DO PARASITES MAKE PREY VULNERABLE TO PREDATION - RED GROUSE AND PARASITES

1. An extensive post-mortem survey of grouse revealed that birds killed by predators in spring and summer had significantly greater burdens of the caecal nematode Trichostrongylus tenuis than grouse shot during the autumn. Furthermore, grouse that appeared to have died through the effects of parasites carried greater worm burdens than grouse killed by predators. 2. The proportion of grouse with high levels of parasite infection increased with the intensity of predator control as measured indirectly through keeper density. These two empirical observations suggest that predators selectively prey on heavily infected grouse. 3. The interactions between parasites and predators were examined experimentally by reducing the worm burdens of female grouse with an oral anthelmintic. Nests of treated and untreated females were subsequently located either by research workers flushing the incubating female or by dogs trained to locate birds by scent. The dogs found significantly fewer of the treated than control birds, suggesting that female grouse with large parasite burdens emit more scent and are more vulnerable to mammalian predation. 4. A modified mathematical model of the grouse-nematode system is described which incorporates the effects of both random and selective predation of heavily parasitized grouse. An analysis of the model illustrates the importance of interactions between grouse, parasites and predators in determining the relative densities of each. In particular, when predators selectively remove heavily parasitized individuals, then low levels of predation can lead to increases in the size of the host (or prey) population.