Many weeds have important roles in ecological restoration.
“Invasive species can help ecosystems and people to adapt to global change by maintaining ecosystem processes such as productivity, carbon storage, and nutrient cycling in a context of climate change or land cover transformations.” (1)
We agree with Chandrasena (2) when he suggests that ‘A set of mandatory question in any new weed science grant (or Landcare) application should be:
- “Have you considered the values of the weeds you are targeting for control? Explain”
- “Have you considered the likely environmental impact (benefit or otherwise) of your proposed weed management actions? Explain”
- “What are the risks and benefits of your proposed weed management actions? Explain”
Large Scale – Climate Change and Landscape
Our ‘modern’ activities continue to add greenhouse gases to the atmosphere not just from fossil fuel burning but also from ongoing deforestation and losses of soil organic matter through poor agricultural and weed management practices. The manufacture, transport, and application of herbicides is also a considerable source of greenhouse gases.
There are two main aspects to the role of plants, including weeds, in augmenting or moderating climate change:
- the carbon cycle
- the hydrological cycle
Changes to the Carbon Cycle
- Adding CO2 to the atmosphere
- through broad-scale removal of vegetation (as eg. with forestry, or large scale weed control), industrial agriculture
- when soils are left bare (as following herbicide application) and the soil micro-organisms damaged
- through losses of organic matter by erosion
- Reducing the loss of carbon from the soil
- by maintaining a vegetative cover
- through photosynthesis
- by incorporation of organic matter into soil (carbon sequestration)
Changes to the Hydrological Cycle
- On bare soil, the increase in the uptake of radiative energy increases the emission of long-wave radiation.
- Clearing vegetation increases soil temperature which causes loss of soil moisture to the atmosphere. Because water has a high heat absorption capacity, this increase in atmospheric water is the dominant variable in natural greenhouse gases.
- Landscape drainage decreasing soil moisture which reduces the ability of plants to transpire and thus cool the air.
We have changed the vegetation so much that it can no longer fulfil its original long-term purpose, the regulation of the climate, using evaporation control and ground water systems. An excellent explanation of the role of vegetation is given in Walter Jehne’s presentation to the Australian Climate Summit of 2013 (3). See also an interesting discussion by Ripl et al. (4)
Currently, global agriculture contributes to roughly a third of all global greenhouse gas emissions. Nearly 15% of Australia’s greenhouse gas emissions come from agriculture, and of that amount, 67.4% is methane from ruminant animals such as sheep and cattle – much of Australia’s lands are used for grazing. Alan Broughton, a biological agriculture researcher at Strathfieldsaye Estate in Victoria, points out that it is the feedlots and unmanaged rangelands that create this problem. (5)
In Australia, there is an increasing interest in reversing this trend with an emphasis on soil carbon (6). There is now considerable information on how loss can be mitigated and, indeed, changed management practices could act to reduce climate change. Many farmers in the grazing industry have recognised the importance of sustainable vegetation management and there have been huge successes using cell grazing (Managed Intensive Rotational Grazing) facilitating natural carbon sequestration from the air to the soil (7).
In developing a different view of weeds, and of the role of water in the landscape, in Natural Sequence Farming Peter Andrews has demonstrated improved soil quality, and other environmental outcomes and that local climates can be improved (8,9,10). He says that as regional climate is entirely dependent on vegetation density, local species may not be the best solution for our changed conditions. Courtney White (11) documents stories of restorative and potentially revolutionary agricultural systems throughout the world.
Reforestation of landscapes using native tree and shrub species, termed environmental plantings, has been recognized as a carbon offset methodology which can contribute to biodiversity conservation as well as climate mitigation. However, Evans et al. (12) consider a more cost-effective option would be through assisted natural regeneration in areas of low to intermediate levels of degradation, where regenerative capacity still remains and little intervention would be required to restore native vegetation.
As the climate changes and becomes less suitable for native species we need to consider the role non-native species will play (as well as natives in new areas) in maintaining diversity and ecosystem structure and functions.
