Invasion of Native Plant Communities by African Olive Olea europaea L. subsp. cuspidata (Wall ex G.Don Ciferri) - key threatening process listing
NSW Scientific Committee - final determination
The Scientific Committee, established by the Threatened Species Conservation Act, has made a Final Determination to list the Invasion of Native Plant Communities by African Olive Olea europaea L. subsp. cuspidata (Wall ex G.Don Ciferri) as a KEY THREATENING PROCESS in Schedule 3 of the Act. Listing of Key Threatening Processes is provided for by Part 2 of the Act.
The Scientific Committee has found that:
1. African Olive, Olea europaea L. subsp. cuspidata (Wall ex G.Don Ciferri), is an evergreen dense crowned shrub or small tree with a centre of natural distribution in eastern Africa. African Olive is part of a tropical wild olive group, which are geographically isolated from their Mediterranean relatives (such as the cultivated European Olive Olea europaea L. subsp. europaea) and adapted to totally different climates within their native ranges (Zohary 1995). It is readily distinguished from European Olive by the presence of a hooked leaf apex and a lower leaf surface which is green or yellowish brown (Harden 1992).
2. African Olive was originally introduced into Australia from southern Africa in the early-mid 19th century and was planted extensively as a hedging plant. It is now well established as a woody invasive plant, particularly in the Cumberland Plain (western Sydney) and Hunter Valley regions of New South Wales, where it is considered a significant environmental weed. In contrast, populations of Olea europaea L. subsp. europaea which are invasive in South Australia are mostly derived from western or central Mediterranean cultivars (Besnard et al. 2007). Hybrids (O. cuspidata x europaea) have been identified in eastern Australia (Besnard et al. 2007) and these may also be invasive (Cuneo 2008), although their longterm viability is yet to be determined (W. Stamp, in litt.).
3. African Olive, Olea europaea
L. subsp. cuspidata,
is an evergreen shrub to 5 m or tree, rarely to 15 m. Leaves opposite, coriaceous, entire, 6-10 cm long, 10-25 mm wide covered with peltate scales, undersurface of leaf green or yellowish brown, often with a hooked apex. Inflorescences axillary, calyx small, 4-lobed, persistent in fruit. Drupe 5-7 mm long, fleshy, glaucous when ripe, purple-black.
4. African Olive is a highly persistent and long lived tree (100 years or more) which fundamentally alters ecosystem structure through the formation of a dense mid-canopy in native vegetation communities. The dense canopy structure (>80% crown cover) of African Olive creates deep shade at the ground level, preventing the growth of native grasses and herbs (Cuneo and Leishman 2006).
5. African Olive produces large crops of small black fruits (>25,000 fruits/tree/year) which are readily consumed and dispersed by a range of native and introduced birds. Native birds that are recorded as dispersers of African Olive fruit include Satin Bowerbird Ptilonorhynchus violaceus and Pied Currawong Strepera graculina. Exotic birds that are recorded as dispersers of African Olive fruit include Blackbird Turdus merula and Common Myna Acridotheres tristis (Spenneman and Allen 2000). Seedlings establish as ‘halos’ around large perch trees. Dense seedling ‘mats’ form in the seed fall zone of mature plants, with densities of up to 950 seedlings/m2 recorded (Cuneo and Leishman 2006). African Olive seedlings are able to remain at this ‘seedling bank’ stage for many years. The dense and persistent canopy of African Olive also results in long term loss of native species from the soil seedbank, limiting the ability of native plant communities to naturally regenerate following the long term (>15 years) presence of African Olive.
6. Mature African Olive is able to regenerate after fire, however seedlings <1m are killed by fire (von Richter et al. 2005).
7. African Olive is adapted to a wide range of environments from dry exposed ridgelines through to saline watercourses. African Olive has a clear distributional preference for shale derived clay soils, and does not readily establish on low fertility sandstone soils. The clay soil preference of African Olive coincides with regions of highly fragmented native vegetation heavily cleared for agricultural production and urban development, such as the Cumberland Plain of western Sydney and the Hunter Valley.
8. The main ecological impact and threat posed by African Olive is through its ability to form a dense mid-canopy in native vegetation communities. Dense shading from African Olive dramatically reduces light levels which prevents the establishment and growth of native grasses and herbs and the establishment of overstorey trees such as Eucalypts. The establishment of native understorey plants is also adversely impacted in the root zone area beyond the African Olive crown. In Cumberland Plain Woodland, the formation of a dense African Olive canopy results in a 77% reduction in native understorey plant species richness (P. Cuneo, unpublished data). Native tussock grasses, which require high light levels, are particularly affected. Cumberland Plain Woodland in the Sydney Basin Bioregion is listed as a Critically Endangered Ecological Community under the Threatened Species Conservation Act 1995.
