Biodiversity and Ecosystem Function research: Part 1 setting the scene

In this post I try to define the framework in which my PhD is into, I will define some key terms that we use a lot and present briefly the field experiment I am working in.

What is Biodiversity:

Biodiversity is the variability of living form on earth, in the Convention for Biological Diversity definition three different levels are defined: i) ecosystem levels ie how heterogeneous your landscape, if you have only forest for thousands of miles this is of course less diverse than a patchy habitat with forest, meadows, rocky slopes, lakes …ii) species levels, this is the classical understanding of biodiversity, it is how diverse the species composition in a particular habitat is, of course defining what exactly is meant by diversity of species is tricky, there are whole books talking on these issues (ie Magurran and McGill). iii) within species, this is the variation of genotypes.

An amazing feature of biodiversity for me is that you can have in small area extremely large numbers of species, for example in the amazonian forest, on one tree you may find some thousands arthropod species. In our field site (european meadow) we find more than 500 arthropod species over approximately 1ha.

What is Ecosystem:

Ecosystems are all the living species (animals, plants, micro-organisms, fungi …) plus the non-living part (soil, atmosphere, water) of a certain place. These two elements (called biotic and abiotic by biologists) are interacting together exchanging minerals and matters (ie plants are sucking up nitrates from the soil, animals are breathing out CO2). Ecosystems are dynamic, if you leave a meadow be in 50 years you have a forest and in a few century you find again a meadow (due to fires). And this is this dynamic that is creating an heterogeneous landscape of different type of habitat at various stage in these successions that is maximising biodiversity.

What is Ecosystem Function:

This is another broad terms that is basically divided into two categories: i) size of compartments, this can be plant species richness, biomass of nematodes (tiny worm like creature in the soil), number of carnivores arthropod. ii) or rates between these compartments, like herbivory of the plant shoots by herbivores, decomposition of the organic matter, parasitism rates etc … A few ecosystem functions are beneficial for us humans, we can derive money from them (ie wood biomass, clean water supply) or it would be very hard to live without them (ie oxygen supply by photosynthetic organisms, pollination by insects), these are then defined in a sub-category as ecosystem goods and services. As this is a human-centered concept, it carries some kind of moral values that is questionable in terms of science (if we define science as aiming at revealing objective truth about the world), I aim at talking more about this in further posts.

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How is biodiversity affecting ecosystem function:

For those of you that are not deterred by the scientific lingua I recommend reading Chapin (2000) Nature article that I will try to summarize in simpler words below.

A key element is to realize that it is the species present in the nature that are responsible for moving these tons of elements between the air, the soil and the water, they are also building a very complex web of interactions (positive or negative) between them. Therefore as you start changing the abundance (how many), identity (which species), or richness (how many species) of a certain ecosystem, its associated functioning will be affected. For example if you start removing wild bees, pollination rates will go down, this is direct effects of biotic variation. There can also be indirect effect, like if you introduce a species able to fix nitrogen in an environment that was beforehand nitrogen poor (like in Hawaii) you will see dramatic changes in the plant community and as a result the functioning of this system will change.

Now humans have been altering biotic communities for some time, putting species at some place where they did not usually occur (species invasion), harvesting at unsustainable rates some others driving them towards local extinction*, turning bogs into agricultural fields with only one plant species (one example of land use change), being responsible for climatic changes (temperature increase, precipitation regime shifts …), and messing up with global cycles of elements by releasing tons of CO2 in the atmosphere or leaving nitrogen from fertilizers leach from fields to rivers. The figure below summarize all this pretty nicely

So this is where BEF research comes into play, we try to understand the links between a changing biotic communities and ecosystem functions.

How do we do this?

There are quite a few approach to this, the basic idea in the first generation of BEF experiment is to experimentally manipulate the diversity (mainly species richness) of different units and to measure some (one or more) ecosystem function on these units and to link the changes in the function with the manipulated gradient of diversity. In the Jena Experiment we are manipulating the species and functional richness of grassland plant species. There are 82 plots (a square of 20x20 meters) that were sown in 2002 with different plant mixture to form a gradient of diversity. Since then we measured a whole bunch of function on these plots and relate them to the plant species richness gradient. Some key results from the Jena Experiment include among other things: that plant productivity (ie Biomass) increased along the diversity gradient (Marquard 2009 Ecology), we also have more arthropod both above and below ground with higher diversity in richer plots (Eisenhauer 2011 PlosOne), loosing plant diversity leads to lower potential of carbon storage in the ground (Steinbeiss 2008 Global Change Biology), you may find the full list of publication associated with the experiment online. A recent meta-analysis (piling up data from a bunch of study to look at general trends) can be found here.

Applications of our results:

A question I get asked quite often by family/non-scientist friends is: what is the use of your research? Well we try to understand how ecosystem will change in their functioning if we keep loosing species due to higher and higher human pressure. These changes in ecosystem functions will affect human populations therefore our research try to understand and predict what will happen if we loose say 50% of the species until 2020. As always in ecology one of the conclusion we reached most often is: it depends, which is not so easy to translate into conservation incentives.

So that’s it for today, in the next post on this series I will talk about what I exactly do, the new generation of theory/experiment in BEF research that started around 2010 with increasing focus on the effect of other dimensions of diversity (ie functional, phylogenetic) on ecosystem functions.

Bibliography:

Two easy to read papers on the impact of diversity loss for ecosystem functions:

Chapin III, F. Stuart, et al. “Consequences of changing biodiversity.” Nature 405.6783 (2000): 234-242.

Naeem, Shahid, J. Emmett Duffy, and Erika Zavaleta. “The functions of biological diversity in an age of extinction.” Science 336.6087 (2012): 1401-1406.

The two papers that started a boom in BEF research:

Naeem, Shahid, et al. “Declining biodiversity can alter the performance of ecosystems.” Nature 368 (1994): 21.

Tilman and Downing. “Biodiversity and stability in grasslands”. Nature 367 (1994) 363-365.

***** If we are interested in the effect of one species on a particular ecosystem function local extinction is the scale of interest. If the only nitrogen-fixing species from a certain system disappear it will have big impacts on the functioning of this system. The fact that populations from this species still live some 1000km away is irrelevant in this context.