Determinants of community structure: community-level consequences of species interactions and loss of biodiversity

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Since 1990 we have asked two primary questions using the understorey of the Boreal Forest and adjacent grasslands: (i) how are plant communities structured, and (ii) how do they function?  These questions have been addressed using well-replicated experimental manipulations of the natural ecosystem.    The first question is addressed by two major studies. First we will determine if the plant community is structured primarily by nutrient limitation, by herbivory, or by their interaction. Our experiments manipulate soil nutrient levels and herbivore levels, and we then monitor the consequences on individual plant species, populations, and communities (e.g. diversity, biomass).  Second, we are testing hypotheses about the consequences of species interactions at an individual-level and community-level. Our studies are designed to determine if competition has significant impacts on community structure and if the magnitude of these impacts change in predictable ways along productivity gradients? In addition we will determine if individual-level patterns "scale up" to community-level patterns, and if the "accuracy of the translation" depends on productivity.    The second question asks how biodiversity determines the functioning of ecosystems, and is again addressed using two major studies.  First, we are using a removal experiment called "a functional group knock-out" in which selected components of a plant community are removed.  By observing the consequent changes in community dynamics and ecosystem function we can determine the role of the removed component in ecosystem function (e.g. above-ground biomass, above and below-ground decomposition rates nutrient supply rates, light interception and soil water content). Second, by simulating species loss from natural systems, in conjunction with seed and seedling additions, we can determine the effect of plant species identity on invasibility of a community, and the effect of the level of the disturbance of the community on subsequent invasion.