Abstract
Previous studies have shown variable relationships between dispersal rate and ecosystem functioning, but the reasons for and mechanisms behind variable dispersal rate – functioning patterns are currently unknown. In this study we used six bacterial lake water communities in a laboratory experiment in order to investigate how dispersal among communities influences community productivity by evaluating three different mechanisms: 1) changes in taxonomic diversity, 2) changes in phylogenetic diversity or 3) changes in the composition of functional traits. The experiment was conducted in two phases; (A) a dialysis bag experiment where the dispersal rate among six communities was manipulated and the subsequent change in bacterial diversity and growth rate was recorded, and (B) a regrowth experiment where we manipulated available resources to study how well a taxon grows on certain organic carbon resources, i.e. their functional traits. From experiment (B) we could thus estimate changes in functional traits in communities in experiment (A). Bacterial production was affected by dispersal, but not consistently among lakes. Neither change in taxonomic or phylogenetic diversity with dispersal could explain the observed dispersal – productivity relationships. Instead, changes in trait composition with dispersal, especially the communities’ ability to use p-coumaric acid, an aromatic compound, could explain the observed dispersal – productivity relationships. Changes in this trait caused by dispersal seemed especially important for bacterial productivity in waters with a high aromaticity of the organic matter pool. We conclude that the effect of dispersal on bacterial communities can affect ecosystem functioning in different ways, through changes in functional key-traits which are important for the local environment.
Highlights
For many types of communities there is a relationship between diversity and ecosystem functioning, i.e. a change in biodiversity may alter local ecosystem functioning [1,2]
A hump shaped relationship between dispersal rate and functioning has been explained by the complementarity effect, i.e. that an initial increase in dispersal adds taxa and thereby functions that contribute to community productivity, while at the highest dispersal rates, richness decreases due to regional homogenization and the consequent loss of functions causes a decrease in productivity [4,5]
There are, two assumptions to be fulfilled for the hump-shaped relationship between dispersal rate and functioning: 1) community richness is highest at intermediate dispersal rate, according to a theoretical model [3] and 2) function is positively monotonically related to richness
Summary
For many types of communities there is a relationship between diversity and ecosystem functioning, i.e. a change in biodiversity may alter local ecosystem functioning [1,2]. One concern is that habitat fragmentation affecting dispersal rates among communities may impact community functioning since dispersal among communities may change local community composition and diversity, e.g., [3]. There are, two assumptions to be fulfilled for the hump-shaped relationship between dispersal rate and functioning: 1) community richness is highest at intermediate dispersal rate, according to a theoretical model [3] and 2) function is positively monotonically related to richness. There are several scenarios for non-monotonic relationships between richness and function which would lead to differing effects of dispersal. Different functional traits may be added or diluted as a consequence of dispersal; there is no obvious relationship between traits of importance for community functioning and dispersal, which could result in basically any relationship between dispersal rate and functioning
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