In our conservation efforts we tend to focus on larger species – vertebrates and vascular plants but, as Jerry Franklin points out far more important may be ‘that large array of what are sometimes called “lesser” organisms but might better be referred to as “smaller” organisms-such as invertebrates, fungi, and bacteria. It is these physically smaller but overwhelmingly more numerous elements of diversity that carry out critical ecosystem functions, such as decomposition and nitrogen fixation.’ (13)
Maarten Stapper, an Australian agronomist, says that “Current practices continue with the use of harsh chemicals and ignore the delicate balance of humus, microbes, trace minerals and nutrients in the soil. Such management has resulted in marked losses in soil organic carbon (including humus) and greatly reduced diversity and abundance of microbes (algae, bacteria, fungi, nematodes, protozoa) and larger organisms (e.g.mites, ants, beetles, worms) in the soil foodweb. This exposes roots to harsh conditions, greatly diminishing the capacity of the soil to feed plants, as well as making roots more sensitive to saline and acid condition and the whole plant susceptible to pests and diseases, and requiring plants to be spoon-fed with fertilisers and protected by chemicals. Disruption of soil biological and chemical processes usually leads to physical problems, such as reduced infiltration, compaction and erosion. As a result, conventional farming is now searching for answers to increasing soil organic matter and microbial biomass.” (14)
While he is discussing farming, the soil effects are equally applicable to herbicide use and other disturbance in conservation areas. Weeds are Nature’s way of covering and healing soil that has become exposed by fire, flood, landslide, clear-cutting, clean tillage, or other disturbance including herbicide use. Pioneer plants—what we call weeds—are those species that can rapidly cover bare soil and begin performing one or more of the following vital ecological functions for soil recovery:
- Protect the soil from wind and rain erosion
- Replenish organic matter which aids soil microorganisms development
- Absorb, conserve, and recycle soluble nutrients that would otherwise leach away
- Improve soil structure
- Absorb carbon dioxide from the atmosphere (carbon sequestration) and transfer it to the soil
Recognising the importance of soil recovery, Peter Andrews stresses that “there is nothing to be gained from removing ‘weeds’ early, because the greater the biomass to harbour increased nutrients and produce organic carbon, the faster the progression to more desirable species. We need to sponsor and replicate natural processes of plant succession as a function of landscape regeneration.” (15) Altieri and Nicholls (16) also stress that fast-growing, colonising plants are crucial as ‘living mulches’ and cover crops for the conservation of soil, water and organic matter.p
Fox et al. provide a good explanation of the global carbon cycle and the soil properties and processes that control soil carbon accumulation and differences between forest ecosystems and carbon storage (17). It is also the focus of Healthy Soils Australia.
One noticeable aspect of much research into ‘invasive’ weeds is the failure to take into account the nature and frequency of disturbance. It is always assumed that it is the plant that is the problem! There is an elephant in this room.
We have already mentioned the role of weeds in soil rehabilitation. Annuals generally function to improve nutrient status: tap-rooted weeds like dandelions and dock also pull nutrients up from deep in the soil and open up the subsoil to water. Over time, they die off, building humus in the soil and making it hospitable to other plants. Clovers and other leguminous plants add soil nitrogen.
Moist Forests: In Australia, considerable work has been done on forest rehabilitation. When moist forests are cleared for pasture which are then abandoned, weed trees and shrubs are often noticeably abundant. These can be considered as part of the normal play of succession (18,19).
These species may facilitate regeneration in a number of ways:
- By suppression of pasture grasses: Species like Camphor Laurel and Wild Tobacco are better able to germinate under the competitive pressure of pasture grasses and the altered soil microbial conditions than are native species. Particularly with Camphor Laurel, the dense canopy will suppress grasses. Native wattles are the most likely of native species to perform this function.
- By augmenting native seed stock: Many weed trees and shrubs, by providing more food resources, will attract fruit eating birds which drop native seeds below – repeated frugivore visits promotes an accumulation of deposited seeds.
