Population structure, flowering and seed production in the endangered Whibley Wattle, Acacia whibleyana (Leguminosae: Mimosoideae)

Acacia whibleyana (Whibley wattle) is a nationally endangered wattle, endemic to the near-coastal region around Tumby Bay on the Eyre Peninsula in South Australia. Two distinct sub-populations, referred to as the Quarry and the Salt-lake populations, each contained plants growing on roadsides and in scrub fragments. Marked differences were observed in the population size structure between the two populations. The absence of juveniles on roadsides in both populations signaled potential limitations to natural recruitment. Studies of reproductive biology of scrub plants over two years (1996-1997) indicated that between 1.1% (Salt-lake) and 6.9% (Quarry) of A. whibleyana inflorescences produced fruit. Each infructescence produced an average of 1.2–1.8 pods, although up to 10 pods per infructescence were observed at the Quarry scrub in 1996. Each pod contained 4–6 seed on average, with a viability of 85%. Plants at the Salt-lake scrub generally set less fruit, produced fewer pods per infructescence, and had lower seed numbers per pod when compared with Quarry plants, differences attributable to the superior vigor of Quarry plants. A. whibleyana seeds were dispersed by ants attracted by the lipid-rich aril. The soil seed bank had a patchy distribution across both sites possibly due to seed concentrating in the vicinity of ant nests. We concluded that the reproductive attributes studied were unlikely to constrain natural recruitment of A. whibleyana as both populations were capable of producing viable seed over the two years of this study.Manfred Jusaitis, Birgitte Sorensen and Lesley Polomk