Objectives

This topic is designed to introduce environmental weeds, their impact and management. At the end of the module you will:

• understand the origins of environmental weeds
• appreciate the ecological impact they have
• be familiar with the management strategies used
• describe the steps involved in revegetation techniques after environmental weed removal

Module Outline

• Introduction
• Source
• Impact of environmental weeds
• Aims of environmental weed management
• Control techniques
• Revegetation techniques
• Case studies
• Conclusions
• Further reading

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Introduction

(Notes based on: Vranjic, JA, Groves, RH and Wills, AJ 2000, ‘Environmental weed management systems’, in Sindel, BM (ed.) Australian Weed Management Systems, RG and FJ Richardson, Melbourne, Victoria, pp. 329-354.)

Environmental weeds are weeds that invade natural ecosystems. They may also include native species growing beyond their natural range (e.g. coastal tea tree). Many types of ecosystems/habitat can be affected, and therefore the management of environmental weeds has to be on a case/by/case basis.

There might be differences in the patterns of invasion in northern as compared to southern Australia. Broad-scale single species invasions often occur in northern Australia, as compared to more multi-species invasions in southern Australia, where climate and topography are more varied.

Definitions

Environmental weed

The common definition of a weed being ‘a plant out of place’ emphasises an anthropocentric view of plants. Environmental weeds mostly do not directly affect human kind nor do they necessarily have the economic impact that can be measured for agricultural weeds.

• Environmental weeds are plants that invade native communities or ecosystems.
• Serious environmental weeds cause major modification to species richness, abundance, or ecosystem function.
• Very serious environmental weeds can totally and permanently destroy an ecosystem (Humphries et al. 1991).

Source

Most environmental weeds were introduced into Australia deliberately for ornamental or agricultural purposes. Some were introduced to meet an environmental need (e.g. dune stabilisation) and then got out of control. Some are introduced accidentally. Alligator weed is thought to have been introduced via the dumping of ballast from an arriving ship.

Dispersal is by abiotic means, animals or humans and can result in spread over great distances. Examples are burrs attached to vehicles or clothing, dumping of garden refuse, and deliberate plantings such as blackberries and St Johns Wort.

Environmental weeds threaten disturbed and undisturbed native vegetation areas. New environmental weeds are still being introduced and spreading.

Ornamental plants, often dumped in bushland or sold by nurseries, compound the problem and public education is needed to overcome this problem and promote more responsible plant selection.

Impact of environmental weeds

Introduction

Environmental weeds affect biodiversity and ecological processes in native ecosystems. Their direct impacts include shading, competition and change in fuel load affecting the fire regime. Their indirect impacts occur through changes in other species, e.g. weed presence may affect abundance of pollinators. Indirect impacts may be just as important in long-term ecosystem processes though they are more difficult to determine.

Impacts on native species

Severe weed invasion has displaced native species through competition and caused extinction of rare taxa. Their impact may be more severe when weeds share similar morphology or physiology with the native species that are being displaced.

Threatened taxa, often found in remnant vegetation areas and degraded habitats, are at increasing risk, as these areas are often subject to the kinds of disturbance that favours weeds.

Seed bank composition changes as weed propagules enter the soil leading to an increased presence of weed seeds.

Reproductive capacity of weeds is often greater than native plants, leading to a change in the seed bank composition over time.

Impacts on native fauna

Environmental weeds cause changes to the composition and structure of vegetation, altering sources of food and shelter for fauna. Weeds can favour some native animals and harm others. Where native animals feed on weeds, management strategies need to supply alternative food sources. Management also must control spread of weeds by feral animals.

Software Environmental Weeds of Australia Select the ‘Environmental Weeds of Australia’ CD and run it on your computer. Go through the tutorial until you understand the operation of the CD. You can use the key to identify examples of environmental weeds (over 1,000 examples are in the database). This software product is available on a semester loan basis from the UQ Gatton Library.

Aims of environmental weed management

The aims of integrated weed management are to effectively contain spread of existing weeds, to manage the environment to prevent new invasions and to rehabilitate the disturbed ecosystem as far as possible.

Ecological information is needed to develop strategies. Such ecological information could be: how the weed is propagated/dispersed, life cycle information (i.e. growth, flowering, fruiting periods), whether there are natural enemies, and response to control methods. Non-target impacts need to be considered, for example a herbicide that kills native plants is not appropriate.

