A comparison of coastal plant trait variability between Maltese endemics and their widespread Mediterranean congeners

Abstract

Identifying the ecological characteristics of narrow endemic species is essential, not only for providing insight on the evolutionary patterns leading to endemism, but also for its contribution to establish guidelines for their conservation. The aim of this study was to investigate plant trait variability between congeneric pairs of endemic and widespread species from two wide ranging taxa, Limonium and Euphorbia, to identify any ecological characteristics unique to endemic species, hence identifying their patterns of endemism on the Maltese islands and, based on these traits, determine the extent of ecological impact associated with landscape modification. Sampling was carried out at 21 sites around the islands of Malta, Gozo and Comino. Environmental parameters related to stress were compared for each species, together with plant competitiveness including coexisting species and ecophysiological parameters pertaining to plant resource acquisition and conservation abilities established through functional leaf traits. Morphological and reproductive analyses including flower density, pollen viability and flower synchrony were similarly carried out for each species. Whilst for the endemic species investigated the ‘specialist’ model seems to be the most likely process leading to their endemism on the Maltese islands, PCA analyses highlighted that both endemic and widespread species correspondingly exhibit adaptations to withstand environmental constraints with the endemic species being not more stress tolerant than their widespread congeners. This, with the exhibition of a lower competitive capacity by endemics has clear implications on their persistence in a competitive environment. Results also demonstrated morphological variations between the congeneric pairs whereby certain morphological traits such as habit and branching angle are plastic, whilst other traits including leaf dimensions and corolla width are evolutionary products of endemism. Reproductive traits including higher flower densities, smaller flowers with less elaborate displays and limited pollen production reflected by a lower pollen viability are unique in endemics. These traits render selfing as the main breeding strategy adopted by endemics which serves as an obstacle for the evolution of adaptations in changing environments and hence raises implications for their long term survival on the Maltese islands.