Although we now have extensive knowledge about how biodiversity affects the functioning of ecosystems, we do not know enough about how these effects are manifest through the process of community assembly involving colonizations and extinctions, as well as changes in population sizes. Two current approaches can be used to study these effects. The first (that we label S-C) is one that identifies how 'selection' (in which dominant species contribute more to function than rare ones) and 'complementarity' (in which coexisting species with distinct niches can contribute more than either could by itself) interact. The other uses the Price Equation to separate the roles of colonizations, extinctions and changes in resident populations. Although distinct, the two are also likely to be interrelated. It also isn’t clear how they relate to particular mechanisms of species interactions.
Here we model how mechanistic resource competition models affect the patterns revealed by these two approaches. We examine the effects of different types of resources, ranging from strongly complementary to strongly antagonistic, and of small-scale environmental heterogeneity, comparing cases with no heterogeneity to cases where heterogeneity facilitates coexistence via habitat segregation. We also compare our model results to analyses of data from the biodiversity-ecosystem function experiment conducted at Jena (Germany).
Results/Conclusions
We find that the nature of resources strongly interacts with environmental heterogeneity to alter these effects. If resources are complementary and there is no environmental heterogeneity, patterns in ecosystem function are strongly dominated by selection effects. Otherwise, ecosystem functioning is affected by a mix of the other community assembly effects. We also find that the S-C and Price equation approaches provide complementary insights about how community assembly affects the functioning of ecosystems.
Our analyses of the data from Jena reveal patterns that are similar in many respects to our model results, particularly for cases involving either antagonistic resources or complementary resources with environmental heterogeneity, and are clearly distinct from scenarios involving complementary resources in homogenous ecosystems. Nevertheless, these data also differ in important ways from the results of our models. This implies that other processes besides resource competition also affect community assembly and indirectly alter the functioning of ecosystems. We argue that the joint use of S-C and Price equation approaches can enhance our understanding of alternative scenarios but will require a more sophisticated approach than one simply focused on resource competition.