Assessing spatiotemporal relationships between atmospheric nitrogen deposition and butterfly species records through statistical modelling
Creators: name: Risser, Hannah
Publisher: name: Lancaster University
Year: 2023

description: Atmospheric nitrogen deposition has been linked with an overall loss of
 plant species richness and homogenisation of semi-natural habitats both in
 GB and elsewhere. We expect that nitrogen-induced changes in plant
 communities will impact invertebrate species through the loss of
 reproductive habitat, food plants and suitable microclimatic conditions
 caused by the shifts in composition of plant communities. Prior to this
 thesis, no quantitative research had been undertaken to assess the
 potential effects of nitrogen on fauna in GB. Butterflies are often used
 as indicator species due to their sensitivity to environmental change, our
 comprehensive understanding of their ecology, and the existence of
 long-term datasets on their abundance and distribution.
 
 
 In this study, I analysed butterfly data from the UK Butterfly Monitoring
 Scheme alongside data on expected driver variables including nitrogen
 deposition, sulphur deposition, temperature, rainfall, land use intensity,
 and elevation. I performed a spatio-temporal analysis on the data for each
 species individually using generalised additive models to understand the
 complex and expected non-linear relationships between butterfly trends and
 their drivers. Model results were summarised to provide an overview of the
 total number of species exhibiting responses to nitrogen. In addition,
 results were summarised by trait groupings such as voltinism, host plant
 category, host plant specificity, and breeding habitat to summarise
 whether any trait groupings may be particularly strongly impacted by
 nitrogen pollution. In addition, I performed further detailed analysis on
 Lasiommata megera, the Wall Brown butterfly, which has been shown to be
 negatively impacted by nitrogen in studies undertaken elsewhere in Europe.
 I ran a similar spatio-temporal analysis to that mentioned above, but with
 the addition of two variables I hypothesised would be key drivers of L.
 megera: temperature in the previous September and elevation. The results
 for this additional analysis were presented separately.
 
 
 I demonstrated that individual butterfly species vary in their
 relationships with nitrogen deposition and highlighted both species-level
 and potential trait level responses. Nine butterfly species were
 negatively correlated with historic nitrogen deposition, and nine were
 negatively correlated with percentage change in nitrogen deposition at the
 site over time. Two species showed significant negative relationships with
 both historic nitrogen deposition and percentage change in nitrogen
 deposition over time: Fabriciana adippe (High Brown Fritillary) and
 Hipparchia semele (Grayling). These findings suggest that there is a
 strong correlative relationship between nitrogen deposition and the
 abundance of many butterfly species in GB. Other key drivers of change
 identified in this analysis were time, rainfall, and temperature in the
 current and previous year. I also demonstrated a strong relationship
 between abundance of L. megera and historic nitrogen deposition using the
 model with more detailed covariates. Initial summaries based on traits
 were inconclusive, not highlighting any particular trait groupings as
 being especially susceptible to the effects of nitrogen pollution.
 
 
 The results of this study present the first correlative link between
 nitrogen deposition and negative impacts on terrestrial fauna in GB. It
 reinforces the importance of continued efforts to reduce emissions to
 protect the natural environment. It also provides a basis for further
 field and lab-based work to be undertaken to better understand the causal
 mechanisms behind the observed relationships.