Impact of nitrogen deposition on larval habitats: the case of the Wall Brown butterfly Lasiommata megera

Nitrogen deposition is considered as one of the main threats to biodiversity and ecosystem functioning. Three mechanisms have been proposed to explain the detrimental effect of excess nitrogen on butterflies: loss of host plants, deterioration of food plant quality and microclimatic cooling in spring. Here, we investigated whether these mechanisms might explain the dramatic recent decline of the Wall Brown butterfly Lasiommata megera. Monitoring data from the Netherlands indeed show a greater decline at higher critical load exceedance of nitrogen deposition. Loss of host plants is not a likely explanation of the decline for this grass-feeding species. In a greenhouse experiment, we only found beneficial effects of nitrogen fertilization on larval performance, which seems to rule out a nutritional cause; application of a drought treatment did not result in significant effects. Microclimatic conditions at overwintering larval sites of L. megera and the related but increasing Pararge aegeria provided a possible clue. In comparison with larval sites of P. aegeria, those of L. megera showed higher temperatures at the mesoscale and less plant cover and more dead plant material at the microscale. L. megera caterpillars were also found closer to the shelter of vertical structures. The greater dependence on warm microclimates suggests that microclimatic cooling through excess nitrogen contributes to the recent decline of L. megera.