The “gold standard” experimental design for asking how do changes in biodiversity change ecosystem function has been to randomly assemble communities of varying species richness, but equal abundance, and examining differences in function from one level of richness to the next.
But let’s be honest. Changes in diversity due to impacts by man will not be random. In many ways, the early (and much maligned) designs of Tilman and colleagues are really more relevant to realistic biodiversity change scenarios than random assembly experiments. Sure, the mechanism behind diversity’s effect may not be as clear, but the end results sure are.
When it comes to changes wrought due to runaway trophic cascades (e.g., overfishing -> release of small predators -> removal of their grazer prey), the recent article by Altieri et al. provides a pretty compelling* example of some of the counterintuitive changes from real-world changes in ecosystem function.
The authors look at the consequences of increasing densities of grazers in intertidal tidepools for 6 months. They find that, while increasing grazer density decreases the biomass of algae (natch!), that total productivity (here defined as mg of Oxygen produced per hour per square meter) stays constant. So, the amount of oxygen produced per hour per gram of algal biomass has actually gone up. Wild, eh? What is more striking is that 1) this appears to be due to pools that have a more even even distribution of species having higher biomass specific rates of productivity (see Figure 2 from the paper below) and that increasing the abundance of grazers actually increases species evenness. It does not, however, effect the number of species per pool.
The authors argue that, by changing evenness in a non-random manner, we can see that realistic changes in biodiversity can indeed alter how ecosystems function. More intriguingly, changes in function may bee counterintuitive. Heck, if I looked at a pool full of lush green algae versus one which, while it had a pretty even mix of species, was grazed down to the dregs, I’d say that the lush pool was more productive. But I’d be wrong.
I do admit, I am curious if this is all just an Ulva (the dominant seaweed) story. But, then again, any competitive dominant is going to alter the function of all other species in the surrounding community. By ensuring that no one species can dominate, grazers boost total function. And what is more, those changes really depend here not so much on the number of species, but how evenly distributed they are across a landscape.
I remain curious as to how work like this can better tie into ecological forecasting. We can often understand some of the basic implications of an human impacts – changes in abundance of key players or the response of sensitive species. I wonder how this work on non-random changes to community properties such as evenness and diversity will enhance our ability to predict and understand the more profound, yet also less intuitive, changes to ecosystems.
* – Full disclosure, I have worked with most of the authors at one time or another, and they are a motley, loveable, and brilliant bunch of folk.
Altieri, A., Trussell, G., Ewanchuk, P., Bernatchez, G., & Bracken, M. (2009). Consumers Control Diversity and Functioning of a Natural Marine Ecosystem PLoS ONE, 4 (4) DOI: 10.1371/journal.pone.0005291