The record dry summer of 2018 collapsed the Glanville fritillary butterfly metapopulation

Research from the University of Helsinki shows that the Glanville fritillary butterfly in the Åland islands, Finland, experienced a tremendous population decline due to the record dry summer of 2018.

Long-term ecological monitoring schemes are of key importance to increase our understanding of the impacts and predictability of climate change.

“When we visited the Åland islands at the end of June in 2018, it became clear that we would experience a very unusual yet interesting year in regards to our long-term monitoring. Even the meadows that would normally be lush and green had only dry and brown host plants that were clearly unsuitable for the caterpillars to feed on,” says Associate Professor in evolutionary ecology,  Marjo Saastamoinen from the HiLIFE Helsinki Institute of Life Science and Faculty of Biological and Environmental Sciences, University of Helsinki.

In a study Marjo Saastamoinen, together with her research group Life-history evolution, and Miska Luoto, a professor in physical geography, aimed to understand the impacts of extreme climatic conditions and the predictability of these impacts in natural populations. They combined climate and satellite data with over 25-years of ecological monitoring data from the Glanville fritillary butterfly in the Åland islands, Finland. The butterfly persists on the archipelago in a network of meadows and pastures in a large metapopulation. 

The research showed that the climatic conditions, as well as the overall productivity of the  vegetation within the butterfly's habitats, were very different from data obtained over the past decades. Utilising mathematical models, the researchers were able to show that the decline in the number of butterflies was strongly linked to the climatic conditions, and the overall decline in population size was predictable due to the extreme drought.  

The population dynamics of the Glanville fritillary butterfly have been extensively studied for decades at the University of Helsinki. The research was initiated by the internationally acknowledged, late academician Ilkka Hanski. The metapopulation of the Glanville fritillary has become a model system in ecology and continues to be studied by several researchers. However, when Saastamoinen and her colleagues tried to utilise already existing information about the extinction risk of local populations rather than the metapopulation as a whole, the mathematical models did not accurately predict the local extinctions in 2018. 

The research highlighted that even though it was evident that the extreme drought would lead to a massive decline in population size, the researchers could not predict which specific populations would survive and which would go extinct. Thus, even in a system with data collected over 25 years, predicting the consequences of rare events is extremely difficult. 

Saastamoinen points out: “New data from each year adds to the value of long-term monitoring schemes. Extreme climatic events are predicted to become more common in the future due to climate change. Long-term monitoring of ecological data is crucial for our aim to understand how biodiversity will be impacted by climate change and how to mitigate further loss.” 

Original article: 

van Bergen E, Dallas T, DiLeo M, Kahilainen A, Mattila ALK, Luoto M & Saastamoinen M (2020). The effect of summer drought on the predictability of local extinctions in a butterfly metapopulation. Conservation Biology, early online.