Population Ecology of the Red Grouse Lagopus lagopus scoticus with Particular Reference to the Effects of the Parasite Trichostrongylus tenuis.

Abstract

Trichostrongylus tenuis is a prevalent nematode parasite of red grouse Lagopus lagopus scoticus. It reduces breeding success and survival of birds, and as a result may be responsible for the cycles in bird density that occur in many red grouse populations. In this thesis I examine this host-parasite interaction, including testing the effectiveness of parasite control, studying the frequency distribution and spatial distribution of parasites in a grouse population, and investigating the nature of parasite-induced cycles in host numbers through theoretical modelling.
Many grouse moors rely on the application of anthelmintic-coated grit for controlling nematode infection in red grouse. This grit is placed on the moor for the grouse to consume, which they do to aid digestion. However, a possible side effect of frequent dosing is the development of parasite resistance to the anthelmintic. I tested for resistance in parasites from 12 different moors in northern England, which varied in the timing of grit treatment and the quantity of grit applied to the moor. Egg hatch assays on T. tenuis eggs showed no evidence of resistance on any of the moors.
Studying the spatial distribution of parasites in the environment, and the degree to which they coincide with the spatial arrangement of the hosts, is fundamental to understanding the host-parasite interaction. A detailed survey of the distribution of T. tenuis on an area of moorland in Teesdale, northern England, supported the hypothesis that the parasite population is not uniformly distributed among the host population: both adult parasites among hosts and eggs among caecal faeces were aggregated. Environmental factors and host characteristics played a role in determining the parasite distribution, with parasite infections being associated with age of birds and location on the moor. However T. tenuis egg concentration in caecal faeces on the moor was only weakly spatially auto-correlated suggesting that further intrinsic or extrinsic factors may be influential. Distribution of eggs on the moor was not related to the density of grouse.
Finally, I developed an individual-based stochastic model, which specifically modelled the red grouse T. tenuis interaction. This showed that the parasite could theoretically cause cycles in grouse abundance, with the spatial distribution of both the host and parasite being important in the occurrence of cycles. Adding density-dependent host mortality to the model, had a stabilising influence on the host population, although the parasite still generated cycles in host numbers. In some cases this density dependence generated damped cycles in host numbers in the absence of the parasite. These cycles were amplified when parasite induced mortality was included, suggesting that the parasite can increase the cyclic tendency of the host population in these cases. Cycle periods were influenced by parasite-related parameters and were similar to those recorded in natural grouse populations.