"A simple model for the dynamics of a host-parasite-hyperparasite interaction".
Reviewed 20/14/11
The study of parasites and their importance in ecological systems is growing. But what about hyperparasites? There does in fact exist parasites of parasites, and it is possible that their role in systems could be just as important, or more so, than any other tri-trophic interaction. The authors of this paper developed a theoretical model for the study of a host-parasite-hyperparasite interaction using a modification of the classic SIR models for epidemiology. The system they applied their model to includes the former chestnut tree populations in America, the ascomycete fungus C. parasitica, and its parasite Cryphonectrica Hypovirus (CHV). There are several strains of the hyperparasite all which affect the fungus in differing ways. The authors also included a section on the vegetative compatibility of viruses with the fungus. Depending on this interaction the number of infected spores produced by the infected fungus could alter.
The basics of the model centered around 4 different character states for the tree: Susceptible (S), Infected with hyperparasite-free fungus (I), Infected with a hyperparasite-bearing fungus (H), and Dead (R). Infected state individuals can die or be turned into the H state. It is assumed that trees infected with hyperparasite-bearing fungi are able to recover and return to the S state, but a transition to dead (R) was not included, assuming that the reduction of fungal pressure on the tree was so largely reduced by its own parasite that it could not cause death in its tree host.
Two conditions were set for hyperparasite establishment to occur. If both are met, but not one alone, than the establishment success of the hyperparasite, and their corresponding role as a method of biological control, depended next upon the initial conditions. One of these metrics includes a term stating that there needs to be less full recovery of the tree host than their is horizontal transmission of hyperparasite-bearing fungus. A certain number of H state trees need to remain in the system in order to provide new sources of hyperparasite. This also relates to the result that hyperparasite establishment is easier if parasite (the fungus) transmission is high. Evolutionarily I think this result is quite interesting. The high level of reproduction that would typically be considered advantageous for the parasitic fungus becomes less so if it also helps its enemy succeed.
Virulence levels for the hyperparasite also play a role in the model. It would seem that a more mid-line virulence would be more successful for endemic virus levels to persist; too high and fungal pressure on the tree becomes so reduced that the tree is able to recover at a faster rate than the hyperparasite can propagate itself in the system.
I found this paper to be very intriguing. It is interesting to explore the role of hyperparasites in natural systems, especially as their presence in the chestnut example may be a key reason for why stability of the tree populations in Europe was obtained after an introduction of the fungus while American populations were decimated to the point of extinction. The authors propose that their models could be altered into a framework useable for invasion into a community by transforming the character states of the hosts into states of patch or niche such as; susceptible to invasion (S), invaded (I), invaded by a enemy-controlled invader (H), etc. This is an interesting mental-exercise, but I am not sure of its application ability. The unit of study at the host level is easier as it tends to function more on a live or die scenario. Patches, or niches, are somewhat more complicated as it becomes more necessary to measure the extent of invasion in a patch, and then there is always the question of defining patch boundaries. Either way, it would seem as if the model presented has some merit for study in hyperparasitic systems in the future.
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