A comparison of the biology and ecology of Euphorbia dendroides and Euphorbia melitensis

Abstract

Comparing congeneric pairs of plants in which one is endemic and the other is widespread may shed light on the conditions which favour endemism to take place. In this study, the biology and ecology of Euphorbia melitensis, endemic to the Maltese Islands, and Euphorbia dendroides, distributed across the Mediterranean, were analysed alongside each other. Eight sites across Malta and Gozo were sampled to determine whether the presence of endemic species is driven by stressful conditions. In three of the studied habitats, both species exist in sympatry, while in the rest, either one of the two congeners can be found. Nine environmental factors relating to stress were compared in each habitat along with vegetation richness. Readings of leaf length, plant width and height of the two congeners were also measured together with a sample of 300 propagules from four of the eight sites. Detrended Correspondence Analysis of the environmental parameters indicated that wind exposure is the leading contributor to the distribution of the two plants based on the nine factors tested. This implied that at higher wind exposures, the endemic species is favoured. Other parameters such as altitude above sea level and distance from the sea were found to be much less effective in the two species’ distribution. The size of the plants and leaves was found to be smaller for Euphorbia melitensis than for Euphorbia dendroides, indicating a reduction in size as a measure to combat stress. These findings may be specific to the genus Euphorbia or to the Maltese Islands and further analysis is required to confirm if these traits are shared among endemic species in different territories.