Loss and degradation of native plant and animal habitat by invasion of escaped garden plants, including aquatic plants - key threatening process listing

The Scientific Committee, established by the Threatened Species Conservation Act, has made a Final Determination to list ‘Loss and degradation of native plant and animal habitat by invasion of escaped garden plants, including aquatic plants’ 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.

NSW Scientific Committee - final determination

The Scientific Committee has found that:

1. Invasion of natural ecosystems by exotic species is recognised globally as a significant threat to biodiversity (Millenium Ecosystem Assessment 2005). Invasive exotic plants can impact on ecosystem structure and function, reducing native species richness, altering hydrological or fire regimes, changing soil nutrient status and modifying habitat (Cronk and Fuller 1995; Lockwood et al. 2007).

2. Invasion by exotic plants has been identified as the primary cause of extinction of four native plant species in Australia, with another 57 species recognized as threatened by competition with invasive exotic plants (Leigh and Briggs 1992). In New South Wales, introduced invasive plants have been recognized as having an adverse impact on 341 species, 14 populations and 64 ecological communities listed as threatened under the Threatened Species Conservation Act 1995 (Coutts-Smith and Downey 2006).

3. The largest source of new plant species introduced into Australia is the horticultural industry (25,360 new species or 94%, Virtue et al. 2004; 25,448 or 97% of introduced species, Randall 2007). Escaped garden plants including aquatic plants are defined as plants that are currently or were historically used in gardens or aquaria for ornamental or utility purposes, which have formed self-sustaining populations in natural or other areas (Blood 2006). Escaped garden plants make up 66% of the 2779 introduced plants that have become naturalised (i.e. developed self-sustaining populations) within Australia (Groves et al. 2005). The largest source of environmental weeds (defined as those introduced plants that have a detrimental effect on native biodiversity) are escaped garden plants that have become invasive (72% of the 1765 listed environmental weeds, Groves et al. 2005).

4. Introduced garden plants that become invasive go through the same process as other introduced plant species that become invasive. The invasion process consists of at least three stages: (1) Introduction, where a species is introduced into a new region; (2) Naturalisation, where an introduced species forms self-sustaining populations, not requiring human intervention; (3) Invasion, where the naturalised species spreads beyond the original site of introduction and may become locally abundant (Richardson et al. 2000). Those invasive species that have a negative impact on biodiversity (either directly or via modification to ecosystem processes) are considered environmental weeds. Those invasive species that dramatically alter the biotic or abiotic environments where they become dominant are termed ‘transformer’ species (Richardson et al. 2000).

5. The chance of an introduced plant establishing naturalised populations and becoming invasive depends on a combination of environmental conditions (for example soil fertility, water availability, physical disturbance), plant traits (for example capacity for rapid growth, large seed production, seed dispersal mode) and propagule pressure (the number of individuals introduced) (Rejmanek et al. 2005). In the case of introduced garden plants, propagule pressure reflects both the species’ biology (seed production and dispersal traits) and its commercial properties in the nursery and aquarium trades (popularity, sales and distribution).

6. Garden plants may escape human-tended gardens and enter bushland and other areas through the dispersal of seeds or vegetative propagules. Natural dispersal vectors of seeds include wind, water, ants, birds and other animals. Many garden escapes are introduced directly into bushland by humans, via dumping of garden rubbish (for example Tradescantia Tradescantia fluminensis, Kikuyu Pennisetum clandestinum) and emptying of aquaria and backyard ponds (for example Salvinia Salvinia molesta, Cabomba Cabomba caroliniana).

7. Invasive escaped garden plants are a serious and expensive management issue in many NSW national parks, conservation reserves and state forests (DPI 2008; Coutts-Smith and Downey 2006). The estimated cost to Australia of weeds (including both agricultural and environmental weeds) is over 4 billion dollars annually in control and lost production (Sinden et al. 2004; NRMMC 2007).

