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It has long been an axiom of environmental science that a diverse ecological community (aka biodiversity) is more resilient and productive because it stabilizes the ecological system as a whole. This should not be confused with a focus on an individual species for which the complex interactions within a diverse ecosystem can lead to drastic fluctuations in abundance. See, for example, https://www.etap.org/demo/biology_files/lesson6/instruction5tutor.html. Thus, it is difficult, if not impossible, to select which individual species to save, let us say, within an ecological community; what should be preserved is the ecological community as a whole. This concept, ironically, also applies to remediation of contaminants in surface and ground waters.
In lab setups that mimicked the "lazy pools" and rushing freshets of freshwater streams, researchers found that a mix of algal species proved more than four times as efficient at taking up nitrates than a single supercleaner species living in the same kind of stream. The key appears to be that diversity can take advantage of niches. For example, when the researchers added a mix of algae to simplified stream setups lacking a variety of living conditions, or niches, the so-called "diversity bonus" did not appear. What gives diversity its efficiency, then, is not some superspecies that happens to be in the mix; instead, the long-noted diversity bonus in nutrient uptake comes from the way a mix of species can expand and take advantage of the many different niches in an environment. This then raises a question about the attempt by many researchers to create a single superspecies to address a particular contaminant; the best strategy would appear to be to provide a mix of organisms that can each take advantage of different niches to execute their remedial effect.
The notion of diverse communities having impacts by expanding into multiple niches and complementing each other has had great appeal, but empirical confirmation has been difficult. Many studies that are advocated as supporting this concept have had "confounding factors". This most recent study goes a long way to supporting the general principle.
The study can be found at http://www.nature.com/nature/journal/v472/n7341/full/472045a.html.