Parasitology Today, vol. 5, no. 9, 1989 Population Biology of Trichostrongylus tenuis, a Parasite of Economic Importance for Red Grouse Management P. J, Hudson and A. F'. Dobson* The caecal nematode, Trichostrongylus tennis, is associated with dramatic decreases in the density of red grouse (Lagopus lagopus scoticus) resulting in a significant loss of sporting income to many upland estates in the UK. In this article Peter Hudson and Andrew Dobson describe the dynamics of the grouse-T, tenuis system, show that the parasites can cause population crashes and present preliminary findings from the development of new control methods. The first detailed studies on Trichostrongy- lus tenuis were conducted at the turn of the century by Edward Wilson, the scientist and artist who subsequently died with Captain Scott on the ill-fated expedition to the South Pole. He an:flysed large num- bers of samples and established the life cycle I of T. tenuis axld published his results in a volume edited by Lord Lovat. While Wilson thought the parasite caused large-scale mortality and a decline in grouse density, Lord Lovat was of the opinion that the quality of the heather food plant was more important. Sub- sequent work on the population biology of red grouse 2-4 assumed the parasite to be a consequence rather than a cause of popu- lation change, particularly since the mor- tality caused by the parasite appeared insufficient to account for changes in grouse density. 2500 The consensus now is that T. tenuis burdens can in fact seriously affect the grouse population but probably do not have a serious effect on other gamebirds in the Western Palearctic and the Nearctic. Grey partridge (Perdix perdix) populations also used to fluctuate like red grouse and earlier studies have suggested that T. tenuis played a significant role in the dynamics of this species although densities of partridges are now much lower than during the 1930s and levels of infection are low. The red grouse The geographical distribution of red grouse is determined by its main food plant, heather (Calluna vulgaris), but the density of grouse on many estates is increased through care of the heather habitat and the control of predatory foxes 2000 1500 c 0 1000 500 E 0 0 0 --500 Z --1000 o do 160 260 August grouse density (No Ikm 2) (~) 1989,Elsevier Science PublishersLtd, (UK) C,169~.7071891503.50 283 REVIEW5 Upland Research Group Game Conservancy Crubenmore Nevvtonmore Inverness-shire PH20 I BE, UK *Department of Biology University of Rochester Rochester NY 14627,USA Fig. I. Output from a simple economic model of grousemanagement relat/ng net profit to grouse density. The point where costs equal profit is reached when grouse density is 138 grouse per 2 km , equivalent to a bag of 35 grouse shot per km2. The figuresare based on a 20 km z estate with fixed overheads that include the costsof one gamekeeper, variable costs according to number of grouse shot, and all grouse shooting hired out at the rates described in the text Note, this figure does not include any net income from sheep- farming.