Dispersal characteristics of old-growth soil arthropods: the potential for loss of diversity and biological function

Management practices of Northwest forests drastically affectabundance and species composition of the small arthropods (insects,spiders, and their relatives) that regulate soil microstructure andelemental recycling. Burrowing arthropods provide channels inwhich roots grow and nonburrowing arthropod species can enterthe soil matrix. Detritus-feeding species facilitate nutrient releaseby increasing the surface area available for microbial attack. In nor-mal densities, microbe-feeding species increase the rate of microbialgrowth and litter decomposition. Predaceous taxa control the com-petitive balance between fungi and bacteria.
Most species of soil fauna show strong preferences for conditionsprevailing at particular stages of forest succession. Studies underwayat the Andrews Experimental Forest (western Cascade Mountains,Oregon) demonstrate that the typical practice of clearcutting andburning reduces total arthropods in the soil by about 90%. Theimmediate effect of this process on species richness is a function ofboth fire intensity and patchiness of the habitat, as well as the fire.Random sampling often reveals a decrease of nearly all the species,but sampling within potential refuges (inside punky [decayed, dry)tree trunks, under heavily decayed logs) reveals that many specieshave the potential to survive the fire in favorable locations. Thesurviving individuals soon are joined by the highly mobile speciesthat characterize habitats disturbed by humans and forest fires. Su-perficial examination of the abundant species reveals few taxa incommon with the old growth until the new canopy is well-formed(20-40 years later).
Many invertebrate species that prefer old growth are flightless—not only the most abundant taxa (e.g., springtails, oribatid mites,predaceous mites), but also groups normally characterized by effi-cient flight. Wingless species of flies (i.e., phorids, cecidomyiids,sciarids, and tipulids), wasps (i.e., diapriids and ceraphronids), bee-tles (i.e., carabids, cicindelids, curculionids, and pselaphids) andbugs (i.e., tingids) that characterize the old-growth fauna havewinged relations that abound in the early successional stages.
When natural disturbances like winter blowdown and forest fireswere the major disturbances to the Vancouveran forest, disturbedpatches frequently were small, and they probably encompassed manyrefugia; the ratio of edge to disturbance area was high and recolon-
ization relatively easy. In the infrequent events when huge forestfires occurred, observation tells us that fire intensity was very patchyand that numerous partially burned enclosures remained. Underpresent conditions of widespread timber harvest with slash burningand very restricted islands of old growth, the distances required forsuccessful immigration may well be limiting. The immigration ratesof soil taxa, the practice of burning litter after clearcutting, the typesand long-term success of refugia during fires, and the practice ofleaving "green islands" (with undisturbed litter) in clearcuts allrequire critical investigation. If old growth is to remain as restrictedislands scattered over the landscape, island biogeography theorytells us that extinctions of taxa are bound to occur. If many of thesespecies cannot re-colonize due to their wingless condition, trans-plantation of litter between sites may be necessary to assure con-tinuing persistence of critical species.