Species diversity and ecosystem functioning...putting the relationships into context

The world is currently experiencing the sixth mass extinction in the history of life on Earth. Human activities are rapidly accelerating rates of species extinction, and this has prompted much concern over the potential environmental consequences of species loss. Recently, scientists have hypothesized that species extinctions may diminish the productivity of ecosystems on which humans rely. In this dissertation, I present evidence that this hypothesis is true, but only under certain sets of environmental conditions. I begin with a conceptual model comparing the relationship between species richness and ecosystem productivity in ecosystems that are spatially heterogeneous and periodically disturbed by events that impose mortality to communities. The model predicts that variation over space and time will generally decrease the effect of species diversity on ecosystem productivity because it minimizes competitive interactions that allow production to be dominated by a single, highly competitive taxon (called ‘selection effects’). I tested this prediction in a simple experimental system and found that while environmental variation does indeed reduce the likelihood of ‘selection effects’, it also induces facilitative interactions between species that cause a diverse community to capture more resources and be more productive than less diverse communities. I end with results from a survey of the relationship between species diversity and primary production in natural ecosystems characterized either by a high or low degree of environmental variation. That study found that ecosystem productivity is strongly related to species diversity in variable environments, in part, because facilitative interactions are stronger and more frequent. The conclusion from my research is that the effect of species diversity on ecosystem productivity changes as environmental variation alters the balance of positive and negative interactions between species. This suggests that the effects of species loss on productivity will not be equal for all types of ecosystems. The challenge, therefore, is to place the diversity-productivity relationship into a broader spatial and temporal context that begins to outline conditions under which diversity does, and does not ‘matter’ for ecosystem functioning. Only in doing so will we move towards the ability to predict the ecosystem-level consequences of biodiversity loss.