Determinants of alpha and beta vascular plant diversity in Mediterranean island systems: the Ionian islands, Greece

Abstract

The Ionian archipelago is the second largest Greek archipelago after the Aegean, but the factors driving plant species diversity in the Ionian islands are still barely known. We used stepwise multiple regressions to investigate the factors affecting plant species diversity in 17 Ionian islands. Generalized dissimilarity modelling was applied to examine variation in the magnitude and rate of species turnover along environmental gradients, as well as to assess the relative importance of geographical and climatic factors in explaining species turnover. The values of the residuals from the ISAR log10‐transfomed models of native and endemic taxa were used as a measure of island floristic diversity. Area was confirmed to be the most powerful single explanatory predictor of all diversity metrics. Mean annual precipitation and temperature, as well as shortest distance to the nearest island are also significant predictors of vascular plant diversity. The island of Kalamos constitutes an important plant diversity hotspot in the Ionian archipelago. The recent formation of the islands, the close proximity to the mainland source and the relatively low dispersal filtering of the Ionian archipelago has resulted in islands with a flora principally comprising common species and a low proportion of endemics. Small islands keep a key role in conservation of plant priority sites.

Endemic and alien vascular plant diversity in the small Mediterranean islands of Sardinia: Drivers and implications for their conservation

“…Such results are not surprising, being similar of those found in other Mediterranean island groups (e.g. Nikolić et al, 2008;Pretto et al, 2012;Valli et al, 2019) and confirming the need of urgent actions for a more effective conservation of the Mediterranean island ecosystems (Médail, 2013).…”

“…We checked for weak co-linearity among the initial set of 19 variables by applying a stepwise trait selection, based on variance inflation factors (VIF) by means of function 'vifstep' in the usdm R package (Naimi, 2015). In order to reduce heteroscedastic biases and improve the linearity of the relationships in the regression models, variables were log10-transformed to normalise their distribution so that they could be compared with bivariate and multivariate regression methods (Valli et al, 2019). The Shapiro-Wilk test, with 95% level of confidence, was used to check the normality of each transformed variable.…”

“…Positive and negative values of Pearson's standardised residuals refer to areas with species diversity components above and below average, respectively (Hobohm, 2003). We used residuals, instead of simple species richness and compositional dissimilarity, because this is a measure of island species diversity that does not reflect the influence of island area (Hobohm, 2003;Moody, 2000;Valli et al, 2019). Such an approach was conceptually conceived to go beyond the common direct correlation between species richness and island area; however, this has been recently applied to other species diversity components, such as compositional dissimilarity, that were clearly correlated with the island size or distance (e.g.…”

“…Such an approach was conceptually conceived to go beyond the common direct correlation between species richness and island area; however, this has been recently applied to other species diversity components, such as compositional dissimilarity, that were clearly correlated with the island size or distance (e.g. Penone et al, 2016;Valli et al, 2019). Summarising, the final four response variables used for regression analyses were the power model residuals of endemic/alien species richness (α-Endemic and α-Alien, hereinafter) and endemic/alien compositional dissimilarity (β-Endemic and β-Alien, hereinafter).…”

“…For instance, if consistent differences in terms of adaptation and colonisation abilities can be identified between invasive and native taxa, these differences might help explain why an invader has become so successful (Daehler, 2003). Furthermore, such a comparison might be used for the prioritisation of actions aimed at the control of alien species in insular areas of particular conservation interest (Carli et al, 2018;Valli et al, 2019).…”