Plant species richness hotspots and related drivers across spatial scales in small Mediterranean islands

Abstract

Small islands represent a common feature in the Mediterranean and host a significant fraction of its biodiversity. However, the distribution of plant species richness across spatial scales—from local communities (alpha) to whole islands (gamma)—is largely unknown, and so is the influence of environmental, geographical, and topographical factors. By building upon classic biogeographic theory, we used the species–area relationship and about 4500 vegetation plots in 54 Central Mediterranean small islands to identify hotspots of plant species richness and the underlying spatial determinants across scales. To do so, we fitted and averaged eight species–area models on gamma and alpha richness against island area and plot size, respectively. Based on positive deviations from the fitted curves, we identified 12 islands as cross‐scale hotspots. These islands encompassed around 70% of species and habitat richness, as well as almost 50% of the rarest species in the data set, while occupying less than 40% of the total island surface. By fitting generalized linear mixed models, we found that gamma richness was mainly explained by island area and was weakly related to mean annual temperature (positively) and annual precipitation (negatively). As for alpha richness, after accounting for the idiosyncratic effect of habitats and islands, plot size and gamma richness remained the only significant predictors, showing a positive relationship. This work contributes to the understanding of the patterns and drivers of plant diversity in Central Mediterranean small islands and outlines a useful methodology for the prioritization of conservation efforts.

Topographic Complexity Is a Principal Driver of Plant Endemism in Mediterranean Islands

“…‘Isolation’ would now be an important factor, since the reduced immigration rate characteristic of isolated islands would be reflected in reduced gene flow from the mainland, also reducing the probability of genetic dilution of island variants. A varied topography can also favour the persistence of relict populations in refugia [ 21 ], promoting the formation of ‘restricted endemics’ and ‘disjunct endemics’ [ 3 , 12 , 22 , 23 ].…”

MEDIS—A comprehensive spatial database on Mediterranean islands for biogeographical and evolutionary research

“…Mediterranean islands have been the subject of a plethora of studies investigating various taxa and ecological topics (see Médail, 2022). Researchers focused on the biodiversity and the ecological dynamics of these islands, analysing the patterns of their unique flora and fauna and determining their underlying drivers (Bonardi et al, 2022;Fattorini, 2006;Fattorini et al, 2017;Médail, 2022;Özden et al, 2008;Sfenthourakis & Triantis, 2017;Testolin et al, 2023). Other studies encompassing particularly well-surveyed islands explored changes in plant species composition across two centuries, likely one of the longest timeframe for studies of island biogeography (Chiarucci et al, 2017).…”

Floristic Richness in a Mediterranean Hotspot: A Journey across Italy

“…Most studies dealing with SAR in the Mediterranean basin deal with insular systems [ 18 , 19 , 20 , 21 , 22 , 23 ], while mainland areas have received less attention (however, see [ 2 ]). Accordingly, the aims of this study are as follows: (a) implement a species–area relationship with respect to vascular plant species on a national scale (Italy) and (b) compare vascular plant richness among Italian administrative regions, which traditionally are the basic units upon which floristic investigations are carried out in this country.…”