University of Helsinki: Weather shifts re­lated to cli­mate change are a threat to but­ter­flies

Climate change is transforming butterfly habitats, but is this beneficial or detrimental to them? According to a doctoral thesis completed at the University of Helsinki’s Faculty of Biological and Environmental Sciences, shifts in weather conditions caused by climate change constitute a threat to butterflies. However, protecting habitats and increasing their biodiversity may help mitigate the risks.
Butterflies undergo a complete metamorphosis: first, the female lays an egg. Once hatched, the tiny caterpillar will multiply its weight several times over before cocooning in a pupa, where the adult butterfly develops. Research into insect ecology often focuses on specific stages in the insect’s lifecycle, which means that the understanding of environmental effects is based on observations from a tiny segment of the insect’s life.

“The duration of the various stages varies widely between butterfly species, but the species in my study, the Glanville fritillary (Melitaea cinxia) and the Clouded Apollo (Parnassius mnemosyne), share the feature that their adult stage only lasts for approximately one month each year. This means that these butterflies spend most of their one-year lifespan in stages that are largely invisible to humans, but crucial for their survival,” explains Susu Rytteri, MSc, of her doctoral thesis.

Rytteri’s research used long-term monitoring data, which for the Glanville fritillary have been collected by the University of Helsinki and for the clouded Apollo by the Finnish Environment Institute. The long-term monitoring datasets were combined with detailed short-term data on the caterpillars of the Glanville fritillary, collected both through monitoring wild populations and manipulating snow conditions in winter. With such diverse data, it was easier to understand how changes in weather during different seasons impacted the butterflies, and which stages of their life cycle were particularly susceptible to change.

In­creas­ing hab­it­ats boosts dis­persal in the warm­ing North
In Finland, climate change is increasing winter temperatures in particular, as well as reducing snow cover and accelerating the arrival of spring. At the same time, the weather is expected to become more volatile, with long dry periods interspersed with seemingly endless rain. The study noted that caterpillars were less likely to survive if there was no snow cover to protect them or if spring arrived exceptionally early. In addition to such direct effects, caterpillars lacked sufficient food during very early springs, as they would awake from their winter hibernation before the plants they eat had had a chance to grow.

To balance these detrimental effects of warmer winters and springs, warmer summers promote the spread of butterflies and other insects further north. The study found that butterflies were most likely to migrate from their native areas to new meadows during flight seasons that had the most sunny and hot days. To boost this dispersal of butterflies, it is particularly important to increase the area of the habitats and make sure they are connected – simply put, to make more habitats available to the butterflies.

“Butterflies and other important pollinators require warm and sunny weather during their adult stage, as heat helps them move and eat efficiently. However, warmer springs will only boost the dispersal of insects when they have somewhere to go: for many insect species, habitats must be increased for them to be able to move to new areas. For example, Finland would need more meadows with diverse plants and insects,” states Rytteri.

Susu Rytteri, MSc, will defend her doctoral thesis entitled “Butterflies in changing weather conditions: implications for ecology and conservation” on 30 March 2021 at 12.00 at the Faculty of Biological and Environmental Sciences, University of Helsinki. The public examination will be streamed online.

Senior Researcher Toke Thomas Høye from the University of Aarhus, Denmark, will serve as the opponent and Associate Professor Marjo Saastamoinen as the custos.

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