Patterns of plant diversity in ironstone ranges in arid south western Australia
Highlights
► The isolated ironstone ranges of arid south western Australia showed high beta diversity both within and between ranges. ► The high beta diversity between ranges was largely due to high species turnover between ranges. ► Species richness patterns were most strongly correlated with broad scale spatial and climate gradients. ► Compositional patterns were correlated with environmental and spatial pattern across a number of different scales. ► Areas of highest conservation concern occur along the arid zone boundary with two hotspots of specialist ironstone taxa.
Introduction
South western Australia represents one of the oldest and most stable surfaces on the planet and the South West Australian Floristic Region (SWAFR – sensus Hopper and Gioia, 2004) has an extraordinarily rich and endemic flora, with a large number of naturally rare species with highly disjunct and fragmented populations (Hopper et al., 1996, Hopper and Gioia, 2004).
The interactions of long evolutionary history, cyclical climatic fluctuations, fire and other stochastic processes have been implicated in the evolution of the this flora and it represents a major exception to the general trend of decreasing species richness with increasing latitude (Mittelbach et al., 2007, Hopper, 2009). Granite inselbergs are a common landform across the SWAFR and studies of the flora of these inselbergs have been pivotal in developing ideas on the role of genetic isolation (James, 1965, James, 2000, Moran and Hopper, 1983, Byrne and Hopper, 2008) and fragmentation (Coates, 2000, Yates et al., 2007) in the evolution of the flora of the SWAFR (reviewed in Hopper and Gioia, 2004), however much less attention has been paid to the isolated ironstone ranges of the adjacent Arid Zone.
Starting near the boundary of the SWAFR, a series of ironstone ranges extend inland for over 750 km. These small ancient ranges are floristically distinct from the surrounding matrix and some, near the border of the SWAFR, have a series of unique vegetation communities strongly correlated with topography (Beard, 1981, Gibson, 2004) and are repositories of taxa endemic to, or with distribution centered on, these landforms (Butcher, 2007, Gibson et al., 2007, Gibson et al., 2010). It has been speculated that these ranges have acted as both refugia during drier climatic cycles and centers of recent speciation (Butcher et al., 2007). They provide an opportunity to examine patterns in plant diversity of edaphic islands across the Arid Zone.
Recent studies of these ranges have been precipitated by a significant growth in the global demand for iron ore which has resulted in increased exploration and mining of these ranges, well away from the traditional large scale mining operations of the Pilbara region in north western Australia (Ye, 2008). If the patterns in endemism and high species turnover reported for a number of ranges close to the Arid Zone boundary (Gibson et al., 2010) are common across the other ironstone ranges then the potential for conflict between resource development and conservation values is likely to be high as has been documented for tropical mountain vegetation on similar geologies in Brazil (Jacobi et al., 2007).
The aim of the present work was to synthesise the results of our recent surveys across 24 ironstone ranges, to elucidate distribution patterns of specialist ironstone taxa (taxa endemic to, or with distribution centered on ironstone) and to examine patterns in both perennial species richness and composition across the region. Specific questions addressed were a) was the high beta diversity reported for ranges on the boundary of the SWAFR common across the arid zone; b) were the patterns in composition change, and richness of the perennial and specialist taxa correlated with broad scale spatial pattern, climatic gradients or local topographic or soil chemistry patterns; c) what were the distributional patterns of specialist ironstone taxa? Answers to these questions would help elucidate the floristic patterning across the ranges and identify those areas of high conservation significance that could be seriously impacted by mining.
Section snippets
Study area
The 24 ironstone ranges surveyed occurred along the Arid Zone boundary and inland for over 750 km, from 25° 30’S to 30° 00’S and from 116° 00′E to 122° 15′E (Fig. 1). The midpoints of the closest ranges were 30 km apart while the most distant ranges were 700 km apart (Appendix 1). Rainfall estimates ranged from 329 mm in the south west dropping to 186 mm in the north east. Rainfall seasonality changed across the region falling predominately in winter (3.3 times summer rainfall) in the south
Patterns in alpha diversity
A total of 906 species in 74 families were recorded from the 1217 plots. The Fabaceae (140 species), Myrtaceae (75 species), and Poaceae (64 species) were the most speciose families. Sixteen lifeform groups were recorded with shrubs (41.5%) and annual herbs (29.4%) being the most common (Table 1). A large number of rarely sampled species occurred in our dataset but neither family composition (of the 10 most common families, accounting for 64% of species), nor the lifeform spectrums of species
Discussion
The patterns in composition and richness found in the ironstone ranges are comparable to those reported for granite inselbergs in Western Australia and elsewhere. In a study of the compositional change across five granite inselbergs in south western Australia, Burgman (1987) reported correlations with the individual rock, substrate, a broad geographic class. A more widespread review found decreasing species richness and endemic taxa with increasing aridity (Hopper et al., 1997). Similar
Acknowledgements
We would like to thank S. Dillon, G. Owen, B. Bayliss, J. Wright, N. Sheehy and Y. Caruso for technical assistance and the staff of the Western Australian Herbarium and the Western Australian Chemistry Centre. Two referees gave very valuable comments on earlier versions of this paper. This projected was funded under the Western Australian Department of Environment and Conservation’s Biodiversity Conservation Initiative.
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