I am going to go out on a limb and suggest that many, if not most, parasite species exploit several to many hosts species, for a given stage of their (i.e., the parasite’s) life cycle. The question of why such parasite species are catholic in their diet (I say diet because, really, the host is a source of food for the parasite’s growth, development and/or reproduction), remains largely an open question. One interesting nuance is that a given species of parasite often has differential fitness or success on different host species (here, we are controlling for time and location of collection of both the parasite and host species). A troubling issue is why the parasite has not evolved discrimination, another is why some host species are better hosts than others, a third is whether there are alleles coding for host species use that are in competition with one another. There are several other questions such as how important is host species relative frequency across space and time in determining whether it is a principal species exploited by the parasite and what, if any, trade-offs exist in using one species over another. We are just starting to get answers to these questions. Sometimes, nature pitches real oddities at you. Consider, two damselfly species where one is totally susceptible to a mite ectoparasite and the other is totally susceptible. If you want to getthe scoop, visit Julia Mlynarek’s blog: Julia was one of my PhD students here art Carleton University.
Recently, one of my former students, André Morrill, got a paper accepted in the Canadian Journal of Zoology as first author on explaining covariation between ectoparasitic mites and endoparasitic eugregarine protozoans of Lestes damselflies. Many previous researchers have addressed the extent to which either mites or gregarines are associated with reduction in fitness of their insect hosts. In these singular studies examining just one parasite taxa or species, there is the implicit assumption that any patterns discovered are attributable to the parasite under study, and not some other parasites species that has not been monitored or enumerated. André’s work shows that there are reasons to expect these parasites might covary. The link to the paper for those interested can be found here. André also has recently senior authored a paper in International Journal of Parasitology concerning how a Poisson distribution of random parasite encounter and a normal distribution of host condition can combine through a few simple rules of condition-linked immunity to produce the oft-seen and oft-cited negative binomial distribution of parasites on hosts. This research suggests that we need not invoke environmental heterogeneity in parasite infective stages to explain aggregated distributions on hosts; such occurrences, however, will make parasite distributions even more aggregated. Read the abstract of that paper here.
Professor & student