Population dynamics of the Red Grouse – Trichostrongylus tenuis system

The University of Calgary Helminthology 690 class was fortunate to have Andrew Dobson, from Princeton University ( http://www.princeton.edu/~dobber/ ) as a guest lecturer this month semester.
During this lecture, we heard about his involvement in a 10 year study of the Red Grouse – Trichostrongylus tenuis system in Northern England, and how they found that the T. tenuis parasites influenced the population cycling displayed by the Red Grouse. Parasite interactions have impacts at the individual level, population level, and on the community level; information gained for each level of interaction, leads to building of models to test other levels.
In a subsequent class, we reviewed three pivotal Dobson et al, papers these papers were some of the first to show that parasites influence population cycles of their host, and are a central driver of population dynamics.
Picture red grouse

Picture T tunuis
The parasitic nematode T. tenuis has a direct lifecycle, parasites eggs released from the feces of infected grouse hatch, and develop through several free living larval stages to reach the infective stage, they then position themselves on the heather buds of the moors and wait to be eaten by their host (the Red Grouse), in order to continue their development to the reproductive adult stage in the caecum of the Red Grouse. T. tenuis has the ability to enter hypobiosis, — that is, if they infect the grouse in the autumn, they can arrest their development (as producing eggs that will only be laid on winter snow is a waste of resources and makes no evolutionary sense), they will then resume development in the spring. The researchers involved in this 10 year study were able to collect detailed data on grouse population fluctuations, parasite burdens, along with grouse predation within their study area.
Picture hunter heather
Results showed that T. tenuis parasite had low degree of aggregation within the grouse population and that there is a delay an parasite recruitment due to hypobiosis . It was also found that T. tenuis had effects on host population: grouse with heavy worm burdens tended to be associated with heavy winter losses and tend to have smaller clutch sizes, and grouse with heavy worm burdens were also more heavily predated upon.
The Red Grouse – T. tenuis system display the conditions required for population cycling: i) there is a parasite induced reduction in grouse breeding production, ii) there is a low degree of parasite aggregation in the grouse population, and iii) there are time delays in parasite recruitment. Model building to describe the populations requires a model with limited arrested larval stages, which will then more closely corresponds to the grouse population data of Northern England.
These seminal papers highlight the importance of host-parasite interactions and their effect on population cycling.

Posted by Susan Stasiuk

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