Eradication of rabbits from islets is essential for conservation of microinsular vegetation and narrow endangered flora: the case of Medicago citrina (Fabaceae) in s'Espartar islet (Balearic Islands, Western Mediterranean Basin)

Abstract

[eng] The introduction of exotic herbivores to islands is one of the most harmful challenges to the conservation of insular biodiversity, causing the extinction or geographical reduction of many plant species worldwide and motivating conservation actions from environmental managers. Here, we evaluated the recovery of plant communities, specifically the endangered Medicago citrina, on a small islet close to Ibiza (Western Mediterranean Basin) after the complete eradication of introduced rabbits (fully eradicated in 2016). To evaluate the variation of plant richness and coverage, five permanent plots were installed in 2015 to record over five years. Additionally, an exhaustive census geolocating all M. citrina individuals was carried out in 2019. Total vegetation cover, plant richness and Shannon's diversity index significantly increased throughout the study period. We counted 2,322 M. citrina individuals over 15 cm tall and a large number of seedlings. Therefore, this population not only successfully recovered, but also established the largest population in the entire geographic distribution of this species. The demographic structure is dominated by young individuals and a few large individuals, and the distribution across the island is heterogeneous since the largest spots of individuals were located in ravines where air currents probably provided seeds from plants located in inaccessible cliffs. However, the appearance of the invasive insect Icerya purchasi now threats the population of M. citrina as it feeds on the tissues of adult plants and compromises its development and survival. This study proves the eradication of herbivores is the most efficient way to conserve vulnerable species.