Species vs. Functional Groups: Which Kind of Diversity Matters Most?

While conservation biologists agree that plant diversity is crucial to ecosystem function, the debate rages over which aspect of diversity is most important. Most biologists focus on species diversity, but two recent papers suggest that diversity of functional groups (such as nitrogen-fixing legumes and soil-stabilizing bunchgrasses) is also critical. The first paper argues that both types of diversity are roughly equally important, and the second paper argues that functional group diversity is more important to ecosystem function.

The first paper is in the October 26, 2001 issue of Science and is by David Tilman, Troy Mielke, and Clarence Lehman of the University of Minnesota in St. Paul, and Johannes Knops and David Wedin of the University of Nebraska in Lincoln.

“Plant species number and functional group composition became simultaneously and approximately equally important in our long-term experiment,” say Tilman and his colleagues.

The researchers seeded 168 9X9 meter experimental plots with 1-16 grassland perennials from five functional groups (warm- and cool-season grasses, forbs, legumes, and woody species). To determine the relative importance of species and functional group diversity to plant productivity and carbon storage, the researchers measured both aboveground and total biomass annually for 7 years.

Tilman and his colleagues found that while the number of functional groups had the greatest impact on biomass initially, by the end of the study, the number of species also had contributed significantly to biomass. The researchers also found that total biomass was influenced by several individual species (such as the legume Lupinus perennis) and functional groups (legumes and warm-season grasses).

“Our results show that ecosystem processes are simultaneously influenced by diversity and composition,” say Tilman and his colleagues. However, they caution that long-term work in other ecosystems is needed to resolve the debate over applying biodiversity measures to ecosystem management. “Both species diversity and functional group composition must be considered to be critical elements controlling ecosystem functioning,” says Tilman.

The second paper on how species and functional group diversities affect ecosystem processes is in the November 2001 issue of Trends in Ecology and Evolution and is by Sandra Diaz and Marcelo Cabido of the Universidad Nacional de Cordoba in Argentina.

“There is a growing consensus that functional diversity, rather than species numbers per se, strongly determines ecosystem functioning,” say Diaz and Cabido.

Most studies of how plant diversity affects ecosystem function are limited because they consider man-made experimental plantings rather than natural plant communities and only address species diversity, say Diaz and Cabido. However, the few studies that also address functional diversity suggest that this is more important to ecosystem function.

“Most often, species are not equally important in their contributions to ecosystem processes, and a few key species can account for a large fraction of ecosystem functioning,” say the authors. For instance, trees with complex branches and extensive roots affect ecosystem processes from climate buffering and animal diversity to soil and water retention. Likewise, the invasion of tall tussock grasses can increase the frequency and intensity of fires in places such as Hawaii, northern Mexico, and Mediterranean Europe.

Furthermore, communities dominated by fast-growing plants (such as meadows on fertile soils) tend to have high resilience (that is, they change and recover easily) and low resistance (that is, they are slow to change and recover). The opposite is true of communities dominated by slow-growing plants (such as arid shrublands).

Diaz and Cabido stress that while conserving species diversity is essential, simply knowing and monitoring numbers of species will probably not be enough to protect key ecosystem processes. “Species diversity is often an inadequate surrogate,” they say, calling for standardizing and implementing ways to assess functional group diversity.

Understanding the links between diversity and ecosystem function is critical to conservation, and Diaz and Cabido advocate assessing both species and functional group diversity. “A novel approach, resulting from cross-fertilization between the species-based and functional type-based approaches, has the potential to contribute to practical management for the conservation of diversity and ecosystem services,” they say.

For more Information
Tilman, D. et al. 2001. Diversity and productivity in a long-term grassland experiment. Science 294:843-845.

Diaz, S., and M. Cabido. 2001. Vive la difference: Plant functional diversity matters to ecosystem processes. Trends in Ecology and Evolution 16:646-655.

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