Garden Heroes are Aiding Butterfly Conservation in the UK

In a new study published in the journal Insect Conservation and Diversity, Plummer and colleagues have shed new light on the significant role of private gardens as a haven for butterfly populations in the UK. Using data collected over 14 years (2007–2020) from the Garden BirdWatch survey, the largest structured bird survey in the UK, the researchers examined trends in 22 butterfly species across nearly 8,000 gardens. This research paints a rich, vibrant picture of insect life in our backyards and uncovers an intriguing fact—butterflies seem to be thriving better in domestic gardens compared to the broader landscape. These findings underscore the importance of our humble gardens as critical sanctuaries for these mesmerising insects.

According to the research, trends in butterfly populations in private gardens closely mirror national trends, pointing to the fact that these garden spaces may be an invaluable habitat for these insects.

Based on data from the Garden BirdWatch project, the study looked at twenty-two of the most common and widespread butterfly species in the UK. Surprisingly, the numbers of these species have generally increased in gardens since 2007, a trend that contradicts global observations of insect decline. Even more fascinating, butterflies appear to be faring better in gardens than in the wider countryside.

In recent years, scientists have been increasingly alarmed by the significant declines in invertebrate populations globally, with particular emphasis on insects, such as butterflies. The primary culprit? Habitat loss, often due to farming and urban development. 

Researchers have proposed that residential gardens, which make up a substantial portion of urban green space, could serve as safe havens for these threatened insect species. This is especially relevant in the UK, where almost a third of urban land comprises of gardens. By providing a diverse range of plants and structures, gardens could act as essential habitats for insects, especially in areas where the surroundings are dominated by monotonous agricultural fields. Consequently, the scientific community, conservationists, and the public have started seeing the value of gardens and their owners in supporting insect conservation.

Yet, how well do gardens actually support insects, specifically butterflies? This is an area where scientists have lacked robust evidence. The UK has a long tradition of monitoring butterflies, yet much of this monitoring is concentrated in natural and rural habitats. Monitoring in private gardens has been limited due to access restrictions and the lack of a structured approach.

However, earlier successes in garden-based monitoring of birds and mammals, through programs like the British Trust for Ornithology‘s Garden BirdWatch and the Garden Moth Scheme, have shown that such efforts are indeed feasible. As butterflies are charismatic creatures that readily capture public interest, it seems reasonable to believe that homeowners could similarly contribute to tracking butterfly populations in their gardens. This research aims to test that belief.

The authors build on an initiative started in 2007, where butterfly monitoring was added to the established Garden BirdWatch project. They aimed to assess how valuable such long-term monitoring is for understanding butterfly population trends in gardens, in comparison to trends observed in the wider countryside. 

In this study, butterfly watchers across the UK were tasked to record the maximum number of butterflies they spotted in their gardens each week, particularly focusing on the period from March to October, when most UK butterfly species are active. To ensure the data collected was reliable, only observations from sites that regularly reported non-zero butterfly counts were included. This method allowed the scientists to track changes in butterfly abundance over time.

To add depth to their analysis, the team also used information about the landscape around each participating site. For instance, what percentage of the surrounding area was forest, farmland, or urban land? This helped to understand how the landscape context might be influencing butterfly numbers.

Each week’s butterfly counts were carefully examined to exclude any extraordinary or outlying values which could skew the results. The species that were recorded in at least 15 sites each season were then selected for further analysis to track population trends over time.

The scientists used a statistical model, known as the Generalised Abundance Index, to estimate annual butterfly abundance. This index was used to generate a flight curve, showing the rise and fall of butterfly numbers over each season. By comparing these patterns year on year, they were able to identify trends in butterfly populations over the long-term.

Out of the 22 species they closely studied, more than half showed significant increases in population. Even more, some species increased their numbers drastically. The Marbled White and Large Skipper species both increased by more than 200%! That’s a significant uptick in butterfly fluttering happening across UK gardens.

Not all news was good though. The Wall Butterfly (Lasiommata megera), although not significantly, had a slight decline in numbers. But, luckily, this was the only one out of the 22 species studied.

Interestingly, the trend wasn’t the same for migratory species. When scientists left these butterflies out of their calculations, the increase was more noticeable in gardens. Non-migrating species showed a greater boost in their numbers within gardens compared to the countryside. This suggests that resident butterflies might be finding something particularly appealing about our gardens.

In an email to Botany One lead author Kate Plummer says: ” I’m not sure that the comparative increase in gardens, compared to elsewhere, surprised me as much as confirmed my belief that garden habitats are (or have potential to be) hugely valuable for wildlife. Our gardens can provide butterflies with the shelter and food they need to thrive, from nectar source for adults to food plants for their caterpillars.”

Citizen scientists have played a significant role in these findings. The Garden BirdWatch project relies on volunteer data collection, which has proven to be a reliable way to monitor butterfly populations in gardens nationwide. Though the study acknowledges the potential limitations of volunteer-based monitoring, such as inaccuracies in species records, the large sample sizes and consistent data collection have helped produce credible results.

What does this mean for the average gardener? Simply put, your garden can make a real difference to butterfly conservation. While the mobile nature of many butterfly species means that trends in gardens may reflect patterns on a wider scale, the differences in population changes in gardens compared to the broader landscape suggest that gardens could be providing essential resources for these insects.

Another important factor for individual gardeners is that, by working together, they can be part of a chain of oases for butterflies to move a landscape. “Connectivity is key to enabling butterflies, and other wildlife, to make use of human-dominated landscapes, like towns and cities,” says Plummer. “Local authorities, landscape planners and policymakers all have a responsibility to ensure that urban greenspaces function as connected pockets of wildlife-friendly habitat, helping all forms of wildlife to live and breed in urban landscapes.”

How can you make your garden butterfly friendly? On their website the BTO say that “The key to attracting butterflies and moths is to understand which flowers they visit and why.” They offer some tips on what to plant and how to manage your garden, including when to trim your buddleijas. However, the butterfly is only the last stage of the invertebrate.

“Butterflies would need to breed in gardens for them to be their primary habitat, which, at the moment, is not commonplace for a lot of species. Appropriate planting can help to increase chances of breeding activity,” says Plummer. So you can make your garden a good butterfly garden, by making it a caterpillar garden. That requires some different planting, says the RHS.

Many of the flowers listed as Plants for Pollinators will attract the more common and mobile species of adult butterfly but most are unsuitable as food plants for the larvae. The caterpillars eat leaves and often have a narrow range of plants. With the exception of the large and small white butterflies, the larval food plants are often wild plants.

RHS

The RHS lists some suggestions, along with what sorts of butterflies are looking for what sorts of plants in the garden. Tolerating or even encouraging caterpillars to feed in your garden won’t just help the butterflies. A good supply of caterpillars will provide food for birds and other animals, with 20,000 needed to raise a clutch of Blue Tits.

In a nutshell, your garden is not just your garden; it’s a mini wildlife reserve contributing to insect conservation. It’s a peek into the effects of environmental change. Every little choice we make can have a huge impact. So, next time you’re in the garden, think about the choices you’re making. You could be a key player in ensuring these magnificent creatures continue to grace our gardens for generations to come.

READ THE ARTICLE
Plummer, K.E., Dadam, D., Brereton, T., Dennis, E.B., Massimino, D., Risely, K., Siriwardena, G.M. and Toms, M.P. (2023) “Trends in butterfly populations in UK gardens—New evidence from citizen science monitoring,” Insect Conservation and Diversity. Available at: https://doi.org/10.1111/icad.12645.

Updated: 2023 June 9 to include quotes from Dr Kate Plummer.