Weed control must facilitate regeneration of native plant species. An integrated strategy would encourage native regrowth and regeneration but also attempt to discourage the replacement of the target weed with another weed species. Replanting of native species may be necessary to achieve this.

Control techniques

Chemical, mechanical, biological

Chemical controls must be applied with proper care but can be very effective (e.g. cut-stump, stem injection, weed wipers to apply herbicide directly). Control techniques must be thoroughly tested through quarantine or field trials for effects on non-target species and residual effects before implementation.

Mechanical methods such as cutting and mowing can be impractical, it is labour intensive and infested areas may be inaccessible.

Grazing may be an option on private land and hand removal by volunteers might be possible near populated areas.

Fire

Use of fire can be an important tool for management of environmental weeds, particularly in natural ecosystems with a fire history and native species regeneration is linked to this.

Factors for consideration in using fire to control weeds are the response of the weed to burning, the type of ecosystem the weed inhabits, the fire regime employed and the season of fire.

Some weeds do not burn well, others respond vigorously, resprouting and regenerating from basal buds after the fire. Bitou bush seeds on the surface are killed while those under the soil are stimulated to germinate by fire.

Fire would not be a good tool in fire-sensitive ecosystems such as rainforests.

The intensity and frequency of the control fires need to be carefully planned to suit the requirements of the native species and the desired outcome for the plant community.

If the fire is implemented in the wrong season there can be increased levels of risk of fire escaping or the seed bank may be depleted, harming regeneration of native species.

Fire can be used in conjunction with other methods (e.g. application of herbicide on new weed seedlings after burning).

Revegetation techniques

Introduction

Revegetation is the introduction of plant species into a degraded habitat. Exotic or native species can be used, but natives are preferred.

Non-native species can be planted but this is not the ecologically preferred option. Problems can occur when the local native species are unsuitable for use in revegetating a habitat (e.g. poor germination or soil no longer suitable) and therefore it may be necessary to use species from nearby locations instead.

Revegetation can help prevent weeds by choosing competitive native plants. This increase in competition means gaps are more likely to be colonised by native plants rather than weeds. The ‘Bradley method’ is commonly used to help choose competitive native plants.

Bradley method

The Bradley method focuses on removing small satellite weed populations before working on the core infestations. It works from minimally disturbed sites in towards the main weed infested areas. It keeps soil disturbance to a minimum and the rate of native plant regeneration dictates the rate of weed removal.

This method is best for sites with pre-existing native soil seed bank as the conditions aim to minimise conditions that favour weeds while favouring natural regeneration.

This method is not suitable for very severe infestations of invasive weeds.

The approach

In areas where weed infestation is severe, there are three main techniques that can be used to initiate revegetation:

• manipulation of the soil seed bank to stimulate the pre-existing seed banks. This is often undertaken by burning;
• sowing of native seed using knowledge of the germination requirement of that seed, or using seed pre-treatments to promote germination. The pre-treatments often required include scarifying the seeds, or using heat or smoke to break dormancy and promote germination. After sowing, other methods may also be necessary (e.g. fire or seed burial). Timing (i.e. season) is also important;
• planting of seedlings of native species. This is much more labour intensive than using seed, but generally gives higher plant establishment rates. Planting should only occur when there is a good chance of survival. Protection from grazing may be necessary.

Case studies

Impact of environmental weeds on native species abundance:

The rare ground orchid Pterostylis arenicola is only known to exist at three sites in South Australia. At one site the population is threatened by bridal creeper (Asparagus asparagoides) which has a similar phenology. At this site the number of orchid plants is decreasing as the number of bridal creeper plants is increasing.

Impact of environmental weeds on ecosystem function

Cyticus scoparius (Scotch broom) is a widespread leguminous weed in southern Australia. It is a nitrogen fixing capacity, capable of altering soil nitrate levels, especially when burnt. Increased nitrate levels may favour growth of Mediterranean grass weeds, which could out-compete the perennial native grasses.

This leads to a change in the understorey and a change in the flammability of vegetation as native summer-green perennial grasses are replaced by annual grasses that grow in spring and ‘hay off’ in summer. This can cause long-term ecosystem changes that are difficult to reverse. Control of one shrub may not solve the problem as another could take over.

