Our research focuses on the population dynamics of plants and how they are influenced by impacts of natural disturbances and global environmental change. We are particularly interested in the interactive effects of fire, grazing and drought in grasslands and woodlands in southern Australia, and how climate change, fragmentation and shrub encroachment affect ecosystems.

Saturday, 24 March 2012

The productivity-richness debate continues.....

A few months ago, Science published a paper by Peter Adler et al. that I helped co-author, examining evidence for a relationship between species richness and productivity. Grime (1973 Nature 242:344) was one of the first to note that a unimodal relationships between local community diversity and local resource productivity existed in grasslands in the UK; such unimodal relationships have attracted attention ever since and, it's probably safe to say, the unimodal diversity-productivity relationship is “widely accepted” in ecology. We challenged this notion by conducting standardised sampling in a replicated, global study. For herbaceous communities (mostly grasslands), in 48 study sites across five continents, we found no clear relationship between productivity and fine-scale richness within sites, within regions, or across the globe, and concluded that "rather than investing continued effort in attempting to identify a general productivity-richness relationship, ecologists should focus on ...investigating the complex, multivariate processes that regulate both productivity and richness".

Now, Science has published two technical comments on the Adler et al. paper, as well as a reply led by Jim Grace that I was a co-author on. These two papers criticise our work, in the context (primarily) of our analyses and assumptions. It's good to see that we've generated some debate about this topic, but I tend to think our study holds up despite these critiques.

Pan et al. claim that, properly analyzed, the Adler et al. data show a strongly linear diversity-productivity relationship, while Fridley et al. claim both that the data are “clearly deficient” as a test for a humped diversity-productivity relationship, and that they show a “clear” humped diversity-productivity relationship.
The links to the papers are: Pan et al.: http://www.sciencemag.org/content/335/6075/1441.1.short  and Fridley, Grime, Huston et al.: http://www.sciencemag.org/content/335/6075/1441.2.short

We reply in the paper by Grace et al. reply: http://www.sciencemag.org/content/335/6075/1441.3.short.

In short, we believe that debate over the productivity-richness relationship has been strongly influenced by the way that scientific motives influence how theories are evaluated. Attachment to particular explanations can result in a tendency to overlook inadequacies and contradictions and lead to a reliance on “theory demonstrations,” which selectively sift through data to find supporting evidence. Both Pan et al. and Fridely et al. seem to have taken this approach in their commentary of our paper. By contrast, “theory investigations” have a different motivation – to evaluate the explanatory adequacy and limitations of theories so as to improve them. This is what our original paper set out to do. Theory investigations are challenged to be either exhaustive in their examination of evidence or to rely on unfiltered samples that represent the variation nature has to offer. Given the scale of our study, and the complete absence of a unimodal relationship, we think we are on fairly solid ground.

I thought I'd leave an "unbiased" assessment of the current debate from one of my favourite ecology bloggers, Jeremy Fox (who writes the Oikos Blog - a really dynamic and informative blog about all things ecological - he's definately worth bookmarking). His commentary is a stark reminder that science should always question dogma or, as he puts it, zombie ideas:    http://oikosjournal.wordpress.com/2012/03/22/trying-to-save-a-zombie-idea/

No comments: