Insularity promotes plant persistence strategies in edaphic island systems

Abstract

Aim Trait‐based approaches are being used increasingly in island biogeography, providing key insights into the eco‐evolutionary dynamics of insular systems. However, the determinants of persistence of plant species after they have arrived and established on an island remain largely unexplored. Here, we used three edaphic island systems (i.e., habitat patches distinguished from the landscape matrix by distinct soil conditions and specialized vegetation) to examine relationships between persistence strategies (those associated with clonality, bud bank, seed mass and life‐form) and insularity. We hypothesized that insularity promotes and selects strategies to persist locally, such that species occurring on small and/or isolated edaphic islands show trait values indicative of enhanced persistence and lower functional diversity. Location Three European systems of edaphic islands in the Western Carpathians, Moravia and the Cantabrian Range. Time period Present. Major taxa studied Vascular plants. Methods For each system, we used linear models to explore persistence‐related plant trait patterns (mean trait values and functional diversity) in relationship to three insularity metrics (island size, isolation and target effect). We focused on patterns of edaphic island specialists because their presence is confined to the islands. Results We found that insularity metrics largely explained the variation of the mean value and diversity of persistence‐related traits of edaphic island plant specialists. Specifically, insularity was positively related to traits supporting local persistence (e.g., more extensive lateral spread) and to a reduced variability in persistence traits. More insular systems showed stronger and more numerous trait–insularity links. Main conclusions Insularity can affect plant species diversity, form and function in edaphic island systems, such as selecting for enhanced and less diverse persistence strategies. Plant species occurring in insular systems might therefore avoid or delay local extinction by promoting adaptive strategies to persist in situ.

Sticking around: Plant persistence strategies on edaphic islands

“…However, lateral spread of clonal species showed no relationships with insularity and very sparse, inconsistent links with soil and microclimate. This seems to contrast with the results obtained when all clonal specialist species were analysed at the insular assemblage level (based on interspecific differences; Conti et al, 2022). In that case, the lateral spread was positively associated with insularity; this suggests that the response at the assemblage level is formed by different, or lack thereof, responses at the species level (Kichenin et al, 2013).…”

“…These species constitute ~45% of the total number of specialists of this vegetation type (Mendez‐Castro et al, 2021)—we could not sample the other species because they either occurred on fewer than five edaphic islands or their conservation status would not allow the destructive sampling required for collecting the selected persistence‐related traits. Edaphic island specialists are considered to be: (1) adapted to the harsh habitats of rocky outcrops (especially in relation to resource‐poor shallow soils; Doležal et al, 2022) and (2) more affected by insularity than non‐specialists (for which edaphic patches should not constitute an island), that is, the surrounding landscape should represent an effective barrier to dispersal and establishment, similar to water for oceanic islands (Conti et al, 2022; Mendez‐Castro et al, 2021). The 13 species belong to different plant functional types (nine forb, two chamaephytes, one grass and one sedge species), life histories (five clonal, eight non‐clonal species; Figure 1) and nine families.…”

“…Here, we focus on edaphic islands constituted by granite outcrops in Central Europe that host temperate dry grasslands rich in perennial plants specialised in resource‐poor shallow soils. Previous studies conducted in the same system have demonstrated the effects of insularity on edaphic islands' specialist species assemblages, namely the decrease in specialist species richness (Mendez‐Castro et al, 2021) and the tendency to have fine‐tuned strategies to persist in situ (Conti et al, 2022) with increasing insularity. However, it remains poorly understood how persistence‐related traits of individual species or species grouped by distinct life histories (i.e.…”

“…Yet, most of this research has focussed on dispersal and resource acquisition traits, neglecting other important functions (e.g. those related to clonality and longevity) that are necessary for a better understanding of how plants may increase their persistence likelihood and therefore possibly offset local extinction (Auffret et al, 2017; Conti et al, 2022). For example, plants occurring on isolated islands experiencing rare immigration and limited gene flow should be characterised by adaptive strategies to persist locally.…”

Biogeographic deconstruction of phylogenetic and functional diversity provides insights into the formation of regional assemblages

“…Importantly, the phylogenetic and functional signature left by species that are restricted to harsh regions (i.e. clustering) may be blurred with that of widespread species inhabiting there and elsewhere, thus hindering the identification of possible drivers responsible for the formation of regional assemblages (Conti et al 2022). To tackle this, we implement here a biogeographic deconstruction approach to examine phylogenetic and functional diversity patterns within regions for entire regional assemblages and biogeographic elements (i.e.…”