Red grouse living on heather moorland in northern England and Scotland show population cycles, with numbers crashing every 4 to 8 years or so. A long-running study by Peter Hudson and colleagues tested whether a parasitic nematode worm, Trichostrongylus tenuis, which lives in the gut of the grouse, causes these crashes.Heavily infected birds are in poorer condition, lay fewer eggs, rear fewer chicks and are easier for predators to catch. The worm larvae are picked up by eating heather shoots, so infection spreads more when grouse are at high density. Parasitism therefore has its biggest effect when the population is large.The researchers carried out a field experiment. In some populations, grouse were caught and treated with an anthelmintic (a drug that kills parasitic worms) in the years when a crash was expected. Other populations were left untreated as controls.The results:
- Untreated control populations crashed as predicted.
- Treated populations crashed much less, or not at all, so the cycle was reduced or prevented.
This is strong evidence that the parasite is a major cause of the grouse cycles. It is a good case study because it uses a manipulated experiment with controls, rather than only a correlation between two sets of numbers. It also shows that a population can be regulated by a parasite that weakens its host without killing it outright.
- Untreated control populations crashed as predicted.
- Treated populations crashed much less, or not at all, so the cycle was reduced or prevented.
This is strong evidence that the parasite is a major cause of the grouse cycles. It is a good case study because it uses a manipulated experiment with controls, rather than only a correlation between two sets of numbers. It also shows that a population can be regulated by a parasite that weakens its host without killing it outright.