INFLUENCE OF CLIMATE, TIME OF DAY AND SEASON, AND FLOWER DENSITY ON INSECT FLOWER VISITATION IN ALPINE NORWAY

This study examines the influence of environmental factors (temperature, light intensity, wind speed, time of day and flowering season, and flower density) on flower visitation activity of insects in a middle alpine area in southwestern Norway. Data were obtained by measuring the environmental factors and simultaneously observing the visitation activity to flowers within squares (1 x 1 m) for 10-min periods. Flies (Diptera) were the only flower visitors. In a simple linear regression analysis, mean daily temperature best explained the variation in mean daily visitation activity within the squares (R2 = 0.66). R2 increased to 0.74 in a multiple regression with all factors included. An unimodal progression in visitation activity was measured through the day, with peak visitation at 1200 h (solar time). Visitation activity was highest early in the flowering season (early July) and decreased as the season progressed (R2 = 0.21). Visitation was positively correlated with the daily mean flower density (R2 = 0.18). The flower density of individual species also affected visitation activity. Visitation reached a maximum level at medium flower densities, whereafter it stabilized, suggesting that the local visitor pool was saturated. The relationship between visitation activity and environmental factors at Finse, Norway, is similar to that found in alpine U.S.A., Chile, and Australia.