The evolutionary story of ecosystem function

Twenty years of research has repeatedly shown that communities with greater diversity result higher functioning -namely greater production of biomass. One of the major mechanisms producing this relationship is that different species use differing resources, such that their complementary use of resources uses the total resource pool more thoroughly, thus converting more resources into biomass. Resource preference is the product of evolution and how organisms have adapted to using various resources can influence the strength of the diversity-function.

In a recent paper in Nature, Dominique Gravel and colleagues test how the evolution of specialization versus general resource use affect the strength of the diversity-function relationship. They use bacteria strains that have undergone evolution on diverse resources (generalist) versus on a singular resource (specialist). The resources in their case are different carbon substrates.

Assemblages of generalists were able to use many available resources and generally had greater productivity than specialist assemblages. Generalists also show an increasing relationship between diversity and productivity, because no generalist used all resources and they still showed some preferences. Combining multiple such generalists meant that more of the total resource pool was consumed. Specialists also resulted in the positive relationship, but a much steeper one. Because specialist use many fewer carbon substrates, additional specialists meant that new resources were tapped into. Thus increasing specialist diversity resulted in more new resources being consumed than with the generalist species.

While these results are logical, they are important for two reasons. First is that the strength of the relationship between diversity and function is mechanistically determined by the resource use efficiency of individual strains, and how many of the total substrates they can use. The mechanisms producing different relationships in previous experiments were hypothesized after the results analyzed, as opposed to being predicted. Second, recent work has shown that evolutionary history seems to be a better explanation of community function than the number of species. These results show how the history of evolution can have important consequences for function.

Gravel, D., Bell, T., Barbera, C., Bouvier, T., Pommier, T., Venail, P., & Mouquet, N. (2010). Experimental niche evolution alters the strength of the diversity–productivity relationship Nature, 469 (7328), 89-92 DOI: 10.1038/nature09592

Experimental test of Darwin's naturalization hypothesis

Among the numerous and still informative ecological predictions made by Darwin, one posits that when species are introduced into regions where they were not formerly found, the most successful tend to not have close relatives already occupying the region. This is known as Darwin's Naturalization Hypothesis, and his logic was that among close relatives, where ecological requirements should be most similar, the struggle for existence is most severe. Thus the modern formulation is that invader success is influenced by the amount of time since two species shared a common ancestor (usually called phylogenetic distance). Tests of this hypothesis have been primarily done on large species inventories, with results from different studies either supporting or refuting it. In a new study by Lin Jiang and colleagues published in the American Naturalist, they cleverly use bacteria with known relatedness to test this hypothesis.

They used four species of bacteria: Bacillus pumilus, B. cereus, Frigoribacterium sp. and Serratia marcescens as residents in every possible 1, 2, 3 and 4-species communities and invaded them with a subspecies of S. marcescens. What they found was that the invader density was highly significantly related to phylogenetic distance, so that the invader reached its greatest density when communities contained only distantly-related species.

Though these types of laboratory experiments are simplistic (I too use these systems), they offer insights into particular mechanisms, which may otherwise be difficult to detect in noisier systems.

Jiang, L., Tan, J., & Pu, Z. (2010). An Experimental Test of Darwin’s Naturalization Hypothesis The American Naturalist, 175 (4), 415-423 DOI: 10.1086/650720