Parasites are Always Late to the (Range Expansion) Party

Looking back at my previous posts, they are all pretty long. So I’m going to shoot for brevity here.

Two kinds of factors determine where one can find an organism, or its range: Abiotic factors, or how often it rains, soil mineral content, etc.; and biotic factors, or what other organisms are around, what they eat, what they defecate, what they do, etc. For a parasite, which needs a host to survive, biotic factors play a dramatic role in determining the parasite’s range. Abiotic factors contribute a great deal as well, but focusing on the biotic side is more than enough for a short blog article.

Take host range effects on parasites as an example. Due to the nature of parasitism, parasites can only exist where their hosts do. This restriction poses a problem for parasites because host ranges can shift over time, and hosts that can shift ranges quickly (mobile animals, for example) can move beyond their parasites (Phillips et al. 2010). How could a host “out-run” its parasite though? Parasites are typically tightly connected with their hosts (e.g., often physically attached to or within the host), and so should move with their hosts.

According to Phillips et al. (2010), hosts can move away from their parasites through range shifts because of chance. Before I go any further, I should say that hosts do not often permanently escape their parasites through range shifts; rather, the parasite shifts its range more slowly than the host, so the host experiences a parasite-free lag time before the parasite catches us. Now, chance can create parasite range shift lag times through two mechanisms. The authors contend that low host population size at the leading edge of range shifts increases the chance that parasites will not be present in that subsample of the population. The authors also argue that the parasites will be less able to maintain successful transmission in the smaller host population at the range edge. These two chance mechanisms are called founder effects, and they rely on probability. In any range, the population on the edge of the range is much smaller than that in the center of the range. Because of its smaller size, the range population has an increased chance of not having parasites that are found dispersed among the larger parent population (Say you have a large bag of marbles, and most are black and a few are red. If you grabbed a few out of the bag, representing your range edge population, the chances that that group would have a red marble are pretty low). Further, the few parasites that are represented within the range edge population have an increased risk of local extirpation because the fewer number of hosts at the range edge mean the parasites may not be able to maintain transmission – by chance the parasites may not encounter a host when their life cycle dictates they must. However, this second mechanism only applies to parasites with density-dependent transmission (the number of hosts present in an area affects the success of parasite transmission). Because of these chance, or founder, effects, parasites will be more likely to die or never have been present at the range edge, allowing the host population at the range edge to expand with fewer parasites than the host population in the center of range. Such a lack of parasites at the range edge creates the lag time between host range expansion (which occurs at the range edge) and parasite range expansion.

Phillips, B.L., et al., Parasites and pathogens lag behind their host during periods of host range advance. Ecology, 2010. 91(3): p. 872-81.