TRANSLATING ACROSS SCALES - SIMULATING SPECIES DISTRIBUTIONS AS THE AGGREGATE RESPONSE OF INDIVIDUALS TO HETEROGENEITY

The mechanistic linkage between movement responses of animals to heter ogeneity and distribution of populations provides a useful framework f or investigating the extent to which fine-scale ecological information can be extrapolated across scales to explain broad-scale phenomena. W e developed a simple, spatially explicit simulation model to explore w hether patterns of species distributions across landscapes emerge as t he aggregate response of individuals to fine-scale heterogeneity. As a n empirical basis for this modelling exercise, we studied two species of acridid grasshoppers (Orthoptera) in the shortgrass prairie in nort hcentral Colorado, USA. Grasshopper distributions were sampled in two pastures that had been subjected to different intensities of cattle gr azing. A large species, Xanthippus corallipes (Haldeman) was patchily distributed across this grassland, whereas the smaller Psoloessa delic atula (Scudder) occurred as a random distribution in both pastures. We produced a grid map of each pasture, in which each grid cell was clas sified according to 3 habitat types representing a gradient of forage abundance for grasshoppers. During model simulations, individuals were randomly distributed across the pasture maps and allowed to redistrib ute according to habitat-specific movement probabilities - the rate th at an individual would leave a particular cell (habitat) type. Movemen t probabilities were extrapolated from observed movement rates of each species within habitats. We were initially unable to simulate realist ic levels of aggregation for the two grasshopper species when extrapol ated rates of movement were applied to the model. Species distribution s thus do not emerge as a linear function of fine-scale movement rates , presumably because movement is constrained by different processes op erating at different scales. Fine-scale movement responses to heteroge neity can be used to provide qualitative predictions of species' distr ibutional patterns in different landscapes, however. For example, P. d elicatula exhibited faster rates of movement through habitats comprisi ng 92% of one pasture; such a high rate of turnover should lead to a r andom distribution, which is what we observed for this species in this system. Xanthippus corallipes had reduced movement in 35% of this sam e landscape, but was able to move rapidly across the remainder. This m ay enable individuals to aggregate within a minor component of the lan dscape and produce the clumped distributions we observed. While the ge neral pattern of distribution can be determined from individual moveme nt responses to heterogeneity, such information is too coarse to quant ify the exact location of individuals and other statistical properties of population distributions (e.g., density).