The effect of the caecal threadworm (Trichostrongylus tenuis) on the population dynamics of red grouse.
The regular fluctuations in animal numbers, known as population cycles, have been described and studied in a wide range of species although the causal mechanisms that generate the cycles are not clearly understood for any species. Numbers of red grouse (Lagopus lagopus scoticus) shot on moorland estates in Great Britain exhibit population cycles although the pattern and period of the cycles varies within the birds distribution (Potts et al. 1984; Williams 1985). The period tends to increase the further north the population is, in the Peak District of Northern England records show fluctuations of about 4 years, in the Borders about 5 years and in the Highlands of Scotland 6 to 7 years, although some populations show no tendency to cycle.
The possible mechanisms that could generate population cycles have been investigated in some detail. Intensive studies of red grouse in north-east Scotland have been conducted for more than 30 years and have concentrated on the role of spacing behaviour and dispersal. These studies have suggested that while extrinsic factors such as food, predation and parasitism can alter grouse numbers, they are non-cyclic components (Moss & Watson 1985) and cycles are caused by intrinsic factors (i.e. those within the birds). It has been proposed that changes in levels of aggression and tolerance lead to fluctuations in numbers. At low density, individuals are tolerant of neighbours so numbers increase until they reach a density threshold, at this point individuals become intolerant and aggressive, cocks take larger territories and many of the birds without territories are forced to disperse, leading to a fall in density. The suggestion is that cyclic changes in animal numbers are a case of adaptive irruptions, an idea currently being tested in north-east Scotland.
In the north of England, studies have taken a different approach since the cyclic changes in numbers are seen as a series of population crashes (see Figure 3) rather than a series of irruptions. Population crashes are frequently associated with outbreaks of the parasitic nematode Trichostrongylus tenuis as noted by Edward Wilson in the report of the Committee of Inquiry on Grouse Disease (Lovat 1911). Analysis of a large number of corpses and a study of the parasite's life cycle led him to suggest that the parasite caused heavy mortality in adult birds and this loss from the population caused numbers to crash. However, studies in Glen Esk found that losses caused by the parasite were insufficient to account for the population crash (Jenkins et al. 1963), Potts et al. (1984) found evidence to suppose that the parasites could reduce the breeding production of grouse and with a simulation model showed that such effects could cause the population cycles observed in the north of England. This paper summarises some recent experimental findings that have demonstrated that the parasitic worm does reduce the body condition, survival and breeding production of grouse and outlines mechanisms for controlling the parasite in managed grouse populations.