Resource partitioning, density and distribution of Perognathus flavus in an arid nutrient and water limited ecosystem
In arid water and nutrient limited ecosystems, consumer fitness is tied to the quality, quantity, and timing of primary production, which in turn influences their distribution and abundance. In our study area, winter/spring rains fuel perennial and annual shrubs and forbs that use the C3 photosynthetic pathway with carbon isotope (d13C) values of -24.5‰. The summer monsoon fuels grasses that use the C4 photosynthetic pathway with d13C values of -15‰. These two components of primary production can be traced up food chains into consumers. The two main objectives of this research were to (1) quantify resource use within Perognathus flavus as either generalists (which track isotopically with C3/C4 forage availability) or specialists (which seek out a preferred food regardless of availability), and (2) Calculate the density and distribution of these mice. Climate models predict a ~20% decrease in winter/spring precipitation in the American Southwest, which will decrease the availability of C3 forbs, the preferred dietary source for our study species due to their larger, more nutritious seeds. Perognathus flavus community assembly may change in response to a changing landscape of primary producers. Mice were trapped monthly, pit-tagged, sexed, aged, bled, and reproductive status determined.
Our plant analysis showed a tight but distinct clustering of values for C3 (d13C -24.5 +/- 2.0) and C4 (d13C –15.1 +/- 1.1). We used a two-source, one isotope Bayesian mixing model to estimate the proportion of assimilated carbon that was derived from C3 versus C4 sources in our plasma. We estimated a δ13C trophic discrimination factor by comparing plasma δ13C values to those of plants. Plasma d13C, d15N, and mixing model output (reported as percent C3) were examined using ANOVAs and ANCOVAs. Differences in d13C and d15N values among rodents and plant functional groups (C3 versus C4) were assessed using statistical tests to adjust for multiple comparisons. We used Bayesian Ellipses to quantify isotope variation temporally across the species as measured by standard ellipse areas in d13C versus δ15N bivariate space. Variation in isotopic space can be linked to individual variation in diet (specialist versus generalist). d15N values indicate the trophic breadth of the community while d13C values represent foraging preferences. By analyzing 1,570 plasma samples with 32 animals that were sampled 5 times or more over an 8 month period, we were able to adequately quantify foraging strategies. Using the MARC mark-recapture program we calculated density and distribution across our study area.