The spider communities of four stages of a successional sere leading to and including spruce forests were studied in northern Utah. Four seral stages were recognized. These include: meadows, aspen (Populus tremuloides) stands, subalpine fir (Abies lasiocarpa) forest, and the climax Engelmann spruce (Pica engelmanii) forests. During the snow-free periods of 1976, 1977 and 1978, 15,987 spiders were collected by three methods including: pitfall traps, by beating vegetation, and with sweep-net samples. Additionally, 1600 15-second intervals of behavioral observations, and measurements of 182 web locations were made. Of 99 species collected, 44 were considered residents of the sere: criteria for assigning the spiders to foraging strategies (3) and guilds (9) are presented. Five spider communities were ostensively defined--one in the ground stratum of each of the stages and one in the tree stratum of the conifer stages. The data were used to compare the guild strategies of the spiders of the seral stages and to address various hypotheses about successional change in animal community characteristics. Increases with maturity as predicted were observed for 6 spider community parameters including: total biomass, species diversity--richness component, species diversity--equitability component, stratification and spatial heterogeneity, mean organism size, and temporal stratification. A life cycle hypothesis (i.e., short and simple life cycles in early stages, long and complex ones in mature stages) could not be tested because, depending on the life cycle type considered, I found diametrically opposed trends (semi-annual and biennial life cycle types both increased with maturity). The spider species of the ground-stratum meadow community were primarily dispersed in a time dimension (seasonal); the spiders of the tree-stratum community were primarily distributed in a spatial dimension (microhabitat). Spiders of the forest ground-strata communities were dispersed in spatial and temporal dimensions. No dimension was ascertained to be of fundamental importance. Distributions of ground-dwelling species with different foraging strategies, and the resident species of the ground-stratum communities were correlated canonically to 8 environmental variables. Spider species of the meadow community were correlated with a bare dirt variable. Spiders of the aspen community were correlated with 2 environmental variables including: grasses and forbs and a low foliage index. Hunting spiders were correlated with the meadow and aspen variables. Ambushing spiders, web-building spiders, and the spider species of the ground stratum spruce community were correlated with 5 environmental variables including: litter depth, canopy cover, tree basal area, dead leaves and needles, and logs.

Doctor of Philosophy / Department of Biology / Anthony Joern / North American tallgrass prairie is a dynamic ecosystem that evolved with variable regimes of fire and grazing interactions (pyric herbivory), and variable mid-continental weather. Combined, these ecological factors create a shifting mosaic of plant communities that create heterogeneous and structurally complex habitats that move around across the landscape in time and space. The overarching goal of my dissertation was to study how bottom-up habitat templates created in response to fire-grazing interactions influence the community structure of spiders, key arthropod predators in grassland food-webs. Spiders are a ubiquitous and diverse group of terrestrial predators that partition their habitat at fine scales with species distributions and abundances that are sensitive to habitat structure. Primary hypotheses examined include: (H1) Spider density, species diversity, species evenness and functional richness of hunting strategies should increase as the spatial heterogeneity of habitat structure and overall habitat productivity increases, as predicted by the habitat complexity and heterogeneity hypothesis. (H2) Pyric herbivory indirectly determines spider community structure through is effect on vegetation structure and spatial heterogeneity, thereby promoting the formation of a mosaic of spider species assemblages that track changes in the distribution of key habitat resources. My research takes advantage of a long-term, watershed-level manipulations of fire frequency and bison grazing across a topographically variable landscape at Kansas State University’s Konza Prairie Biological Station, a tallgrass prairie research site near Manhattan KS. Spider communities were sampled for three years at 23 sites representative of multiple habitat types ranging from low-stature grass-dominated sites to grassland-gallery forest transition zones. In addition, a field experiment was performed to test the hypothesis that vegetation structure contributes directly to web-builder abundance and web-type richness of spiders in open grasslands. Here, the availability of structure for web placement was increased by adding dead woody stems along transects in three watersheds that differed in burn histories and existing habitat structure in the absence of grazing. Results were consistent with the three key hypotheses. Species diversity and the functional diversity of spiders increased as the spatial heterogeneity and overall structure of habitat increased in response to fire-grazing interactions. Vegetation heterogeneity influenced spider community responses most strongly in the summer. Structural complexity of vegetation influenced spider diversity, species evenness and richness of hunting strategies throughout the growing season, becoming most important by the end of the growing season. The transitional ecotone between grasslands and woodlands supported a hotspot for spider density, species diversity and richness of hunting strategies along vegetation gradients (H1), and among habitat types (H2). Increasing the availability of web-anchoring structures in open grasslands led to increased web-builder density in open grassland, particularly for small and medium sized orb-web species that took advantage of increased physical structure. Disturbance from pyric herbivory indirectly promoted dynamic and malleable assemblages of spider species that coexisted in syntopy through effects on vegetation structure and its availability in time and space. Changes in habitat structure and heterogeneity as spatially and temporally shifting mosaics of habitat type increased the overall spider diversity at the landscape scale.