Prolonged isolation and persistence of a common endemic on granite outcrops in both mesic and semi‐arid environments in south‐western Australia

Abstract

Aim Granite outcrops may be able to act as refugia for species during adverse climate change, owing to their topographic complexity. We assessed this hypothesis by examining phylogeographical patterns in a common, geographically widespread granite endemic, Stypandra glauca (Hemerocallidaceae). Location Granite outcrops of the Southwest Australian Floristic Region, Western Australia. Methods Twenty‐four tetraploid individuals of the granite endemic Stypandra glauca were sampled from each of 12 granite outcrops: 7 from a mesic environment and 5 from the semi‐arid region. Phylogenetic reconstruction and divergence‐dating was achieved using Bayesian and parsimony analyses of chloroplast haplotypes from 90 individuals. Nuclear diversity and population differentiation were analysed across all individuals using 10 microsatellite loci. Results Stypandra glauca exhibited high (chloroplast) or moderate (nuclear) levels of divergence among, and low diversity within, outcrops. Haplotype diversity was high in both sampling regions, and each haplotype was unique to one outcrop. There was little correlation between geographical and genetic distance. Both nuclear and chloroplast diversity were higher in southern (mesic) outcrops than in northern (semi‐arid) outcrops, although the level of chloroplast divergence among outcrops was similar for both climatic regions. Main conclusions The levels of divergence and low diversity revealed in S. glauca support a scenario of prolonged isolation and persistence on granite outcrops in both mesic and semi‐arid climatic regions, with no evidence of contraction–expansion dynamics across the outcrop network. The higher levels of diversity in the southern populations may result from the maintenance of a larger effective population size in southern regions, which retained more mesic climates during drier glacial periods. Although the climatic conditions differ between outcrops in this study, our results indicate that outcrops in both regions have harboured S. glauca throughout climatic changes, accentuating the value of these habitats to biodiversity conservation under future changing climate.

Refugial debate: on small sites according to their function and capacity

“…Sampson et al 1988;Byrne and Hopper 2008), rather than contraction/local extinction and recolonization. The more common and geographically widespread K. pulchella was also recently described as displaying a pattern of contraction and expansion only within particular outcrops but not between them (Tapper et al 2014).…”

“…(Myrtaceae) is a bird-pollinated evergreen shrub endemic for the South-West Australian Floristic Region and adjacent Eremean province (Tapper et al 2014). The species actually grows within granite outcrops where relatively moist microhabitats occur.…”

“…Firstly, the genetic pattern within the actual restricted range of the species, typified by high divergence among populations (Tapper et al 2014), suggests that multiple microrefugia are present, characterized by the long-term isolation between populations rather than a single macrorefugium. Secondly, molecular dating, showing the divergence among populations (Tapper et al 2014), indicates a correlation with the major aridification phases of the mid-Pleistocene; this is similar to the trends observed in other regional endemics (Byrne 2008) and generally reflects a changes in oscillation patterns during the Milankovitch cycles.…”

“…Since that time, the species has been observed in isolated populations in a microrefugial equilibrium within rocky outcrops, thus providing genetic divergence the time to evolve. Currently occurring populations can be defined as stabilized relict populations whose local abundance can fluctuate only within granite outcrops according to climate changes as suggested by Tapper et al (2014); this can be identified as the fluctuation dynamics within microrefugia.…”

A Climate-Oriented Approach to Support Decision-Making for Seed Provenance in Ecological Restoration

“…Mbogga et al (2010) and IsaacRenton et al (2014) also suggest that field-collected or remotely sensed-data of climate-driven plant health decline (especially relevant for trees) could be used to advocate for a provenance approach that increases the resilience of restoration plantings through assisted gene flow. In contrast, if the populations of a widespread species are isolated by topographic or environmental features (e.g., mountain ranges, granite outcrops in a matrix of a different geology; Tapper et al, 2014), then a more cautious approach is needed. This might include consideration of the likely role of those features on the partitioning of genetic diversity between populations.…”