Abundance of day-flying Lepidoptera along an air pollution gradient in the northern boreal forest zone¹
Day-active lepidopterans were counted in the summers of 1991-1993 on transects of 5 x 100 min 12 localities representing five zones of pollutioninduced forest deterioration in the Kola Peninsula, northwestern Russia. A total of 671 specimens representing 19 species was observed during 696 counts. Two butterflies (Clossiana euphrosyne, Vacciniina optilete) and three day-active moths (Rheumaptera subhastata, Ematurga atomaria, Sympistis heliophila) were used in the analysis; the remaining 14 butterfly species were too scarce for the statistical treatment. At early stages of pollution-induced forest damage (mean annual SO2 concentrations 20-40 µg/m3 ), the densities of the monitored species increased by a factor of 1.5 to 5, but then declined with increase in pollution. Since the host plants of the monitored species, except that of C. euphrosyne, were found in all localities surveyed, the decline could be attributed to the SO2 toxicity rather than to the lack of larval food. Although transect counts did produce valuable information about the impact of pollution on subarctic forest ecosystems, the method is poorly suited for routine bioindication of pollution in northern regions.