Emergence of population dynamics and community structure from interactions between organisms and environmental processes: Experiments with protist microcosms and desert rodents

My doctoral research explored how population and community level phenomena derive from interactions between the scale of environmental variability and the responses of organisms to that variability. I first addressed this topic with two protist microcosm experiments on resonance, which I defined for ecological systems as an interaction between a periodic environmental perturbation and another periodic process or a nonlinear organismal response that either amplifies or dampens the abundance or variability of a population. In the first experiment the form of environmental variability was a resource perturbation between high and low nutrient concentrations. The average density of the population was 20% higher in the treatment with a perturbation frequency in between the reproductive rate and the starvation rate of the organisms than it was in the constant control at the average nutrient concentration. In the second experiment, the form of the periodic perturbation was a density perturbation with organisms alternately added and removed from the populations in conjunction with pulsed resource renewals at varying treatment frequencies. Population variability was lowest when the density perturbations occurred at the same frequency as the pulsed resource renewals.; Next I addressed scale-dependent interactions between organisms and their environment at the community level with a food addition experiment on granivorous heteromyid rodents in the Sonoran Desert. Reproduction and biomass increased dramatically, but there was no change in the density of adult rodents. This suggests that animals maintained the same home range size when resources were abundant even where they had previously been limiting. There was also a decrease in species diversity and a corresponding increase in the presence of the dominant pocket mouse species with the addition of resources. Such results would be expected in a community with a hierarchical distribution of resources. A hierarchical distribution of resources occurs when those individuals that initially have access to more resources are in turn able to further monopolize resources. This experiment indicates that individual-level processes can affect community diversity. Overall, resonance impacting population abundance and home range size impacting diversity are both examples of scale-dependent interactions between organisms and their spatio-temporal environment that significantly alter population dynamics and community structure.