Re-introduction of Spanish moon trefoil in Illa Grossa, Columbretes Islands, Spain

Summary

Spanish moon trefoil (Medicago citrina), a Critically Endangered shrub found on only ten Mediterranean islets with under 2,000 total plants, went extinct on Illa Grossa in Spain's Columbretes archipelago by the 1960s through overgrazing by introduced rabbits and overexploitation for lighthouse-keeper fuel, surviving only on two nearby islets. Once the archipelago was transferred to Valencia's regional government and protected as a Natural Park in 1987, the Service of Biodiversity hunted 213 introduced rabbits with traps, bows and arrows to avoid disturbing seabird colonies, removed over 400 young invasive prickly pear plants while deliberately preserving mature ones as bird and pollinator food, and between 1988 and 1996 planted more than 800 Medicago citrina specimens grown from seed collected on the two donor islets, with supplemental irrigation for the first two to three years. A 1997 outbreak of the invasive scale insect Icerya purchasi, carried in on migrating birds, destroyed nearly 66% of both the native and reintroduced population within two weeks, prompting the annual spring release of a predator beetle, Rhodolia cardinalis, as ongoing biological control. By the mid-1990s the first reintroduced plants flowered, and the population has grown to 220 adult plants with active natural recruitment, sustained partly by an unexpected pollinator relationship with insects that also use the invasive prickly pear's decaying fruit. About 40% of young plants died from prolonged summer drought in the program's first five years. The project rates itself Successful, crediting sustained rabbit and weed eradication plus continuous management by Nature Reserve keepers.