Aim: How introduced mammal herbivores affect insular flora is still under study. Also, disentangling which particular traits, that plants might develop from exaptations, are functional to avoid herbivory remains mainly unknown. This study aims to assess if the flora of continental islands with historic native herbivores has exapted to the introduction of new mammal herbivores and to predict the potential vulnerability of endemic species from islands where mammal herbivores have not been introduced yet.

Location: Balearic Islands

Taxon: 96 Balearic endemic plant species

Methods: We investigated whether the endemic flora on continental islands maintains functional traits that resist introduced mammal herbivores by analysing the chemical and morphological traits related to plant resistance of five individuals per species (n=480). Also, we measured plant-size variables to assess plant escape strategies. Overall, we combined these traits with the accessibility to goats. Predictive models were generated for species that inhabit islands where goats have not been introduced to assess their potential vulnerability.

Results: Endemic species may defend against new herbivores (e.g., goats) if they contain highly toxic compounds (alkaloids, glycosides, coumarins), spinescent and urticating structures, or specific plant architecture (low plant size, high specific leaf area). If such traits are absent, the species may become extinct—unless they inhabit areas inaccessible to goats. On continental islands, some endemic species are expected to resist the introduction of herbivores, while others may be significantly affected.

Main conclusions: Part of the endemic flora may have previously adapted to ancient herbivores on the islands. Even though the ancient connection with the mainland, these traits may allow the plants to resist the presence of introduced herbivores. However, non-exapted species could be threatened by the introduction of non-native ungulates.

The methodological information can be found in the original article: "Exaptation and vulnerability to introduced mammal herbivores on Balearic endemic flora"