Friday, March 13, 2009

Why do I have so many parasites?

This week's paper (Bordes et al 2009) looked for forces that influence the parasite diversity or parasite species richness (PSR) among mammals. While it may seem almost impossible to think that there might be a single factor, there have been many different proposed influences (e.g. body size, geographic range, population density). The host home range, "area used in daily and seasonal movements" (Bordes et al 2009), could be related to the parasite diversity in two distinct ways. Their first prediction is that as home range increases so will PSR because it will result in an increased contact with diverse habitats (and therefore parasites). Their "spatial dispersion model of parasite acquisition" uses parasite transmission and host density to get this relationship to predict the opposite relationship. The results of their analyses supported the second prediction.

The group found the methods and results of this paper relatively straight forward. The use of independent contrasts to control for the effect of phylogeny was very appropriate in this paper. We did find one area of the analysis confusing with respect to the host sampling number. It is well known that sampling intensity may bias the number of parasites found on a host . The more hosts one searches the more parasites will be found up to some saturation point. The authors controlled for the bias by using the residuals of parasite richness and host number. However, we were then confused by the inclusion of "Host sample size" in the regression analyses. While other variables in the regression analyses were significant, it was hard to determine the impact of this highly significant variable on the total fit of the model. We were left wondering how much of the variation in PSR did the home range explain?

The main conclusion of this paper is to confirm a roll for epidemiological factors (density and transmission) on the relationship between home range and PSR. The results show a negative relationship between home range and PSR which is consistent with their second prediction. The strong negative relationship between home range and host density relates their effect to how this can influence the number of parasites. It seems that home range not only describes a complicated trait of a host species, but is perhaps influenced by a complicated set of other factors.

Today's group speculated on broader potential relationships of host traits and parasite diversity. Could there be a more universal law that predicts the parasite species load? This study and many of the citations contained within focus on animals and their macroparasites. Maybe there is a rule that works across such taxonomic divisions? The paper cited previous work on the role of body mass (Arneberg 2002, Lindenfors et al 2007) and parasite diversity with larger hosts being home to a larger number of parasites. Can this relationship be scaled up to incorporate host density? What about the total mass of a host species?


Bordes, F., S. Morand, D. A. Kelt, and D. Van Vuren. 2009. Home Range and Parasite Diversity in Mammals. The American Naturalist 173:467-474.

Arneberg, P. 2002. Host population density and body mass as determinants of species richness in parasite communities: comparative analyses of directly transmitted nematodes of mammals. Ecography 25:88–94.

Lindenfors, P., C. L. Nunn, K. E. Jones, A. A. Cunningham, W. Sechrest, and J. L. Gittleman. 2007. Parasite species richness in carnivores: effects of host body mass, latitude, geographical range and population density. Global Ecology and Biogeography 1:1–14.

Bordes, F., Morand, S., Kelt, D., & Van Vuren, D. (2009). Home Range and Parasite Diversity in Mammals The American Naturalist, 173 (4), 467-474 DOI: 10.1086/597227

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