Dispersal, determinism, and the structure of a desert grassland grasshopper community

Whether neutral dispersal limitation, deterministic niche differentiation, or both structure biotic communities remains an important question in community ecology. We studied the spatial structure of a local grasshopper community in central New Mexico and tested for species sorting based on interspecific interactions and environmental variables, as well as community wide dispersal limitation. We achieved this by contrasting the change in species relative abundance and composition along an environmental gradient against community change across relatively homogeneous space. We found a significant decrease in pairwise community similarity along both environmental and distance gradients, though the interaction was strongest for environmental gradient. This result indicates that both niche assembly and dispersal limitation potentially structure the local community. To determine if dietary niche differentiation drives local spatial distributions, we performed laboratory feeding trials to investigate the dietary preferences of two coexisting congeners (Psoloessa texana and P. delicatula). Locally, P. texana and P. delicatula segregate, somewhat, based on their differential preference for B. eriopoda and B. gracilis, which constitute the dominant plant cover in our study site. However, much variance in local Psoloessa distribution remains unexplained by Bouteloua cover, implying the presence of other significant variables. Thus, while deterministic niche differences have the potential to structure local communities, dispersal limitation may be important to community assembly. Our results imply that dispersal limitation may override determinism when environmental variables are not autocorrelated, though explicit tests are needed.