Do parasites make prey vulnerable to predation? Red grouse and parasites

Lagopus lagopus scoticus killed by predators in spring and summer had significantly greater burdens of the caecal nematode Trichostrongylus tenuis than those shot during the autumn. Grouse that appeared to have died through the effects of parasites carried greater worm burdens than grouse killed by predators. The proportion of grouse with high levels of parasite infection increased with intensity of predator control. These two empirical observations suggest that predators selectively prey on heavily infected grouse. Female grouse with large parasite burdens may emit more scent and may be more vulnerable to mammalian predation. 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. 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. -from Authors