Islands in the Sky
The granite peaks of the Sierra de Gredos, west of Madrid, are "sky islands," isolated, high-altitude ecosystems separated by areas of lower, warmer terrain. During the Pleistocene ice ages, these mountains became glacial refuges. While glaciers filled the valleys, the peaks remained as ice-free sanctuaries, trapping populations of plants and animals. This isolation, allopatric speciation, spurred the evolution of unique species. Today, the Sierra de Gredos has many endemic butterflies, with several species and subspecies found only in its high-altitude grasslands and rocky slopes.
These butterflies are relics of past climate change. As glaciers advanced, they pushed cold-adapted species south. When the ice retreated, some populations moved back north, while others ascended the mountains, becoming permanently stranded. Genetic studies of these butterflies confirm their divergence from lowland relatives, a process driven by thousands of years of geographic separation. The Sierra de Gredos is a natural laboratory for studying evolution in response to climate shifts.
High-Altitude Specialists
Life above 1,800 meters (5,900 feet) presents unique challenges, and the butterflies of Gredos have adapted accordingly. Many species here, like the Piedmont Ringlet (Erebia meolans), are restricted to the alpine grasslands that appear after the pine and oak forests give way to scrubland. These habitats exist between 1,400 and 2,000 meters. One endemic subspecies is the Gredos Apollo (Parnassius apollo hispanicus), a large white butterfly with red eyespots on its hindwings. Apollo butterflies are typical of high altitudes, requiring cold winters and sunny summers to complete their life cycle.
The Spanish Argus (Aricia morronensis) is an Iberian endemic with strong populations in Gredos. It thrives in open, rocky areas above 1,300 meters where its specific host plant, Erodium carvifolium, is abundant. The larvae of the Spanish Brassy Ringlet (Erebia hispania) feed on high-altitude grasses of the Festuca genus. The short flight season, typically from late June to August, is an adaptation to the brief alpine summer.
The conservation of these specialized insects is a concern. As climate change warms the planet, these cold-adapted species are forced to move to ever-higher elevations, a strategy that has limits when you run out of mountain. More than 40% of Europe's endemic butterflies are now considered threatened or near-threatened. Long-term monitoring projects in the Sierra de Gredos track population changes, providing data on how these unique glacial relicts are responding to a warming world.