Hierarchies of cause: understanding rarity in an endemic shrub Verticordia staminosa (Myrtaceae) with a highly restricted distribution

Abstract

Verticordia staminosa C.Gardner & A.C.George subsp. staminosa is an extremely rare shrub occurring as an isolated population of ~1200 plants on a granite outcrop in the semi-arid agricultural region of Western Australia, separated from its closest relative V. staminosa subsp. cylindracea by 400 km. We aimed to determine a hierarchy of causes for explaining the extremely restricted distribution of subsp. staminosa, and to determine the genetic relationships among populations within both subspecies. We measured allozyme variation in all known populations of the two subspecies. There were exceptionally high levels of genetic divergence between subsp. staminosa and subsp. cylindracea, including an apparent duplication of the gene encoding phosphoglucomutase, leading to an additional gene in subsp. cylindracea. These findings combined with UPGMA analysis indicate a very long period of historical separation, perhaps originating in the early Pleistocene. Genetic variation was partitioned mostly between rather than within populations, with very low levels of genetic variation within populations of both subspecies. For subsp. staminosa we quantified seed production for three consecutive years and demography for five consecutive years. We used transition matrix models to describe the shrub’s population dynamics and stochastic simulations to explicitly compare the effects of low rainfall and disturbance on population viability. Verticordia staminosa subsp. staminosa produces large numbers of seeds each year and has flower to fruit ratios greater than reported for related rare and common congeners. Seedling recruitment occurs in most years, with pulses in the wettest years. The mean finite population growth rate was 1.031. Elasticity analyses showed that population growth rate was more sensitive to stasis of established plants than to seedling recruitment. Population viability declined with lower rainfall and increased fire-related mortality of adult plants. Rarity in subsp. staminosa is best explained by evolutionary history and the interaction of climate change and disturbances such as fire that kill plants. Climatic fluctuations since the late Pliocene might have led to stochastic extinction episodes of populations on other granite outcrops, resulting in the currently restricted distribution. We discuss the implications of our findings for management of the species.

Genetic connectivity and diversity in inselberg populations of Acacia woodmaniorum, a rare endemic of the Yilgarn Craton banded iron formations

“…The degree of genetic structure at Jasper Hill may be influenced by the effects of genetic drift under a scenario of limited pollen production and dispersal among populations of smaller effective size Despite the suitability of inselberg habitats for studies assessing genetic connectivity and the occurrence of inselberg habitat around the world, detailed studies of endemic inselberg flora are limited. Species occurring on granite outcrops in WA have shown higher levels of genetic differentiation than observed in A. woodmaniorum, for example, Eucalyptus caesia (Byrne and Hopper, 2008) and Verticordia staminosa subspecies cylindacea (Yates et al, 2007). Populations of these species are distributed over wider geographical ranges and have greater levels of geographic isolation however.…”

“…For these species, the surrounding landscape typically comprises a matrix of unsuitable habitat that acts as an effective physical barrier to dispersal and the size and spatial arrangement of populations assumes an important role in the structuring and maintenance of genetic diversity. Limitations to connectivity result in a degree of demographic isolation and genetic isolation among populations (Slatkin, 1987), and produce the phylogeographic and genetic structuring observed in many habitat specific, rare, endemics (Yates et al, 2007;Byrne and Hopper, 2008;Butcher et al, 2009). Limited connectivity among disjunct populations can result in reduced levels of genetic diversity as a result of increased levels of inbreeding and via the negative impacts of genetic drift (Slatkin, 1987;Ellstrand, 1992), both of which are further enhanced in small populations.…”

“…A number of studies have described high levels of genetic structuring among populations of these species (Coates, 2000;Yates et al, 2007;Byrne and Hopper, 2008;Millar and Byrne, 2013). Historical rarity, geographic restriction, disjunct population distributions and genetic structuring in the WA flora is thought to be largely the result of a diverse mosaic of soils (Anand and Paine, 2002), localised stochastic extinction events driven by climatic fluctuations, an overall increasing aridity of the environment during the Pleistocene, and the actions of fire across the landscape over long time frames (Hopper, 2009).…”

Soil-vegetation relationships on a banded ironstone 'island', Carajás Plateau, Brazilian Eastern Amazonia

“…There is a clear vegetation gradient from herbaceous campo rupestre to montane forest accompanying the gradient of soil depth: herbaceous campo rupestre<shrubby campo rupestre<capão forest<montane forest. Yates et al (2011b) similarly identified species habitat preferences associated with soil depth for bandediron plant communities in Western Australia.…”

Degraded or just dusty?: 150 years of ecological change in inland eastern Australia

“…This pattern of species becoming 'stranded' on rock outcrops has been documented elsewhere in Australia (Gibson et al 2012). Genetic studies could shed more light on the relative period of historical separation between populations (Yates et al 2007). …”