A nitrogen index to track changes in butterfly species assemblages under nitrogen deposition

The impacts of nitrogen deposition (N) on animal communities are still poorly understood in comparison to plant communities. Long-term monitoring of community changes may contribute to this understanding, complementing experimental studies on underlying mechanisms. Butterflies are particularly suitable for such analyses, because the different species cover a broad gradient of productivity, their ecological traits are well-known, monitoring data are available in a growing number of countries, and the short life history of butterflies ensures a rapid response to changing environmental conditions.Here, we use species-specific nitrogen optima to develop a community nitrogen index (CNI) for butterflies in the Netherlands. Over a 25-year period (1990-2015), data from the Dutch Butterfly Monitoring Scheme reveal a significant increase in the CNI in response to high nitrogen deposition levels. However, the rate of increase is declining, in close parallel with reduced nitrogen deposition loads. The continuing increase indicates that nitrogen deposition still exceeds the critical nitrogen load of butterfly communities in the Netherlands. Overall, the relative increase of butterflies from more productive environments reflects the advantage, under high nitrogen availability, of mobile and multivoltine species with high reproductive capacity, rapid larval development and hibernation as pupae or adults. We discuss the perspectives and limitations in applying the CNI at both national and local scales. We propose that, when taking the critical nitrogen load of the examined butterfly community into account, the CNI may prove a valuable tool to track changes of biotic communities in relation to nitrogen deposition.