1.6. Goshawk’s impact on grouse populations

Due to the high proportion of grouse in the diet, the goshawk, being a relatively common raptor, might have a fairly high impact on grouse populations. Depending on the strength and timing of the impact, the predator can limit the prey population or even regulate it (May 1973, 1981, Hanski et al. 1991, Korpimäki 1993, Murdoch 1994, Korpimäki & Krebs 1996). A typical feature of many northern species, periodic fluctuation of abundances, also considers grouse that show multiannual fluctuation, with a cycle length of 6-7 years in most parts of Finland (Linden 1988). A somewhat shorter cycle length, 3-5 years, is found in northern Finland and Scandinavia (Myrberget 1984, Angelstam et al. 1985, Linden 1988, 1989). There are two hypotheses presented to explain grouse cycles by predation. Angelstam et al. (1984) brought up and defined an old theory originally proposed by Hagen (1952) and Lack (1954), the so called alternative prey hypothesis (APH), which explains population cycles of small game to be formed by varying predation pressure, caused by vole-eating predators. Population fluctuation of grouse may also be driven by a specialist predator, according to the general predation theory (Rosenzweig & MacArthur 1963, May 1973, Hanski et al. 1991, 1993). Although known as a generalist predator, the goshawk is relatively specialised on grouse in northern conditions. If a predator were to cause cyclic fluctuations in the prey population it firstly should be relatively specialised on it e.g. show no functional response for prey. Secondly, it should respond numerically with a time-lag to prey population and thirdly, predation pressure should be highest during low phase of prey population and there should be a negative correlation between the kill rates of the predator and the change in the prey population (Korpimäki et al. 1991, Nielsen 1999).