So, we know that species are going extinct at a pretty stunning rate. Mostly by human activities. The natural question is, will this affect the function of the natural world? You may well say ‘Duh! Of course!’ as a first instinctive pass, but, the issue isn’t so clear cut – will species that survive simply take up the slack? What’s the value of a ‘species’ anyway?
Starting in the late ’90s the field of diversity-function research has tackled this topic, largely using manipulations plant species number. And the results are pretty conclusive – what you change plant diversity, you affect how the natural world works.
But note I said plants.
A bunch of us in the early to mid ‘aughts wondered if changes in the number of top predator species, or herbivores, or intermediate predators, or other species other than plants and algae might also alter the way the natural world worked in an analogous way. Emmett Duffy outlined a number of reasons we could expect changes in diversity at different trophic levels to produce either the same results as changes in plant diversity or maybe not!
So we went out and did the experiments, and found – well, sometimes diversity affected function one way, sometimes another. It all seemed to depend on something about each individual experiment. I was involved in this by examining about whether losses of predator diversity affected the impact of herbivores on their plant or algal prey – so called trophic cascades.
And in looking at the relationship between predator diversity and trophic cascades we really did see every kind of result one could imagine.
Fortunately, there seemed to be some predictability here which can be seen both by comparing different experiments or looking at some of Deborah Finke’s awesome work in a variety of systems. That one could predict the effect of changing levels of diversity if they knew the relative number of omnivores, specialists, or within-trophic level predation (so called intraguild predation). But these insights were all pretty qualitative. There’s no real quantitative guide here as to when diversity will do what.
A head scratcher.
Leaving this, I went off and did some work looking at how climate change may alter the network structure of food webs. And promptly did a palm-to-the-forehead. Food web network ecology has done a brilliant job of deriving metrics to describe the structure of, well, food webs. And the structure of food webs seems to influence the effects of species going extinct on trophic cascades or any other function one would care to measure.
Clearly, these two fields needed to come together.
So this is what I’m doing for my postdoc here at NCEAS. I am slowly but slowly trying to figure out how to link food web network theory with biodiversity-ecosystem function.
What’s my goal? Simple. Look at the food web to the left. In it, different trophic levels have different colors. But, heck, even within a trophic level, we may split things into finer trophic groups based on their diet and types of interactions – something we do all the time qualitatively (e.g., by saying there’s a brown food web of detritivores and a green food web of consumers of living tissue) and can even now do quantitatively.
What I want to be able to do is say, let’s take this food web. If we know some structural properties of the web, can we then predict the effects of a change in diversity within any trophic group on the flow of energy and control of consumption within the web.
For example, if some number of species go extinct in F, will consumption of A increase or decrease? If some number of species in C go extinct, will G accumulate more or less energy?
I realize this doesn’t take some important things into account – interaction strengths or the abundance of each individual species. I think the former can be folded in later. I’d also note that I am trying a very different approach than qualitative modeling and think that problems of indeterminacy in predictions may be dealt with by using a probabilistic framework from the start. With respect to abundances – I’m hoping the results can translate into predictions of biomass, but, we shall see…
In the coming weeks, I’m going to try and open up my lab notebook, and lay out the theory I’m developing to answer these sorts of questions. I’ll be honest, I’m doing this for myself as much as anything. I have a lab notebook full of scribbles – some blind alleys, some promising leads. Writing it out will force me to focus my arguments and spot weaknesses (or have you spot weaknesses)! I’ve got some of this nailed, and some of it I may stumble around a bit with. And, heck, I’m always game to hear thoughts from the peanut gallery.
As I answer different pieces of the puzzle, I’ll put links to them in this entry. So, let’s start this open notebook and see where it goes!
Pieces of the Puzzle
1) How do changes in predator richness in a 2-level predator prey web affect the probability that prey will be eaten and energy will be transferred to predators?
2) Can we examine how the statistical properties of a predator-prey food web will affect the consequences of extinction, rather than looking at a single specific food web structure?
3) How do changes in prey species richness affect the probability of energy being transfered to predators?