A phylogenetically controlled test does not support the prediction of lower putative anti‐herbivore leaf traits for insular woody species
Résumé
Aim : It is predicted that insular plant taxa have evolved reduced defences in response to lower herbivore pressure on islands. However, the few studies testing this hypothesis have addressed variation in individual defensive traits, without paying attention to patterns of correlated trait expression (i.e. defence syndromes).
Location : Balearic and Canary Islands.
Taxon : Ninety-one woody plant species.
Methods : We tested whether plant species with contrasting histories of insularity (namely, endemics, non-endemic natives and exotics) differed in their defensive syndromes using a suite of morphological, physical, and chemical traits putatively associated with herbivory. For this, we measured eight leaf traits of 42 endemic, 29 native non-endemic and 20 introduced species for which specimens were sourced from botanical gardens found in two archipelagos: the Balearic and Canary Islands.
Results : We conducted phylogenetic-controlled analyses which showed that, contrary to predictions, insular taxa (endemics and non-endemic natives) across both archipelagos were more defended (thicker, smaller leaves with less nutrients) than exotic species. There were no differences in chemical defence (phenolic compounds) between endemics, non-endemic natives and exotics. Finally, we also found different defence syndromes between archipelagos: whereas species from the Balearic Islands were more physically defended, on average, those from the Canary Islands had higher chemical defences.
Main conclusions : Overall, these results point to a defence syndrome based on low-nutrient and physically defended leaves characteristic of insular plant taxa that is indistinct for endemic and non-endemic taxa, relative to introduced species, as well as quantitative and qualitative differences in defences between archipelagos owing to changes in species composition and likely also to different histories of biotic or abiotic pressure.