8. Introduced garden plants may become an even greater threat in the future. Climate change may result in many introduced species becoming naturalised and many naturalised species expanding their range to become invasive. ‘Anthropogenic Climate Change’ is listed as a Key Threatening Process under the Threatened Species Conservation Act 1995. Pollination, and hence reproduction and spread, of invasive garden plants may be linked to subsequent invasion of exotic pollinators such as the Large Earth Bumblebee. ‘Large Earth Bumblebee, Bombus terrestris (L.)’ is listed as a Key Threatening Process under the Threatened Species Conservation Act 1995. Furthermore, the continuing importation, sale and distribution of new plant species presents a significant future risk to native biodiversity (Groves et al. 2005).

9. Escaped garden plants are recognized as posing a serious threat to native biodiversity. Of the 20 listed Weeds of National Significance, 10 were introduced originally for horticulture and a further 6 have been cultivated for horticulture (Randall & Kessal 2004), although all Weeds of National Significance have been banned from sale in NSW since 2005 (Johnson pers. comm. 2011). Fifty-seven percent of the 49 naturalised introduced plant species that impact on rare or threatened native plant species are invasive garden plants (Groves et al. 2005). Coutts-Smith and Downey (2006) identified Lantana (Lantana camara), Bitou Bush (Chrysanthemoides monilifera subsp. rotundata), Blackberry (Rubus fruticosus agg.), Kikuyu (Pennisetum clandestina) and Scotch Broom (Cytisus scoparius) as the five environmental weeds most commonly cited as threatening biodiversity in New South Wales, all of which are escaped garden plants. Groves et al. (2005) lists Banana Passion Fruit (Passiflora tarminiana), Broom (Cytisus spp.), Cat’s Claw Creeper (Macfadyena unguis-cati), Glory Lily (Gloriosa superba), Holly Leaved Senecio (Senecio glastifolius), Hybrid Mother of Millions (Bryophyllum daigremontianum X B. delagoense), Lippia (Phyla canescens), Madeira Vine (Anredera cordifolia), Mother of Millions (Bryophyllum delagoense) and Yerba de Hicotea (Hygrophila costata) as the ten most important garden plants available for sale in New South Wales, excluding those that are not already widespread in the wild. The negative impacts of many currently commercially available plant species may not be apparent for many years as they have not yet reached their invasive potential (Mulvaney 2001).

Escaped garden plants have adverse impacts on a large number of threatened species and ecological communities listed under the Threatened Species Conservation Act 1995. These include, but are not confined to: (1) FLORA: Acacia terminalis subsp. terminalis, Chamaesyce psammogeton, Endiandra floydii, Pimelea spicata, Thesium australe, Zieria granulata, Zieria prostrata; (2) FAUNA: Mountain Pygmy-possum Burramys parvus, Eastern Bristlebird Dasyornis brachypterus, Beach Stone-curlew Esacus neglectus, Bathurst Copper Butterfly Paralucia spinifera and Little Tern Sterna albifrons; (3) ECOLOGICAL COMMUNITIES: Blue Gum High Forest of the Sydney Basin Bioregion, Cumberland Plain Woodland of the Sydney Basin Bioregion, Eastern Suburbs Banksia Scrub in the Sydney Basin Bioregion, Littoral Rainforest in the NSW North Coast, Sydney Basin and South East Corner Bioregions, Lowland Rainforest on Floodplain in the NSW North Coast Bioregion, Swamp sclerophyll forest on the coastal floodplains of the NSW North Coast, Sydney Basin and South East Corner Bioregions. The following Key Threatening Process Determinations identify a large number of species and communities listed under the Threatened Species Conservation Act 1995 as threatened by escaped garden plants: Invasion, establishment and spread of Lantana (Lantana camara L. sens. lat); Invasion and establishment of exotic vines and scramblers; Invasion and establishment of Scotch Broom, Cytisus scoparius; Invasion of Native Plant Communities by Chrysanthemoides monilifera.

