Isolated Czech butterfly population cycles through booms and busts on an 11-year rhythm

In a small woodland reserve amid the intensively farmed Elbe river alluvium of Central Bohemia, one of Europe’s most endangered butterflies has been quietly performing a remarkable demographic routine. Twenty consecutive years of monitoring show that the Czech Republic’s last autochthonous population of the scarce fritillary, Euphydryas maturna, swings through regular boom-and-bust cycles with a peak-to-peak period of 11 years, its numbers collapsing to barely a tenth of their maxima before recovering again. The finding, published in the journal Nota Lepidopterologica, resolves long-standing puzzles about the species’ elusive status and carries a stark lesson for conservation: a strongly cycling insect cannot be saved within a single site, no matter how well that site is managed.

Euphydryas maturna, the scarce fritillary, is a Eurosiberian woodland butterfly distributed from eastern France across Central Europe and southern Scandinavia to the Urals and beyond, toward the Baikal region. It depends on open-canopy broadleaf woodlands with species-rich herb layers, habitats that once thrived under historical forestry practices such as forest pasture and coppicing. When those practices were abandoned across much of Europe during the twentieth century, the woodlands darkened and the butterfly retreated. In the Czech Republic the decline was especially severe: the species was historically recorded from 29 mapping squares out of 675, but today survives at a single locality. By the late twentieth century it was even considered nationally extinct, until the naturalist Vrabec discovered a hitherto unknown population roughly 12 kilometres from a historically known, then already unoccupied site.

That discovery set the stage for one of the longest continuous monitoring efforts devoted to a single butterfly population in the region. Since 2001, experienced surveyors equipped with binoculars have systematically combed the Dománovice forest reserve each summer, counting the communal silken larval nests that the species builds on ash saplings and low-hanging branches between mid-July and mid-August. Every nest detected is GPS-recorded, and the resulting maps are handed to woodland managers to guide their work. The full survey takes six to ten person-days per season. In four years, mark-recapture studies added estimates of adult numbers, allowing researchers to relate flying butterflies to the nests they produce.

The 20-year time series tells a dramatic story. Two population peaks occurred, reaching 164 larval nests in 2006 and 202 in 2017. Each peak was preceded by a multi-season plateau with more than 100 nests, and each was followed by a season with diminished but still high numbers, then by a bust in which nest counts fell to roughly eight percent of the peak values. The lowest counts ever recorded were just 11 nests in 2010 and 7 in 2020. The gap between the arithmetic mean of 92.6 nests per year and the harmonic mean of 36.3 reveals just how skewed the distribution is toward the bust years. The mark-recapture data showed that about four adults are needed to produce each larval nest, a ratio that varied little with the stage of the cycle.

What makes the pattern scientifically valuable is the population’s complete isolation. Immigration into the Dománovice forest is practically impossible, so the observed dynamics cannot be explained by individuals moving among local populations with independent local dynamics, a factor that often obscures cyclic patterns elsewhere. The 11-year period is long, but it falls within the range known for cyclically fluctuating woodland moths, which spans from three to eight years in species such as the gypsy moth to 12 to 20 years in some budworms. Similar fluctuations have been documented in scarce fritillary populations in Salzburg in Austria, in Steigerwald in Germany, and in Hungary, with the Austrian data showing amplitude and period remarkably similar to the Czech population.

The authors attribute the cycling to pressures of natural enemies, the classic suspect behind abundance cycles in woodland defoliating insects. The butterfly’s communal egg batches and gregarious young larvae attract large numbers of predators and parasitoids, and microbial or fungal infections are also possible. Until recently, the species’ rarity and legal protection precluded experimental quantification of infestation levels. An indirect line of support comes from the spatial distribution of the nests. During peaks, the nests spread evenly across the inhabited forest, concentrating at suitable open-canopy patches, whereas during busts most nests occupy marginal, peripheral positions. Contagion-like threats such as natural enemies or diseases tend to affect populations from their centroids toward the peripheries, sparing the edges, and it may also be harder for searching enemies to locate peripherally situated hosts. A trade-off may thus exist for the butterfly between developing in half-shaded mature forest, which is microclimatically suboptimal but less exposed to enemies, and developing at microclimatically suitable but infestation-exposed clearings.

Notably, the trajectory of increase, boom and bust followed an identical shape before and after 2011, when active management for the species began. Favourable management probably heightened and prolonged the boom phase, contributing to the longer plateau above 100 nests and the higher amplitude of the second peak, but it did not alter the overall pattern. The second cycle nonetheless ended with a steeper decline, a 90 percent reduction from 2018 to 2019, and the authors suggest that unfavourable weather deepened this bust. The year 2018 was historically the hottest ever recorded, and in ex-situ breeding a third of the larvae produced by unusually early-eclosing 2018 adults pupated and generated a partial second generation of 25 adults in September. In natural conditions such late-season adults would perish from scarcity of nectar and larval diet in the advancing autumn. The mild 2018 winter was followed by an Arctic spell in May 2019 that killed half of the pupae during a dozen nights with freezing temperatures, suggesting a lack of pupal cold hardiness in this species.

The cycling pattern also resolves contradictions in earlier reports based on shorter time frames. Alarm calls prompted by the low counts of 2002 and again in 2009 appeared unnecessarily alarmist shortly thereafter, when numbers recovered, but those warnings accelerated the adoption of more favourable site management and propelled the establishment of ex-situ rearing in 2017. Understanding that single-site conservation is doomed in the long term for strongly fluctuating insects, conservationists have already used captive-bred progeny to re-establish a population at the Libice forest, 11 kilometres away, releasing thousands of larvae between 2018 and 2020; searches there yielded 24 egg batches in 2019 and 16 larval nests in 2020. A further transfer is planned for the Žiželice forest, just 800 metres from the Dománovice forest, where open canopy structure is being restored.

The strategy follows metapopulation theory, which predicts that colonies connected by transfers of individuals should achieve mutually asynchronous abundance dynamics, stabilising the entire system against regional extinction. If all goes well, the remnant deciduous woodlands of the Elbe river alluvium will be re-settled by the scarce fritillary, with individual woods rather than clearings within them functioning as local colonies. A looming threat is ash dieback, caused by the fungus Hymenoscyphus fraxineus, which is already visibly declining ashes at all the Czech sites; planting ash saplings and increasing the abundance of the alternative host plant Lonicera nigra are considered possible rescue measures. For now, the 20-year record from Dománovice forest stands as a vivid demonstration that nature’s rhythms can defeat even the most dedicated single-site rescue, and that saving a cycling species means thinking in terms of woods, not clearings.

Subject of Research: Cyclic abundance fluctuations in a completely isolated population of the endangered butterfly Euphydryas maturna

Article Title: Cyclic abundance fluctuations in a completely isolated population of Euphydryas maturna

Article References: John, V., Pavlíčko, A., Vrabec, V., Rybová, V., Andres, M., & Konvicka, M. (2021). Cyclic abundance fluctuations in a completely isolated population of Euphydryas maturna. Nota Lepidopterologica, 44, 213-222. https://doi.org/10.3897/nl.44.69153

Image Credits: AI Generated

DOI: 10.3897/nl.44.69153

Keywords: Euphydryas maturna, checkerspot butterflies, population cycles, natural enemies, metapopulation, butterfly conservation, Czech Republic, larval nest monitoring, habitat management, ex-situ breeding, ash dieback, Cyclic