INTRODUCTION Viable seeds that fail to germinate when provided with adequate water, oxygen and suitable temperatures are termed dormant (Baskin & Baskin 1998). Dormancy is considered to be an adaptation restricting germina- tion to periods favourable for establishment and growth of the plant. In unpredictable environments such as those of arid Australia, germination represents a high-risk event as rainfall sufficient to allow germi- nation may be insufficient for seedling establishment (Jurado & Westoby 1992). Consequently, there is a ten- dency for the seeds of many arid zone species to spread the risk of germination and avoid depletion of the seed- bank following a single large rainfall event (Grice & Westoby 1987; Phillipi 1993; Auld 1995). Risk spread- ing is evidenced by having a high proportion of dor- mant seeds within the seedbank (Grice & Westoby 1987; Jurado & Westoby 1992). Such a strategy, how- ever, poses the problem of when to break dormancy, particularly if we consider that a species may miss opportunities for successful germination and recruit- ment by having a high proportion of dormant seeds. It is essential, then, that desert plants also have mech- anisms that allow germination to occur under appro- priate conditions, however unpredictable they may be (Phillipi 1993; Clauss & Venable 2000). The Australian Acacia species possess a hard imper- meable seed coat that renders the seed dormant until the seed coat is broken and there is adequate moisture available to allow imbibition of water and hence ger- mination (Clemens et al. 1977; Auld 1986a, 1995). In the case of Australian arid zone Acacia species, the proportion of dormant seeds produced and length of dormancy vary widely (Auld 1995). Under natural conditions, dormancy-breaking mechanisms of Acacia species include scarification of the seed coat in the soil, passage through the gut of vertebrates, unsuccessful seed predation and fire (Glyphis et al. 1981; Gill 1985; Auld 1986a). In addition to the breaking of dormancy, the estab- lishment of Acacia seedlings is influenced by the site of germination and, thus, their method of seed dis- persal (Davidson & Morton 1984). Australian Acacia species can be placed into three dispersal syndromes on the basis of their diaspore structure: arillate bird- dispersed species, arillate ant-dispersed species and non-arillate species (O’Dowd & Gill 1986). Arillate bird-dispersed species possess a large, fleshy, yellow or red aril, while ant-dispersed arillate species possess a relatively small, white aril. Despite this classification scheme, dispersal syndrome is not exclusive. The diaspores of bird-dispersed species are also dispersed by ants and ant-dispersed species are occasionally dispersed by birds (Forde 1986; O’Dowd & Gill 1986; French & Westoby 1996). Purported adaptive advan- tages for having these seed dispersal syndromes include Austral Ecology (2000) 25, 368–374 Bet-hedging and germination in the Australian arid zone shrub Acacia ligulata MIKE LETNIC,* CHRISTOPHER R. DICKMAN AND GAYLE MCNAUGHT Institute of Wildlife Research, School of Biological Sciences, University of Sydney, Sydney, New South Wales 2006, Australia (Email: mletnic@bio.usyd.edu.au) Abstract The diaspore of the Australian arid zone shrub Acacia ligulata is dispersed by birds and ants. To investigate the benefits of providing a dispersal structure attractive to both groups, we compared the germination response and viability of seeds eaten by birds, handled by ants or collected from trees to simulated precursors of germination: scarification, fire and rainfall were simulated. Seed germination and viability were related to the degree of preheating disturbance to the seed coat. Heating increased the germinability of seeds not scarified or eaten by birds. In the absence of heating, ingestion by birds increased germinability. Heating increased the mortality of seeds. Our results suggest that ingestion of seeds by birds may break seed dormancy and hence enable some seeds to germinate soon after dispersal. Alternatively, seeds not eaten by birds are likely to remain dormant until sufficiently scarified by soil or stimulated by fire. Consequently, in areas such as the Simpson Desert, A. ligulata may be able to use a range of seedling establishment ‘windows’ provided by monsoon rains, post-fire environ- ments and unseasonal winter rains, and also spread the risk of unsuccessful seedling establishment by retaining dormant seeds in the seedbank. Key words: Acacia ligulata, bet-hedging, desert plant, dormacy, life history, seed germination, variable environments. *Corresponding author. Accepted for publication November 1999.