H. J. Andrews ExperimentalForest in western Oregon during 1986. Foliage-bearing branches inold-growth Douglas-fir and western hemlock harbored five timesas many forest canopy arthropod species (75) and twice as manyfunctional groups (6) as did young planted Douglas-fir.
Old-growth foliage supported a variety of herbivores, which col-lectively caused negligible (30%) foliage injury and loss byaphids. Black-stain root disease and its insect vectors have causedserious mortality in young managed Douglas-fir forests. Relatedstudies show similar contrasts in pest abundances between youngmonocultures and diverse older forests in eastern deciduous forestsand southern pine forests. Predator diversity and abundance were particularly reduced inplanted stands, potentially limiting ability to control pest popula-tions. Predator species present in old growth, but absent in adjacentplanted forests, may require habitats, host resources, or moderate
climatic conditions provided by old growth.
Loss of invertebrate species through old-growth harvest wouldeliminate medical and pest management resources. Invertebratesproduce an enormous variety of chemicals to detoxify food, attractmates, and discourage enemies. Relatively few species have beenexamined for useful chemicals, but these have provided drugs fortreating leukemia, arthritis, warts, and infection, as well as naturalinsecticides, fungicides, and repellents. Biological control of pestsdepends on natural or augmented abundances of predators. Reducedpredator diversity as a result of forest conversion would limit bio-
logical control options.
Large-scale conversion of tropical forests has fueled concern forloss of diversity and promotion of pests in crop systems. Conversionand fragmentation of native forests in the Pacific Northwest alsowill decimate rich and largely unknown species assemblages and
promote forest pests.