Ecosystem Function and Biodiversity

Accelerating rates of species extinction have prompted researchers to formally investigate the role ofbiodiversity in providing, maintaining, and even promoting 'ecosystem function'. Typically, studies experimentally modify species diversity and examine how this influences the fluxes of energy and matter that are fundamental to all ecological processes. In many cases, studies are designed to document the effects of species richness on the efficiency by which communities produce biomass, although the effects of species diversity on other ecosystem functions such as decomposition rates, nutrient retention, and CO2 uptake rates have also been examined. Several seminal studies report a positive relationship between biodiversity and ecosystem function. Yet, the generality of the results, and the mechanisms driving them, have provoked considerable debate and several counterexamples exist.

At the crux of the debate lies a question with deep historical roots: do some species exert stronger control over ecosystem processes than others? Imagine two distinct positive relationships between biodiversity and ecosystem function (Figure 9). In type A communities, every single species contributes to the ecosystem function measured, even the rare species. By contrast, in type B communities, almost all of the ecosystem function measured can be provided by relatively few species, suggesting that many species are in fact redundant. Few empirical studies support type A relationships, rather, empirical evidence points to the prevalence of type B relationships. In fact, a recent meta-analysis of 111 such studies conducted in multiple ecosystems on numerous trophic groups found that the average effect ofdecreasing species richness is to decrease the biomass of

Biodiversity

Figure 9 Type A communities: every single species contributes equally to ecosystem functioning. Type B communities: ecosystem function is provided by only a few species.

Biodiversity

Figure 9 Type A communities: every single species contributes equally to ecosystem functioning. Type B communities: ecosystem function is provided by only a few species.

the focal trophic group, leading to less complete depletion of resources used by that group. Further, the most species-rich polycultures performed no differently than the single most productive species used in the experiment. Consequently, these average effects of species diversity on ecosystem production are best explained by the loss of the most productive species from a diverse community. These results could be considered consistent with what has become known as the 'sampling effect'.

Critics argue that a positive relationship between species diversity and ecosystem function is a sampling artifact rather than a result of experimentally manipulated biodiversity per se. Such a 'sampling effect' can arise because communities comprising more species have a greater chance of being dominated by the most productive taxa. Yet, controversy surrounding the 'sampling effect' itself exists given the duality in its possible interpretation: is this a real biological mechanism that operates in nature or is it an experimental artifact of using random draws of species to assemble experimental communities? To add to the ecosystem function-biodiversity debate is the critical issue that many of these studies focus on a single trophic level and neglect or dismiss multiple trophic-level interactions, such as herbivory and other disturbances well known to alter ecosystem processes, calling into question the generality of these results.

Despite the controversy, these studies generally reinforce the notion that certain species exert much stronger control over ecological processes than others. However, identifying which species these are in advance of extinction remains a challenge. Nonetheless, identifying the mechanisms driving ecosystem functioning is an important conservation priority given that human well-being relies on a multitude of these functions.

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