The contribution of species richness and composition to bacterial services

On the beech Despite their importance, we are only beginning to understand how mixed communities of bacteria operate. There is a good reason for this: the microbial world is remarkably complex and dynamic so it is difficult to design experiments that ask the right questions. Laboratory microcosms are useful but involve small numbers of species in unreal situations. A natural ecosystem that can be manipulated experimentally is available, however. Rainpools that form in bark-lined depressions at the base of European beech trees are communities of up to 72 species, rather than the thousands found in, say, pond water. In this rainpool ecosystem the number of bacterial species (the biodiversity) strongly influences the rate at which the community provides a particular service (in this case, respiration). On this scale at least, species richness determines the level at which an ecosystem can function. Bacterial communities provide important services. They break down pollutants, municipal waste and ingested food, and they are the primary means by which organic matter is recycled to plants and other autotrophs. However, the processes that determine the rate at which these services are supplied are only starting to be identified. Biodiversity influences the way in which ecosystems function 1 , but the form of the relationship between bacterial biodiversity and functioning remains poorly understood. Here we describe a manipulative experiment that measured how biodiversity affects the functioning of communities containing up to 72 bacterial species constructed from a collection of naturally occurring culturable bacteria. The experimental design allowed us to manipulate large numbers of bacterial species selected at random from those that were culturable. We demonstrate that there is a decelerating relationship between community respiration and increasing bacterial diversity. We also show that both synergistic interactions among bacterial species and the composition of the bacterial community are important in determining the level of ecosystem functioning.