Functional diversity: the number of functional roles represented in an ecosystem.
The statistical results show that functional diversity and composition significantly affected the six response variables to a greater extent than species diversity. By themselves, all three factors significantly affected ecosystem processes and also influenced each other. The mechanisms and degree by which they influenced each other are unclear. It is important to note that the Tilman article doesn’t purport to have the definite answer. Uncertainty is implied in the major conclusions of the paper: “ . . . the number of functionally different roles represented in an ecosystem may be a stronger determinant of ecosystem processes than the total number of species, per se. However, species diversity and functional diversity are correlated. . .” This study implies that to progress, scientists on both sides of the diversity-function debate must develop a holistic model that acknowledges the inextricable relationship between diversity and function.
In the fourth installment of the Bulletin of the Ecological Society of America’s “Issues in Ecology” series, the Tilman et al. study was used to support the argument for a positive correlation between diversity and productivity. Following its release, the “Issues” authors were accused of being deliberately misleading, presenting controversial findings as fact. Wardle charged that the diversity experiments reported in Tilman et al., despite being stated in “Issues” as solid fact, were confounded by an experimental design that assumed “that biological communities are randomly assembled with regard to the ecosystem property being investigated.” This phenomenon is termed the “selection probability effect.” The “Issues” article, however, does not present this opinion as fact. It discusses the “sampling effect” as a possible mechanism, not as an irrefutable mechanism. Furthermore, its conclusions are marked by uncertainty: although species diversity and functional diversity are correlated, functional diversity may play a larger role in ecosystem processes than the total number of species.
Despite the uncertainties in our understanding of ecosystem processes, our reliance on biological systems is such that we cannot afford to wait for definitive answers. The importance of ecosystem processes and services necessitates the precautionary preservation of biodiversity. Some of the vital services provided for by ecosystems include the components used in fabricating food, clothing, medicine, and energy production. Recreation and passive ecosystem services are significant as well. These include fishing, hunting, hiking, birding, camping, water filtration/purification, climate moderation, flood mitigation, erosion prevention, and pest management.
As reported in "Expert Estimates About Effects of Biodiversity on Ecosystem Processes and Services," ecosystem process rates correlate strongly with biodiversity, and these processes are crucial for ecosystem services. While the scientific community is not providing us with an exact prescription for conservation, it is indicating an overwhelming need for biodiversity protection. In light of this, the “Issues” report is on target, but possibly not bold enough. The “Issues” report may underestimate the role of biodiversity in ecosystems. It is hoped that as our understanding of ecosystem processes increases, we will be able to develop more specific and effective biodiversity conservation measures. Until that time, scientific uncertainty must not subvert caution and restraint. It is better to err on the side of protecting ecosystems than to proceed recklessly under the guise of “progress.”