Fact file Q
Working for water ▲
What’s wrong with alien invaders?
A few ‘007’ control agents
They may
■
■
use too much water and block out light for other plants
■
pollute drinking water (e.g. water hyacinths)
■
reduce streamflow and yields from dams and the ability of catchments to store water, and they clog irrigation canals and pumps
■
reduce grazing lands and land productivity
■
increase the use of potentially harmful chemical herbicides and pesticides
■
accumulate or redistribute salt in soil
■
change living space or habitat, sediment, light, and water, reducing abundance and diversity of native plants and birds
■
prevent replenishment of sand on beaches, leading to beach erosion
■
accumulate litter and increase biomass (woody material and foliage), creating fire hazards, soil erosion, and flooding from rapid runoff after rain
■
shade river banks, strangle forest/woodland canopy (e.g. cat’s claw creeper), and act as ‘ladder fuels’ (e.g. triffid weed) carrying fire to tree canopies
■
reduce capacity for boating, canoeing, waterskiing, and fly fishing
■
reduce freshwater input to estuaries and change the frequency of opening and closing of the mouth, making estuaries unsuitable for nursery sites for important marine fish species
■
disturb indigenous insect populations, for example, dragonflies, of which many species are endangered (especially by black and long-leaved wattle alongside rivers and lakes: shade trees prevent growth of grasses and bushes where dragonflies lay eggs, and their habitat is disturbed by cattle seeking the aliens’ shade).
■
■
Plant-eating cochineal insects (used for making food colouring) were introduced in 1913 and since then have provided sustained, day-by-day, cost-free control of drooping prickly pear (Opuntia monacantha). The cactus moth, Cactoblastis cactorum, was mass-reared near Uitenhage and released against sweet prickly pear (Opuntia ficus-indica) in 1933. A cochineal insect (Dactylopius austrinus) was released in 1935 to control jointed cactus (Opuntia aurantiaca). Mass-reared near Uitenhage, these insects are provided free of charge for manual re-distribution as needed.
■
Two species of gall-forming pteromalid wasps have been used successfully against long-leaved wattle (Acacia longifolia) and golden wattle (A. pyncantha).
■
A fungus has been deployed against the Port Jackson willow.
■
A cone-feeding weevil species with potential to reduce seeding in cluster pine (Pinus pinaster) is being tested in quarantine.
■
The leaf-feeding beetle (Chrysolina quadrigemina) and bud-galling midge (Zeuxidiplosis giardi) have brought St. John’s wort (Hypericum perforatum) under control in the Western Cape.
International trade and travel creates pathways for invaders (e.g. marine organisms in ship ballast water, contaminated soil and seed lots).
■
Growing demand for forestry products increases plantation areas of invasive tree species (e.g. pines, eucalyptus).
■
Internet and global databases spread information about useful plants, prompting horticulturists and others to import species without considering potential invasiveness.
■
Road and rail networks transport seeds, which can also be carried by earth-moving and construction equipment, and in construction materials.
■
Horticultural fashions drive demand for new species of flowers, bushes, shrubs, trees, and ground covers, many of which are weeds.
■
Laws, policies, and regulations to control, import, plant, and manage known and potential invader species are either not in place or are not implemented or upheld.
‘Conflict-of-interest’ alien species Not everyone wants to get rid of: the jacarandas that purple our cities and gardens; pine trees, which underpin large commercial activities, such as plantation forestry; eucalyptus (gum) trees, which provide timber, poles, firewood, nectar and pollen for honey production, and pollinators for the deciduous fruit industry.
Benefits for people Working for Water has: created jobs; funded poverty relief; encouraged small enterprises of contractors to help to clear land, sell firewood, and make products from wood and bark; spread HIV/AIDS education; forged partnerships with organizations able to employ, train, and manage workers in alien plant-clearing; given training; emphasized gender redress; re-integrated former prison inmates; and raised public awareness of destructive aliens.
Biological control How biological control agents attack invaders: depending on the type, they can – chew and devour plant tissue, or mine or bore into the plant; suck the sap; induce plant-galling (growths); produce necroses (dead tissue) and rust spots; eat the seeds; and stunt the growth of young plants, which then take longer to mature and produce fruit.
How humans help the aliens
■
Modern industries can increase atmospheric CO2, encouraging alien grasses that photosynthesise more efficiently.
Some biological control agents: weevils and their close relatives; families of beetles; sucking bugs; butterflies and moths; flies; pathogenic fungi; plant-feeding wasps.
■
Fertilizers and atmospheric pollution deposit more nutrients in natural ecosystems, benefiting alien invaders more than native species.
They act by damaging vegetative tissues (leaves, petioles, stalks, stems); reproductive tissues (buds, flowers, seeds, pods); sap drained from the phloem vessels.
■
Growing human populations demand more food (encouraging more use of fertilizers, imported species, and increased areas under production).
10 Quest 1(1) 2 0 0 4
■
People change environments so that native species decline or become extinct, making room for alien invaders to move in.
Do’s and Don’ts ■
Do plant indigenous plants and buy from certified indigenous plant nurseries.
■
Don’t import potentially invasive plants and don’t plant evergreen trees, or ‘thirsty’ trees and reeds near streams, or grasses such as pampas grass, fountain grass, Spanish reed, and feather-top (all these are prohibited Category 1 weeds in South Africa).
■
Do remove these weeds from your garden: Cape gooseberry, Chinese elm, common privet, cosmos, elephant’s ear, eucalyptus, evening primrose, forget-me-not, Formosa lily, giant reed, guava, lantana, moon cactus, moonflower, morning glories, New Zealand bottlebrush, oleander, pampas grass, potato creeper, purple pom-pom weed, queen of the night, redstar zinnia, Spanish broom, sword fern, syringa, thorn apples, tree daisy, weeping willow, white or grey poplars, white or common mulberry, white ginger lily, wild tobacco, yellowflowered Mexican poppy. ■
For more see South African Journal of Science, vol. 100, no. 1–2 (2004) and consult R.P. Randall’s A Global Compendium of Weeds (Melbourne: R.G. and F.J. Richardson, 2002).