350 Introduced Plants, Negative Effects of Spartina species are perennial, deep-rooted (30 cm), rhizomatous, sward-forming erect grasses that mostly occupy low- to midtidal mudflats They are well adapted physiologically to tolerate saline conditions with special salt excretion, dilution, and restriction mechanisms A high wateruse efficiency, due to their C4 photosynthetic system, and the unusual ability to maintain higher rates of photosynthesis than other C4 and many C3 species under cool temperate conditions (5–10 1C), increases their environmental range The native geographic distribution of the genus centers on the east coast of North and South America, with fewer species on the west coast of North America, Europe, and north Africa Spartina alterniflora, native to the Atlantic coast of North America, was accidentally introduced to the United Kingdom early in the 19th century, and produced rare hybrids with the local congener Spartina maritima, which is restricted to western Europe and Africa The male sterile F1 hybrid Spartina x townsendii subsequently produced, via doubling of chromosomes, a new fertile species Spartina anglica It quickly became apparent that Spartina townsendii, and particularly Spartina anglica, were markedly more invasive than either of the parent species Estuarine habitats in southern England were transformed from exposed mudflats into tall-grass meadows In less than a century Spartina anglica, via seed dispersal, clonal spread, and human plantings, covered approximately 10,000 of intertidal salt marsh along the coast of Britain Early this century, estuarine stabilization to maintain commercial waterways and reclamation for industrial and agricultural development were seen as important goals of coastal management Experience in the United Kingdom resulted in Spartina alterniflora and its derivatives (Spartina x townsendii, Spartina anglica) being widely planted in parts of North America, in regions occupied by native Spartina species, and in areas well outside its native range (e.g., Australia, and New Zealand) where they were similarly successful, especially Spartina anglica The heightened competitive vigor and environmental range of Spartina anglica appears to derive from a greater genetic heterozygosity, due to hybridization and polyploidy, and expanded phenotypic plasticity Spreading Spartina species appear to have fewer insect and avian herbivores in their new ranges Invasive Spartina species induce major geomorpholgical changes in tidal habitats by enhancing rates of sediment accumulation Sediment accretion rates of up to 17 cm yrÀ1 (average 5–10 cm) have been recorded within the tall grass sward, and in southern England Spartina caused elevation of mudflat surfaces by 1.8 m in 37 years Declining tidal influence, and the accumulation of biomass (dry weight up to 7500 kg haÀ1), transforms salt marsh areas into nonestuarine ecosystems Impacts on biodiversity are no less dramatic At the pioneer phase, establishment via seed or planting produces circular clumps that expand at rates of between and m yrÀ1, eventually coalescing to form extensive meadows on sand and mud substrates in the low- to midtidal range (Figure 5) Spartina anglica physically displaces many native indigenous low-growing halophytes (e.g., Selliera, Salicornia, Schoenus, Puccinellia) and modifies habitats formerly occupied by a range of bird, fish, and invertebrate species In southern New Zealand, the loss of fish and other food sources in estuaries colonized by Spartina anglica threatens the persistence of 34 species of foraging shorebirds in these habitats Figure Colonizing patches of Spartina alterniflora on mudflats at Wilapa Bay, Washington, USA Photo: Glen Miller The hybridization of native and introduced Spartina species during the 19th century in the United Kingdom is presently occurring in western North America, where the recently established Spartina alterniflora (introduced) is genetically assimilating the common Spartina foliosa (native) through introgressive hybridization Abundant Spartina alterniflora pollen improves seed set in Spartina foliosa, producing fertile hybrids that also cross with the native species Locally, the introduced Spartina species and progeny could readily displace the native species and colonize lower reaches of the mudflats, currently beyond the environmental tolerance of the Spartina foliosa Because of growing appreciation of the biological values of estuaries, new plantings of Spartina alterniflora have largely ceased, and the species is now considered a serious weed Extirpation of existing Spartina meadows has, however, been difficult, but containment and the removal of outliers has been achieved using systemic herbicides Opuntia (Cactaceae) Cacti are prized as ornamentals for their novel form, spectacular flowers, edible parts, and medicinal uses Opuntia (prickly pear) a genus of about 90 species native to North and South America, has been distributed globally over the past two centuries Sixty species have become naturalized, and at least 15 are considered major weeds Opuntia species provide some of the earliest and most spectacular examples of invasive plants in dry environments and the effectiveness I of insects as biological control agents In the major centers of Opuntia invasion (Australia, South Africa, and India), several species have displaced horticultural crops, reduced the grazing potential of rangelands, and threatened the conservation values of native grasslands, scrublands, and woodlands Eradication may be unattainable, but some invasive Opuntia species are now at low population densities, while others are continuing to spread Opuntia species are fast-growing perennial succulents with thickened, often flattened, segmented cladodes, usually supporting spines Most Opuntia will regenerate from seed, cladode fragments, and underground tubers Ranging in size from