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POPULATION DYNAMICS OF COTTON RATS ACROSS A LANDSCAPE MANIPULATED BY NITROGEN ADDITIONS AND ENCLOSURE FENCING
Citation:
Clark, J. E., E. C. Hellgren, AND E E. Jorgensen*. POPULATION DYNAMICS OF COTTON RATS ACROSS A LANDSCAPE MANIPULATED BY NITROGEN ADDITIONS AND ENCLOSURE FENCING. Presented at Wildlife Society Annual Meeting, Bismark, ND, 09/01/2002.
Description:
Nitrogen additions in grasslands have produced qualitative and quantitative changes in vegetation resulting in an increase in biomass and decrease in plant species diversity. As with plants, we theorize that animal communities will decrease in species richness and become dominated by a few successful competitors that respond to increased nitrogen availability. We examined how cotton rats (Sigmodon hispidus), the dominate species of small mammal at our study site, responded to habitat changes resulting from nitrogen additions to an old-field grassland in central Oklahoma. The experimental design consisted of 2 treatments (nitrogen addition and enclosure fencing to manipulate herbivory and predation) randomly assigned to 16, square 0.16-ha plots in a 2 ? 2 factorial arrangement. Small mammals were sampled at 3-5-week intervals from July 1999 to December 2000 using Sherman live-traps and mark-recapture techniques. We used minimum number known alive (MNKA) to estimate abundance of cotton rats and tested for differences in MNKA between treatment combinations using a 2-way ANOVA with repeated measures. We observed significant 2-way interactions between fenced plots and time and between nitrogen and fenced plots. Nitrogen-fenced plots had higher abundances than other treatment combinations, and fenced plots, regardless of nitrogen additions, tended to have higher abundances across time. We used a multi-strata model in Program MARK to investigate models with varying treatment (i.e., nitrogen and fence) and sex effects in survival and transition probabilities. Variation in apparent survival probabilities across our treatment plots was best explained by the presence or absence of a fence (i.e., fence effect), regardless of nitrogen additions, for both males and females. Thus, we conclude that variation in population parameters of cotton rats across our study plots was primarily attributed to fence effects rather than nitrogen additions. These results are consistent with concomitant data on short-term vegetation response to nitrogen additions.