PUBLISHED VERSION Jusaitis, Manfred; Sorensen, Birgitte; Polomka, Lesley Population structure, flowering and seed production in the endangered Whibley Wattle, Acacia whibleyana (Leguminosae: Mimosoideae), The Open Conservation Biology Journal, 2009; 3:4-13 © Jusaitis et al.; Licensee Bentham Open. PERMISSIONS http://www.benthamscience.com.proxy.library.adelaide.edu.au/open/toconsbj/MSandI.htm COPYRIGHT: Authors who publish in Bentham OPEN Journals retain copyright to their work. Submission of a manuscript to the respective journals implies that all authors have read and agreed to the content of the Covering Letter or the Terms and Conditions. It is a condition of publication that manuscripts submitted to this journal have not been published and will not be simultaneously submitted or published elsewhere. 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Articles are licensed under the terms of the Creative Commons Attribution non-commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted, non- commercial use, distribution and reproduction in any medium, providing that the work is properly cited. th 14 of December 2012 http://hdl.handle.net/2440/74637 4 The Open Conservation Biology Journal, 2009, 3, 4-13 Open Access Population Structure, Flowering and Seed Production in the Endangered Whibley Wattle, Acacia whibleyana (Leguminosae: Mimosoideae) Manfred Jusaitis*,1, Birgitte Sorensen2 and Lesley Polomka3 1Botanic Gardens of Adelaide, Department for Environment and Heritage, North Terrace, Adelaide, SA 5000, Australia 2Strategic Policy Division, Department for Water, Land and Biodiversity Conservation, GPO Box 2834, Adelaide, SA 5001, Australia 3Native Vegetation Assessment Branch, Environment Division, Department of Industry and Resources, 100 Plain Street, East Perth, WA 6004, Australia Abstract: 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 concen- trating 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. Keywords: Demography, rare, reproduction, seed bank, seed dispersal, South Australia, threatened plant. INTRODUCTION pool, and/or combined effects of natural field conditions (microsite, predation/grazing, competition, edaphic factors) Whibley wattle, Acacia whibleyana R.S. Cowan & on germination and growth. In this study we analyzed the Maslin (Leguminosae: Mimosoideae: subgenus Phyllodi- effects of the former, namely limitations in seed supply. neae: section Plurinerves) is a nationally endangered shrub Seed limitations may occur at the level of seed set, matura- with an extremely restricted distribution near Tumby Bay on south-eastern Eyre Peninsula, South Australia. Until the tion or dispersal [4]. The scattered nature of these small, early 1990s, it was only known from a single population of fragmented subpopulations suggests that cross-fertilization less than 50 plants growing on road verges in a very local- may be restricted, potentially leading to in-breeding and ized agricultural area [1]. In the mid 1990s, extensive sur- population decline. Habitat fragmentation has a major nega- veys around the region revealed the existence of two disjunct tive influence on pollination and plant reproduction gener- populations (Salt-lake and Quarry populations) about 15 km ally [5, 6] and for several species of Acacia specifically [7- apart [2]. The majority of plants were mature individuals and 10]. A few studies have also looked at the effect of fragmen- there appeared to be little evidence of regeneration. The area tation on seed dispersal, but these have mostly been re- had been extensively cleared leading to fragmentation and stricted to tropical forest species [11 and references therein, modification of natural ecosystems, with the only remaining 12]. Clearly, without ongoing recruitment of new individuals suitable habitat to be found in small native remnants on road of A. whibleyana, these few remaining populations will be reserves or verges and on private land. A recent survey re- lost as mature plants die. corded 98 and 255 plants at the Salt-lake and Quarry respec- This paper reports on the results of investigations to tively [3], 16% occurring on roadsides, 67% in scrub sites, evaluate and compare seed production of A. whibleyana at its and the remainder on farm land. two population centers by examining flowering, seed set, The paucity of A. whibleyana juveniles in these frag- seed dispersal and the soil seed bank. The demographic mented populations suggested recruitment limitations result- structure of populations was quantified and the demographic ing from either constriction in supply of the natural seed stage structure between roadside and scrub sites at the two populations was compared. Our aim was to estimate past recruitment trends and to determine whether seed supply was *Address correspondence to this author at the Botanic Gardens of Adelaide, a limitation to regeneration. Department for Environment and Heritage, North Terrace, Adelaide, SA 5000, Australia; E-mail: [email protected] 1874-8392/09 2009 Bentham Open Population Structure and Seed Production in Acacia whibleyana The Open Conservation Biology Journal, 2009, Volume 3 5 MATERIALS AND METHODS bata, Bromus spp., Lycium ferocissimum, Asparagus as- paragoides, Marrubium vulgare, Asphodelus fistulosus and Study Species Hordeum leporinum. A. whibleyana is a dense shrub, growing to 2.5 metres A. whibleyana plants were also found on roadsides in the high and 4 metres across [13]. Phyllodes are elliptic to oblanceolate, asymmetric, and terminate in a rostriform tip. vicinity of these two population centers. Roadsides were 5 m The plant is inconspicuous during most of the year, but very wide and generally more weedy than the spatially larger Salt- visible when flowering between August and September. lake and Quarry scrub populations. No evidence of roadside Globular, bright golden heads (2.5–3 mm diameter) cover management activities that may have been disruptive to the plant during this time, and fruits mature between Decem- roadside populations were observed during the study period. ber and January. Phenology, Growth and Population Structure Study Sites The annual cycle of growth, flowering and seed produc- Studies were performed at the two population centres for tion in A. whibleyana was monitored over three years (1995- A. whibleyana, referred to as the Salt-lake scrub (Lat: 1997). Eleven plants representative of the size range of plants at the Salt-lake scrub site were selected. On regular 34°27.079S; Long: 136°02.144E, Altitude 10 m) and visits (three per annum) to the site, each plant was measured Quarry scrub (Lat: 34°19.270S; Long: 136°02.984E, Alti- in three dimensions (H = height, W1 = maximum width, W2 tude 150 m) populations. The Salt-lake scrub consisted of a = orthogonal width) and the presence or absence of repro- small (~ 0.8 ha) triangle of land forming part of a road inter- ductive structures (buds, flowers, fruit) was recorded. Plant section. The site was fenced prior to commencement of the size-indices were calculated for each plant as SI = study to prevent entry of vehicular traffic and to exclude (H+W1+W2)/3 and means were plotted against time to show rabbits and stock, although no evidence of herbivory was the seasonal pattern of growth. Annual rainfall at Tumby observed on translocated or wild plants at either site. Native Bay (average annual rainfall = 338 mm) during the study vegetation was dominated by Pittosporum angustifolium, period was 426 mm (1995), 325 mm (1996) and 309 mm Melaleuca acuminata, Enchylaena tomentosa, Lasiopetalum (1997). Annual rainfall at Hillview (average annual rainfall = baueri and a ground flora of samphires. Parts of the site were 366 mm) was 478 mm (1995), 397 mm (1996) and 359 mm infested with weeds, including Asparagus asparagoides,