Arthropod communities in pear are conceptualized as hierarchically
organized systems in which several levels of organization or subsystems
can be recognized between the species population level and the community
as a whole. An individual tree is taken to be the community habitat with
arthropod subcommunities developing on leaf, fruit and wood subcommunity
habitats. Over a hundred species are reported to colonize one or more
subcommunity habitats. Each subcommunity is composed of trophically
organized systems of populations. Each system of populations is comprised
of a guild of arthropods that use the habitat primarily for feeding but
also for overwintering or egg deposition, and associated guilds of
specialized predators, parasitoids and hyperparasitoids. Understanding
community organization entails understanding how subsystems and their
environments interact so as to be incorporated into a unified whole. Along
with the relatively specialized interrelationships so important in
organizing systems of populations, higher level community subsystems are
coupled through the activities of phytophagous and predaceous generalists
which feed in or take their prey from more than one subcommunity. In
meeting the habitat requirements of each of its life history stages,
several members of the pear community move from one subcommunity to
another during the course of community development and serve to integrate
these subsystems. Additional habitat, trophic and life history aspects of
subsystem incorporation and interpenetration are discussed.
Community development or change in organization through time is
conceptualized as being jointly determined by the development of the
habitat and the organization of the species pool. Seasonal development of
the pear arthropod community is described in terms of changes in the
species composition, size, and spatial distribution of guilds and
subcommunities. Community habitat development is taken to be the primary
determinant of changes in community structure and organization. As the
season progresses, changes in the kinds and biomass of developmental
states of each subcommunity habitat are accompanied by changes in the
kinds, number or biomass, and distribution of associated community
subsystems. Although the influence of habitat development on community
development is emphasized, it is from the the species pool that arthropods
colonize pear. The species that colonize, and their abundance and time of
arrival is partially determined by the organization of this system of
communities