- The Stinking Laurel (Cryptocarya foetida) listed as Vulnerable (C’wealth, NSW, Queensland) is abundant under Brazilian Cherry (Eugenia uniflora) in Crown Land at Brunswick Heads
- Many native species are found below mature trees of Camphor Laurel (Cinnamomum camphora) – since the destruction of much of the subtropical rainforest of northern NSW and southern Queensland, this has been an important food source for rainforest pigeons (Photo)
- In the same area the berries of Lantana (Lantana camara) also attract fruit-eating birds
- By providing architectural complexity: This gives greater provision of shelter, food diversity, nesting opportunities not just for fruit-eating birds but for a range of other species as well.
- By acting as protective shade trees for the recovery of native species. Improving micro-climatic and soil conditions provides a more favourable environment recruitment and survival of native rainforest species of later successional stages. Pioneering nurse species include lantana, willow, gorse, blackberry, broom.
- By providing protection from drying sun/winds: Where there are no protective trees or shrubs, we have also found that native species which germinate protected by smaller weed species are more likely to survive long dry periods.
Manning et al (20) found scattered trees are keystone structures, providing important ecological functions and that their contribution to ecosystem functioning is disproportionately large given the small area occupied and low biomass of any given tree, and the low density of scattered trees collectively. They point out that “Because scattered trees fulfil unique functional roles in a wide range of scattered tree ecosystems, their loss may result in undesirable ecological regime shifts”.
In India, researchers Murali and Siddapa Setty (21) found that species richness and abundance in plots infested with Lantana was high. They argue that Lantana simply occupies the moist habitats that most other species also prefer. Similarly in a study in Zambia, Lantana did not change tree/shrub diversity (22). This has also been found in Australia on chemical-free sites (pers com. D.Drinkwater, Huonbrook).
Streams, Wetlands, Water in the Landscape:
Haikai Tane, Director Watershed Systems in New Zealand maintains that weeds (as well as native vegetation) are critical in the process of aquifer recharge and discharge (23). His method is very different to the standard approach of control of streams and flooding with extensive land drainage. At times, in changed environmental circumstances, weeds are likely to be better adapted than natives and may pave the way for native plant recovery. He cites the work of Peter Andrews who has demonstrated the benefits of non-native plants in retaining water in the landscape. Peter has done considerable research into the role of Willows to control stream erosion (24). Tao Orion, reviewing the practice of ‘invasion ecology’ (25), also outlines the ecological problems of demonising and removing non-native species. In one example, she follows the ‘invasion’ of the Salt Cedar along the Colorado River, and clarifies that it is the salinization from upstream dams and over-allocation of water which makes the river banks now unsuitable for native species. Her gentle and holistic approach is a model for the new paradigms we need to develop.
Some land uses mean that non-native vegetation will be critical in initial rehabilitation. Dryland salinity affects every Australian state and, while it is usually thought of in terms of loss of agricultural productivity, in fact, it is a disaster for remnant vegetation and stream sides. CSIRO’S Dr Tom Hatton, discussing the West Australian wheat-belt, says:
“Eighty per cent of the remnant native vegetation on farms and fifty per cent on public lands is at risk. The South West of WA is one of the great biodiversity centres on the planet, it is particularly well endowed with plants and animals. Many of those species are restricted naturally to places in the landscape which we will lose to salt. Most of the river beds and banks are degraded, and over half our usable river water is already saline, brackish or marginal.” (26) Acid mine drainage and other mining pollutants in altering both the groundwater and surface flow, also affect the ability of native plants to participate in ecosystem recovery.
Other species may also be extremely important on stream sides for bankside erosion protection.
Many aquatic weeds provide significant natural ecosystem services, particularly water purification, absorbing excess nutrients from polluted waterways thus improving aquatic habitat in wetlands and streams. For example, Water Hyacinth is both an indicator of nutrient pollution and a remediator. It is well established in many countries for wastewater treatment. Similarly, native wetland species may be pollution indicators and have also been used to treat pollution yet may be regarded as weeds in many parts of Australia; e.g. Typha spp (many parts of Australia, 27), and Phragmites (West Australia, 28). It is thus important when determining rehabilitation strategies to assess whether these species are ‘invading’ into increasingly nutrient dense communities.
