In many ecosystems, seasonal shifts in temperature and precipitation induce pulses of primary productivity that vary in phenology, abundance and nutritional quality. Variation in these resource pulses could strongly influence community composition and ecosystem function, because these pervasive bottom-up forces play a primary role in determining the biomass, life cycles and interactions of organisms across trophic levels. The focus of this research is to understand how consumers across trophic levels alter resource use and assimilation over seasonal and inter-annual timescales in response to climatically driven changes in pulses of primary productivity. We measured the carbon isotope ratios (d13C) of plant, arthropod, and lizard tissues in the northern Chihuahuan Desert to quantify the relative importance of primary production from plants using C3 and C4 photosynthesis for consumers. Summer monsoonal rains on the Sevilleta LTER in New Mexico support a pulse of C4 plant production that have tissue d13C values distinct from C3 plants. During a year when precipitation patterns were relatively normal, d13C measurements showed that consumers used and assimilated significantly more C4 derived carbon over the course of a summer; tracking the seasonal increase in abundance of C4 plants. In the following spring, after a failure in winter precipitation and the associated failure of spring C3 plant growth, consumers showed elevated assimilation of C4 derived carbon relative to a normal rainfall regime. These findings provide insight into how climate, pulsed resources and temporal trophic dynamics may interact to shape semi-arid grasslands such as the Chihuahuan Desert in the present and future.
This research was conducted on the Sevilleta LTER, located 100 km south of Albuquerque, New Mexico, which is an ecotonal landscape of Chihuahuan desert shrub and grasslands (Muldavin et al. 2008). Data were collected from a 0.9 x 0.5km strip of land that encompassed a flat bajada and a shallow rocky canyon of mixed desert shrub and grassland dominated by the creosote bush (Larrea tridentata) and black grama grass (Bouteloua eriopoda).
Tissue collection & sample preparation for stable isotope analysis:
From May to October of 2005 and 2006 we collected plant, lizard, and arthropod tissues for carbon stable isotope analysis. During mid-summer of 2005, we randomly collected leaf and stem samples from the 38 most abundant species of plants; these species produce over 90% of the annual biomass on our study site (see Appendix Table A). Approximately 3.5 mg of plant material was then loaded into pre-cleaned tin capsules for isotope analysis.