Assessing rarity and threat in an arid zone flora

Jenny Silcock, School of Biological Sciences, University of Queensland

Jenny Silcock with a healthy specimen of the Vulnerable Rhaphidospora bonneyana in the Wallaroo Range.
Photo © Rebecca Richardson, DERM

Biological rarity means being few in number and/or having a limited distribution. It may also mean a higher chance of extinction, so rare species are central to conservation biology. However, across the vast spaces of arid and semi-arid Australia (70% of the continent), threatening processes are diffuse and often poorly understood. In the inland’s ‘boom and bust’ environment, plants and animals can be scarce (or even absent) for decades. However, given ideal seasonal conditions, they can appear almost overnight in spectacular profusion. The chance of a collector's visit coinciding with these 'booms' is slight, especially in remote and inaccessible areas. Clearly, in arid environments, the concept of rarity involves special considerations.
 
My study focuses on the Mulga Lands, Mitchell Grass Downs and Channel Country bioregions of western Queensland, where forty-four plant species are currently listed as Near Threatened, Vulnerable or Endangered under Queensland’s Nature Conservation Act. To start my project, I systematically assessed the 1800 species that occur across south-western Queensland, looking for those with restricted ranges, few populations, lack of recent collections, or facing known threatening processes. Using Queensland Herbarium specimen notes, field guides, online herbaria and interviews with numerous experts, I identified an additional 61 species not currently listed, but of possible conservation concern. This gave me a total of 105 ‘candidate species’. Most were known from a handful of collections, often decades old, with almost no information on their life histories, population size or potential threats.

Yang Yang Range near Yaraka – part of the northern Grey Range system, which harbours the highest concentration of rare plant species in western Qld.
Photo © Jenny Silcock

My research questions are straightforward: how are the vulnerable elements of the arid zone flora faring under current land use, and what factors are influencing patterns of rarity and threat? The only way to answer these questions is to spend a lot of time driving and walking around seldom-visited corners of western Queensland, searching for, counting and measuring rare species.

After thousands of dusty kilometres and over 500 search-hours, and blessed with the best season in thirty years across most of western Queensland, some answers – and, of course, more questions – are beginning to emerge. Many species are doing well. Large, healthy populations of 50 candidate species have been found, while a further 24 have been found in low numbers.

Next year will be spent analysing data on grazing pressure and distribution of rare plants in the northern Grey Range, the area with the highest concentration of rare species in western Queensland; investigating the seed bank ecology of two 'boom-bust' rare species, Sclerolaena walkeri and Ptilotus pseudohelipteroides, which were known from only a handful of collections but have carpeted large expanses of western Queensland in the past three years; and continuing measurements on grazing exclosures built around the Vulnerable sub-shrub Xerothamnella parvifolia.

One of six exclosures set up on Idalia National Park to monitor the effects of grazing on the Vulnerable Xerothamnella parvifolia.
Photo © Jenny Silcock

I will also intensify efforts to find and assess populations of species that appear to be genuinely rare and/or threatened. These include 11 long-lived shrubs and trees that tend to be heavily grazed by macropods, goats and/or cattle, and a handful of species that have so far been found only in low numbers – or not at all – despite many hours of systematic searching.

This research will provide detailed information required to achieve on-ground conservation outcomes in western Queensland and, on a broader level, assist in understanding plant rarity and conservation consequences in arid environments. Focussing on the rarest and most vulnerable elements of the flora can also help to answer bigger questions about land degradation and resilience of the arid zone vegetation.

Photo © Tracy Wattz, DERM	Photo © Jenny Silcock	Photo © Jenny Silcock
The bizarre succulent Euphorbia sarcostemmoides, stunning Hakea maconochieana (both listed as Vulnerable) and delicate Calandrinia sp. all inhabit barren plateaus of western Queensland.

Photo © Jenny Silcock	Photo © Peter McRae, DERM
(Left) The Endangered Ptilotus brachyanthus – known from just six historical records, two populations totalling 48 plants have been found in over 20 hours of targeted searching since 2007; (Right) The Regal Pumpkin Burr, Sclerolaena walkeri, listed as Vulnerable and not collected since 1941, carpeted large areas of the Paroo and Bulloo River floodplains from 2006-2008.