Thursday mini-review: Papers on diversity-productivity relationships

So, over the last couple years (decades!), it seems like ecologists have had a small fire ignited beneath their tails to keep on explaining the plant productivity-diversity relationship as it exists in terrestrial ecosystems.  This is a fine endeavor, one that pulls the mind back to the initial years of LTER data and glasshouse experiments with different functional groups.  This fall has been a big time in the debate with semi-competing papers coming out in both Science and Nature.


First off, last month had Forest Isbell and others publish in Nature on September 8th, arguing that high diversity is required to maintain our planet's critical life support systems.  The authors meta-analyzed from 17 different experiments, cataloging 147 grassland species. They concluded that 84% of the species at hand enhanced the provisioning of ecosystem services at least once. This isn't the usual "diversity drives productivity" argument that scientists have been trying to make for year, but a claim that most species contribute to some set of ecosystem functions at least some of the time. Think of a hippie summer camp counselor: "Everyone is good and special in their own way, man..." The authors conclude their paper advocating for use of the precautionary principle in removing species from systems since in some contexts each species may provide a disproportionate benefit to a system. My take away for restoration is that some species matter more than other depending on context, so pick wisely and hedge your bets when looking at novel conditions or extreme gradients. Personally, I take away the same points that Dunwiddie et al (2009) made about restoration and global change.

A few short weeks later, Peter Adler dropped a major product from his NUTNET project, declaring that biodiversity and productivity had no relationship.  Adler took uniform data from across grassland systems, circumnavigating meta-analytical frameworks, and concluded that there is not really a relationship between diversity and productivity in grasslands. Analyses were carried out across a range of human-dominated and managed systems on 5 continents, and still no trends were found between productivity and richness at local, regional or global scales.

Interestingly, Adler et al. exonerate the meta-analytical frameworks that received much criticism from Whittaker (2010). Whittaker chalked up at least some portion of the blame for inconsistent results across studies due to differing sampling scales and schemes. While these effects undoubtedly exist, Adler et al (2011) illustrate that grassland systems do not exhibit the productivity-richness relationships shown elsewhere in the literature, and that methods aren't the driver behind this lack of a relationship. For restoration, I conclude that interspecific diversity may have an effect on productivity, but that other factors should be confronted first e.g. site availability and performance and individual species performance (potentially including intraspecific performance gradients).

This debate is far from over when you consider time, space, diverse management strategies and histories, and novel environmental conditions, but at least last month was a good time to watch the discussion move forward. 

References:



Dunwiddie, P. et al. 2009. Rethinking conservation practice in light of climate change. Ecological Restoration 27: 320-329.

Isbell, F. et al. 2011. High plant diversity is needed to maintain ecosystem services. Nature 477: 199–202.


Whittaker, R. J. 2010. Meta-analyses and mega-mistakes: calling time on meta-analysis of the species