Community structure changes over relatively short time periods. Short-term variation in community structure reflects interactions among species responding differently to fluctuating abiotic conditions and species interactions. Relatively few studies measured effects of seasonal or annual changes in arthropod communities over extended periods. Several studies represent annual to decadal dynamics in arthropod communities.
Fluctuating weather conditions and disturbances can cause appreciable changes in arthropod community structure. Changes in precipitation pattern can elicit differential responses among arthropod species. Schowalter et al. (1999) found that particular arthropod species, as well as the entire arthropod community, associated with creosotebush, Larrea tridentata, in southern New Mexico showed distinct trends in abundance over an experimental gradient in precipitation volume. Abundances of several species increased with moisture availability, whereas abundances of others declined with moisture availability, and some species showed nonlinear or nonsignificant responses. Multivariate analysis indicated distinct community structures on plants subjected to different amounts of precipitation.
Polis et al. (1997b, 1998) studied community changes on desert islands in the Gulf of California during a 5-year period (1990-1994), which included an El Niño event (1992-1993). Winter 1992 precipitation was 5 times the historic mean and increased plant cover 10-160-fold. Insect abundance doubled in 1992 and 1993, compared to 1991 levels, with a significant shift in dominance from detritivores supported by marine litter to herbivores supported by increased plant biomass. Spider densities doubled in 1992 in response to prey abundance, but declined in 1993, despite continued high plant and prey abundance, as a result of increased abundance of parasitoid wasps, partially supported by nectar and pollen resources. These changes were consistent among islands throughout the archipelago, indicating that general processes connecting productivity and consumption governed community dynamics in this system.
Changes in precipitation pattern in western Oregon, United States, between 1986 and 1996 altered the relative abundances of dominant folivore and sap-sucker species in conifer canopies (Fig. 10.1). In particular, western spruce budworm, Choristoneura occidentals; sawflies, Neodiprion abietis; and aphids, Cinara spp., were abundant during a drought period, 1987-1993, but virtually absent during wetter periods. A bud moth, Zeiraphera hesperiana, was the dominant folivore during wet years but disappeared during the drought period.
Schowalter and Ganio (2003) described changes in arthropod community structure in tropical rainforest canopies in Puerto Rico from 1991 to 1999. Hurricane Hugo (1989) created 30-50-m diameter canopy gaps dominated by early successional shrubs, vines, and Cecropia schreberiana saplings. Several
1986 1992 1996
1986 1992 1996
□ Pollen and seed feeders
| Temporal change in arthropod abundances in old-growth Douglas fir canopies at the H. J. Andrews Experimental Forest in western Oregon; 1989 and 1996 were relatively wet years; 1992 was in the middle of an extended drought period (1987-1993). Z, Zeiraphera hesperiana; Ch., Choristoneura occidentals; N., Neodiprion abietis; Ci., Cinara spp.; A., Adelges cooleyi; Co., Coccoidea (4 spp.). Note the log scale of abundance. Data from Schowalter (1989,1995 and unpublished data).
species of scale insects and a phytophagous mirid bug, Itacoris sp., were significantly more abundant on foliage in canopy gaps, compared to nongaps, in 1991 and again following Hurricane Georges (1998), suggesting positive response to storm disturbance. Scale insect and folivore abundances were significantly more abundant during a record drought (1994-1995), compared to intervals between disturbances, providing further evidence of responses to disturbances.
Factors that increase competition or predation can reduce population sizes of particular species. Some species may become locally extinct, whereas others show population irruptions. Changes in species abundances affect interactions with other species. Both the strength and direction of interaction can change greatly. Herbivores that have little effect on their hosts at low abundances can interact in a more predatory manner at high abundances. Reduced abundance of one member of a mutualism can jeopardize the persistence of the other.
Changes in species composition and abundance alter species diversity, food web structure, and functional organization. Change in abundance of species at one trophic level can affect the diversity and abundance of species at lower trophic levels through trophic cascades. For example, reduced predator abundance usually increases herbivore abundance, thereby decreasing plant abundance (Carpenter and Kitchell 1987,1988, Letourneau and Dyer 1998).
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