Arthropod Diversity in Northwest Old-Growth Forests
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Arthropod Diversity In Northwest Old-Growth Forests John D. Lattin Old-growth coniferous forests of the disturbed areas. We estimate that Pacific Northwest are a paradox: they only about half of the arthropod spe- are characterized by long-term persis- cies present on the site have been col- tence and stability but at the same lected. By way of comparison, there time are the product of two or more are 143 vertebrate species and 460 centuries of change. Jerry F. Franklin, vascular plant species on the site. Cur- a professor of ecosystem analysis at rently, the arthropods represent about the University of Washingtons Col- 85 percent of all the species found in lege of Forest Resources, refers to these three categories. Few arthro- these forests as Douglas-fir / western pods appear to be found exclusively on hemlock forests, but they are not pure the old-growth portions of the H. J. stands, and typically contain an as- Andrews Forest, but many species oc- sortment of other tree, shrub, and forb cur there in greatest abundance, and Recent clearcuts of Douglas fir interior forest The area shown is in species. The precise assemblage we still have much to learn about their the western foothills of the Oregon Cascade Mountains. Photo- varies with the moisture gradient, soil distribution. graph by Spencer B. Beebe type, site aspect, and disturbance his- If you examine biological diver- tory. The result is a mosaic of diverse sity along a succession of communities neasuring the well-being of managed matic mega vertebrates will probably plant and animal communities, an whose later states include the old- tnd highly perturbed systems, espe- still drive much of the publics inter- ecosystem that is complex in struc- growth forests, you see an increase in ially as we begin the next-century est. Some might argue that myriad ture, organization, and function. plant species richness through time, ask of ecosystem restoration. invertebrates will be conserved where Arthropods are an integral part from bare ground, through the forb As the indicators of ecosystem spotted owls are protected. But as con- of this system —from the deep litter- and shrub stage, to the time when iealth, little things will have special servationists move towards a more soil layer and the extensive deposits of young trees emerge above the sur- mportance. Signs of ecosystem stress whole systems / whole earth approach, coarse woody debris to the tree canopy rounding vegetation and arthropod nay first be observed in populations of to the extent that invertebrates can be 70 meters above the ground. On the richness responds accordingly. Below nicroorganisms in lake and stream shown to help point the way toward H. J. Andrews Experimental Forest (a ground the story is similar, but species )ottoms and soils. Species may dis- priority ecosystems and ecosystem 6,400 hectare, Long-Term Ecological richness develops more slowly because tppear, and, after these structur- health, the brave voices of the Xerces Research site on the west slope of the it takes much longer for the litter layer al changes occur, nutrient cycling Society will be heard worldwide. Oregon Cascade Mountains), we have to develop. When the trees become hrough the system could be modified. found more than 3,400 species. large enough and the canopy closes It may be difficult to convince a To date, all primary habitats of (between 20 and 40 years), plant diver- arge public that invertebrates are Spencer B. Beebe is Conservation Inter- the forest have been sampled: high ele- sity declines because much of the ;etting too little attention on the nationals Senior Vice President for vation meadows (about 1,500 meters), understory is shaded out—and arthro- ,rowing conservation agenda. Charis- Mexico / North America, rocky ridges, streams, springs, and pod diversity generally declines as WINGS SUMMER 1990 7 well. But in the litter layer beneath Becky Fichter of Oregon State Univer- the trees, which is still developing, ar- sity have reported densities of oribatid thropod diversity continues to in- mites as high as 120,000 per square crease. When the stand approaches meter in the H. J. Andrews Forest. early old-growth (between 150 and 200 More than 90 genera have been col- years), natural thinning occurs and lected there. (See page 11 of this issue.) openings appear, allowing further de- Once the forest matures, its per- velopment of the understory. This adds sistence is determined largely by the to the structural complexity of the for- disturbances to which it is subjected. est, and, in turn, arthropod diversity According to Franklin, fire is the pri- increases. mary natural disturbance, wind a sec- .1 Y,..-0,4••••■ David Voeghtlin (now with the Il- ondary factor. While smaller under- • \z• linois Natural History Survey) recov- story wildfires are not uncommon in t • 11 • ered more than 500 species of arthro- these forests, only catastrophic stand- .