Historic exposure to herbivores, not constitutive traits, explains plant tolerance to herbivory in the case of two Medicago species (Fabaceae)

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)

“…This event is not rare, as many plant communities in other areas increased after eradication of herbivores (Bullock et al 2002;Beltran et al 2014;Chapuis et al 2004). The impact of herbivores is more severe under insular conditions, especially for range-restricted species without defences (Bowen and Van Vuren 1997;Cubas et al 2019;Capó et al 2021), and can also negatively affect other plant-animal interactions (Traveset and Richardson 2006). Moreover, species richness in the permanent plots had doubled four years after the total eradication of rabbits, indicating a clear expansion of less-abundant species across habitats, as observed in other areas (Kessler 2001).…”

“…In addition, these observations are compatible with the hypothesis that M. citrina may have had a wider distribution in the past and that an ancient introduction of herbivores constrained its distribution to smallest islets where rats, goats or other animals never were introduced. Moreover, other species endemic to the Balearic Islands has been ecologically restricted to rupicolous habitats or islets without mammal herbivores (Capó 2021). Indeed, some plants endemic to the Balearic Islands recolonized shrublands on sa Dragonera islet after eradication of rats such as Hippocrepis balearica, Cheirolophus intybaceus, Helichrysum fontanesii or Lomelosia cretica (pers.…”

“…Though herbivory by alien interferes with the whole ecosystem, range-restricted species are considered to be the most vulnerable species, especially if they lack defenses against herbivory or have high palatability (Bowen and Van Vuren 1997;Cubas et al 2019;Capó et al 2021). Therefore, range-restricted species may become extinct or constrain their distribution if alien herbivores are uncontrolled (Pisanu et al 2012;Doherty et al 2016;Fenu et al 2016;Bellard et al 2016a;Oliveira et al 2021).…”

“…Therefore, range-restricted species may become extinct or constrain their distribution if alien herbivores are uncontrolled (Pisanu et al 2012;Doherty et al 2016;Fenu et al 2016;Bellard et al 2016a;Oliveira et al 2021). Many species have been threatened by alien herbivores on archipelagos such as the Hawaiian Islands (Eijzenga 2011;Barton and Hanley 2013;Barton 2016), Galápagos Archipelago (Campbell et al 2004;Carrion et al 2011), Santa Cruz Island in California (Beltran et al 2014), Canary Islands (Nogales et al 2006;Garzón-Machado et al 2010;Cubas et al 2019) and Balearic Islands (Latorre et al 2013;Capó et al 2021).…”

“…Therefore, herbivores could have been the main reason of this so constricted distribution. Indeed, a recent study reported that M. citrina has lower tolerance to herbivory than the closely related and widespread species Medicago arborea (Capó et al 2021). Like many other Mediterranean range-restricted species, M. citrina is well adapted to drought conditions and exposure to high salinity (Juan and Crespo 1999;Lefi et al 2004), which has allowed it to effectively compete against other plants under the hard environmental conditions of the small islets (Juan and Crespo 1999).…”