Introduction
Islands are dynamic entities with continuously evolving geographical
settings influencing the distribution and genetic evolution of organisms
they host. At deep time scales (Ma), an island’s ontogeny is controlled
by geological processes (Whittaker et al. 2008), while on shorter time
scales climatic processes dominate. Climate drives major sea-level
fluctuations causing islands to shrink and expand, fragment and merge,
or even disappear and emerge (e.g. Simaiakis et al. 2017).
Late-Quaternary sea-level changes have left their imprint on insular
species diversity patterns (Ali and Aitchison 2014, Rijsdijk et al.
2014, Weigelt et al. 2016, Ávila et al. 2018, 2019, Norder et al. 2019).
All of these did however focus on volcanic oceanic islands, or did not
make a distinction between oceanic volcanic and continental insular
systems and lumped them in their analyses (Weigelt et al. 2016, Veron et
al. 2019). By disregarding the specifics of continental archipelagos,
relevant biogeographical processes are potentially overlooked (Ali 2017,
Simaiakis et al. 2017, Whittaker et al. 2017).
Continental shelf islands (sensu Ali 2017) are characterized by
sitting on a continental shelf and may include “land-bridge” islands,
that were connected to the mainland in the past when sea levels were
lower. Another characteristic is the proximity of the mainland often
surrounding continental islands (Weigelt and Kreft 2013). The
geo-spatial effects of sea-level dynamics on continental island biota
have been studied extensively (e.g. Diamond 1972, Cardillo et al. 2008,
Itescu et al. 2020). The Aegean archipelago in the Mediterranean Sea is
located between the Greek and the Anatolian peninsulas, and is one of
the largest archipelagos on Earth (Blondel et al. 2010). Its complex
geological history and high environmental heterogeneity contribute to
its high biodiversity and endemism, thus rendering an ideal stage for
biogeographical studies (Strid 2016). The Aegean islands form a typical
continental shelf archipelago with a high geological heterogeneity and
numerous land-bridges due to sea-level change. Unsurprisingly, its
biogeography has been studied intensively (e.g. Triantis et al. 2018).
Although providing crucial biogeographical insights, most studies remain
limited in scope by focusing on a single explanatory factor of interest:
i.e., by not distinguishing between native and endemic species (Itescu
et al. 2020), by considering a single species group or a limited part of
the archipelago (e.g. Panitsa et al. 2010). The combined effect of area
change, fragmentation and connectivity driven by sea-level changes on
insular species diversity has never been investigated for the Aegean
archipelago; while we think that such a combined analysis is crucial to
interpret the relevant biogeographic processes correctly (see
Kougioumoutzis and Tiniakou 2014, Norder et al. 2019). Moreover, the
influence of continental land-bridge islands on biodiversity patterns in
different chorotypes as reflected in multivariate models and Island
Species-Area Relationships (ISARs) has never been quantitatively
assessed (e.g. Triantis et al. 2008, 2012). Since we have recently
quantified the paleogeographic change of islands in the Aegean Sea
(Simaiakis et al. 2017) and its biota are well studied, this setting
represents an ideal study system to test our hypotheses.
Our aim is to investigate the combined impact on continental insular
species diversity of current as well as past island area and
connectivity for the Aegean archipelago for two well-studied taxonomic
groups, angiosperms and centipedes. Our first hypothesis
(H1) is that more native species occur on land-bridge
islands than on true islands, reflecting the higher establishing rates
of native species on land-bridge islands (Simaiakis et al. 2017). Our
second hypothesis (H2) is that endemism is negatively
influenced by land-bridge islands, as allopatric speciation is
suppressed by repetitive gene-flow from the continents. Our third
hypothesis (H3) concerns the effect of the duration of
the archipelagic configurations as a result of sea-level drop affecting
richness patterns. On oceanic islands, the median archipelago
configuration could largely explain the endemism richness patterns than
the extreme and short-lasting Late Glacial Maximum (LGM) configuration
(Norder et al. 2019). Thus on land-bridge islands, the long-lasting
median glacial configuration may have largely influenced the species
richness patterns, rather than the short lasting LGM extreme state,
assuming that short-lasting spatial configurations do not provide the
time needed for speciation to occur. To test our hypotheses, we analyse
the effects of land-bridge islands on species richness of different
endemism levels. We investigate H3 by analysing species
richness patterns in relation to two paleogeographical settings: 1)
during LGM, and 2) the median sea level during the last
glacial-interglacial cycle (Figure 1).