The effect of biodiversity on ecosystem functioning: The Patagonian steppe as a model

Human activities are drastically altering global biodiversity, with potentially severe consequences for ecosystem functioning. The first tests of the biodiversity ecosystem functioning hypothesis used controlled artificial ecosystems. The broadest objective of my dissertation is test the biodiversity-functioning hypothesis in natural ecosystems, which arguably matter most. The Patagonian steppe with its low natural diversity was an ideal model ecosystem for experimental research. In a field experiment, where I manipulated plant-species richness while maintaining all other conditions constant, I found that primary production increased with biodiversity. The main mechanisms were niche complementarity (the differential use of resources by different species) and positive interactions (benefit to at least one species in the presence of another). Remarkably, the effect of biodiversity in ecosystem functioning in the natural ecosystem was significantly larger than previously reported using artificial ecosystems.;Further, I investigated niche complementarity among species in the Patagonian steppe. Using field and laboratory experiments, I assessed species traits that affect ecosystem processes. The most abundant life forms, grasses and shrubs, were complementary in morphological related traits. Among species, niche complementarity resulted from differences in traits affecting carbon, water, microclimate, and nitrogen dynamics. Niche complementarity was most affected by traits that were evenly distributed among species, which in Patagonia were rooting depth and potential soil nitrification. Trait diversity successfully explained net primary production, with only eight traits accounting for 70% of the plant-species richness and net primary production relationship.;Finally, I explored the interaction of niche complementarity and positive interactions, which are often lumped together because of the difficulty in separating their effects in field experiments. I developed a simulation model of the Patagonian steppe where resource partitioning and facilitation could be controlled. In the model, biodiversity increased primary production through resource partitioning, facilitation, and sampling effect (the higher chance to include the most dominant species with diversity). The biodiversity effect increased in magnitude with resource partitioning and facilitation. Maximum facilitation occurred at low levels of resource partitioning and sampling effect was only important with low levels of resource partitioning and high levels of facilitation.