A consequence of Darwin's 'principle of divergence' is that loss of species can harm the functioning of ecosystems. A study of algal communities in artificial streams suggests that he was right. See Letter p.86
'Could do better!' This was last year's disappointing report on attempts to meet the Convention on Biological Diversity's goal of slowing the rate of global biodiversity loss, in an assessment for the 2010 United Nations International Year of Biodiversity1. The outlook for this century suggests that we're set for a biodiversity crisis that may, within a few centuries, join the ranks of the previous 'Big Five' mass extinctions2. Many ecologists are busy trying to work out what this could mean for the ecosystem services that we benefit from but largely take for granted, and on page 86 of this issue, Cardinale3 presents one of the latest advances in this area. Using laboratory experiments with artificial stream ecosystems, he has shown that complex freshwater habitats require diverse communities of algal species, occupying different environmental niches, in order to be productive and to maintain water quality.
Human activities now fix more nitrogen into the biosphere than all natural processes combined. Eventually, through the run-off of fertilizers from agricultural fields and similar processes, much of this nitrogen and other nutrients will end up in our freshwater and coastal environments. This excessive nutrient loading can have negative impacts both on the health of humans and on the environment, and necessitates costly clean-up mechanisms. It is against this backdrop that Cardinale asks what part algal diversity plays in maintaining water quality by taking up nitrogen.
Cardinale used a high-tech set-up of artificial streams, which varied in the complexity of their physical environments in terms of frequency of disturbance and by having variable or constant stream-flow velocities. This set-up generated experimental ecosystems that had a relatively high or low number of different environmental niches. He inoculated these experimental streams with algal communities of varying diversity, and studied how productive the resulting model ecosystems were and how much nitrogen they captured from the water column.
His results provide some of the strongest support so far for a hypothesis that dates back to Charles Darwin. In his 'principle of divergence'4, Darwin proposed that species evolve into different niches through adaptation to different environmental conditions. He thought that this should lead to the evolution of communities of complementary species and an ecological “division of labour” — by analogy with the economic division of labour noted in pin manufacturing by Adam Smith — that can increase overall resource capture and productivity5,6. As a consequence, biodiversity loss can have negative effects on ecosystem functioning by leaving ecological niches vacant or underused. Support for this idea was provided recently in a study7 of marine microbes that were experimentally evolved into either specialists (which exploit narrow environmental niches) or generalists (which thrive under a broader range of conditions). The relationship between biodiversity and ecosystem functioning was found to be stronger for the communities of specialists tightly adapted to particular niches.
Cardinale's research3 provides further evidence of the detrimental effect of biodiversity loss on ecosystem functioning. In his complex stream environments, only a diverse community of complementary algal species could exploit all the available niches (see Fig. 2 of the paper3). When some species were absent, their ecological niches were left vacant or underused because their absence could not be fully compensated for by competitors adapted to other niches. The presence of vacant or underused niches led in turn to reduced nitrogen uptake and decreased productivity of communities.
Identifying the detailed biological mechanisms responsible for the effects of biodiversity on ecosystem functioning has proved difficult, particularly for complex communities of plants and their bacterial and fungal partners and adversaries. Most studies so far have therefore had to settle for identifying general classes of proximate mechanisms using a variety of statistical approaches8,9, as indeed Cardinale did in his study3 to separate species complementarity and selection effects. But Cardinale's experimental system was able to address biological mechanisms more directly than most previous studies, for two reasons. First, the preferred environmental niches of the algal study species were already quite well defined. Such definition is sorely needed in other investigations of biodiversity and ecosystem functioning, and in community ecology in general. Second, the number of environmental niches could be changed directly by creating stream ecosystems of varying complexity.
The combination of this experimental manipulation of habitat-niche complexity with the more customary manipulation of community diversity allowed Cardinale to provide an unusually direct demonstration of how the two factors interact (Fig. 1). In simple habitats, he observed that a low diversity of species is sufficient to occupy all of the available niches as long as appropriately adapted species are present. But complex habitats require diverse communities of complementary species to use the environment more fully, as proposed by Darwin's principle of divergence. When Cardinale simulated rapid (in evolutionary terms) species loss by using low-diversity communities, this led to vacant or underused niches in the complex stream environments.
Critics of the idea that biodiversity influences ecosystem functioning, and of Cardinale's experiments, may argue that our current and future homogenized environments might function adequately with low biodiversity, as long as the species that are left are adapted to the prevailing conditions. The assumption that we will be left with low-diversity communities of species that function well is risky, but may sometimes be true, at least if we ignore such considerations as the stability of ecosystem functioning over time10 and the need for ecosystems to perform multiple functions11. For example, although Cardinale's simple stream environments could host only low-diversity algal communities, these ecosystems were roughly as productive, and captured as much nitrogen, as more complex stream habitats that supported a wider range of species. In fact, in the simple environments, a monoculture of one species was the most productive of all (compare the top and bottom rows of Fig. 1a and 1b in the paper3). This suggests that low-diversity algal communities may be adequate to support a small number of functions in the homogeneous, managed freshwater systems that currently characterize much of our world, at least over the short term. But Cardinale's study also shows that such communities may not be capable of fully using all the available niches if we progress to a post-Anthropocene world containing more varied and complex environments, similar to the natural systems where these species evolved.
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