The purpose of the study is to investigate how pulses of precipitation translate into pulses of plant aboveground net primary productivity (NPP) and how the small mammal community responds to such changes also in relation to shrub gradient across the landscape. Particularly we are interested in how the energy flows through the ecosystem in response to pulses of rain, how the small mammal community partition resources (in terms of C3 (forbs and shrubs) and C4 (grasses) plants) and how the genetic structure of some species (i.e.: Dypodomis spp.) is affected by their population dynamics.
1) According to the concept that desertification leads to lows of productivity, NPP should be positively related to precipitation and negatively related to shrub cover.
2) Small mammal abundance should respond positively to precipitation and NPP.
3) On a temporal scale, the small mammal energy use should show parallel fluxes along the shrub gradient.
4) The small mammal community should consume C3 and C4 plants according to their availability (or NPP).
5) At low population density, dispersal should be limited and the genetic variance will be distributed among populations rather than within (i.e., Fst will trend towards higher values). After pulses of rain and NPP, population densities will be greater, dispersal prevalent, and the genetic variance of populations will be distributed within populations (i.e., Fst will approach zero) as dispersal homogenizes populations.
Five ecotone sites (reduced to three in 2008 due to budget cuts) were established at the JER and the Chihuahuan Desert Rangeland Research Center (CDRRC). At each site, three rectangular trapping grids (300m X 100m, 16 X 6 traps, 20m apart) were permanently marked in each of the three habitat types characterizing the ecotone (shrubland, grassland, and the transition zone between them) where small mammal trapping and vegetation measurements took place. Trapping occurred for 4 consecutive nights using Sherman-type live traps (one at each station) baited with a seed mix. Rodents were uniquely marked with ear tags and released unharmed at the point of capture. Data on reproductive condition, weight, sex, and age (adult/young) also were recorded and the minimum number of small mammals known to be alive (MNKA) was calculated. Hair samples were clipped from the animal's backs and stored for Carbon and Nitrogen stable isotope analysis in order to assess the proportion of C3 and C4 plants ingested and assimilated. Ear snips were also collected and placed in vials containing 95% ethanol, and stored at -80 ºC. All field procedures followed the ethical guidelines proposed by the IACUC (Institutional Animal Care and Use Committees; NMSU IACUC No. 2004-019). Vegetation cover was estimated using the line-point intercept (LPI) method at all grids in 2003 and 2009. Measurements were conducted every 10 cm on five 50 m transect lines distributed in a staggered pattern across each grid. Plant live aboveground biomass was measured each spring and fall on two transects of 32 permanent, 1-m2 quadrats located inside each grid in order to estimate annual NPP.