Determinants of spider community structure and function : a look at competitive effects and optimal functioning

Three paradigms for identifying the mechanisms which determine community structure were considered: 1) proximate competition, 2) evolutionary competition and proximate stochastic processes, and 3) optimal functioning. Each of these had associated with it a number of predictions concerning specific aspects of community structure. The general trends which were predicted for each paradigm were: 1) proximate competition: little overlap among similar species, competitively inferior species forced into poorer resource condition areas, and a richer and more diverse community structure where there were greater resource opportunities for either avoiding competitive encounters and/or for partitioning resources; 2) evolutionary competition and stochastic processes: niche partitioning through guild development suited to the region and no variability in community structure among habitats in this region; and 3) optimal functioning: differences in community structure among habitats which were related to the favorability of resource conditions for spider functioning.

Forty-five percent of spider community structure in these habitats appeared to reflect the outcome of evolutionary competitive processes associated with resource availability in the region. These trends concerned inter- and intraguild development in which similar patterns for guild richness, diversity, patterns of dispersion and species representation were observed in all of the communities studied. Other structures reflecting an evolutionary history of competition for available resources and proximate stochastic processes included similar trends in the body sizes represented, in the breadth of the spatial niches of the species, in their interspecific associations, and in the role of non-web spiders as spatial generalists and prey size specialists.

Thirty-three percent of the community structure observed reflected resource preference and the associated optimal functioning of spiders. The following community structures varied with the microclimatic and microhabitat conditions existing in the different habitats: species composition, richness and diversity, spider density both in general and within guilds, spider body sizes, the spatial niche overlap observed, and the degree to which spiders found over a wide range of habitats varied in their utilization of microhabitats. Both spider body size and the representation of mature individuals in a community varied with prey constancy.

Only 22 percent of the community structure trends suggested the conditions under which proximate competitive interactions may shape community patterns. Proximate competitive effects appeared where the variability in the distribution of preferred resources was large relative to the potential for spider emigration and immigration. Based on the findings of this study, a hypothetical model predicting the relative strengths of evolutionary competition, optimal functioning, and proximate competition in determining the community structure of other organisms was formulated.