Biodiversity can promote ecosystem efficiency

In a theme issue on "Human influences on evolution, and the ecological and societal consequences," published by the Royal Society (UK), two review articles are devoted to fish: the first discusses adaptive capacities in fish exposed to pollution, while the second -- an Eawag contribution -- examines the effects of lake eutrophication on fish biodiversity. The authors show that the increase in primary production caused by eutrophication can lead to changes throughout the food web. Changes in productivity alter the physico-chemical environment, which has further effects -- e.g. via selection processes -- on lake fauna and flora. Such changes can also affect habitat availability, thus eroding differences in habits and behaviour which had previously contributed to the separation and genetic differentiation of species. Eutrophication thus commonly results in reduced ecological specialization and genetic and phenotypic homogenization of species, both among lakes and among niches within lakes.

Essentially, these findings reflect those of an earlier Eawag study of whitefish (published by Vonlanthen et. al. in Nature in 2012), as well as studies of other fish in other lakes. Here, however, the phenomenon of "eco-evolutionary feedback" has been further investigated. Taking the example of whitefish, the authors not only studied the effects of eutrophication on biodiversity but also, for the first time, analysed the relationship between current fishery yields, nutrient availability and functional diversity. The latter was measured in terms of the range of a key functional trait -- the number of gill rakers: sparsely rakered fish are better adapted for sediment feeding but cannot filter plankton effectively, while for densely rakered fish the converse is true. Fishery yields relative to lake productivity were shown to be higher in lakes where whitefish diversity is higher. In Lakes Thun or Lucerne, for example, which were not subject to heavy eutrophication and which still harbour relatively diverse communities, the whitefish yield per unit phosphorus is higher than in, say, Lakes Zug or Geneva. According to the researchers, this indicates more efficient utilization of the trophic resources available in the lakes.