Islands within islands: a frontier for eco-evolutionary studies?
The use of simplified subterranean ecosystems as a model allowed us to disentangle the role of interactions from that of confounding effects, such as the shift of traits related to ecological successions or trait-dependent extinction risk that has been described in other types of island habitats (Karadimou et al. 2018; Gray 2019). Conversely, given that the subterranean environment through time and across islands filters for simplified communities bearing a relatively narrow set of traits, we avoided problems associated with selecting meaningful traits across a large sample of organisms. However, the main advantage of our model system is that there is no migration across subterranean habitats between different islands, warranting that most changes that we observe in the trait space of each island depends on in-situ processes. In essence, subterranean habitats in each island are independent and only linked by the common geological history of the archipelago.
In a sense, including further archipelagos in our analyses would have increased the number of confounding factors, since different archipelagos across the world have largely different species pools with different evolutionary trajectories. Archipelagos consisting of non-oceanic islands, for example, are expected to follow different geological evolution generating different age-dependent diversity patterns (Whittaker et al. 2017). Furthermore, most oceanic archipelagos in the world are much younger than the Canary Islands and cover a shorter time span hindering comparability of results (Borregaard et al. 2017). There are few exceptions, such as Cabo Verde and Madeira (Florencio et al. 2021). However, very fragmentary data exists on the subterranean animals of Cabo Verde, whereas only non-specialized species have been found in the Selvagens and Porto Santo, the oldest island in Madeira (Oromí, 2004).