The study of phenology – cyclical patterns of biological events – has long been used to investigate relationships between climate and natural phenomena.  In this way, events such as bird migrations, animal breeding periods and plant flowing can be predicted, knowledge which has numerous economic and conservation applications.  However, climate change can alter these timings, with dramatic consequences for a range of species.  To illustrate this, a new study in Journal of Animal Ecology has examined the effects of changing temperatures on the spring emergence of butterflies.

Butterflies are one of the UK’s best-studied insects.  As a result of their short life cycles and close relationship with environmental conditions, butterflies are increasingly being recognised as valuable environmental indicators of climate change.  Yet research shows a serious, long-term and ongoing decline of UK butterflies, with 76% of species declining in either abundance or occurrence (or both) since 1976.  Previously, research into their phenology has focused on the emergence of adult butterflies in association with spring temperatures.  But the latest research suggests this may have been ignoring the effects of winter variables.

This study by three researchers from Stockholm University, Sweden, used citizen science data to examine the phenology of spring butterfly emergence in the UK and Sweden.  Previously, butterfly emergence has been predominantly studied in terms of spring temperatures.  “For spring phenology of wild species, the focus has been on the impact of spring temperature,” says lead author Dr Sandra Stålhandske.  “This has caused other variables to be neglected in many studies.  Though it has been shown that winter climate has an effect on plants, agricultural as well as plant species, winter effects have been largely overlooked in analyses of phenology of wild animal species”.  To address this, Dr Stålhandske and her colleagues considered both spring and winter climate.  Five butterfly species that overwinter as pupae across the UK were examined using data from 1976 to 2013, whilst in Sweden one species was assessed using data from 2001 to 2013.

Results showed that warmer springs led to earlier emergence of adult butterflies in all species considered, whilst milder winters delayed the emergence of three of the five study species.  This delaying effect of warm winters is particularly pronounced in data from the last ten years, as winters have decreased in duration.  Since the 1950s, spring temperatures have increased and winters have become shorter, with European winters predicted to be the season that changes most with climate change.  Therefore, the impact of winter conditions on phenology is likely to have an increasing impact in the future, particularly in northern regions.

For Dr Stålhandske, the main implication of this research is the interpretation of response to climate change.  “A warm spring advances butterfly phenology and a warm winter delays it,” she says.  “A species that has been reported to change little in response to a warmer spring may actually have a very strong response to spring warmth, but this change has been counter-acted by the warmer winter.”  So by ignoring the effect of winter duration, incorrect conclusions may be drawn about phenological responses to climate change.

However, such discoveries require long-term datasets – which is where the contributions of citizen scientists show their amazing value.  “Citizen science data allows for analyses across temporal and spatial scales that would otherwise be inconceivable for a given research project to collect,” says Dr Stålhandske.  “Such projects involve the public in current research and creates a dialogue between scientists and the non-academic world that both parties benefit from.”

To participate in the citizen science projects associated with this study, visit the UK Butterfly Monitoring Scheme and Artportalen (Swedish Species Observation System) websites.

Read the full paper

Stålhandske, S., Gotthard, K. and Leimar, O. (2017) Winter chilling speeds spring development of temperate butterflies.  Journal of Animal Ecology.  doi: 10.1111/1365-2656.12673

Sarah Marley
Blog Editor