Trends in butterfly populations in UK gardens - new evidence from citizen science monitoring

Trends in butterfly populations in UK gardens - new evidence from citizen science monitoring - British Trust for Ornithology

Butterflies are among the most visible garden invertebrates, and the garden habitat has been recognised as a potential refuge for these charismatic species. However, we know very little about the changing fortunes of garden butterfly populations because gardens are not well-represented in traditional monitoring schemes. This study highlights the role that BTO Garden BirdWatch could play in identifying and reporting on annual trends in butterfly populations.

BTO Garden BirdWatch (GBW) is the UK’s largest structured bird survey, delivering over 25 years of weekly bird counts from more than 14,000 gardens, predominantly occupying suburban and rural locations. Since 2007, a subset of GBW participants has additionally recorded the weekly abundances of butterflies, and it is these data – drawn from 7,971 gardens over the period 2007 to 2020 – that have been used to produce garden-specific abundance trends for 22 widespread butterfly species.

The team of researchers, bringing together staff from BTO and Butterfly Conservation, used a statistical approach developed for butterfly data, first estimating seasonal flight curves – the pattern of abundance throughout the flight period – for each species and calculating the mean relative abundance across sites for each year, before then modelling the long-term trend for the species concerned.

The results showed that half of the species investigated had increased significantly in abundance in gardens between 2007 and 2020. Conversely, only one species – Wall – showed a marginal reduction, though this change was not statistically significant.

Importantly, a strong, positive association between these new, habitat-focused trends and those for UK butterflies more broadly, previously reported by the UK Butterfly Monitoring Scheme (UKBMS), indicates that patterns of abundance in gardens are largely a reflection of the changes that are occurring nationally. However, the results indicate that butterflies appear to be faring better in gardens compared to the wider landscape, with GBW recording significantly greater increases over time than UKBMS.

The study demonstrates that GBW butterfly recording can produce reliable and informative population trends, and it also provides important evidence of the significant role gardens play in sustaining butterfly populations across a wider area.