Wednesday, 12 June 2013

Parasites and Networks - Food Webs, Epidemiology

When we think about how parasites fit into nature - their close interaction with their hosts, their reliance on specific hosts in a specific sequence, their ubiquitous presence in the environment - a network (or web) context makes sense. Recently, two articles  have come to my attention; they represent parasites in webs in similar ways but for different reasons.

Parasites Affect Food Webs

This food web for a mosquito would be much
more complex - and of much more relevance
to humans - if it included parasites.
Illustration by Tyler Rubley. CC BY-SA 3.0



The first is "Parasites Affect Food Web Structure Primarily Through Increased Diversity and Complexity." by J. Dunne and co-authors (PLoS Biol 11(6): e1001579), published on June 11, 2013. These researchers added parasites to aquatic food webs and asked whether the changes that resulted (a much more complex food web) could have been caused by the addition of any large group of organisms, or whether food web structure was altered by parasites in unique ways.

Beyond the central findings of that study, the article raised (or re-raised) a question for me: if we know that parasites are an important component of an ecosystem, if we know that they affect food webs in both generic and unique ways, why do they never appear on endangered species lists? For every endangered species, there should be a list of parasites that depend on that host species for survival. And why do conservationists routinely rid endangered animals in captivity of their parasites, knowing that if that species ever returns to the wild, it will do so without its specialist parasites, potentially to its detriment?

Parasites Predict Disease


The other paper is "Centrality in Primate–parasite Networks Reveals the Potential for the Transmission of Emerging Infectious Diseases to Humans" by José María Gómez and co-authors , published in PNAS (110:19 2013).  Like the paper by Dunne et al, this research builds a food web, but this web has parasites as the main consumers - a network of nonhuman primates that share parasites (or that are consumed by the same parasites, to look at it from the parasites' viewpoint).

The point is to examine whether this web can predict where diseases of primates are likely to jump to humans. The authors "found that primate species having higher values of centrality in the primate–parasite network harbored more parasites identified as EIDs [emerging infectious diseases] in humans and had parasite communities more similar to those found in humans." These species, then, are more likely to be the source of emerging infectious diseases in humans. Another good reason to include parasites in food webs.

Food webs tell us where parasites fit, and I think that is something we really need to understand, for our own sake and for theirs.