Butterflies are important sentinels of climate change. Both caterpillars and adult butterflies are very sensitive to weather conditions – they can move long distances to track their preferred climate and they can reproduce rapidly to set up new populations in more favourable conditions. They can also rapidly decline from an area with unfavourable conditions. These characteristics mean that they are one of the groups of species with the quickest responses to our changing climate.
The 35 species of Irish butterflies are just a small fraction of butterfly diversity. There are about 19,000 species of butterflies worldwide. A recent study by Chowdhury and colleagues analysed the areas occupied through time by 1,758 butterfly species worldwide, almost 10 per cent of the butterflies that exist. They found that in five European countries (Czechia, Finland, Luxembourg, Spain and Sweden) at least 50 per cent of the butterfly species showed some change in their recorded range.
The Irish Butterfly Atlas showed that half of our resident species also showed some change in their ranges. Nearly a quarter increased the area in which they occur and just less than a quarter decreased in range, occurring in fewer parts of Ireland through time. This matches the findings for the five European countries in the global study.
Butterflies are important pollinators, including of crops such as cotton, and their numerous caterpillars are eaten by many birds and other animals, making them a critical part of the food web. Butterflies are widely monitored by citizens and scientists so there is relatively good information on their numbers and locations through time.
The Irish Butterfly Atlas documents the locations of Ireland’s 32 resident butterfly species, that live here year round, and three regular migrant species from Europe and northern Africa that spend their summers in our mild climate. The National Biodiversity Data Centre’s butterfly monitoring schemes record changes in the numbers of butterflies in the same locations every summer.
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When faced with changes to their environmental conditions, individual organisms can move to track their preferred environment, or they can stay where they are and either decline or tolerate unfavourable conditions. Individual movements, births and deaths are the foundations of population ecology and determine where a species can occur and how abundant it becomes. Individual movements and tolerances therefore determine where populations and species can survive and thrive.
Human-caused climate change has multiple impacts, including increases in temperature, changes in the amount and timing of rainfall and reduced occurrence of frosts. These changes affect the movements, births and deaths of caterpillars and adult butterflies. This complexity of timing and effects of climate change on weather patterns means some species may benefit and others may lose out.
As our weather becomes more variable through climate change, it becomes hard to predict how butterfly species will respond. Butterflies often have specialist relationships with plant species that they feed on as caterpillars or adults and even other insect species such as ants. The caterpillars of some species feed ants with their “honeydew” excretions – a sweet, sticky byproduct of digestion – in return for protection from predators. This interconnected web of species can be very sensitive to a change in climate conditions that knocks out one of the partners, or a change in the timing of seasonal conditions can misalign their life cycles.
While butterflies are changing where they live due to many pressures including agricultural intensification, land use change, invasive species and habitat destruction, the most commonly cited cause of range changes in global butterfly distributions was climate change and severe weather.
The beautiful scalloped winged comma butterfly is a notable new resident in Ireland. It was first spotted in 2000 in Wexford and confirmed as a breeding resident in 2014. The comma has expanded its range to Ireland from Europe due to climate change. Nine of our butterfly species have disappeared from part of their range in Ireland, according to the Irish Butterfly Atlas and importantly, according to Jesmond Harding of Butterfly Conservation Ireland, no resident Irish butterfly species has increased its population since 2008.
Despite a flurry of butterfly activity in this warm summer, the combination of climate change and habitat degradation will continue to reorganise where butterflies call home and to mess with their finely tuned responses to the environment.
Yvonne Buckley is professor of zoology at Trinity College Dublin and codirector of the Co-Centre for Climate + Biodiversity + Water