Impact of environmental weeds on flora and fauna

Tamarix aphylla (tamarisk) has become naturalised along the Finke River system in central Australia. The measured impacts it is having include:

• decreased regeneration of formerly dominant Eucalyptus camaldulensis;
• changes in the understorey composition;
• reduced numbers of reptiles;
• increased number of insectivorous birds;
• no effect on granivorous birds;
• increased litter, shade and riverine salt levels compared to native vegetation.

Detrimental effects of herbicides

The endangered species Grevillea iaspicula occurs in seven sites in NSW, some of which are threatened by blackberry invasions. Attempts to control blackberry by cutting and spraying the stumps with herbicide resulted in stunted shoot growth and leaf chlorosis of some of the G. iaspicula plants. This has delayed the recovery potential of these populations.

Positive effects of herbicides

Trials of herbicide spraying along NSW coast tested the responses of bitou bush (Chrysanthemoides monilifera subsp. rotundata) and other (mostly native) woody species. Low rates of glyphosate applied in winter kills bitou bush shoots with no long-term impact on many native species. The research resulted in aerial application of glyphosate along large sections of coastline. Impact on other flora is unknown (e.g. ephemeral annuals).

Host-specificity of biological control agents

St Johns wort (Hypericum perforatum) was introduced into Australia over 100 years ago and became a major pasture weed. A beetle was released in the 1930’s even though there are two native species from the same genus which might be affected.

One of the native species co-occurs with the introduced weed in grasslands and grassy understoreys. The weed was such a problem that little importance was given to the possible effects on the native species. Sixty years later a mite Acalus hyperici was introduced after extensive testing. Some success was found in controlling the weed and with no apparent effect on native species, despite its close relationship with the weed.

Integrated management of a temperate coastal weed

Bitou bush was deliberately planted as a dune stabilizer in NSW in the 1940s, but by the 1960’s had become very invasive. Deliberate plantings ceased but by then the bush had spread from Tathra on the south coast to the Queensland border, and had invaded other dune systems, coastal heathlands and woodlands.

In comparison with the dominant native species Acacia longifolia subsp. sophorae (coastal wattle), bitou bush has a much longer flowering and fruiting season, a greater seed production and seed bank and higher seed survival and faster seedling growth.

Bitou bush can therefore compete against and displace coastal wattle despite its dominance. The fruits of bitou bush are spread by birds and foxes, and may be influencing the composition of the bird population and aiding the survival of the foxes.

Bitou bush responds to the following control methods as follows:

• cutting or grazing shoots causes regeneration of basal buds;
• spraying with glyphosate causes shoot death but plants can survive if contact with plant and herbicide is poor e.g. when leaves are covered with dust;
• fire kills most adult shoots and seedlings but regeneration from basal buds may occur, and fire stimulates germination of the seed;
• several biocontrol agents have been released and established along the coast.

The bitou tip moth, Comostolopsis germana, which destroys the apical shoots and a seed fly Mseoclanis palana, which destroys developing seeds, have established along the coast.

The biological information and responses to control measures show that long-term management should aim to reduce seed-set and exhaust the seed bank without harming native plants.

Two important ecological aspects are that the winter application of herbicides can kill adults without impacting on natives and fire can stimulate massive seed germination events of the bitou bush as well as natives, providing competition for weed seedlings and depleting bitou bush seed bank.

A management plan can centre on a spray-burn-spray regime. This consists of aerial spraying of the invaded vegetation with low rates of herbicide in winter, leaving some patches for biocontrol agents; burning off of the sprayed bitou bush once it is dead, to stimulate seed bank germination. Competition can be enhanced by sowing native seeds just before and after the fire (for fire sensitive species). Allow colonisation of seedlings by biocontrol agents, spot spray seedlings in winter and ensure removal before flowering to prevent new seeds. Continue to monitor and apply follow-up removal treatments.

Integrated management of a tropical weed

Giant sensitive plant (Mimosa pigra) is a prickly shrub that originated in tropical America and has spread in Australia from Darwin to the South and East Alligator Rivers and Kakadu National Park.

It mainly infests damp areas such as wetlands, billabongs and riverbanks, and forms dense thickets. Seeds are buoyant and spread by water, also spread by cattle and people. Giant sensitive plant has replaced native vegetation affecting fauna and Aboriginal food-gathering. Research is investigating the use of herbicide, biological control, fire and sowing of grass seeds. There is currently no effective integrated strategy for this and many other species.

