Glanville fritillary

The metapopulation theory was previously largely based on the assumption that all suitable habitats for a species are of the same size and distance of each other. However, the Glanville fritillary was observed to live in a network of dry meadows in Åland, where the size, shape, quality, and distance between meadows vary across the region. Data collected from the autumn survey of the Glanville fritillary revealed that the probability of a meadow being occupied depends on its size and its connectivity to other occupied meadows. According to the so-called incidence function model, the risk of local population extinction decreases as meadow size increases, and the probability of colonization of a new meadow increases with decreasing distance to the nearest occupied meadow.

The Åland data has been used to develop a model that can calculate extinction thresholds for other species. The model accounts for species-specific traits such as mobility and reproductive capacity. If the loss or excessive fragmentation of suitable habitats causes the extinction threshold to be crossed, the population in the area will eventually go extinct. However, extinction will not occur immediately; instead the remaining habitat will continue to sustain the population for a time. In metapopulations, local extinctions become more common and new colonisations less likely until the entire metapopulation goes extinct. This phenomenon is called extinction debt.

The research has also examined which types of habitats contribute positively to the viability of the metapopulation. The knowledge produced by this research has made it easier to target conservation efforts to key areas and to implement them more effectively.

The Åland system shows that the increased frequency of extreme weather conditions has had an effect on the population dynamics of the Glanville fritillary. The average population size of the butterfly has stayed relatively constant, but the year-to-year fluctuations have become more prevalent. A single unfavourable year can cause dramatic changes in the population size of the butterfly. The effect was evident in 2018 when the unusually hot and dry summer caused a crash in the population size of the Glanville fritillary. 

The Åland Islands research quickly expanded to include the interactions between multiple species in the food web. The butterfly’s host plant ribwort plantain occurs throughout the island, while spiked speedwell is found only in the northwestern part of the Åland Islands. The regional variation in the host plants has been observed to influence the female butterfly’s choice of oviposition plant, which in turn affects local colonisations and extinctions.

Two species-specific parasitoids of the Glanville fritillary larva (Cotesia melitaearum and Hyposoter horticola) were included in the surveys already in the 1990s, and they were found to follow their own population dynamics within the system, in which meadows occupied by the butterfly provided suitable habitats for the parasitoids. Both parasitoids attack and kill Glanville fritillary larvae before the butterfly develops into an adult. Cotesia melitaearum occurs very locally in the Åland Islands, whereas Hyposoter horticola typically parasitizes about one-third of all larvae in a Glanville fritillary nest. Hyposoter horticola has also been observed to have a hyperparasitoid, Mesochorus stigmaticus, which parasitizes the H. horticola larva inside the Glanville fritillary.

The viability of a metapopulation depends not only on ecological and environmental factors but also on genetic factors. Our research has focused on how habitat structure and population-biological factors together influence a population’s genetic diversity and local genetic adaptations. The genetic structure of populations has been studied in the Glanville fritillary in the Åland Islands, as well as at higher trophic levels – in this case, the species-specific parasitoids of the butterfly larva. These studies have shown that genetic diversity decreases as habitat fragmentation increases, including when moving up in trophic levels.

One of the earliest examples from scientific research demonstrating the harmful effects of inbreeding in natural populations was found in the Glanville fritillary data from Åland in the late 1990s. The findings showed that inbreeding reduced larval viability, adult butterfly lifespan, and hatching success of egg clutches. This was found to reduce population growth potential and increase the risk of local population extinction. The effects of inbreeding in natural populations were revisited in the 2000s using the same system, and these studies showed that the harmful effects of inbreeding can be mitigated within a single generation if gene flow can be established between local populations through dispersal.

Dispersal enables the metapopulation to remain viable in a highly fragmented habitat. The Glanville fritillary is a weak flyer, capable of traveling only a few kilometers over its lifetime. Studies have shown substantial variation in the flight ability and the Pgi gene among adult individuals. The Pgi (phosphoglucose isomerase) gene codes for an enzyme essential in metabolism, and its different variants have been associated with  variation in the flight performance in both field and laboratory experiments. Habitat structure has also been observed to influence the distribution of different Pgi alleles within populations. This gene does not explain all the variation in flight ability, but it affects also other species traits such as adult lifespan and reproductive success. For these reasons, variation in the Pgi gene can account for up to 30% of the fluctuations in metapopulation size.

After the mapping of the butterfly’s whole genome, research has aimed to identify which regions of the genome influence key fitness-related traits of the species. These links between the butterfly’s genome and phenotype have been studied at the Lammi Biological Station using various experimental setups the individuals collected from the Åland Islands.