Community composition influences ecosystem resistance and productivity more than species richness or intraspecific diversity

Matthew Bowker, Cristina Rengifo-Faiffer, Anita Antoninka and Henry Grover, School of Forestry, Northern Arizona University, Flagstaff, AZ, Kirsten K. Coe, Biology, Middlebury College, Middlebury, VT, Kirsten Fisher, Biological Sciences, California State University, Los Angeles, Los Angeles, CA, Brent D. Mishler, Integrative Biology, University of California, Berkeley, Berkeley, CA, Melvin J. Oliver, Division of Plant Sciences, University of Missouri, Columbia, MO, Lloyd Stark, University of Nevada Las Vegas

Background/Question/Methods
Biodiversity describes the variety of life and may influence properties and processes of ecosystems, such as biomass production and resistance to perturbation. We investigated the effects of multiple facets of biodiversity – species richness and composition of the community, and intraspecific diversity in two key species – on both production and resistance of experimentally-assembled biological soil crusts (biocrusts). We grew experimental biocrusts featuring a focal species (Syntrichia caninvervis or S. ruralis), in addition to 1-4 additional biocrust species randomly drawn from a species pool. We crossed this gradient with a gradient in intraspecific diversity within Syntrichia, by varying the number of Syntrichia clones added (2, 5, or 7). We tracked NDVI as a measure of production during a favorable growing period, and its resistance in response to an experimentally-imposed stressful climate disturbance. We used structural equation models to partition the influence of species richness, intraspecific diversity, their interaction, and community composition on production and resistance.
Results/Conclusions
We found that productivity was most strongly influenced by community composition (variation in the presence and relative proportions of community members), and weakly positively influenced by species richness. Intraspecific diversity, encompassing both the richness and composition of clones, had a neutral effect on community productivity within one focal species (Syntrichia caninervis), but a moderately negative effect on productivity within the other focal species (S. ruralis). Resistance was also most strongly influenced by community composition, although different sets of species contributed most to resistance and production. Resistance was not affected by either species richness or intraspecific diversity in either focal species. Our findings supported our general expectation that community-level facets of biodiversity would be more influential than intraspecific diversity within a species because trait variability among species is usually greater than that within. These results also indicated that although species richness is often thought to strongly affect both productivity and resistance, the presence, absence and relative abundance of particular species may be more influential to both. Simultaneous manipulation of multiple facets of biodiversity in experiments may lead to a more complete understanding of the variety of ways in which biodiversity may regulate ecological systems.