Drivers of plant and arthropod diversity and community structure in a grassland ecosystem

Forces operating at different spatial scales are known to influence species coexistence and community organization, although the relative importance of these forces is debated. While niche-based models emphasize the effect of local processes on community structure, regional species pools and dispersal abilities of organisms are also thought to constrain species distributions and community diversity. To discern the roles of local and regional processes in shaping ecological communities, this thesis examines the importance of factors operating at multiple spatial scales in governing plant diversity and arthropod community structure. Plant diversity is generally hypothesized to be limited by local competition/productivity ("niche limitation hypothesis", "NLH"), and/or by regional species pools/dispersal ("species pool hypothesis", "SPH"); however, these factors likely interact and vary over periods of community development, i.e. succession. For my first chapter, I test the relevance of the SPH and NLH in limiting diversity, in addition to testing how the importance of these factors may shift from seed/dispersal constraints to local competition/productivity over gradients of 1) productivity and 2) time. Species availability and soil nutrient (N and P) availability were manipulated via seed and fertilizer additions, respectively, and diversity responses were measured over the first 11 years of succession. Diversity increased with seed addition and increased species availability, in support of the SPH; and decreased with nutrient addition and the amplification of local competition, in support of the NLH. Species availability became less important in determining species richness as local productivity and pressures of competition increased. Also, positive effects of seed addition dampened with time, suggesting that species/dispersal constraints became less important and local competition/microsite availability became more important over succession. Additionally, species availability mediated effects of soil resources/competition over time. These results suggest that limits to diversity act at both local and regional scales, and shift in their degree of influence over gradients of successional time and habitat productivity. The diversity of basal resources is generally predicted to determine diversity at higher trophic levels, as are additional characteristics of local resources (e.g. abundance) and regional spatial processes (e.g. dispersal, species availability). Yet, the relative importance of these processes is unclear. For my second chapter, I test the importance of basal resources (i.e. plants) and spatial processes on community structure of higher trophic levels (i.e. consumers). I examine arthropod responses to management/prairie restoration regimes in old-field plant communities, and predict that more diverse and abundant arthropod communities will result from 1) increased resource diversity ("resource diversity hypothesis"), 2) increased resource availability ("resource abundance hypothesis"), and/or 3) increased plant litter density ("plant litter hypothesis"). Additionally, I test arthropod community responses to plant species composition, spatial processes, and direct effects of treatments; and examine responses at the whole community level and within nested microhabitats. I found support for both the resource diversity hypothesis and plant litter hypothesis, and detected effects of plant species composition and spatial processes on community structure. Assemblages occupying distinct microhabitats (and presumably different niche space) varied in their responses to these factors, suggesting that niche-specific processes influence community organization. These results suggest that both characteristics of local resources and regional spatial processes significantly influence consumer community structure; however, as considerable variation is left unexplained, other unaccounted factors are likely to be important in structuring consumer communities. Overall, this thesis demonstrates that community development and organization of ecological communities are influenced both by regional processes of dispersal and species availability and by local processes of competition and resource gradients. These processes interact and vary over time and space, highlighting the importance of spatial and temporal scales in understanding determinants of community structure