“For all other animals, except humans, weeds are undoubtedly a great resource. Most animals cannot be choosy, and they are generally adept at exploiting any resource available for food and shelter. Nearly all insects, fish, birds and foraging herbivorous animals use colonising plants as resources. Birds, bees, ants and other insects derive sugary food from the flowers and fruits of species, such as Lantana (Lantana camara L.), considered an obnoxious pest (Gosper and Vivian-Smith 2006). Similarly, bumblebees, the great pollinator of field crops, rely heavily on weeds for sugary nectar. Macro invertebrates and small fish, living in our streams, thrive on food in the root zones of large macrophytes, such as Typha angustifolia L. and Phragmites australis (common reed), which are also often regarded as problematic aquatic weeds.” (1)
Tim Low says that all over Australia native animals are learning to live with weeds and that a great many rare and threatened species have also become weed dependent for food, shelter, nesting.(29) This points to the importance of developing ecologically holistic rehabilitation strategies. Below are a few examples of adaptation. I am sure that you, the reader, will know of many more.
Bacteria: Non-native plants perform some surprising rehabilitation functions. Tao Orion outlines the role that Salt Cedar plays in increasing precipitation in a desert environment. While transpiring, plants also release cloud-seeding bacteria. It seems that Salt Cedar harbours more bacteria, both in number and diversity than its non-salt accumulating native neighbours. (30)
- Bees: Stingless and Blue-banded Bees forage nectar and pollen from introduced species. Dead canes in clumps of Lantana or Blackberry are favoured nesting sites of Reed Bees (Exoneura spp). Dollin et al. provide the process by which the nests can be identified and removed if the Lantana must be destroyed. (31)
- Freshwater invertebrates and ants: Douglas et al. (32) investigated the effects of exotic grass invasion in Northern Australia and found that although there was a clear change in the composition of wetland and terrestrial flora (diversity and biomass) following weed invasion, freshwater macroinvertebrate communities and terrestrial ant communities showed little or no response. They posit that “the response of some faunal groups will be determined largely by the degree of structural change”.
- Litter invertebrates: clearly the removal of vegetation providing leaf litter and woody debris will affect litter invertebrates.
- Butterflies and moths: Some 55 introduced species have been identified as butterfly host plants in S.E.Queensland and Northern N.S.W. (i.e. the larvae can develop from eggs to emerge as healthy adults) and they are used by some 61 species of butterfly. (33) In addition, many, like Lantana, are excellent nectar resources – Swallowtail and Birdwing Butterflies are notably attracted to Lantana flowers. The Monarch Butterfly which is endangered in its home country (USA) has established healthy populations here on the weedy milkweeds.
- Small birds: “Bad weeds do make good habitat! In fact, in many urban areas, they are the only habitat small birds have left. When a habitat pocket, that is, an area of vegetation which is being used by small birds, is discovered, it should be protected – even if it is 100% weeds. It needs to be protected until alternative native plant habitat has been created and has been seen to be in use by the small birds for at least an entire year, including a breeding season. To keep safe, small birds generally live in and take refuge in dense, multilayered vegetation.” (34) This is echoed by Holly Parsons, Project Officer, ‘Birds in Backyards Program’ for Birds Australia. (35)
Fruit-eating birds: Exotic plants like Camphor Laurel, Tobacco Bush, and Lantana often form the first woody vegetation that grows on abandoned farmland. These plants, as well as providing safe living space, attract vertebrate frugivores which also disperse the seeds of native plants. Neilan et al. believe that “Careful management of regrowth dominated by fleshy-fruited exotic invasive trees can provide an opportunity for broadscale reforestation in extensively-cleared landscapes.” (36)
Despite public pressure, the Australian Government concluded that Camphor Laurel was not a Threatening Process. Amongst their points was: “A number of fruit-eating pigeon species occur in the rainforests of northern New South Wales. It has been recognised that these species declined in abundance after forest clearing causing the loss of seasonally important food sources and that their future survival in New South Wales may depend, at least in the short term, on the retention of rainforest remnants and exotic fruiting plants. At lower elevations, where the abundance of native fruits has been reduced by clearing of rainforest, the Camphor Laurel may in fact be the major winter food for Topknot and White-headed Pigeons.”(10) Tobacco Bush is an important food source for Brown Pigeons.
