Responses of a grassland spider community to habitat structural heterogeneity driven by fire and bison-grazing disturbances interactions

Printer-friendly version

An overarching hypothesis in community ecology is that increased habitat heterogeneity increases species diversity and alters the species assemblage composition at multiple trophic levels across the landscape.  In grassland ecosystems, interactions among fire, grazing, topography and climate create shifting mosaics of plant communities and associated habitat structural complexity. Accordingly, the habitat complexity and heterogeneity hypothesis predicts that overall abundance and species diversity of consumers increases with spatial heterogeneity of habitat structure.  We test the hypothesis that bottom-up, fire-grazing interactions structure ubiquitous and diverse spider assemblages to create a shifting mosaic of habitats at multiple scales that in turn drive the habitat heterogeneity/ species diversity relationship. Spiders are important predators in grasslands and provide an ideal ecological model to study how diverse predator assemblages respond to spatio-temporal variability in habitat structure driven by fire-grazing interactions. Unique long-term manipulations of fire frequency and bison grazing at watershed levels since 1972 have resulted in a mosaic of habitat types at Konza Prairie Biological Station. Spider were sampled using sweep nets along a habitat gradient ranging from low-stature grass-dominated sites to gallery forest to test hypotheses related to structural heterogeneity. Spider species richness increased over the growing season. Spider abundance and species richness increased with increased time since the last fire and moderate to low bison use of the sites. In general, spider abundance and diversity increased with increasing habitat structural complexity and heterogeneity from grass dominated site to those with a greater mixture of grasses, forbs and woody vegetation in response to fire-grazing interactions.  The transitional ecotone between grasslands and woodlands supported a hotspot for spider abundance and species richness.  The mosaic of grassland types from unique long-term watershed level manipulations of fire frequency and bison grazing at KPBS sustain multiple, distinct spider species assemblages, supporting the conclusion that habitat structural heterogeneity is critical for maintaining diversity in these important invertebrate predator assemblages.