Continental-shelf and oceanic archipelagos
The geographical setting of the LGM is an extreme configuration that was represented only sporadically and for short time-periods throughout the Pleistocene, with only 2% of the last 800 Ka BP estimated to have a similarly low sea level (Norder et al. 2019). Therefore, the duration of these episodes may have been too short to have had a significant impact on the diversity and composition of insular biota on oceanic islands (Porter 1989, Heaney et al. 2013, Norder et al. 2019). Our results show however, that in a continental-shelf archipelago, most of the Aegean angiosperm endemic richness patterns are best explained by LGM configurations, and the richness of native non-endemic angiosperm species and to a lesser extent, centipedes still shows an impact of the same configuration. We explain this as the result of extreme connectivity that was achieved during the LGM between the expanded Aegean true islands, land-bridge islands and the surrounding continental land mass. This connectivity with the mainland, even though short-lasting, promoted both conditions of native supersaturation on islands that were once connected to the mainland and conditions of lower endemism levels in the Aegean island biota. The ΙSAR z-values for total angiosperm richness fall into the z-value range for continental island settings. On the other hand, the endemic richness z-values are much lower than those on oceanic islands, pointing to reduced endemism on continental islands. The mean adjusted species richness of natives on land-bridge islands point to oversaturation of natives on land-bridge islands. We also observed that the high degree of Aegean island fragmentation by Pleistocene sea-level oscillations led to MIEs sharing much more islands (> 5) than is observed for oceanic archipelagos. The multiple island angiosperm endemism manifest in the Aegean today is indeed largely shaped by the fragmentation of large landmasses into smaller islands by the present high sea level. Lastly, we noted that deep time pre Quaternary geological island fragmentation left a distinct signature on endemism. Native species supersaturation and reduced endemism are thus essential processes that distinguish continental and oceanic island from the perspective of historical biogeography. It remains to be tested in how far maximum connectivity conditions are exclusively related to the lowest short-lasting sea-level stand, or whether for most biota maximum connectivity was reached earlier and lasted longer than the LGM. Clearly this depends on the geometry and depth of the basin under study, influencing the sea‐level thresholds at which island area and connectedness change significantly (Norder et al. 2019). We can conclude there is a crucial difference between oceanic and continental islands: the rate of area loss, as well as the degree of fragmentation, is far greater in continental islands and as such, the recent sea-level changes might have influenced more intensely their biota mediated through increased connectivity with the nearby mainland during marine low-stands. Moreover, speciation as a result of isolation and ecological speciation (through vacant niche space) is the main driver of species richness on oceanic islands, in contrast to continental islands where supersaturation and species disequilibria due to greater extinction debt are seen as prevalent. Single island endemism is generally supressed on continental islands, and when endemism does manifest, it is then the result of higher variability in topography or geodiversity offered by the continental islands (e.g. Panitsa et al. 2010).