1. Changing competitors and dynamics in a desert rodent community Glenda M. Yenni, Department of Biology, Utah State University Abstract: Recently, attempts have been made to reconcile and synthesize neutral and niche-based models to describe the universal processes behind coexistence. The importance of niche or neutral processes may be a gradient and depend in part on the stabilizing mechanisms working in a system. One stabilizing mechanism is resource partitioning. Avoidance of competition through resource partitioning is well documented in desert heteromyid rodents. At a long-term study site near Portal, Arizona, the rodent community has been altered by the local extinction of a major competitor (Dipodomys spectabilis) and colonization of another (Chaetodipus baileyi). This site also has experimental plots removing a subset of the rodent community. I used the natural and experimental manipulations on the rodent community to examine how changes in the dominant competitors affected Chaetodipus penicillatus, a small granivore at the site. For this species, I examined relative abundance, survival selective dispersal, and frequency-dependant growth. I found that relative abundance of C. penicillatus changed significantly after the extinction and colonization events and differed between controls and manipulations. However, there was no significant effect on survival and only a small percentage of individuals disperse between plots. Frequency-dependant growth on the treatment plots indicates strong stabilization. Thus, the majority of population changes must be due to within-plot dynamics. In the future, I intend to determine on what level(s) heteromyids at Portal are partitioning resources to achieve these dynamics. Microhabitat selection and nutrient ratio preferences will be explored first as possible mechanisms of stabilizing coexistence. Relative abundance shifts on treatment types following the change in community composition Survival shows no pattern of response to experimental or natural perturbations Negative frequency-dependant growth is a sign of strong stabilizing mechanisms Dispersing individuals show no clear preference for plot type Redundancy analysis confirms that abundance patterns in the community have shifted Conclusions: Results from a redundancy analysis suggest that the change in community composition has resulted in a reorganizing of species interactions. The significant change in community dynamics in the second period of this analysis would indicate that C. baileyi is, in fact, a significant driver in this system. The sign of strong stabilization in the relationship between abundance and growth rate is a likely confirmation of resource partitioning. Further analysis is required, however, to determine what aspects of life history are affected in each of the community members to translate into a change in abundance on the experimental plots. Within-plot dynamics other than survival, such as fecundity, must be examined for their sensitivity to abundance of major competitors. Also of interest is the identification of what niches these species occupy to allow direct measurement of niche overlapping. Among these closely related species, resource partitioning may be occurring on one (or more) of many levels, and several candidates will be considered. Microhabitat selection and nutrient ratio preferences will be explored first as possible mechanisms of stabilizing coexistence. To describe microhabitat selection, GIS work on aerial photographs, supplemented by vegetation sampling, will be used to map microhabitats at trapping locations to quantify trapping frequency in each microhabitat type. If an indication of microhabitat preference is found, direct measurements such as radio telemetry can then be used for confirmation. Some ecological stoichiometry solutions will be explored by measuring differences in nutrient ratios in diet items. Again, a more direct measurement, such as giving up density trials, may then be required for confirmation. Quantifying levels of resource partitioning in this way should develop a good picture of what subtle shift in interspecific relationships allows coexistence of the major species to continue. ○ Dipodomys spp ∆ Dipodomys spectabilis + Chaetodipus penicillatus x C. baileyi Proportion of individuals caught once (1 catch), caught multiple times on the same plot (1 plot), and caught on more than one plot (disp) for three species of kangaroo rat (DM, DO, DS) or two species of pocket mouse (PB, PP) and the time period 1988-1994 (88) or 1995-2006 (95). Proportion of dispersing individuals (‘disp’) dispersing to other plots of the same type (1 type) or plots of a different type (btw type). And proportion of dispersing individuals (‘disp’) representing each dispersal type : CO-within controls, EX-within kangaroo-rat exclosures, CO-EX-from control to exclosure, CO-RE-from control to total exclosure, EX-CO-from exclosure to control, EX-RE-from exclosure to total exclosure. This method focuses on the amount of variation in a species abundances matrix that is explained by an environmental/experimental matrix (Palmer 2007). Note the change in species score for C. penicillatus (PP). Dipodomys abundance PBAbundancePBAbundance Literature Cited Adler et al. 2007. A Niche for Neutrality. Ecology Letters. Palmer, Michael. 2007. The Ordination Web Page. http://ordination.okstate.edu/http://ordination.okstate.edu/ Acknowledgments S. K. Morgan Ernest for advising and support Katherine Thibault for work on C. baileyi Diversity Fellowship, Utah State University The Portal project is currently supported by NSF grant DEB-0702875 C. Penicillatus relative abundance Estimated survival Year After the introduction of C. baileyi, it and C. penicillatus both show a negative relationship between their relative energy use in the community and their population growth rates, indicating stabilizing mechanisms at work in the relationship between these species. An aerial photograph of the site shows the 24 treatment plots. Note differences in vegetation on different plot types.