Introduced Plants, Negative Effects of low-growing shrubs to small trees, the species characteristically occupy dry habitats with seasonal water deficits As with all cacti, Opuntia species have a distinctive photosynthetic system (crassulacean acid metabolism or CAM), which enables them to fix carbon at night when evaporative stress is reduced, as well as during the day, if adequate moisture is available A high water-use efficiency, coupled with a large internal water-holding capacity and the ability to restrict water loss by tightly shutting off stomata, enhance drought tolerance In North and South America Opuntia species are usually sparse and restricted regionally, and several are considered threatened in the wild The major invasive weed species come from a range of native regions: Opuntia stricta (common or erect prickly pear) eastern North America, West Indies and adjacent South America; Opuntia aurantiaca (tiger pear, jointed cactus) temperate South America; Opuntia ficus-indica (sweet prickly pear, Indian fig) Central America; and Opuntia vulgaris (drooping prickly pear, Barbary fig) eastern South America Initially Opuntia species were distributed outside the New World as ornamental plants, but a range of uses rapidly emerged, especially as stock fodder, hedges, and as fruits or vegetables A novel use was the production of carminic acid, a commercially important red dye, derived from the crushing of dried cochineal beetles fed a diet of cacti Opuntia can provide an important source of food and income for local communities, and attempted extirpation is not universally accepted Introduced into Australia in the early 19th century, Opuntia stricta had by 1925 infested more than 25 million of eastern Australia and was spreading at the rate of 100 per hour, aided by the dissemination of vegetative and seed material by floods, humans, and feral and wild animals Areas most susceptible were pastoral and arable land cleared amongst Acacia and Casuarina woodland, although the cacti also penetrated the woodland understorey The Australian flora has few native succulents, and none with the environmental tolerance of Opuntia Approximately 10 years after initial establishment at a site, ground cover became dominated by impenetrable thickets of Opuntia reaching densities of 16,000 plants and a biomass of 250,000 kg, per hectare The weed frustrated the farming ambitions of European settlers and caused the ruin of early rural economies Opuntia ficus-indica, a tall shrub, was established in southern Africa at least 250 years ago By the early 20th century, infestations occupied 900,000 ha, mainly in the Eastern Cape region, occupying grassland, succulentkaroo, and the savanna biome (Figure 6) Opuntia aurantiaca, a low-growing species, which may be of hybrid origin, was introduced much later (early 19th century), spreading originally from garden plantings around Cape Town A total infested area of about 400 in the 1890s has grown this century to around 830,000 ha, mainly in eastern parts of South Africa It is replacing important pastoral plants in grasslands and savanna, injuring domestic and feral animals, and degrading natural rangelands A single cladode can produce up to 145 new cladodes over a 200-day growing period, and the potential for vegetative spread is enormous Currently Opuntia aurantiaca is considered to be South Africa’s most expensive weed Opuntia vulgare has also been an important weed in the latter part of the twentieth century, mainly in western coastal areas 351 Figure Opuntia ficus-indica invading rangeland in the Eastern Cape Province of South Africa Photo: John Hoffmann In Australia and South Africa, the control of Opuntia assumed top priority for land management agencies early in the 20th century, when various herbicidal and mechanical methods were attempted Poisoning, using a combination of arsenic pentoxide and sulfuric acid, and mechanical cutting were successfully used to control small infestations of Opuntia in open lands but were very expensive and achieved little at a regional scale However, spectacular success has been achieved using plant-sucking cochineal insects Dactylopius species and the cladode-eating larva of the moth Cactoblastis cactorum Indications of the potential of insects as biocontrol agents first appeared in 1795, when the accidental introduction of Dactylopius ceylonicus resulted in widespread death of Opuntia vulgaris in India Subsequently, deliberate introductions of several Dactylopius species have drastically reduced Opuntia weed infestations in southern Africa, and Cactoblastis has been similarly used to diminish population densities of Opuntia stricta in Australia Overall, biological control agents have lowered population densities of Opuntia by 90%, especially in drier climates Opuntia remains a widespread and invasive weed in these countries, but the economic impact on agricultural land uses has diminished hugely, especially in Australia Fallopia Japonica (Polygonaceae) The human passion for gardens increases biodiversity in urban areas, often creating large source populations, which initiate the effective dispersal of introduced plants into adjoining landscapes One garden ornamental that has become an invasive weed is Fallopia japonica (Asiatic knotweed), a large perennial herb native to the Far East In the 20th century the species has infiltrated much of central and western Europe, North America, and several southern temperate countries, where it excludes native plant species in artificial and highly disturbed habitats, riparian areas, and in open woodlands In the United Kingdom, Fallopia japonica is presently the tallest and most aggressive common herbaceous species Fallopia japonica is a rhizomatous, clump-forming, perennial native to China, Japan, Korea, and Taiwan In these countries it is an early successional species, growing to less