9. The formation of African Olive canopy causes changes in woodland bird assemblages through changes in vegetation structure and fruit availability. The Speckled Warbler Pyrrholaemus saggitatus listed as Vulnerable under the Threatened Species Conservation Act 1995 is likely to be adversely affected by vegetation structural changes resulting from African Olive invasion (DECC 2007). Non-indigenous bird species that are favoured by African Olive invasion include the Common Starling Sturnus vulgaris, Blackbird and Red Whiskered Bulbul Pycnonotus jocosus. Native species that may utilise African Olive-infested areas include Bell Miner Manorina melanophrys, Pied Currawong, Silvereye Zosterops lateralis, Eastern Yellow Robin Eopsaltria australis, Red-browed Finch Neochmia temporalis and King Parrot Alisterus scapularis (T. Beshara, unpublished data based on DECC Wildlife Atlas records).
10. There is estimated to be > 4000 ha of dense African Olive canopy now established across the western Sydney region (Cuneo and Leishman 2006), with > 1900 ha of dense African Olive infestation mapped from satellite imagery in a 721 km² study area in the southern Cumberland Plain (Cuneo et al. 2009). The Camden – Picton region is the original introduction location, and a major centre of African Olive infestation. African Olive poses a major threat to the long term existence of several western Sydney Endangered or Critically Endangered Ecological Communities listed under the Threatened Species Conservation Act 1995, including Western Sydney Dry Rainforest in the Sydney Basin Bioregion (25% invaded), Cumberland Plain Woodland in the Sydney Basin Bioregion (8.5% invaded) and Moist Shale Woodland in the Sydney Basin Bioregion (28.5% invaded) (Cuneo et al. 2009). African Olive occurs to some extent in almost all reserves in the Sydney Region, particularly those that contain Cumberland Plain Woodland, including Mulgoa Nature Reserve, Leacock Regional Park, Scheyville National Park, Kemps Creek Nature Reserve, Prospect Nature Reserve, as well as Sydney Harbour National Park, Botany Bay National Park, Cattai National Park, Bents Basin SCA, and Gulguer Nature Reserve (DECC 2008).
11. African Olive is considered to be one of the most serious environmental weeds in the Central Hunter region (Peake 2006), with dense infestations appearing over the last 15 years (G. Lyons, in litt.). African Olive occurs within all Hunter LGAs, with core infestations occurring in the Maitland, Singleton and Cessnock LGAs (G. Lyons, in litt.). Major infestations occur in the Lochinvar – Maitland district where African Olive forms dense monocultures. Vegetation mapping for the Central-Upper Hunter region (Peake 2006) describes four Spotted Gum/Box/Ironbark grassy woodland communities of regional conservation significance which are under threat from African Olive. African Olive is considered to be a serious threat to seven Vulnerable or Endangered Ecological Communities in the Hunter region that are listed under the Threatened Species Conservation Act 1995: Central Hunter Grey Box-Ironbark Woodland in the NSW North Coast and Sydney Basin Bioregions, Central Hunter Ironbark-Spotted Gum-Grey Box Forest in the NSW North Coast and Sydney Basin Bioregions, Hunter Valley Footslopes Slaty Gum Woodland in the Sydney Basin Bioregion, Lower Hunter Spotted Gum – Ironbark Forest in the Sydney Basin Bioregion, Lower Hunter Valley Dry Rainforest in the Sydney Basin and NSW North Coast Bioregions, Swamp Oak Floodplain Forest of the NSW North Coast, Sydney Basin and South East Corner Bioregions, and White Box – Yellow Box – Blakely’s Red Gum Woodland (Peake 2006, G. Lyons, in litt.).
12. African Olive is found in the Kiama, Dunmore and Bombo districts in the Illawarra region and is listed as one of the significant woody weeds that threaten the long-term viability of the Endangered Ecological Community Illawarra Subtropical Rainforest in the Sydney Basin Bioregion, listed under the Threatened Species Conservation Act 1995. African Olive may also threaten the Endangered plant species Zieria granulata and Cynanchum elegans and the Endangered Ecological Community Illawarra lowlands grassy woodland in the Sydney Basin Bioregion, listed under the Threatened Species Conservation Act 1995.
13. African Olive is also found in all Local Government Areas of the Sydney region (DPI 2008), in the Tamworth and Lismore regions and in the Wellington Caves area (W. Stamp, in litt.), and is likely to be found elsewhere in NSW.
14. African Olive has been identified as a widespread weed having a high impact on biodiversity in a total of 25 Endangered Ecological Communities, 13 threatened flora and four threatened fauna species within the Sydney Metropolitan, Hawkesbury-Nepean and Hunter-Central Rivers Catchment Management Authorities (A. Leys, in litt.), and potentially further north (e.g. Scientific Committee 2004 a and b; Coutts-Smith and Downey 2006).
15. The Invasion of Native Plant Communities by African Olive Olea europaea L. subsp. cuspidata (Wall ex G.Don Ciferri) is eligible to be listed as a Key Threatening Process as, in the opinion of the Scientific Committee:
(a) it adversely affects threatened species, populations or ecological communities, or
(b) it could cause species, populations or ecological communities that are not threatened to become threatened.
Dr Richard Major
Proposed Gazettal date: 01/10/10
Exhibition period: 01/10/10 – 26/11/10
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