A trip along the diversity effects highway: a new and exciting fork in the road

The dynamic dune ecosystem along Lake Michigan was where I first came to appreciate nature as a young child (granted, I was in the water most of the time). Blissfully unaware of the pioneering work of Cowles (1899), it was my time wandering forward and backward through successional time as I matured into a young man that put me on the road to studying ecology. So, when I came across an awesome study done along the shores of Lake Michigan (Crawford and Rudgers 2013, Ecology), I had to check it out. Ecology has many winding, intersecting roads and this paper also intersects with the road I’m currently on, studying how species and genetic diversity affect ecosystems. So,  a little background . . .

Let’s start with species. The effects of species diversity on community structure and ecosystem function are well studied. Ecology set out down this road at least in the 1950s, but it was first paved with the work of Tilman and others in the 1980s and 1990s. For example, greater primary producer biodiversity is often correlated with higher levels of functions like invasion resistance and primary productivity, and greater abundance and diversity in higher trophic levels. The evidence from experimental communities is especially convincing. (Diversity effects in natural systems are a bit rougher road to travel.)

But the road gets improved. Before this simple version of ‘diversity begets diversity and function’ could become ossified in the ecological canon, however, researchers began asking whether species diversity was too blunt of an instrument and was only shorthand for something else. Studies comparing functional group diversity, trait diversity, phylogenetic diversity, and species diversity (e.g., Cadotte et al. 2009) paint a more precise picture of the phenotypes responsible for the supporting and stabilizing effects of diversity. Likewise, similar results have been reported for experiments that manipulate genetic diversity in the constituent species. According to recent studies, genetic diversity in primary producers may promote species coexistence and therefore maintain diversity, and may also support more diverse communities in higher trophic levels. At this point, the diversity effects highway is a six-lane interstate, and the traffic is dense.

So, okay, diversity is important. And, apparently it is fairly easy to find support for the importance of diversity at all kinds of levels. In natural communities, however, all levels of diversity – phylogenetic, functional, species, genetic – can vary independently. What are the consequences of this? Does one level of diversity impact how another level affects community structure and function?

Ammophila breviligulata with sunsetCrawford and Rudgers (2013, Ecology) recently published the results of an experiment that posed this question in a Lake Michigan sand dune community. They investigated the effect of plant species diversity, genetic diversity, and their interaction on the diversity and structure of associated arthropod communities. To do this, they constructed independent plots with three levels of plant species diversity (1, 3, and 6), and plots with three levels of genetic diversity (1, 3, and 6) in the dominant species, the grass Ammophila breviligulata. Of course, these plots travel the well-worn highway of recent decades. In order to break new ground, to understand the context-dependence of different diversity effects, they constructed plots where species and genetic diversity are crossed.

What did they find?

1. In independent plots, genetic diversity and species diversity did not affect arthropod communities, with one difficult to interpret exception – plots with 3 genotypes supported lower arthropod richness than 1- and 6-genotype plots.

2. In crossed plots, there was no statistically significant interaction between genetic and species diversity.

3. In crossed plots, however, genetic diversity was still important. Here’s how:

“Interestingly, although there was no interaction among levels of species diversity and genetic diversity, the influence of genetic diversity was strongly context dependent. In the presence of other species, genetic diversity in A. breviligulata significantly affected arthropod abundance and overall community structure. However, in the absence of other species, genetic diversity in A. breviligulata had no effect on arthropod abundance or community structure.” (pg. 1032)

Why is it cool?

1. The design is simple and repeatable (if a bit laborious – but this is science, folks).

2. The research was done on the shores of Lake Michigan.

3. Most importantly, is an important ‘next step’ on the diversity effects highway that not only incorporates insights from across ecology and evolution, it also captures more natural variation. It is more real.

Crawford and Rudgers 2013. Genetic diversity within a dominant plant outweighs plant species diversity in structuring an arthropod community. Ecology, 94(5) pp. 1025-1035.

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