Published online 22 September 2011 | Nature | doi:10.1038/news.2011.553


Plant biodiversity theory debunked

Light, not productivity, may control species richness

US grasslandWhy so many species?NHPA/Photoshot

Why are some environments teeming with different plant species whereas others support only a few?

Ecologists had thought that it all depended on primary productivity — the amount of biomass produced through photosynthesis. In the Arctic tundra, for example, where productivity is low, there is a smaller range of species, whereas tropical rainforests provide the greater resources needed to sustain a multitude of plant forms.

According to biology text books, the relationship between species richness and productivity is hump-shaped. Biodiversity first rises as resources increase, until the area becomes so fertile that larger, fast-growing species begin to crowd out the smaller slow-growing plants, and biodiversity then declines.

But work published in Science today1 debunks this theory. Peter Adler, a plant ecologist at Utah State University in Logan, together with an international network of collaborators show that, on the small scale at least, there is no consistent relationship between biodiversity and productivity, although the link may still hold on regional or global scales.

"This relationship does not deserve its place as one of these text-book patterns that students are introduced to," Adler told Nature. Disproving the relationship at small scales is important, he says, "because this is the scale at which we test our theories of plant species interactions."

What's more, many plant ecologists had assumed that ecological patterns are built from the bottom up. "They look at what is going on at the plant scale and build that up to a global relationship," says Robert Whittaker, a biogeographer at the University of Oxford, UK. The new work shows that such a relationship may not always hold.

Careful comparisons

The team sampled plant biodiversity at 48 sites containing different herbaceous plant communities across five continents, including the mixed grasslands of the mid-western United States and the semi-arid grasslands of Australia.

Previous meta-analyses2 also questioned the relationship between biodiversity and productivity but were criticized for not using standardized sampling procedures. So Adler and his team used exactly the same sampling methods at every site, observing 1-metre-square plots in blocks of 10, and estimating above-ground production from the live biomass of the plants at peak growing season. "Conclusive evidence didn't exist before the data from our network," he says.

"They have used standardized protocols, and there is no variance in their sampling method, timing and how they calculate biomass," agrees Whittaker. "They show there is no consistent relationship. It's a nice demonstration."

Given the small size of the plots sampled, Whittaker questions whether the study captures the true extent of the local biodiversity. But Adler is sure of their findings. "I am confident that we would have found the same thing if we had increased our plot size," he says.

Now that the sheer quantity of biomass has been struck off the list as a driver of biodiversity, Adler and his team are investigating what other factors might be responsible. The availability of light, water, and nutrients in the soil, might all be key, he suggests. He hopes that a more sophisticated understanding of what drives species richness can help to improve predictions about how environmental change will affect biodiversity. 

  • References

    1. Adler, P. B. et al. Science 333, 1750-1753 (2011). | Article | ChemPort |
    2. Whittaker, R. Ecology 91, 2522-2533 (2010). | Article | PubMed | ISI |
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