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).