10. Escaped garden plants that become environmental weeds may have an adverse impact on native biodiversity through a variety of mechanisms. Invasion by escaped garden plants may directly affect vegetation structure and composition. Escaped garden plants may also indirectly affect native species richness and diversity by modifying resource conditions. Some may create deep shade at the ground level, preventing the growth and recruitment of native plants (e.g. African Olive Olea europaea subsp. cuspidata; Cuneo and Leishman 2006). Invasion by escaped garden plants can result in surface-soil nutrient sequestration and also in an increase in soil nitrate (e.g. Lantana camara; Lamb 1988 cited in Swarbrick et al. 1995; Gentle and Duggin 1998). Invasion by fire-promoting species such as Scotch broom (Cytisus scoparius) and Gamba grass (Andropogon gayanus) can increase the fuel loads in native vegetation, resulting in changes to natural fire regimes and fire behaviour (Downey 1999, 2000, 2002; Robertson et al. 1999; Rossiter et al. 2003). Invasive garden plants may also affect geomorphological processes such as soil erosion. For example invasion of watercourses by Lippia (Phyla canescens) can result in increased soil erosion as the root system of Lippia is less effective than those of native grasses in binding cracking clay soils (Groves et al. 2005). Invasive aquatic plants can choke waterways (e.g. Salvinia molesta) as well as modify water chemistry, including nutrient concentration, pH, salinity and available oxygen, which may adversely affect native aquatic species.

11. Escaped garden plants that invade natural vegetation communities may alter the availability of habitat and food resources for native fauna, resulting in reduced faunal abundance and diversity (French and Zubovic 1997; Low 1999; Maron and Lill 2005). For example, invasion of Little Penguin (Eudyptula minor) nesting sites by Kikuyu (Pennisetum clandestina) has been found to reduce nest site density (Weerheim et al. 2003). Escaped garden plants can degrade the aquatic food chain via toxic leachates from leaves in streams, which can poison fish and aquatic invertebrates (Low 1999; Davies 2004; Llewellyn 2005). Interactions involving escaped garden plants can have other disruptive effects on ecosystems. For example, fruits of escaped garden plants including Privet (Ligustrum sp.), Hawthorn (Crataegus mongyna) and Camphor Laurel (Cinnamomum camphora) increase the winter food supply of Pied Currawongs (Strepera graculina), leading to high densities of this nest predator and resultant declines in small bird populations (Debus 2006). Fruits of escaped garden plants also support populations of introduced pests such as Common Starlings (Sturnus vulgaris), which compete with native birds for nest hollows.

12. Some invasive escaped garden plants have previously been identified in the following Key Threatening Process Determinations listed under the Threatened Species Conservation Act 1995:

Invasion and Establishment of Exotic Vines and Scramblers

Invasion and establishment of Scotch Broom, Cytisus scoparius

Invasion of Native Plant Communities by Chrysanthemoides monilifera

Invasion, establishment and spread of Lantana (Lantana camara L. sens. lat)

Invasion of Native Plant Communities by Exotic Perennial Grasses

Invasion of Native Plant Communities by African Olive Olea europaea L. subsp. cuspidata (Wall ex G.Don Ciferri)

This determination does not preclude the listing, as a Key Threatening Process of individual species or groups of species which adversely affect threatened species or may cause non-threatened species to become threatened.

13. Some plant species commercially available now through the nursery industry are recognised as garden escapes (for example Asparagus fern Protasparagus aethiopicus) or may escape to become a threat to biodiversity in the future. Extensive effort has been made by the Nursery and Garden Industry Association (NGIA) to educate the public and control the spread of escaped garden plants (for example, through the NGIA’s Grow-me-Instead campaign). However, only voluntary controls are in place for many species, and funding to assess future weeds is scarce. Non commercial exchange of plant material is also an important mechanism for dispersal of many horticultural species. Public education measures to address this form of dispersal are not yet well developed. Improved quarantine assessment is required to stop the introduction of new plant species that are likely to become naturalised and invasive. National co-ordination of education and control of escaped garden plants is required to achieve effective change.

14. Loss and degradation of native plant and animal habitat by invasion of escaped garden plants, including aquatic plants 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
Scientific Committee

Proposed Gazettal date: 26/08/11
Exhibition period: 26/08/11 - 21/10/11


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