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Weeds, including escaped garden plants, pose serious threats to native flora and fauna. Control is costly and preventing new introductions is critical.
A weed is a plant growing where it is not wanted. Weeds can be an annoyance or have more serious consequences for gardens and agriculture, for the health of humans or stock, or for the natural environment. Environmental weeds are a group of invasive plants that can affect native plants and animals adversely, pushing some species towards extinction. After land-clearing, invasive plant and animal species are the greatest threats to biodiversity. Weeds and IslandsWeeds are problematic in most parts of the world. Possibly because islands have relatively simple floras, including much early successional vegetation (with naturally weed-like characteristics), islands seem to be particularly prone to weed problems. The same islands may have high biodiversity values, and islands where weeds cause major impacts include Hawaii, New Zealand, the Galapagos and Madagascar. The principle extends to the island continent of Australia. The Australian Weed Cooperative Research Centre has recognized 28,000 introduced plant species, of which more than 2,739 species have naturalized. How Weeds Affect BiodiversityWeeds affect other plants very directly by competing for light, water and nutrients. They may compete for pollinators and seed dispersers, alter nutrient cycles, chemically inhibit establishment and growth of other plants, affect fauna species and change fire regimes. Where Do Weeds Come From?The globalised world provides many opportunities for the accidental and deliberate movement of plants, seeds and other plant parts. Historically, plants have been transported wherever humans have travelled and traded. Once in a favourable new environment, some plant species grow and reproduce rapidly. Seed predators, herbivores and diseases that would keep them in check in their original home environment may not be present. For instance, the Galapagos Islands are home to thickets of Rose Apple Syzygium jambos, native to South-east Asia. In Western Australia, Bridal Creeper Asparagus asparagoides from South Africa smothers native vegetation and the neotropical Lantana Lantana camara is established in many parts of Australia and the world. Accidental introductions occur when food, ornamental or agricultural seeds are contaminated by weed seeds. Soil and plant material can be moved by machinery. Most introductions are deliberately or unthinkingly human-mediated, however. Plants brought in for agriculture or ornamental plantings can be spread by wind, water, animals or vegetative growth to neighbouring vegetation, be dumped with garden rubbish on roadsides or wasteland or deliberately planted or sown. Managing WeedsOnce established, it can be very difficult and expensive to eradicate a weed species. Often, the presence of the weed must be accepted and its impacts contained to the best extent possible. Mechanical or chemical methods of control are most common, but biological control methods have been developed for a small subset of weed species. Strategic approaches that concentrate on removing seed sources and working back from expanding edges of new infestations can be developed on a case by case basis. Preventing Weed ProblemsAs management of weeds is expensive and problematic, avoiding new introductions and containing spread of existing infestations is an obvious way to leave scarce resources available for more productive purposes. Lists of potential weed species are available from many local authorities, who recommend that they not be planted or sold (it may be illegal to do so). Plant a local native or a non-invasive exotic species instead. Garden rubbish should always be disposed of carefully, as seeds, stems and roots segments capable of shooting and other weed propagules may be present.
The copyright of the article Environmental Weeds in Plant Ecology is owned by Barbara Stewart. Permission to republish Environmental Weeds in print or online must be granted by the author in writing.
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