09 March 2009

Parasites like their hosts clustered

ResearchBlogging.orgIn epidemiology the importance of ecological and evolutionary processes comes into sharp relief: questions about the networks of interactions between species in a community, or about the evolution of parasite specificity, virulence, and contagiousness have immediate implications for human health, as well as in animal husbandry and conservation. One of the most basic of these questions is, what determines the community of parasites that infect a species? One answer is in this month's issue of The American Naturalist, where a neat meta-analysis shows that the size of mammals' home ranges shapes the number of parasite species they attract [$-a].

A tapeworm parasitic worm
Photo by pinkcigarette.
For mammals, we already know that parasite communities are shaped by the host's body size, geographical range, and population density. In this new study, Bordes et al. propose another factor: the host's home range, the area that a single individual occupies. There are two major ways that home range might shape the diversity of parasites infecting host. Greater home range could mean that the host encounters a broader array of habitats, and opportunities for infection, so that home range and parasite diversity are positively correlated. Alternatively, hosts with smaller home ranges effectively live at higher density, which should create more opportunities for parasite transmission between hosts, generating a negative correlation between home range and parasite diversity.

Bordes et al. test these hypotheses by collecting published studies of the number of parasitic worm (helminth) species infecting mammals, and then performing regressions (corrected for phylogenetic relationships between host species) of parasite species richness on a variety of possible causal factors, including home range. They find that host home range is a stronger predictor of parasite species diversity than host body size, and that home range is negatively correlated with parasite diversity.

In a way, then, this result confirms the importance of host density in host-parasite interactions. But it's not an obvious outcome - it is intuitive that more densely populated hosts should be more susceptible to parasitism in general, but not that they should also be attacked by a wider array of parasites. Maybe dense host populations are more productive habitat to parasites, so that there's ecological "space" to support a greater diversity of parasites. Or maybe these dynamics are a result of the specific biology of helminth parasites, many of which have different hosts for different parts of their life cycle.


F. Bordes, S. Morand, D.A. Kelt, D.H. Van Vuren (2009). Home range and parasite diversity in mammals The American Naturalist, 173 (4), 467-74 DOI: 10.1086/597227


  1. Are you sure that is a tapeworm? With setae? I don't see any segments. It looks more like a Chaetogaster to me.

  2. Whoops. This is what I get for relying on Flickr photo descriptions. Based on Wikipedia, it looks like this is not a tapeworm - Chaetogaster is a better guess than I could come up with. Thanks for the heads-up.