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.

Floristic data

The dataset contains floristic information about three different edaphic island systems in Europe: 1) calcareous spring fens in the Western Carpathians (Slovakia and the easternmost Czech Republic; hereafter fens), 2) acidic alpine grasslands in Cantabrian mountaintops (north-western Spain; hereafter mountaintops), and 3) shallow-soil acidophilous grasslands in Moravian granite outcrops (southern Czech Republic; hereafter outcrops). We worked with vascular plant specialist species of each focal habitat type (edaphic island). These species are exclusively or tightly associated with the edaphic islands, unable to establish viable populations elsewhere in the landscape matrix.

In fens, data was collected using a single 4 m x 4 m plot located at the central part of each island. This species inventory was then completed by a census of the whole edaphic island. In mountaintops, a total of 284 vegetation plots (size between 10 and 40 m²) were used to sample alpine grasslands in isolated patches with acidic bedrock, with a number of plots per island associated with the island area. In outcrops, sampling was performed using four 0.5 m x 0.5 m plots per island and complemented by a census of the whole edaphic island, similarly to what was done for fens. We gathered data on 49 edaphic islands for fens, 25 for mountaintops, and 20 for outcrops. Expert-based selection of habitat specialists was carried out in each study system.

Biogeographic data

We identified and delimited the edaphic islands by combining different techniques. In fens, all known patches found in the Western Carpathians were manually georeferenced using a GPS device. For mountaintops, we built a map by selecting edaphic islands above the regional treeline (1800 m a.s.l.) occurring on acidic bedrock only. We differentiated alpine grasslands from rocky and shrub areas based on the Normalized Difference Vegetation Index (NDVI) taken from Sentinel 2 (USGS 2019). For outcrops, the location of edaphic islands was obtained through a field survey using a GPS device (Garmin eTrex 30x) and a vegetation map provided by the Nature Conservation Agency of the Czech Republic. All the GPS points, satellite data, maps, and polygon layers were processed and analyzed using QGIS desktop. All distance-related metrics were calculated using direct aerial Euclidean distance without considering differences in the terrain elevation.