I was fairly excited today when I popped my Google Reader open and saw that there are two new early view papers up at Oikos about food webs. The first is a methodological paper that investigates the role that aggregating species together in webs (effectively reducing resolution) plays in impacting topological properties and function. The second uses several published food webs with parasites included to test how consumption affects predators’ interaction with parasites.
This paper was very interesting, especially because a great deal of food web studies have used “trophic species,” that is they have lumped species together that feed on similar prey species. As far as I can tell the trophic species lumping is in part the result of a need to be able to compare real webs to models like the niche model, which is unable to predict two species that have the same prey. But the results of this paper are very positive for those who like to lump species together, and especially for those food web people who like to go out and collect the empirical data (that those of us who are theory people LOVE to play with). Basically, they showed that the results of a previous study on the effects of fish predator foraging strategy are fairly robust to aggregation of species by trophic and taxonomic similarity.
What was interesting, however, is that lumping species by size class was not as good. I thought that was strange, especially in this type of aquatic system in which size structure is so important to trophic interactions. The authors note that this does in fact make sense, especially when you consider that organisms of the same size may have different foraging strategies, e.g. visual foragers consume larger prey than similar-sized filter feeders.
But the point of the study is still pretty spectacular. The results of the particular study they were referencing (Lazzaro et al. 2009) were robust to approximately 2/3 reduction in number of nodes on average, the best aggregation they did was based on expert knowledge they lumped 74 species into 17 nodes, and found only 2 errors compared to the fully resolved 74 species web. This pattern held for 15 topological properties! I find that astounding.
Parasites in food webs are quickly becoming a big deal (in no small part [my opinion] due to the efforts of Kevin Lafferty and this NCEAS project). I have talked about parasites and food webs before (a little bit) here and here. I have to admit, I was a little annoyed at this paper, but only because they came very close to answering some questions I have been hoping to start working on with regards to parasites role in structuring communities. Nonetheless I enjoyed reading this paper.
Their focus here was on predation of parasites, and the role that predators may play in transmission. Questions they asked included:
1) Does predator generality increase the chances of concomitant predation and are generalists more vulnerable to parasitism through trophic transmission?
2)How vulnerable are parasites to predation?
3)Is there a link between parasite vulnerability, trophic transmission, and species richness of predators in the food web?
I think these are all important questions to be asking to determine what role parasites may play in both structure and dynamics of the communities to which they belong. To answer these questions the authors used 8 well resolved webs, with a majority of nodes resolved to the species, and parasites that were resolved to the life stage. Additionally, they benefited from these datasets having defined the types of interactions and included many different predator-parasite interactions (e.g. trophic transmission and concomitant predation).
The findings in this paper are fairly interesting, and I highly suggest reading it, because they can articulate their points far better than I. Although if you do decide to read through it I must also suggest you check out this paper by Thompson et al. (2013) (also in Oikos interestingly enough) where the impact of energy flow on parasite diversity and abundance was investigated. Thompson and colleagues used a flow-based food web with parasites to test the relationship between the amount of energy flowing through a particular node as well as its degree (number of links to and from the node), and the number of parasite species and their relative abundances associated with that node. These two papers, I think, go hand in hand. Especially since the discussion of the more recent paper calls for more studies such as the one done by Thompson.