The interaction between the parasites and predators of Red Grouse Lagopus lagopus scoticus.

Abstract

Both parasites and predators can reduce the survival and productivity of bird species and have serious consequences for avian population dynamics. Long-term population studies of Red Grouse Lagopus lagopus scoticus have concentrated on the importance of different natural enemies throughout the geographic range of this subspecies. Examination of birds found dead coupled with long-term studies show that the relative importance of parasites and predation varies both temporally and spatially, but both are important as significant sources of mortality. The parasitic nematode Trichostrongylus tenuis reduces the survival and breeding production of Red Grouse, and these effects are sufficient to generate long-term cycles in Red Grouse abundance. Predation rates on grouse appear to be high in some areas, particularly in Scotland where numbers of Foxes Vulpes vulpes have increased dramatically over the past 30 years. Parasites cause morbidity, and an experimental test illustrates how parasites may influence grouse scent emission and make the birds more vulnerable to predation. Mathematical models demonstrate that small numbers of predators selectively removing heavily infected individuals may allow the size of the Red Grouse population to increase since the predators effectively reduce the regulatory role of the parasites. However, moderate and high levels of predation result in suppression of the Red Grouse population.