In this post, and others to come every Thursday I will choose a random paper from my stash of 1500 or so, read it and review it.
Today’s paper is: Food web topology and parasites in the pelagic zone of a subarctic lake
Added bonus, it is open access (click the image to go get the paper)
This paper starts out with an amusing anecdote about Carl von Linne on an expedition in 1732 to subarctic Lapland. On his visit, apparently he found the local Arctic charr to be so infested with parasitic worms that he would not eat them. Thus we have the impetus for this paper, what role do these parasitic organisms play in structuring the food web.
The main point of this paper is to describe the food web of a freshwater lake and to determine how including parasitic interactions will impact the structure of the web. It seems as if describing food webs with parasites has become quite popular in the last ten years or so, since Lafferty published several papers on the potential impacts and ubiquity of parasites in natural communities. There are data for several such webs stored in the Ecological Archives (and I have talked about them before), and this is just adding to pile, which in my opinion is always a good thing. And for now, I am going to ignore the recent paper by Dunne et al. demonstrating that there is nothing inherently special about adding parasites to food webs that changes the network structure, rather it is the same as simply increasing the size of the web.
On to the web!
Amundsen and colleagues are describing the food web of lake Takvatn (is that redundant? as the authors note “vatn” means “lake”, oh well), a subarctic, oligotrophic, and dimictic lake in Norway. Annual studies since 1979 have yielded information about the population dynamics, feeding ecology, and parasite ecology of the fish community. These studies allowed the authors to create a source based food web (although I would consider a source web to have only a single source, where here they have several species of phytoplankton at the base of the web). Their web is spatially and temporally restricted to the open water limnetic zone of the lake, and only during the ice-free season.
Each node was (to the best of their ability) classified to the species level, which I think was a good decision. While there have been studies that show that lumping species into categories by trophic/taxonomic/functional groups doesn’t impact overall structural properties that much (provided it is applied evenly), I think that if what we are ultimately interested in is the dynamics of the community, it is a waste to throw away useful information, and potentially important complexity. But then again, the most popular food web models (e.g., the niche, cascade, nested hierarchy) are only able to produce trophic species webs (which is what I think drives their popularity). Of course, that is neither here nor there. I also liked that they did not split the parasites up into different life stages. At the end they wound up with a food web with 50 nodes (37 without parasites) and 432 links (198 without parasites).
Of course, while they include species lists in their supplemental material, they do not include the adjacency matrix for their food web, so I am unable to reproduce their results, or incorporate their food web, which seems great, into bigger questions…
I found it interesting that they described their food web as “wasp-waisted” with high diversity in the upper and lower levels, while intermediate levels were of relatively low diversity. I have not heard this term used before, but it does make me want to look through my collection of food web datasets to see if any of them are like that.
Their main results were that including the parasites increased connectance, omnivory, link density, and nestedness. It was interesting to find out that before and after the inclusion of parasites, the most connected species remained the most connected species. I also liked that they found a relationship between the number of predator-prey links a given host species had, and the number of trophically transmitted parasites links it had. Although, perhaps we may say that is relatively unsurprising. I do wonder if that is merely an artifact of probability, if you eat more things you are more likely to get parasitized. One could imagine a simple test using two null models, one that preserves degree distribution and one that did not. If, compared to randomized webs with the same degree distribution you find the same pattern, then it may be simple probability, you would not expect to see the pattern in a randomization that does not preserve the degree distribution.
While it may be (as I said above) that parasites do not alter the structure significantly differently than if they were any other functional group (more predators perhaps), that does not necessarily mean that there is no use in including them in food web studies. There is, to me, no reason to say well, they are not all that structurally important so let’s just pretend there are fewer species in the system (although I don’t think anyone is making quite that argument).
I think that what will become important in the future will be the implications of parasitic links towards the dynamics of the food web. We already know from Allesina and Tang (2008) that increasing the number of antagonistic links in a network has a stabilizing effect, so the fact that including parasites increases connectance somewhat significantly could help to explain how such complex systems are stable. I am definitely looking forward towards being a part of the development of these kinds of ideas as I progress through my dissertation.
One thing I cannot avoid mentioning is that in their discussion the authors cite Begon, Townsend, and Harper (2006). For those who don’t know this is one of the most widely used General Ecology textbooks. Seeing this as a citation in a scientific paper really grated on me. There is no original research in this book (unless I seriously misread it, and correct me if I am mistaken). If you want to cite something from a textbook, stop and just don’t. It is not that challenging to find the source from which the information came and cite the original research article in your paper. Citing textbooks is something you do in high school when you don’t know any better. Sure, using a textbook as a starting point is a good place to find a collection of sources on your topic, but you should always go to the original source. It is paramount to citing Wikipedia, albeit a bit more credible. But that is my opinion.
Overall, I enjoyed reading a paper about how researchers put together a food web. I think I will definitely have to read more of these in the future.