Habitat loss and climate change are the two culprits most regularly blamed for the decline of butterflies and moths, but now a third suspect, nitrogen deposition, is increasingly coming under the spotlight as a key driver of decline.
Over time nitrogen has built up in the atmosphere from the burning of fossil fuels and intensive farming. Transport, power stations, industry, farm fertilisers and livestock are all major sources of harmful, nitrogen-based emissions.
Deposited via the air and in rain, nitrogen enriches the soil, creates acidic conditions with damaging effects to our environment
Nitrogen deposition first came to major prominence in the 1990s when high levels of nitrogen recorded in Dutch chalk grasslands led to invasion by coarse grasses (1). Since then much research has been carried out on the impact on the nitrogen deposition on a range of plant communities (3).
In 2006 further research found that butterflies dependent on dry, nutrient-poor habitats in Sweden were much more prone to extinction than those on nutrient-rich sites. A similar situation was discovered in the San Francisco area (4).
Since then scientists have suggested that nitrogen, perhaps in tandem with warmer weather, could be a cause of the decline of butterflies whose caterpillars depend on very warm microclimates (5). These hotspots are provided by bare ground, very short turf or dead plant material, and in the sunshine can reach temperatures of 30-35°C, while the adjacent grassland may only be around 10°C. Nitrogen, coupled with warmer temperatures, is allowing much greater vegetation growth and the loss of these hotspots is very bad news for the caterpillars that need high temperatures to function properly and to grow fast to avoid predators and disease.
Several of our scarce species that need open sunny habitats, such as the Grayling, Wall and Pearl-bordered Fritillaries, are undergoing further declines. Recent work on the High Brown Fritillary in Cumbria has revealed dramatic losses on nature reserves despite years of seemingly appropriate management (6). The sites are becoming grassier, and the butterfly foodplants, violets, are losing out. So it is likely that these butterflies, already scarce due to habitat loss, and now suffering from the impact of both climate change and nitrogen pollution as well. Studies on the impact of nitrogen on developing caterpillars themselves has shown serious negative effects (7).
While Butterfly Conservation and others can try and manage key habitats as beneficially as possible, there is little we can do about air pollution - except by demanding policies and laws that aim to reduce it, which is one reason why we need an Environment Act for Scotland.
Such an Act could embed in Scots law the four key principles that guide EU policy-making on the environment:
- The precautionary principle: operates where there are reasonable grounds for concern that an activity could cause harm, despite there being some uncertainty.
- Polluter pays: those who produce pollution should bear the costs of cleaning it up.
- The rectification at source principle: policies should tackle the root of the problem rather than just tackling its consequences.
- Preventive action: this is the need to address problems today rather than leave them for future generations to solve.
35 environmental charities from across Scotland, including Butterfly Conservation, have come together to ‘Fight for Scotland’s Nature’ and foster support for a Scottish Environment Act. Join us in calling for a Scottish Environment Act to protect and enhance our nature now and in the future!
Scotland Director, Butterfly Conservation
- Bobbink, O. Effects of Nutrient Enrichment in Dutch Chalk Grassland. 28:28-41
- Plantlife (2017) We need to talk about nitrogen. Plantlife UK, Salisbury. ISBN: 978-1-910212-49-3
- Ockinger, E., Hammarsted, O., Nilsson, S. & Smith H. (2006) The relationship between local extinctions of grassland butterflies and increased soil nitrogen levels. Biological Conservation 128 564-573.
- Weiss, S. (1999), Cars, Cows, and Checkerspot Butterflies: Nitrogen Deposition and Management of Nutrient-Poor Grasslands for a Threatened Species. Conservation Biology 13:6 1476-1486.
- WallisdeVries, M. & Van Swaay, C. (2006) Global warming and excess nitrogen may induce butterfly decline by microclimatic cooling. Global Change Biology 12 1620–1626.
- Ellis, S., Wainwright, D., Dennis, E.B., Bourn N.A.D., Bulman, C.R., Hobson, R., Jones, R., Middlebrook, I., Plackett, J., Smith, R.G., Wain, M. & Warren, M.S. (2019) Are habitat changes driving the decline of the UK’s most threatened butterfly: the High Brown Fritillary Argynnis adippe (Lepidoptera: Nymphalidae)? Journal of Insect Conservation
- Kurze, S., Heinken T. and Fartmann, T. (2018) Nitrogen enrichment in host plants increases the mortality of common Lepidoptera species. Oecologia 188 1227–1237.