Why focus on continental archipelagos?
Although oceanic islands are less numerous compared to the non-oceanic, they dominate ecological and evolutionary theory, compared to continental island systems, which have received much less attention than oceanic islands and archipelagos (e.g., Whittaker et al., 2017). Thus, continental archipelagos, such as the Aegean, provide exceptional opportunities for the refinement and further development of island theory, especially those parts mainly based on oceanic islands.
The Aegean archipelago
The present-day Aegean archipelago has been shaped by the differential influences of tectonic activity, volcanism and eustatism. Except of the islands of the South Aegean Volcanic Arc, the Aegean Islands are of continental origin Francalanci et al., 2005 and differ significantly in their isolation history (e.g., Kapsimalis et al., 2009; Sakellariou & Galanidou, 2016). The Aegean Sea is located on the margin of the African and the Aegean-Anatolian tectonic plates, whose intermittent connection over geological time [e.g., the Mediterranean desiccation during the Messinian Salinity Crisis (MSC) and the Pleistocene glaciations] has enabled biotic exchange between the several Aegean biogeographical regions (e.g., Poulakakis et al., 2015; Kougioumoutzis et al., 2017; Sfenthourakis & Triantis, 2017). This complex palaeogeographical history of the Aegean has had a profound impact on the distribution (Kougioumoutzis et al. 2017) and diversification of several plant lineages in the region (e.g., Crowl et al., 2015; Jaros et al., 2018 ) and has contributed significantly to the emergence of this global biodiversity hotspot.
Intensive field work has taken place in the Aegean phytogeographical regions of Greece (see Dimopoulos et al., 2016) and this has culminated in a wealth of research papers dealing with the species richness patterns of the Aegean archipelago (e.g., Panitsa et al., 2006, 2008, 2010; Trigas et al., 2008, 2013; Kallimanis et al., 2010, 2011), providing valuable insights regarding the factors driving the native and endemic species richness patterns in the Aegean archipelago.
Aegean biodiversity in numbers
The Aegean biogeographical regions host ca. 3300 plant taxa, 685 of which are Greek endemics and 478 Aegean endemics1. But bear in mind that since then, several new taxa have been described from the Aegean region.] (i.e., occurring exclusively on the Aegean islands). The richest and the poorest - in species richness terms - region is that of the East Aegean Islands and the Cyclades, respectively. Since endemism rates in Greece diminish in a N-S axis, the Aegean could not be an exception to this rule: the highest endemism rate (17.7%) is observed in the biogeographical region of Kriti-Karpathos, while the lowest (ca. 3%) in the biogeographical region of the North Aegean Islands2.
The biogeographical region of the Cyclades (Kik) has been traditionally considered as floristically impoverished (see Phitos et al., 1995), due to its lower species richness, compared to the other Aegean biogeographical regions. That view is now overturned: Kik emerges as the second richest Aegean phytogeographical region in terms of Greek and Aegean endemic plant species richness and no longer stands out as the poorest Aegean phytogeographical region (see Kougioumoutzis & Tiniakou, 2014 and Figure 4 in Panitsa et al., 2018).
Kriti constitutes the most important endemic hotspot not only in the Aegean, but in the Mediterranean Basin as well (e.g., Medail, 2017). Anafi - a rather smal, remote, highly geodiverse island - stands out the most important plant diversity hotspot, not only in the Cycladic archipelago, but also in the whole South Aegean Sea. Finally, the South Aegean Volcanic Arc (i.e., SAVA) seems to be not only one of the most significant geological structures in the Mediterranean area, but also of great conservation value, as the areas comprising it host in their vast majority well above average native and endemic taxa.