Conclusions

Indirect costs of environmental weeds include biodiversity loss, loss in aesthetic values, and lower wilderness quality in addition to direct costs of weed management.

Weeds cause economic loss to the Australian community - $3 billion per year for agricultural and pastoral weeds alone.

Integrated management of environmental weeds requires application of general weed management principles that take into account the particular situation.

Proper management requires detailed ecological knowledge of the weed and the native plant community.

Go to Activity 6-1

Further reading

Baird, AM 1977, ‘Regeneration after fire in King’s Park, Perth, Western Australia’, Journal of the Royal Society of Western Australia, vol. 60, pp. 1-22.

Bass, DA 1996, ‘Pied currawongs and invading ornamentals: what’s happening in northern New South Wales’, Proceedings of the 11th Australian Weeds Conference, pp. 362-365.

Blood, K 2001, Environmental Weeds: A Field Guide for SE Australia. CH Jerram and Associates - Science Publishers, Mt. Waverley, Victoria.

Carr GW, Yugovic JV and Robinson KE 1992, Environmental Weed Invasions in Victoria: Conservation and Management Implications, 1st edn. Department of Conservation and Environment, Melbourne, Victoria.

Cheal, D 1991, ‘The impact of environmental weeds on rare or threatened plants in Victoria’, Plant Protection Quarterly, vol. 6, pp. 23-125.

Crawley, MJ 1987, ‘What makes a community invasible?’ in AI Gray, MJ Crawley, & PJ Edwards (eds), Colonization, succession and stability, Blackwell Scientific Publications, pp. 429-453.

Groves, RH 1986, ‘Plant invasions of Australia: an overview’ in RH Groves, JJ Burdon, (eds), Ecology of Biological Invasions, Cambridge University Press, Cambridge, UK, pp. 137-149.

Groves, RH 1998, Recent incursions of weeds to Australia, CRC for Weed Management Systems, Technical Series No. 3, Glen Osmond, Australia.

Hobbs, RJ 1991, ‘Disturbance as a precursor to weed invasion in native vegetation’, Plant Protection Quarterly, vol. 6, pp. 99-104.

Humphries, SE 1993 ‘Environmental impact of weeds’, 10th Australian and 14th Asian- Pacific Weed Conference, vol. 2, pp. 1-11.

Humphries, SE, Groves, RH & Mitchell, DS 1991, ‘Plant invasions of Australian ecosystems. A status review and management directions’, Kowari, vol. 2, pp. 1-127.

Hussey, BMJ and Wallace, KJ 2003, Managing Your Bushland. Department of Conservation and Land Management (CALM), Kensington, Western Australia.

Johnstone, IM 1986, ‘Plant invasion windows: a time-based classification of invasion potential’, Biological Reviews, vol. 61, pp. 369-394.

Krebs, CJ 1984, Ecology, the experimental analysis of distribution and abundance, 3rd edn, Harper & Row, New York.

Muyt, A 2001, Bush invaders of south-east Australia: A guide to the identification and control of environmental weeds found in south-east Australia, RG & FJ Richardson, Meredith, Victoria.

Noble, IR 1989, ‘Attributes of invaders and the invading process: terrestrial and vascular plants’ in JA Drake (ed.), Biological Invasions: a Global Perspective, pp. 301-313.

Panetta, FD & Lane, DW 1996, ‘Managing weeds in Australia’s public lands and forests’, Proceedings of the 11th Australian Weeds Conference, pp. 391- 397.

Radford, IJ and Cousens, RD 2000, ‘Invasiveness and comparative life history traits of exotic and indigenous Senecio species in Australia’, Oecologia, vol. 125, pp. 531-542.

Weiss, PW 1986, ‘The biology of Australian weeds. 14. Chrysanthemoides monilifera (L.) T. Norl.’ Journal of the Australian Institute of Agricultural Science, vol. 52, pp. 127-134.

Williams, JA and West, CJ 2000,. Environmental weeds in Australia and New Zealand: issues and approaches to management. Austral Ecology vol. 25, pp. 425-444.

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Self assessment Do you know? what is meant by an environmental weed and a serious environmental weed the types of native plant communities that are invaded by weeds the invasion process and the factors that favour invasion the effects of environmental weeds to Australia’s unique flora?