Species richness patterns
Late Quaternary sea-level changes have left their mark on island species richness patterns (e.g. Weigelt et al. 2016, Norder et al. 2019), following theoretical expectations (Fernández-Palacios et al. 2015). Recently, the question arose whether LGM paleogeographical configurations or those more representative of the Pleistocene better explain endemic species richness (Norder et al. 2019). It seems that on oceanic islands the intermediate sea-level configuration shapes SIE and MIE diversity patterns (Norder et al. 2019). Our study aimed to investigate the possibility that different patterns might be observed on continental islands, which have not been investigated with regard to these questions so far.
Our power-function model had high predictive capacity (Table 2) and our results are in line with Kreft et al. (2008), as well as the with values reported for several Greek island phytogeographical regions (e.g. Kagiampaki et al. 2011, Kougioumoutzis and Tiniakou 2014, Valli et al. 2019: 82–95%). Our results corroborate H1 and H2: land-bridge islands host more native non-endemic taxa, yet significantly fewer endemics (Figure 3). This pattern is also reflected in ISAR z-values (Table 1; Appendix 3). The overall angiosperm z-value (z = 0.329) falls within the range for continental shelf settings (Triantis et al. 2012) and is similar to that of the East Aegean islands (z = 0.326; Panitsa et al. 2010), which are land-bridge islands. Aegean land-bridge islands host significantly more native taxa and fewer endemics than true islands, thus implying that the former were probably supersaturated in native taxa, after sea levels dropped. This could result from the high geospatial connectivity states prevailing during the Quaternary glacial sea-level low-stand periods, promoting continuous influx of native taxa (e.g., Panitsa et al. 2018). These biota are currently experiencing a relaxation period and have not reached equilibrium (Simaiakis et al. 2017), a pattern reflected on their endemic ISARs as well (low R2 values – Table 1). The opposite trend is observed for all the endemic chorotypes – being more pronounced for SIEs - as the ISAR slope is steeper for the true islands (Appendix 3), confirming H2. Nevertheless, the SIEs ISAR slope is significantly lower than reported from oceanic islands (z = 0.8; Triantis et al. 2008), pointing to a state of endemic underrepresentation on continental islands. The frequent reconnections with the mainland during the glacial-interglacial cycles, enabled gene flow between previously isolated insular and continental source populations repeatedly re-established, thus hampering speciation and consequently leading to lower endemism levels and species numbers (Whittaker and Fernández-Palacios 2007).