Does biodiversity reflect ecosystem function?

Posted on June 4, 2011

Do ecosystems where biodiversity is high also score well in terms of ecological function and productivity?  It’s been argued that, in principle, the larger the number of complementary species or functional feeding groups in an ecosystem, the more efficient and productive the use of resources should be.  Wetlands and shallow lakes tend to occupy alternative states dominated by emergent or submerged plant species.  Do wetlands in these alternative states differ significantly in terms of  biodiversity, and if so, does this affect their ability to carry out ecological functions such as nitrogen extraction?  Because wetlands are widely used to reduce levels of potentially polluting nitrogen in receiving waters, the answers to these questions have a bearing on the way that wetlands should be managed.  Swedish researchers monitored biodiversity and nitrogen retention in 18 wetland basins, each 40 m2 in size, over a four-year period.  In six of the basins emergent plants such as Phragmites and Glyceria were introduced, while submerged forms such as Elodea and Myriophyllum were established in another six basins.  The remaining six basins were unplanted and left to develop freely.  Nitrogen retention was estimated by subtracting the total nitrogen in the wetland outlet from the amount in the inlet water.  In all, 62 aquatic plant species were recorded.  During the first year after establishment plant diversity was significantly higher in the “emergent” basins than in the other two treatments, but over the next two years the pattern was reversed, with plant diversity being lowest in the emergent basins.  However, this trend of decline in the emergent basins wasn’t echoed by  macroinvertebrate diversity, and levels of nitrogen retention were consistently highest in the emergent wetlands over the whole period.  In other words, plant biodiversity wasn’t a good predictor of nitrogen retention.  The fall in plant diversity in the emergent wetlands was probably the result of competitive exclusion by dense, tall plants that progressively shaded out other species.  The dominant emergent species created good conditions for nitrogen removal by supplying denitrifying bacteria with food and attachment surfaces in the form of litter, by shading the water and thus limiting photosynthesis and oxygen levels, and by taking up nitrogen themselves.  Although wetlands are often managed by encouraging conditions that promote high species richness, it seems that this approach can actually compromise the aim of nitrogen removal.

Reference:  Weisner, S.E.B. & Thiere, G.    2010.  Effects of vegetation state on biodiversity and nitrogen retention in created wetlands: a test of the biodiversity–ecosystem functioning hypothesis.   Freshwater Biology 55, 387–396.