- Manatees: Water Hyacinth is being used in Kings Bay, Florida, to maintain manatee populations. The Kings Bay Adaptive Management Phytoremediation Demonstration Project organized by the Howard T. Odum Florida Springs Institute is being led by Bob Knight. Garry Hamilton documents the complexity of interactions. Toxic algae which has flourished in Kings Bay have caused manatee death. Water Hyacinth provides habitat for a rich diversity of algal grazers, reduces nutrients and provides shade which also reduces algal growth. (38)
- Koalas: Under national environmental law Koalas are listed as a ‘Vulnerable’ Threatened Species in Queensland, New South Wales and Australian Capital Territory. A study of coastal koala habitat in Byron Shire showed that Camphor Laurel was the fourth most heavily used feeding tree of the common trees. This would indicate its importance in a heavily degraded environment, particularly in providing connectivity between remnants. (39)
- Macropods: “Lantana (Lantana camara) one of our most bountiful weeds, has become a valued resource for wildlife. Wallabies and bandicoots shelter among its thickets.” “The marram grasslands behind beaches furnish feed for wombats, shelter for small marsupials.” “As a protection against foxes, rare southern brown bandicoots use blackberry brambles and southern barred bandicoots hide in gorse.” Tim Low gives many examples of the importance of weeds as animal food and shelter. (29)
- Tassin Jacques & Christian A. Kull (2014). Facing the broader dimensions of biological invasions. Land Use Policy, 42, 165–169]. (Abstract)
- Chandrasena, N, 2014. Living with weeds – a new paradigm. Indian Journal of Weed Science 46(1): 96–110, 2014 (PDF)
- Walter Jehne (Healthy Soils Australia) at the CCN Australia’s 2013 Climate Summit (Youtube)
- Ripl, W., Pokorny, J. and Scheer, H., 2006. Memorandum on Climate Change: The necessary reforms of society to stabilize the climate and solve the energy Issues. Natural Sequence Farming workshop (PDF)
- Broughton, A. 2016. Ruminants and methane: Not the fault of the animals. Green Left Weekly.
- Interest in farming for soil carbon in Australia.Healthy soils; Soils for Life; Carbon Farmers of Australia; Future Farmers.
- Savory Institute: Holistic Management.
- Andrews, P., 2006. Back from the Brink: How Australia’s Landscapes Can Be Saved, ABC Books, Sydney.
- Andrews, P., website.
- White, Courtney 2014. Grass, Soil, Hope: A Journey through Carbon Country. Chelsea Green Publishing, Vermont.
- Evans, M.C., Carwardine, J., Fensham, R.J., Butler, D.W., Wilson, K.A., Possingham, H.P., Martin, T. 2015. Carbon farming via assisted natural regeneration as a cost-effective mechanism for restoring biodiversity in agricultural landscapes. Environmental Science & Policy,Volume 50, Pages 114–129.
- Franklin, Jerry F. 1993. Preserving Biodiversity: Species, Ecosystems, or Landscapes? In Ecological Applications, Vol. 3, No. 2. (May, 1993), pp. 202-205. (PDF)
- Stapper, Maarten, Soil Fertility Management –Towards Sustainable Farming Systems and Landscapes. ABC Australia Story.
- Andrews, Peter. Management of Soil Hydrology. Soils for Life blog post . See also Rattan Lal, Professor of Soil Science at Ohio State University (Youtube)
- Alterieri, M.A., & C.Nicholls, 2005. Agroecology and the Search for a Truly Sustainable Agriculture. University of California, Berkley. (PDF).
- Fox, T., Markewitx, D., Wynne, R., 2012. Soil properties and processes that control soil carbon accumulation; forest and carbon storage; managing forests in the face of an uncertain climate. PINEMAP Distance Graduate Course Webinar, January 24.
- Elgar, A.T., Kylie Freebody, K., Pohlman, C.L., Shoo, L.P., and C. P. Catterall 2014. Overcoming barriers to seedling regeneration during forest restoration on tropical pasture land and the potential value of woody weeds. Front Plant Sci. 2014; 5: 200.
