The role of species interactions on the diversity and dynamics of arid grass- and shrub-lands in the southwestern US

Background/Question/Methods

The goal of this study was to investigate how species interactions influence plant communities (structure, diversity and dynamics), and some aspects of ecosystem functioning in the Chihuahuan desert in the south western US. We evaluated how species interactions including competition, facilitation, and herbivory affect plant communities at the Sevilleta National Wildlife Refuge and LTER site, in central New Mexico. To achieve this goal, we conducted three independent studies that combined the analyses of long-term datasets and field work. First, we evaluated the effects of competition and facilitation on the diversity and dynamics over a 10-year period of plant communities by comparing grasslands dominated by a native competitive grass (Bouteloua eriopoda), with areas invaded by a predominantly facilitative shrub (Larrea tridentata). Second, we assessed the relative influence of bottom-up (i.e., rainfall) vs. top-down control exerted by herbivorous/granivorous small mammals on plant community diversity, structure and dynamics in grass and shrub- vegetation over 10 years. Finally, we investigated whether herbivory by small mammals affects soil nutrient availability (nitrogen) in three long-term small mammal exclosure experiments located at the Sevilleta LTER, Jornada LTER, and Portal LTBR sites, in the southwestern US.

Results/Conclusions

Our results indicated that facilitation and competition were important species interactions that influenced the structure, dynamics and of the plant communities at the Sevilleta NWR. Plant communities structured by facilitation were unstable and less species diverse compared to communities structured by competition. Thus, interactions imposed by Larrea appear to have the potential to feedback to the species pool through changes on plant community dynamics. Trophic interactions, on the other hand, did not affect the composition, structure, diversity or dynamics of the plant communities at the Sevilleta NWR. Chronic low levels of productivity appear to have set this system in a bottom-up dominated stage, at least during the period of this study. Finally, across our three study sites, we found a positive effect of small mammals on soil N availability. Although herbivory did not exert a direct influence on plant community parameters at the Sevilleta NWR, it did have a consistent positive effect on soil nutrient availability across the three research sites. Hence, we conclude that species interactions are important determinants of various aspects of the plant communities, and of soil nutrient availability in this desert ecosystem.