Population Dynamics of Scottish Rock Ptarmigan Cycles

Abstract

Scottish Rock Ptarmigan (Lagopus mutus) showed unstable population dynamics, with density-dependent features differing between areas on rich and poor soils. The study was conducted on arctic-alpine land on four submassifs, three of nutrient-poor granite and one of richer schist. Ptarmigan numbers cycled with a period usually of ϳ10 yr, but sometimes 6 yr. Delayed density dependence was detected in populations on granite submassifs, but not on schist, despite significant cyclicity there. Spring densities and the number of young reared per brood were higher on schist than on granite. Population change from one spring to the next was related to intervening breeding success on both granite and schist, but to summer loss only on granite. The same factors that determined variations in clutch size among years also largely affected breeding success. Population change, breeding success, summer loss, and clutch size showed delayed density dependence on granite, but not on schist, where numbers fluctuated more erratically. On granite, but not schist, early plant growth was followed by larger clutches, breeding success was related to June air temperature, and June temperature tended to peak 1-2 yr before ptarmigan peaks. Cyclic June temperature may have entrained the timing of ptarmigan cycles on granite. Results on both schist and granite were inconsistent with expectations from predator-prey hypotheses for cycles. In years of rising numbers, hens laid more eggs and reared more chicks per extra egg laid than in years of decline, and the proportion of surviving young recruited into the spring population was probably greater. Our hypothesis is that Scottish Rock Ptarmigan show unstable dynamics, driven by intrinsic positive feedback between recruitment in one year and the next, regulated by negative feedback between recruitment and density, and modified by the physical factors of weather and soil fertility that affect female nutrition and breeding success.

Demographic consequences of age-structure in extreme environments: population models for arctic and alpine ptarmigan

“…High elasticity values for S 0 are consistent with the current understanding of ptarmigan population dynamics: spring numbers are often correlated with the survival and recruitment of young birds (Watson et al 1984;Martin et al 2000;Moss and Watson 2001), although correlations with annual fecundity can also be strong (Bergerud et al 1985;Watson et al 1998). Land management practices applied to moorlands in Scotland, including prescribed burning of heather and predator control, are primarily aimed at increasing the annual fecundity of red grouse but may impact juvenile survival as well (Tharme et al 2001).…”

Seasonal Movements of Gyrfalcons (Falco rusticolus) Include Extensive Periods at Sea.

“…In Kangerlussuaq, limited numbers of land-based prey were observed close to Gyrfalcon nests, and a larger home-range was expected as a result of falcons having to travel generally further for food. However, Rock Ptarmigan numbers have been shown to fluctuate cyclically throughout the Arctic (Gudmundsson 1960, Weeden and Theberge 1972, Watson et al 1998, Moss and Watson 2001, including Greenland (Salomonsen 1950, Vibe 1967), and we do not know at what point in this cycle the local population was during our study. If marked changes in Rock Ptarmigan densities occurred in the Kangerlussuaq area, the size of Gyrfalcon breeding home-ranges could have been affected.…”

Ecology and Biology of Gyrfalcons in Greenland.

“…Rock Ptarmigan have been shown to have cyclic population fluctuations throughout the Arctic (Gudmundsson 1960, Weeden and Theberge 1972, Watson et al 1998, Moss and Watson 2001, and specifically in Greenland (Salomonsen 1950). Based on hunter kills, Vibe (1967) found ptarmigan populations to fluctuate in Greenland on approximately 11-year cycles from 1886 to 1954.…”