We monitored the biodiversity of plants, adult butterflies and leaf-miners in a Dutch nature reserve over a period of six years (1994–1999) within the International Co-operative Programme on Integrated Monitoring on Air Pollution Effects (ICP-IM). Butterfly abundance decreased steadily over the period, indicating a negative diversity trend, while the number of leaf-mining larvae of Microlepidoptera remained fairly constant. Also the concentration of pollutants (NH4, NO3, SO4, Cd, Cu and Zn) was determined in air, leaves, litter, throughfall and stemflow. We have no reason to expect a negative impact of acidification in rainwater or climate change, as temperature and ozone show no significant trends across the six years.
It is shown that the nectar-plants of adult butterflies are much more sensitive to heavy metals than the nectar-plants of moths and other pollinating insects. It is hypothesized that the butterfly decline is a secondary effect of heavy metal stress on local plants, not resulting in a decrease in the number of host-plants, but in a selective pressure of pollutants on the plant vigour, subsequently affecting their pollinators (p < 0.001). An alternative explanation, such as the possible coexistence of a direct effect of xenobiotics on the adult Lepidoptera occurring in the study area, is not supported by our data (p > 0.05). Copyright © 2005 John Wiley & Sons, Ltd.