Biological Conservation 65 (1993) 165-176 THE IMPACT OF GRAZING ON REGENERATION OF THE SHRUB Acacia carnei IN ARID AUSTRALIA Tony D. Auld National Parks and Wild~fe Service NSW, PO Box 1967, Hurstville, NSW 2220, Australia (Received 19 May 1992; revised version received 5 October 1992; accepted 6 October 1992) Abstract Thirty-seven populations from across the known range of Acacia carnei were sampled to determine the level of recent regeneration in this wattle. Thirty populations showed no evidence of regeneration, while the remaining seven showed some limited regeneration. Successful seed production is restricted to one population near Kinchega National Park, NSW and a second, less fecund, popula- tion, within Kinchega National Park. The majority of seeds are non-dormant when released. The production of vegetative suckers is widespread in populations through- out the distribution of A. carnei. Caging experiments at Kinchega National Park indicated that rabbit Oryctola- gus cuniculus grazing was primarily responsible for the observed lack of recent regeneration. The eradication of rabbits from populations of A. carnei is essential for its long-term conservation. Key words: Australia, rabbits, acacia, grazing, regeneration. INTRODUCTION The successful conservation of a plant species involves the active management of that species both inside and outside a reserve system. This can only be achieved through an understanding of the life cycle of a plant and the factors which influence the passage of individuals through that life cycle. Population size may be largely dependent upon the relative success of individuals in particular critical life cycle stages (e.g. fecundity, dispersal, germination, establishment and survival). Critical life cycle stages may be examined in detail and the results used to direct management options. In arid communities, the recruitment of individuals into the adult population is a critical determinant of long-term population numbers and stability (Obeid & Seif el Din, 1970; Crisp & Lange, 1976; West et al., 1979; Lange & Graham, 1983; Chesterfield & Parsons, 1985; Woodell, 1990). Since white settlement, arid and semi-arid Australia have been subject to a changed grazing regime. This has involved a reduction or elimination of medium- sized mammal species (Morton, 1990), combined with Biological Conservation 0006-3207/93/$06.00 © 1993 Elsevier Science Publishers Ltd, England. Printed in Great Britain an increase in large kangaroos (Caughley et al., 1987) and the introduction of several grazing species, notably sheep, cattle, rabbits, goats, camel, donkeys and horses. This altered grazing regime will influence plant popula- tions through the level of recruitment into populations. For long-lived perennial plants, some recruitment, however sporadic, is necessary to replace those adults that senesce and die. Evidence suggests that recruit- ment is not occurring in many species under the current grazing regime (Auld, 1990). This paper examines the conservation of purple- wood wattle Acacia carnei Maiden (Fabaceae), a large shrub from arid Australia, by focusing on three aspects: (i) The size distribution of a range of populations. This approach attempts to detect the presence of recruitment events in the recent past (cf. Chesterfield & Parsons, 1985; Cooke, 1988). (ii) The fecundity and germination characteristics of A. carnei. This allows an estimate of the likeli- hood of seedling recruitment events in the field. (iii) The recruitment of vegetative suckers (ramets) into populations and their survival through time under a range of grazing regimes. This allows quantification of vegetative regeneration events and a comparison of the survival of these re- cruits in the presence and absence of herbivores. As seed germination in the field was not ob- served, no attempt was made to examine grazing pressure on seedlings. Plant nomenclature follows Harden (1990, 1991). 165 STUDY AREA AND METHODS Acacia carnei is a large rounded shrub usually under 5 m in height and several metres in diameter. Its distribu- tion is limited to several locations in a small area of western New South Wales and eastern South Australia (Fig. 1) and hence it is included in the Rare or Threat- ened Australian Plant list (Briggs & Leigh, 1988). Within this distribution, it is largely confined to red sand dunes, alluvial accumulations or occasionally level sandy areas as a dominant or associated with Alectryon oleifolius, Casuarina pauper, Maireana pyramidata or very occasionally with Callitris preisii. Annual rainfall in this region varies from 150 to 250 mm (Bureau of