Persistence and stochasticity are key determinants of genetic diversity in plants associated with banded iron formation inselbergs

Abstract

The high species endemism characteristic of many of the world's terrestrial island systems provides a model for studying evolutionary patterns and processes, yet there has been no synthesis of studies to provide a systematic evaluation of terrestrial island systems in this context. The banded iron formations (BIFs) of south-western Australia are ancient terrestrial island formations occurring within a mosaic of alluvial clay soils, sandplains and occasional granite outcropping, across an old, gently undulating, highly weathered, plateau. Notably, these BIFs display exceptionally high beta plant diversity. Here, we address the determinants and consequences of genetic diversity for BIF-associated plant species through a comprehensive review of all studies on species distribution modelling, phylogenetics, phylogeography, population genetics, life-history traits and ecology. The taxa studied are predominantly narrowly endemic to individual or a few BIF ranges, but some have more regional distributions occurring both on and off BIFs. We compared genetic data for these BIF-endemic species to other localised species globally to assess whether the unique history and ancestry of BIF landscapes has driven distinct genetic responses in plants restricted to this habitat. We also assessed the influence of life-history parameters on patterns of genetic diversity. We found that BIF-endemic species display similar patterns of genetic diversity and structure to other species with localised distributions. Despite often highly restricted distributions, large effective population size or clonal reproduction appears to provide these BIF-endemic species with ecological and evolutionary resilience to environmental stochasticity. We conclude that persistence and stochasticity are key determinants of genetic diversity and its spatial structure within BIF-associated plant species, and that these are key evolutionary processes that should be considered in understanding the biogeography of inselbergs worldwide.

Conservation of old individual trees and small populations is integral to maintain species' genetic diversity of a historically fragmented woody perennial

“…Nonetheless, the effective size of E. caesia populations is substantially smaller than theoretical benchmark values ( N e of ≥100/1,000) for widespread continuously distributed or recently fragmented diploid sexual organisms to retain population viability in the short and long term (Frankham et al, ; Ralls et al, ). Multiple modes of reproduction, including asexual reproduction and therefore a capacity for extreme longevity of localized genets, are almost certainly paramount to the persistence of some small, naturally fragmented plant populations with small effective population sizes (Byrne et al, ; Peakall, Ebert, Scott, Meagher, & Offord, ; De Witte & Stöcklin, ).…”

“…Population census size is suggested to be equivalent to effective population size in E. caesia, particularly for the smaller populations where most trees appear to produce seed and thus contribute to subsequent generations (Moran & Hopper, 1983) and in trees in general (Petit & Hampe, 2006 (Frankham et al, 2014;Ralls et al, 2018). Multiple modes of reproduction, including asexual reproduction and therefore a capacity for extreme longevity of localized genets, are almost certainly paramount to the persistence of some small, naturally fragmented plant populations with small effective population sizes (Byrne et al, 2018;Peakall, Ebert, Scott, Meagher, & Offord, 2003;De Witte & Stöcklin, 2010).…”

“…Around the globe, historically fragmented rocky habitats such as quartzite, ironstone and granite outcrops often contain unique assemblages of biota when compared with those of the surrounding landscape (Byrne et al, 2018;Jacobi, Do Carmo, Vincent, & Stehmann, 2007;Porembski & Barthlott, 2012). Granitoid, or granitic/gneiss, inselbergs are dome-like rock outcrops that are globally significant endemism hot spots (Porembski & Barthlott, 2012;Porembski, Seine, & Barthlott, 1997).…”

High species diversity and turnover in granite inselberg floras highlight the need for a conservation strategy protecting many outcrops

“…Analysis of evolutionary patterns in floras of granite and BIF inselbergs in the SWAFR shows that these landscape features are areas where species can persist through harsh climatic conditions. This leads to both diversity within, and differentiation among, inselbergs at both species and genetic levels (Byrne & Hopper, 2008;Byrne et al, 2018;Tapper et al, 2014aTapper et al, , 2014b.…”

High‐resolution distribution modeling of a threatened short‐range endemic plant informed by edaphic factors

“…While relatively little land area outside of the existing populations was predicted as habitat highly likely to support R. brevis, there were nevertheless five distinct uplifted BIF regions of approximately 1,500 km 2 that had a high modeled likelihood of supporting R. brevis (Figure 4a). Given that a number of very highly suitable habitats occur in the broader region that are uninhabited by this species, it seems likely that stochastic local extinctions have played a substantial role in the distribution of R. brevis, as with other flora species associated with BIF ranges in the region (Byrne et al, 2018). In this regard, the initial species distribution modeling approach based entirely on edaphic features, independent of the usual climatological data that informs correlative SDMs (e.g., Hijmans et al, 2005;Kearney, Isaac, et al, 2014) appears entirely suitable for the production of insightful high-resolution SDMs for SREs.…”