Weather And Synchrony In 10-Year Population Cycles Of Rock Ptarmigan And Red Grouse In Scotland - Statistical Data Included

Ecology, August, 2000 by A. Watson, R. Moss, P. Rothery

A. WATSON [1,3]

R. Moss [1]

P. ROTHERY [2]

Abstract. Rock Ptarmigan (Lagopus mutus on two adjacent submassifs, and Red Grouse (Lagopus lagopus scoticus) on lower ground between them, showed largely synchronous [sim]10-yr cycles during a [sim]50-yr study on the infertile Cairngorms massif of Scotland. Adult birds of both these Lagopus species were counted along transect walks. Both species showed the very low mid-1940s trough previously recorded for tetraonids in much of northwest Europe. Each of five subsequent peaks in all three populations fell within a year of one another, and 1-2 yr after cyclic high June temperatures at a nearby village. Troughs were less synchronous. A model with lagged June temperatures and fourth-order delayed density dependence, with no input from observed bird numbers after the first 4 yr, gave a good postdiction of Rock Ptarmigan numbers on the bigger submassif for 49 yr, suggesting a weather cycle entraining a Rock Ptarmigan cycle. However, June temperatures had little explanatory value for Rock Ptarmigan numbers on the s maller submassif. Indirect evidence suggested that synchrony between the two Rock Ptarmigan trajectories may have been due partly to emigration from the bigger to the smaller submassif. The population trajectory of Red Grouse resembled that of Rock Ptarmigan on the smaller submassif more closely than the two Rock Ptarmigan trajectories resembled one other. Hence synchrony depended more on local circumstances than on species.