Abstract
Nitrogen (N) deposition caused by agriculture and combustion of fossil fuels is a major threat to plant diversity, but the effects on higher trophic levels are less clear. In this study we investigated how N deposition may affect species richness and abundance (number of individuals per species) in butterflies. We started with reviewing the literature and found that vegetation parameters might be as important as climate and habitat variables in explaining variation in butterfly species richness. It thus seems likely that increased N deposition indirectly affects butterfly communities via its influence on plant communities. We then analysed data from the Swiss biodiversity monitoring program surveying species diversity of vascular plants and butterflies in 383 study sites of 1 km2 that are regularly distributed over Switzerland, covering a modelled N deposition gradient from 2 to 44 kg N ha −1 yr−1. Using traditional linear models and structural equation models, we found that high N deposition was consistently linked to low butterfly diversity, suggesting a net loss of butterfly diversity through increased N deposition. At low elevations, N deposition may contribute to a reduction in butterfly species richness via microclimatic cooling due to increased plant biomass. At higher elevations, negative effects of N deposition on butterfly species richness may also be mediated by reduced plant species richness. In most butterfly species, abundance was negatively related to N deposition, but the strongest negative effects were found for species of conservation concern. We conclude that in addition to factors such as intensified agriculture, habitat fragmentation and climate change, N deposition is likely to play a key role in negatively affecting butterfly diversity and abundance.
Article Impact Statement Nitrogen deposition negatively affects butterfly species richness and butterfly abundance, particularly in species of conservation concern.
Data accessibility and reproducibility of results Data and R-scripts to reproduce the results of this manuscript including figures and tables are provided at https://github.com/TobiasRoth/NDep_butterflies. Raw data for analyses are provided in the folder “data”, and the folder “R” contains the R-Script that was used to export the data from the BDM database and to produce the figures and tables of the manuscript.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
We conducted an additional analysis in which we examined the effect of nitrogen deposition on the abundance of each butterfly species separately. We added a table (appendix A5) that gives the estimated effect size for each species. We further included a figure showing that the average effect size was strongly negative for vulnerable and near-threatened species, intermediate for target species for which Swiss agriculture has particular responsibility of conservation, and weakest (but still negative) for the remaining species.
