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).