Parasites and climate synchronize red grouse populations:
We show that in specific years the size of red grouse populations in northern England either increases or decreases in synchrony. In these years, widespread and correlated climatic conditions during May and July affect populations regionally and influence the density-dependent transmission of the gastrointestinal nematode Trichostrongylus tenuis, a parasite that reduces grouse fecundity. This in turn forces grouse populations into synchrony.
Coevolution of the parasite and the grouse synchronize their population sizes, and regulate both populations.