Abstract
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.
Highlights
Biological invasions are the second leading cause of extinction worldwide (Gurevitch and Padilla 2004; Bellard et al 2016b) and disturb the function and stability of ecosystems (Capizzi 2020)
The objectives of the present work were (i) to assess how plant communities responded to the eradication of rabbits from s’Espartar, (ii) to evaluate the whole population structure of M. citrina after the eradication of rabbits from the islet, and (iii) to assess the threat of I. purchasi to the recolonization of M. citrina on the islet
Plant community coverage and richness increased after the eradication of rabbits Analysis of the five permanent plots between 2015 and 2019 revealed a significant increase in the total vegetation cover (Fig. 4)
Summary
Biological invasions are the second leading cause of extinction worldwide (Gurevitch and Padilla 2004; Bellard et al 2016b) and disturb the function and stability of ecosystems (Capizzi 2020). Insular plants and vertebrates have been affected by alien species (Diamond 1989; Hänel and Chown 1998; Bellard et al 2016b). Introduced herbivores such as rats, rabbits and goats interact with the native vegetation via direct impacts such as browsing and indirect impacts such as alteration of the soil composition or destruction of habitats (Coblentz 1978; Desender et al 1998; Campbell and Donlan 2005). A 30-year eradication program was performed on La Pinta Island, Galápagos, Ecuador (Campbell et al 2004), and a 12-year eradication program was carried out on Dirk Hartog Island, Australia (Heriot et al 2019) among others, including 107 successful attempts in the Mediterranean islands (Capizzi 2020)
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