Review
A fresh framework for the ecology of arid Australia

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Abstract

A revised set of propositions about ecology in arid Australia is presented, based on research literature since publication of Stafford Smith and Morton (1990). Fourteen propositions distil our argument that most features of the Australian deserts are explicable in terms of two dominant physical and climatic elements: rainfall variability, leading to extended droughts and occasional flooding rains; and widespread nutrient poverty. Different landscapes within the arid zone show these features to varying degrees, and so it is important to think about different places separately when considering our propositions. Plant life-histories strongly reflect temporal patterns of soil moisture; because Australian deserts receive more variable rainfall than most others, there is a distinctive spectrum of life-histories. Low levels of phosphorus (together with abundant soil moisture on irregular occasions) favour plants producing a relative excess of carbohydrate (C). In turn, C-rich plant products sometimes lead to fire-prone ecosystems, assemblages dominated by consumers of sap and other C-based products, and abundant detritivores (particularly termites). Fluctuations in production due to variable rainfall provide openings for consumers with opportunistic life-histories, including inhabitants of extensive but ephemeral rivers and lakes. Most consumer species exhibit some dietary flexibility or utilise more dependable resources; these strategies give rise to greater stability in species dynamics and composition of assemblages than might first be imagined under the variable rainfall regime. Aboriginal people have had long-standing ecological influence as they accessed resources. For each proposition we suggest the extent to which it is ‘different’, ‘accentuated’ or ‘universal’ in comparison with other deserts of the world, recognising that this categorisation is in need of critical testing. Further tests of each proposition are also suggested to fill the many gaps that still exist in our knowledge of the structure and functioning of Australia’s deserts.

Research highlights

► Revised propositions about arid Australia are presented based on research since Stafford Smith and Morton (1990, Journal of Arid Environments 18, 255-278. ► Australian plant life-histories distinctively reflect variable temporal patterns of soil moisture. ► Low phosphorus levels favour plants producing a relative excess of carbohydrate, leading to fire-proneness and abundant detritivores. ► Irregular fluctuations in production due to variable rainfall encourage consumers with opportunistic life-histories.

Introduction

Ecological research in arid Australia has particular significance. The major environmental themes that are writ large in Australian deserts – soil infertility and highly variable rainfall – are actually characteristic of the continent as a whole. Consequently, the expressions of these two features across Australia have produced exemplar ecosystems and processes that have deeply informed much global ecological synthesis (e.g. Andrewartha and Birch, 1984, Krebs, 2008, Orians and Milewski, 2007, Pianka and Vitt, 2003, White, 2005). The North American and Mediterranean arid zones, from which the bulk of the scientific literature comes, tend to have relatively predictable rainfall and higher soil fertility. Australian deserts, along with those of South America and Southern Africa, provide a valuable counterpoint.

Stafford Smith and Morton (1990) framed ideas about arid Australian ecosystems in a set of inter-linked ‘propositions’. At the time, evidence for their 16 propositions varied from strong to slight. Twenty years later, the present paper revisits those propositions to consider whether new knowledge necessitates revision or rejection of them, to identify where fresh understanding suggests a new proposition and, where research has not yet confirmed or rejected a proposition, to suggest critical tests.

Section snippets

Background

Overall, three gaps are evident in the ideas underpinning Stafford Smith and Morton (1990). Firstly, the role of deep time in forming the Australian deserts of today was hardly mentioned. As a result many particularities of the Australian biota – such as the domination of marsupials among mammals and of Myrtaceae and Mimosaceae among woody plants – were not sufficiently emphasised. In hindsight, the original text tends to read as if Australian deserts had inevitably arrived at the present point

Physical history

Australia’s tectonic stability relative to other continents has strongly influenced its deserts; the regolith is often an ancient inheritance from the Tertiary or earlier (Pillans, 2007). In some places Australian desert soils have also developed over relatively long time periods, but this does not mean that they have stopped changing, as exemplified by continuing emergence of features such as carbonate horizons. Cracking clays (vertisols) are prominent, as are strongly weathered red earths

Evolutionary history

Eucalyptus and Acacia rose to prominence in the Miocene as drying and burning encouraged development of sclerophyllous woodlands. Chenopod shrublands and grasslands developed during the early Pliocene. Drier glacial phases of the Pleistocene must have forced many plant species into refugia, after which expansion by survivors occurred during interglacials (Martin, 2006). Consequently, species arose most often from nearby coastal localities, rather than by speciation within a specifically

Evolutionary history

Among Australian mammals, marsupials radiated from 50 M years BP; bats are of similar antiquity, but rodents arrived from Asia only 6 M years BP (Merrick et al., 2006). Birds are largely of Gondwanan stock (e.g. parrots and honeyeaters), with admixed descendants of Asian immigrants from the late Tertiary; the deserts contain a moderate number of endemic species (Schodde, 2006). Reptiles are a mixture of Gondwanan lineages and northern arrivals in the Miocene (Hutchinson and Smith, 2006), with

Conclusion – the unusual combination of features in arid Australia

On one hand the present review has provided confidence in the outlines of the first framework of Stafford Smith and Morton (1990), while allowing for more effective re-stating of the initial propositions. On the other, it is notable that considerable areas of ecology covered by the framework have not advanced significantly by empirical analysis or quantification, such as analysis of soil moisture dynamics, soil–nutrient relationships, and ecology of invertebrate assemblages, to choose just a

Acknowledgments

The ARC–NZ Research Network for Vegetation Function, supported by the Australian Research Council and Landcare Research New Zealand, provided vital sponsorship to bring the authors together for the present paper. Staff members of CSIRO’s Alice Springs laboratory provided support for the initial workshop as well as a congenial home for SRM as a Visiting Scientist during preparation of the manuscript. Alan Andersen, Craig James and Brian Walker commented constructively on the manuscript, Dick

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    Present address: CSIRO Ecosystem Sciences, Private Mail Bag, PO, Aitkenvale, Queensland 4814, Australia.

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