The drought is having very negative environmental effects in much of Catalonia. Butterflies are also affected by the lack of rainfall that has been recorded in some regions for three years. The lack of water and the disappearance of humid areas are not the only causes of the alteration of the populations of these lepidopterans, but they do stand out as obvious immediate reasons, according to the evaluation of the 2023 season of the Catalan Butterfly monitoring project. Monitoring Scheme (CBMS).
“2023 will be the third consecutive year of widespread drought in Catalonia and butterfly populations suffer its consequences,” highlights the annual report of this citizen science project, which is celebrating its thirtieth anniversary this year. In large numbers, the CBMS evaluation indicates that in 2023 it was detected that “56% of the species have decreased and 44% have had a population increase.” In fact, despite the very negative effects of long periods with scarcity of rains, “there have also been species capable of taking advantage of the periods in which the rains are concentrated, such as May 2023 in much of the territory,” detail the experts of this entity coordinated by the Museu de Ciències Naturals de Granollers.
The species that decreased the most in 2023, compared to the previous year, was the dark striped (Polyommatus ripartii), with a change of 90% of its specimens in a single year. This is an example of the great decline in populations of butterflies that are specialists in Mediterranean mountain areas.
The next positions on this list of butterflies that lost the most population in 2023 are occupied by the peat bog pearl (Boloria eunomia), the levana (Araschnia levana) and the striped moray eel (Eumedonia eumedon). The copper mantle butterfly (Lycaena hippothoe) and the two-pointed ino butterfly (Brenthis ino), also species of Pyrenean peatlands, are also among the 10 most affected species in the 2023 season.
The CBMS annual report indicates that in addition to the collapse in the populations of peat bog species, in the subalpine region there is also a large decline in species such as the serrana woodpecker (Carcharodus flocciferus) and the spiny enameled (Plebejus argus). In the humid Mediterranean region, the decline of the blackthorn leaftail (Satyrium acaciae) stands out, inhabiting open oak forests in central Catalonia, where drought has also played an important role.
In 2023, a total of 28 new local extinctions have also been confirmed in the CBMS network. We consider a population extinct when the species has not appeared in an itinerary for at least 4 years after being present for a minimum of 4 years (it therefore takes 8 years to determine this). Extinctions have occurred throughout the territory, but they have been especially concentrated in different itineraries in the Mediterranean area, with long series of the Natural Parks Network such as Collserola, Corredor, Sant Llorenç del Munt and Montseny. Also in central Catalonia, on the Sallent itinerary, we have confirmed the extinction of three specialists in a single year.
Constantí Stefanescu, scientific coordinator of the CBMS, explains in statements to La Vanguardia that, first of all, “drought means that the quality of nutritional plants is greatly reduced, which causes much higher mortalities in the caterpillars that they feed on.” Especially the youngest caterpillars, from the first stages, are very sensitive to the hardness of the leaves and, in addition, they easily dehydrate and die. More subtly, Stefanescu explains, “drought means that the leaves of many plants close their stomata to avoid transpiring and losing water, which increases the temperature at the leaf level, in some cases reaching the limits that caterpillars tolerate.” .
On the other hand, drought means much less flowering in general, and flowers are a basic element for the survival of adult butterflies: they need nectar to carry out activities and, in the case of females, to produce eggs. Furthermore, of course, drought is very often associated with very high temperatures, which reduce the longevity of adults and can jeopardize the survival of caterpillars when thermal tolerance limits are reached, explains Constantí Stefanescu.
A clear example of how drought affects a Mediterranean butterfly known as the honeysuckle maiden (Euphydryas aurinia), which is in strong decline in Catalonia. “In this species, the caterpillars are born at the beginning of summer or late spring, and must manage to reach the fourth instar to be able to enter diapause until the end of the following winter. If there is a lot of drought, the plants (honeysuckle) dry out and the caterpillars They are not able to reach the stage in which they can enter diapause, details the scientific coordinator of the CBMS.
Furthermore, if temperatures are anomalously high during winter, diapause may be altered and caterpillars consume resources during the peak of winter that do not allow them to survive the rest of the winter months afterwards. In general, we have seen that warm winters are very negative for butterflies, possibly largely because they mean a consumption of reserves that, under normal conditions, would not take place, says Constantí Stefanescu.
