What defines insularity for plants in edaphic islands?

The Theory of Island Biogeography postulates that size and isolation are key drivers of biodiversity on islands. This theory has been applied not only to true (e.g. oceanic) islands but also to terrestrial island-like systems (e.g. edaphic islands). Recently, a debate has opened as to whether terrestrial island-like systems function like true islands. However, identifying the effect of insularity in terrestrial systems is conceptually and methodologically challenging because recognizing species source(s) and measuring isolation is not as straightforward as for true islands. We contribute to the debate by proposing an approach to contextualize the definition of insularity and to identify the role of isolation in terrestrial island-like systems. To test this approach, we explored the relationship between insularity predictors and specialist species richness of edaphic islands in three systems in Europe (spring fens, mountaintops, and outcrops). We detected that insularity affected specialist richness of edaphic islands through island size and target effect (i.e. an emergent property of islands depending on their isolation and size). As predicted by the Theory of Island Biogeography, species richness decreased with increasing isolation. Given the comprehensiveness and ease of implementation of our approach, we encourage its extension to other island-like systems.