Uniform weather conditions across the Åland Islands threaten the Glanville fritillary

The Glanville fritillary butterfly inhabits a network of meadows in the Åland Islands that has been studied for nearly thirty years. The continuity of the research is particularly important now that the climate is changing rapidly.

The Glanville fritillary took its place in the general mosaic of ecology after 1991, when Ilkka Hanski, who would later become an Academy of Finland Research Fellow, arrived in the Åland Islands to construct a model for the purpose of testing the metapopulation theory. The Åland Islands include approximately 4,000 meadows that make suitable habitats for butterflies, and each meadow has been surveyed in both spring and autumn since 1993.

The most recent results of the long-term monitoring indicate that weather conditions in the different areas of Åland have become uniform, which is alarming news as regards the local butterfly populations.

Butterflies follow rhythmic climate variation

Plant and animal habitats, including forests and bogs, become fragmented when humans extract peat, clear land for cultivation and build urban centres and roads. Separated habitat fragments form a kind of “archipelago”.

These habitat archipelagos as well as climate change are typical characteristics of constantly increasing global change. However, we still have very little information about how climate change affects different species in fragmented habitats.

The Glanville fritillaries of Åland can help to address this problem. The meadows inhabited by butterflies in Åland form a natural test system that allows researchers to examine life in a fragmented habitat.

The most recent research results from Åland show that after a gradual transition over the past two decades, the Glanville fritillary populations occupying the different parts of the islands have begun to vary in synchrony. In other words, butterfly numbers are following the same cycle everywhere in Åland. This makes the butterfly population very vulnerable.

“In peak years, all the meadows inhabited by butterflies are teeming with them. In bad years, the numbers plummet everywhere. Such swings significantly increase the risk of local extinction,” says postdoctoral researcher Aapo Kahilainen of the Faculty of Biological and Environmental Sciences of the University of Helsinki.

In the light of recent research results, it is likely that the dynamics of the Glanville fritillary butterfly are connected to weather patterns.

“The weather used to vary from area to area in Åland: the sun could be shining in the East while it was raining in the West. Now the weather conditions are quite uniform everywhere in Åland,” says Kahilainen.

“Bad weather conditions can collapse all local populations in Åland simultaneously, leaving no repository of substituting butterflies.”

The economic impact of synchrony

Glanville fritillary populations are hardly the only ones to vary according to the weather. We can assume that the increased synchrony of weather conditions will have a similar impact on many natural organisms, because most species are at least indirectly dependent on specific weather conditions.

“Pest insects are a good example of this. If the variation of pest insect populations were to become more synchronised, similarly to butterflies, it might lead to peak years where mass populations cover a larger area than previously and cause more economic losses,” says Kahilainen.

The gypsy moth (Lymantria dispar) is a widely studied pest insect and offers an example of population synchrony regulated by weather conditions. In peak years, mass occurrences of gypsy moth larvae can eat the trees bare of leaves in a large area, seriously damaging tree stands.

However, Kahilainen points out that swings can also be part of natural variation. The monitoring material collected on the Glanville fritillary over nearly 30 years is unique, but covers a brief period from the perspective of studying the weather. However, it does introduce new elements to research.

“So far, climate change research has mainly focused on changes in mean values or any variations from one year to the next. In other words, we have been studying, for example, how temperature variation from year to year affects different organisms. Our research focus is on any changes in the way weather varies from one area to the next, which can have a significant impact on natural populations.”

Continued surveying is important in a changing climate

Academician Ilkka Hanski, who passed away in 2016, called butterfly research the particle accelerator of population ecology, comparing it to an expensive device that can be used to answer many questions that ordinary smaller research projects cannot.

Two research teams are currently in place to carry on Hanski’s legacy. Researcher Marjo Saastamoinen is in charge of the actual butterfly project and Professor Anna-Liisa Laine’s team has studied the interaction between the ribwort plantain, which is a food plant of the Glanville fritillary, and its parasites.

However, in spite of the two teams’ efforts, the future of the project is not guaranteed.

“It is very difficult to find funding for long-term monitoring projects. We grapple with financial issues year in, year out,” says Saastamoinen. “So far, we have collected over twenty years’ worth of data on the variation of the butterfly population inhabiting the “archipelago” of meadows in Åland. Ensuring an uninterrupted series of observations is particularly important now that the climate is changing so rapidly.”

Many biology students have observed butterflies as part of their studies after butterfly monitoring was first introduced. Åland has offered a suitable environment for learning how to conduct fieldwork and the basics of experimentation. More students will be engaged as project trainees so that they can develop a thorough understanding of the topic through lectures and other studies.

“Students would suffer if we ran out of funding, because they would lose an opportunity for true on-the-job learning,” says Saastamoinen.

 

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Studying the butterfly population “archipelago" in Åland

A population comprises all the organisms of one species living in a particular area. A metapopulation consists of a group of separated local populations, connected by dispersal.

To better understand the concept of metapopulation, we can imagine a cluster of islands in a sea, each occupied by the same organism. Populations go extinct on different islands at a steady rate. However, the vacated islands are quickly inhabited from occupied islands.

This colonization-extinction balance within a metapopulation ensures that a similar percentage of the available habitat is  inhabited from year to year. The likelihood of local extinction and the transfer of organisms between islands is regulated by the size of the islands and their distance from each other. Small islands located far from other islands are relatively inaccessible. Small islands can also provide for a smaller number of organisms than large islands. In other words, large islands can maintain larger populations than small islands.

Human efforts to cut down forests, cultivate land and build urban centres lead to habitat fragmentation, where patches of habitats form an “archipelago”.

Over thousands of years, human activity has created a similar fragmented living environment for the majority of species inhabiting the Earth. The fields of the Åland Islands form an archipelago of habitat patches naturally suitable for the Glanville fritillary butterfly (Melitea cinxia). This is why the network of meadows in Åland is naturally suitable for testing the metapopulation theory. 

A focus on droughts and land use in relation to accelerated climate change

In the autumn, Glanville fritillary larvae spin a winter nest for diapause. Later in the season, a large group of students – up to sixty – locates the spun nests and marks their locations. In the spring, a smaller taskforce of postgraduate students returns to see how many nests have survived the winter.

A large number of variants have been studied in the patches over the years, including size, distance from other patches, food quantity for larvae and land use in the surrounding areas. The study has been expanded to include questions related to butterfly genotype as well as behavioural experiments in the testing facilities constructed in connection with the Lammi Biological Station.

The “Glanville fritillary particle accelerator” is a unique environment and has so far produced research data on, for example, the causes of changes in a species, the effects of habitat destruction and the significance of parasites for organisms. In compliance with research results published in the spring of 2018, the accelerator will be increasingly used to identify the effects of climate change.

Climate-related research topics include, for example, the impact of extended periods of drought on fragmented natural populations and their evolution. Studying droughts is important because they are expected to become more common as climate change intensifies, and their long-term effects on natural populations may be unpredictable.