Different is Better

I found a nice complement to my More is Better post from January where I reported the results of a new meta-analysis demonstrating how higher species evenness and diversity engendered greater forest productivity – great empirical evidence for the so-called diversity-productivity relationship.

The latest paper adding convincing evidence regarding the important role of species diversity in maintaining ecosystem function comes from Marc Cadotte and colleagues published online early in Ecology. The paper, Phylogenetic diversity promotes ecosystem stability, looks at the problem from a slightly different angle.

If you recall from Zhang and colleagues, forest plots composed of many different species were more productive than single-species stands, and more ‘even’ (i.e., a metric which includes relative abundance of each species in system) stands were more productive, and better at explaining the variance in productivity than species richness alone.

Of course, species richness is considered only a blunt instrument to measure ‘biodiversity’, with evenness providing only a slight improvement. Ideally, we should be talking about genetic diversity considering this is the fundamental unit on which most of evolutionary processes operate (i.e., genes and gene complexes).

So Cadotte and colleagues measured genetic diversity within experimental plots of grassland savanna species established in Minnesota, USA (i.e., consisting of C3 grasses, C4 grasses, legumes, non-legume herbaceous forbs and two woody species) and compared this to ecosystem ‘stability’ (i.e., above-ground biomass divided by inter-annual standard deviation). They measured genetic diversity using four different metrics:

1. the sum of the phylogenetic branch lengths represented by a set of co-occurring species
2. the mean nearest taxon distance = the average of the shortest phylogenetic distance for each species to its closest relative
3. the mean pairwise distance = the average of all phylogenetic distances connecting species in the sample; and
4. an entropic measure based on the relative distribution of evolutionary distinctiveness, measured as the amount of a species’ evolutionary history that is not shared with other species

And indeed, they found a positive correlation between these phylogenetic distance measures and biomass stability.

While this isn’t necessarily ground-breaking work, it is a wonderful empirical demonstration that we need to maintain genetic diversity in our ecosystems if they are to persist through the worst ravages of human modification of the biosphere.

CJA Bradshaw

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