• • A pods from the canopy of three old- replacement fires, which occur ap- growth Douglas-fir trees whose ages proximately every 400 years, reset the were estimated to be about 450 years. system and re-initiate secondary suc- Many of these species are found not cession. Because not all of the old for- Cychrus tuberculatus, a flightless ground beetle, is a only in the canopy, but elsewhere in est burns, the resulting landscape is a typical inhabitant of the mature forest It is a specialized the forest. On the ground, the litter mosaic of different-aged stands. This predator of terrestrial molluscs. Photographed in the H. J. layer becomes deep and complex as the intricate environment is reflected by Andrews Experimental Forest by Andrew Moldenke. forest matures. The diversity of ar- the arthropod diversity, perhaps as thropods in the litter layer approaches great as 8,000 species. arthropod species. Some of these spe- west and other habitats whose ability the greatest number to be found any- The long persistence of some old- cies probably predate the dominance to fly has been lost through wing re- place in the Northern Hemisphere— growth forest stands and the habitats of the present old-growth forests, and duction or wing loss; more such species sometimes reaching 250 species per they contain have given rise to an in- may extend back five million years. certainly exist. Loss of flight in insects square meter. Andrew Moldenke and teresting faunal component, flightless The dynamic aspects of succession, is often associated with habitats that even over the long term, require that have remained stable through time. arthropods have some degree of mobil- The region that contains the old- ity, but the persistence and stability of growth forests appears to be an area of the climax stage allows accumulation such stability, and the arthropod of arthropod species whose ability to fauna reflects this in the number of disperse is limited. In an article in the flightless species present. Further, September 1989 issue of Scientific many normally wingless arthropod A merican, Xerces Society President groups (spiders, mites, millipedes, and E.O. Wilson pointed out that the per- springtails, for example) are richly sistence of an environment may allow represented in these forests, espe- development of diversity through spe- cially in the litter-soil layer. cies accumulation. However, such as- An example of an arthropod semblages are not necessarily stable group encountered in the forest is the and robust enough to resist various true bug family Tingidae (lacebugs), A snail-feeding ground beetle of the genus Scaphinotus. types of disturbance. whose members are plant feeders and Photographed in the H J. Andrews Experimental Forest We have found insect species in who often show bizarre body shapes. by Trygve Steen. old-growth forests of the Pacific North- The family contains some flightless 8 WINGS SUMMER 1990 9 species. The genus Acalypta contains predators of these terrestrial mol- moss-feeding species found through- luscs. Besides beetles, other orders out the Northern Hemisphere; three of insects contain flightless species occur in the H. J. Andrews Forest. found in the forest, and we will cer- Acalypta lillianis, apparently a tainly discover more. One Hundred Twenty Thousand Little Legs relict of colder times, is found in The old-growth forests of the Pa- mosses growing on the cool, high, cific Northwest, with trees of enor- rocky ridges. While the only specimen mous height and mass, diverse shrubs Andrew Moldenke collected from the forest is flightless, and forbs, and large accumulations of winged individuals of this species are coarse, woody debris, provide many known to occur elsewhere. The second habitats. The highly varied and deep Nowhere are the critical roles of in- No one has ever counted the num- species, A. mera, is found at lower ele- litter and soils supply resources for a sects and other invertebrates easier to ber of kinds of bacteria and fungi un- vations in mosses growing on the diverse and characteristic fauna. understand, yet more poorly investi- der a single tree in the forest; no ecolo- ground in more open sites. It is usu- Meadows, openings, ridges, streams, gated, than in forest soil. Proper gist knows just how many chemical ally flightless, but about five percent and springs add to the variety. The growth of forest trees depends on re- transformation processes are neces- of the individuals of both sexes are number of arthropod species conser- ceiving appropriate nutrient levels sary for the full recycling of nutrients. fully winged and able to disperse vatively represents more than three- and water from the roots. The meta- We do know, however, that in undis- should the necessity arise. The third quarters of all species of higher life bolic activity of fungi and bacteria turbed forests there are 200 to 250 spe- species, Acalypta saundersi, has been forms (plants, invertebrates, and ver- liberate nutrients through litter de- cies of invertebrates per square meter found only in the flightless state.