The butterfly population has decreased by 40% in the last three decades in Catalonia. “The figures are worrying and so clear: where before we found 100 butterflies, now there are only 60,” explains Constantí Stefanescu, scientific coordinator of the Catalan diurnal butterfly monitoring program, in an educational manner, referring to the average number of specimens observed. a citizen science initiative that receives the official name of Catalan Butterfly Monitoring Scheme (CBMS) based on the work methodology (Butterfly Monitoring Scheme, BMS) developed in the 1970s by the British biologist Ernest Pollard and currently extended by almost a hundred countries.

The CBMS was launched in 1994 at the initiative of a group of scientists, naturalists and volunteers, among whom the biologist Francesc Giró stands out as an inspiration, a knowledge of the system devised by Pollard from its origins, as Constantí Stefanescu recalls.

More than 250 people collaborate in collecting data and preparing studies in this program, which is based at the Museu de Ciències Naturals de Granollers and has the support of the Generalitat de Catalunya and the Xarxa de Parcs Naturals de la Diputació de Barcelona. . The simplified balance of the long history of the CBMS (“30 years of counting butterflies”, the official motto of the celebration) includes the counting of more than three million butterflies, of 190 different species.

Initially, the CBMS monitoring network consisted of 11 sampling stations located in the northeast quadrant of Catalonia. In 2023 (once the 30th season is over) there were already 240 stations providing data. Geographic coverage has also improved drastically, and data is currently collected in much of Catalonia, the Balearic Islands and Andorra. In Andorra there is its own monitoring program, the BMSAnd, although the information it provides is integrated with that of the CBMS in a common database.

The BMS work system is based on the creation of several transects, itineraries or fixed routes that are periodically traveled by the participants in the program (naturalists or volunteers prepared for this type of monitoring) with the aim of collecting data on the presence of diurnal butterflies (number of specimens and name of the species observed). The interesting CBMS page on the internet explains in detail the methodology for studying butterflies. The first itinerary with the BMS model in Catalonia was launched in Cortalet, in the Parc Natural dels Aiguamolls de l’Empordà, in 1988.

Taking a walk through nature and with instruments as simple as binoculars, a species recognition guide or manual, a butterfly net (entomological aerial net, for experts) and some field cards (to take note of observations), Participants travel through the itineraries they have signed up for and finally transmit the information to the program. The data systematically accumulated over years helps to know the presence and conservation status of the species and, together, draw conclusions about their evolution.

“The people who participate in the project accumulate a lot of experience, some have been doing the same route for 30 years, and they can identify and count butterflies with ease, but in some cases the butterfly net is used to be able to observe some specimens in more detail, because there are species that they are very similar to each other; In all cases they end up being released, we do not hunt butterflies,” Constantí Stefanescu tells La Vanguardia.

CBMS field work is carried out from March 1 to the end of September, “although in recent years, with the increase in temperatures, the observation period is extended until October,” explains Stefanescu.

Climate change is undoubtedly one of the factors that is causing the decline in diurnal butterfly populations. “In Catalonia specifically, the increase in temperatures, heat waves and drought in recent years have caused very significant losses in some species and areas. In some cases, the problem is the decrease in vegetation on which to feed, but in others, temperatures exceed the thermal regulation capacity of these insects; Butterflies literally burn to death,” explains Constantí Stefanescu, researcher at the Museu de Ciències Naturals de Granollers (coordinator of the BiBIO research group) and associate researcher at CREAF.

The other two major threats to butterflies are the inappropriate use of insecticides and changes in land use, mainly due to the loss of meadows and pastures. “Many species of butterflies depend on the existence of open spaces and vegetation such as that found in meadows, and in Catalonia traditional agriculture and livestock are being lost, so forests and closed vegetation are gaining ground,” remembers Stefanescu. The change in temperatures and land use also produces the colonization of some species in areas where their presence was not common but, on many more occasions, the reality is that a decline in specimens and diversity is occurring.

In Catalonia the presence of 203 species of diurnal butterflies has been described so far. At the moment, there is no evidence of any extinction or disappearance of species throughout the territory, “but we do have documented local extinction of species and in the 30 years of existence of the CBMS we have calculated that, on average, there has been a 40% decrease in the butterfly population in Catalonia,” concludes Constantí Stefanescu.