DOI: 10.5061/dryad.qz612jmfh
title: What defines insularity for plants in edaphic islands?

description: 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.
description: <em>Floristic
 data</em> 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 m<sup>2</sup>) 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. <i>Biogeographic
 data</i> 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.