Biogeographical affinity
Biogeographical affinity (i.e., the random effect in our models)
explained only a small fraction of the variance for the native taxa.
This can be attributed to the fact that a large portion
(~40%) of the present Aegean flora has reached the
Aegean islands as a result of human action in prehistoric or early
historic times (Greuter 1979). The presence of these naturally or
artificially distance-insensitive species clearly justifies the less
significant role of biogeographical affinity in the diversity patterns
of the native plant taxa occurring in the Aegean archipelago. On the
other hand, the largest proportion of variance for all the endemic
species richness metrics is attributed to biogeographical affinity. This
is in line with previous studies stating that in the Aegean,
bioregionalization is primarily a result of the region’s complex
paleogeographical history (Kougioumoutzis et al. 2017) This is also due
the high proportion of narrowly-ranged species and consequently of high
species turnover in Aegean island plant communities: nearly
~45% of the endemic taxa occurring in the Aegean are
SIEs, the vast majority of which occur in Crete and Evvia (Panitsa et
al. 2018). Other factors such as climate and geodiversity probably play
an important role in shaping current endemic diversity patterns in the
Aegean, as is actually the case for the central (Kougioumoutzis and
Tiniakou 2014), eastern (Panitsa et al. 2010, Panitsa and Tzanoudakis
2010) and southern (Kagiampaki et al. 2011) Aegean islands. In addition,
favourable climatic conditions most probably permitted a relict flora to
persist in the southern (i.e., Crete, Karpathos, Rodos) and eastern
(e.g., Ikaria: Christodoulakis 1996a, b) Aegean archipelago (Runemark
1969, 1971). In topographically complex islands, some of the old MIEs
formed neo-endemic SIEs through allopatric speciation (Runemark 1969,
1971, Bittkau and Comes 2005, 2009, Comes et al. 2008, Jaros et al.
2018). Geographical isolation through sea-level oscillations may have
supported the recent diversification of neo-endemic species, especially
in the central Aegean where several non-adaptive radiations occurred
(e.g. Campanula , Nigella , Erysimum – e.g. Comes et
al. 2008, Jaros et al. 2018), as a result of a turbulent and
labyrinthine sequence of island-splitting events in a short time-period,
since most of the geographic configuration change occurred during marine
transgression in the Aegean within 5000 years between 16 and 11 Ka BP
(Simaiakis et al. 2017). Especially when considering the repetitive
character of fragmentation over the last 2 My, the fragmented and
disconnected state existed during every glacial-interglacial interval
for ~20 ka disrupting the longer lasting glacially
connected state, leading to cumulative genetic divergence between
populations (Aguilée et al. 2009). One prominent example for this is the
differentiation of the Nigella arvensis species complex which is
due to non-adaptive radiation and random genetic drift as a result of
several vicariant events during the Pliocene/Pleistocene (Bittkau and
Comes 2005, Bittkau and Comes 2009).