Model system and rationale
We selected the Canary Islands as a study system for two reasons: first,
we have a nearly complete knowledge of their terrestrial cave-adapted
diversity (Oromí et al. 2021); second, they cover a time span of 21 Ma
(Gillespie & Clague 2009), being the only studied volcanic oceanic
archipelago in the world with such a broad range of ages (Borregaard et
al. 2017). The eastern islands (Lanzarote and Fuerteventura) are very
close to each other and about 100 km from the continent; the remaining
five islands span towards the West over ca. 400 km. Except for Lanzarote
and Fuerteventura, each island represents an independent volcanic
building separated by oceanic depths, so they were never superficially
connected to one another or to Africa (Troll & Carracedo 2016).
Therefore, the Canary Islands can be regarded as oceanic islands that
have been biologically colonised across the ocean, mainly through the
marine channel separating the islands from North Africa (Florencio et
al. 2021). Most subterranean species in the Canary Islands are
single-island endemics, with speciation trajectories that happenedin situ from surface ancestors (Arnedo et al. 2007). All islands
harbour extensive subterranean habitats, including caves, lava tubes,
and MSS (Oromí et al. 2021). Regarding age, Lanzarote and Fuerteventura
are the oldest islands, having undergone a long erosive period only
interrupted by two historical volcanic episodes in Lanzarote. They are
here considered in their senescent ontogenetic stage. The central
islands of Gran Canaria, Tenerife, and La Gomera represent the islands
with the highest structural complexity, although La Gomera and Gran
Canaria have fewer caves than Tenerife, which might imply a smaller
carrying capacity than expected by their altitude (Oromí et al. 2021).
Lava tubes usually become destroyed within a few hundred thousand years
only persisting in relatively young parts of the islands, thus being
very few on Gran Canaria and La Gomera where volcanism has been scarce
or even absent in the last 2 Ma (Oromí, 2004). These three islands are
considered as geologically mature. La Palma and El Hierro are the
youngest islands, and those that have concentrated most of the volcanic
episodes in historical times (Longpré & Felpeto, 2021). A summary of
the most relevant features of each island is in Table S1.