- Goosem, S., & N.Tucker, 2013. Repairing the Rainforest. 2nd edition. Wet Tropics Managament Authority and Biotropica Aust. P/L, Cairns.
- Manning, A., Fischer, J., Lindenmayer, D.B., 2006. Scattered trees are keystone structures – Implications for conservation. Biological Conservation, Volume 132, Issue 3, pp 311–321
- Murali, K.S., and R. Siddapa Setty, 2001. Effect of weeds Lantana camara and Chromelina odorata growth on the species diversity, regeneration and stem density of tree and shrub layer in BRT sanctuary. Current Science, Vol. 80, No. 5, pp 675-678.(PDF)
- Lwando, Catherine, 2005. Effect of Lantana camara on Plant diversity in Zambia. WWF: Russell E. Train Education for Nature Program.(PDF)
- Tane, Hakai, 2006. Restoring Watershed Systems by Converting to Natural Sequence Farming. Workshop.(PDF)
- Andrews, Peter, 2014. Peter Andrews management of vegetation and soil hydrology. Soils for Life (PDF)
- Orion, Tao 2015. Beyond the War on Invasive Species; a Permaculture Approach to Ecosystem Restoration. Chelsea Green Publishing, White River Junction, VT.
- Murphy, Justin, 1999. Salinity our silent disaster. ABC 7.30 Report; Australian Bureau of Statistics. Measures of Australia’s Progress 2010: Salinity; Australian Government> Fact Sheet: Salinity and Water Quality. (PDF)
- Department of Agriculture and Fisheries. Cumbungi: Bullrush or Cat’s Tails Factsheet.(PDF)
- Biosecurity Queensland. Phragmites australis. Weeds of Australia, Queensland Govt.
- Low, T., 1999. Feral Future. Penguin Books Australia Ltd., Ringwood, Victoria.
- Orion, Tao 2015. Beyond the War on Invasive Species; a Permaculture Approach to Ecosystem Restoration. Chelsea Green Publishing, White River Junction, VT.
- Dollin, A., Batley, M., Robinson, M., Faulkner, B., 2000. Native Bees of the Sydney Region. Australian Native Bee Research Centre, N. Richmond, NSW.
- Douglas, MM, Setterfield, SA, O’Connor, RA, Ferdinands, K, Rossiter, NA, Brooks, KJ, Ryan, BJ, Parr, CL. 2006. Different weeds, different habitats, same effects: exotic grass invasion in tropical woodlands and wetlands. Eds. Preston, C, Watts, JH, Crossman, ND, 15th Australian Weeds Conference: Managing Weeds in a Changing Climate.
- Moss, J.T., 2002. Butterfly Host Plants: of South-east Queensland and Northern New South Wales. Butterfly and Other Invertebrates Club, Runcorn, Qld.
- Stevens, S., Debrincat, B., and Brodie, L., n.d. An introduction to creating small bird habitat. (PDF)
- Parsons, Holly, n.d. Best Practice Guidelines for Enhancing Urban Bird Habitat:Scientific Report. Birds in Backyards Program, Birds Australia.(PDF)
- Neilan, W., Catterall, C.P., Kanowski, J. and McKenna, S. (2006) Do frugivorous birds assist rainforest succession in weed dominated oldfield regrowth of subtropical Australia? Biological Conservation 129, 393-407.
- Threatened Species Scientific Committee, 2003. Camphor Laurel listing. Advice to the Minister for the Environment and Heritage from the Threatened Species Scientific Committee(TSSC) on Amendments to the List of Key Threatening Processes under the Environment Protection and Biodiversity Conservation Act 1999 (EPBC Act). Australian Government, Department of Environment. (PDF)
- Hamilton, G., 2014. Striking a Deal with the Weed from Hell. Conservation. University of Washington.
- Phillips, S., Hopkins, M., 2012. Byron Coastal Habitat Study. Report to Byron Shire Council. Biolink Ecological Consultants, Uki, NSW.