Getting over the Hump

For decades, ecologists have toiled to nail down principles explaining why some habitats have many more plant and animal species than others. Much of this debate is focused on the idea that the number of species is determined by the productivity of the habitat. Shouldn't a patch of prairie contain a different number of species than an arid steppe or an alpine tundra?

Exactly how biodiversity relates to productivity, however, has been a highly controversial issue in ecology over the past few decades. Initially, for example, many researchers believed that the relationship between species richness and net primary productivity (often expressed as the number of grams of carbon produced within a square meter of an ecosystem over a year) could be visualized as a 'hump-shaped' curve, with richness first rising (as stress is alleviated) and then declining with increasing productivity (due to increasing effects of competition).

I've just been involved in a global study that tries to shed light on the hump-shaped relationship between species richness and productivity in grasslands across the globe. Our paper in Science has just been published and the results were, to put it mildly, rather surprising.

 

Locations of the 48 sites that provided data for this study



In a multiscale assessment of 48 herbaceous plant communities on five continents (including four Australian sites), we demonstrate that the modal productivity-diversity pattern is quite rare in nature, rather than the dominant relationship. This is really interesting given that so much attention had been devoted to this theory.



Our study shows no clear relationship between productivity and the number of plant species in small study plots when you look at data collected from across a range of grassy vegetations from across the globe. These range in temperature, rainfall and geological and evolutionary history. Indeed, the evidence for the predicted 'humped-back' reponse is rather weak within most of the sites studied regardless of these big differences in history.



Within-site relationships between productivity, measured as peak live biomass (dry weight)

and species richness. The inset shows the frequencies of relationships that were

nonsignificant (NS, thin dashed lines), positive or negative linear (thick dashed lines),

and concave-up (+) or -down (–) (solid curves). The marginal histograms show the frequency

 of species richness and peak live biomass across all sites.



The findings suggest that ecological understanding may advance more rapidly if ecologists focus on exploring a range of topics that are germane to the productivity-diversity relationship in a changing world (e.g. how will loss of species affect productivity, stability and resilience of ecosystems to change), rather than continuing the search for a dominant pattern ("the Holy Grail"). Our work not only sheds light on this classic question, it also demonstrates the power of a network approach. Working in a network with ecologists who share your passion and interest in theoretical ecology is a great way to facilitate insights into the functioning of nature, insights that aren't possible in a focus on individual ecosystems.

Here is a link to the paper:

And an accompanying Perspectives piece

And an NSF press release: