Invasive Species

CHAPTER 7 Invasive species

Daniel Simberloff

Aninvasivespeciesisonethatarrives(oftenwith world—especially islands (see Box 7.2)—intro- human assistance) in a habitat it had not previ- duced species comprise a large proportion of all ously occupied, then establishes a population species. For instance, for the Hawaiian islands, and spreads autonomously. Species invasions almost half the plant species, 25% of insects, 40% are one of the main conservation threats today of birds, and most freshwater fishes are intro- and have caused many species extinctions. The duced, while the analogous figures for Florida greatmajorityofsuchinvasionsarebyspecies are 27% of plant species, 8% of insects, 5% of introduced from elsewhere, although some na- birds, and 24% of freshwater fishes. Not all intro- tive species have become invasive in newly occu- duced species become invasive, however. Many pied habitats (see Box 7.1). In some areas of the plant species imported as ornamentals persist in

Box 7.1 Native invasives Daniel Simberloff

Although the great majority of invasive species with the introduction of European genotypes are introduced, occasionally native plant as a forage crop in the 19th century (Lavergne species have become invasive, spreading and Molofsky 2007). rapidly into previously unoccupied habitats. The second category of native invasives arises These invasions fall into two categories, both from human modification of the environment. involving human activities. In the first, a native For instance, in western Europe, the grass species that is rather restricted in range and Elymus athericus, previously a minor habitat is supplemented with introductions component of high intertidal vegetation, from afar that have new genotypes, and the began spreading seaward because of increased new genotypes, or recombinants involving the nitrogen in both aerial deposition and runoff, new genotypes, become invasive. An example and it now occupies most of the intertidal in in North America is common reed (Phragmites many areas (Valéry et al. 2004). The plant australis), which was present for at least apparently uses the nitrogen to increase its thousands of years and is probably native, but tolerance or regulation of salt. In various which spread widely, became much more regions of the western United States, Douglas common, and began occupying more habitats fir(Pseudotsuga menziesii) and several other beginning in the mid‐ nineteenth century. This tree species have invaded grasslands and invasion is wholly due to the introduction of shrublands as a result of fire suppression, Old World genotypes at that time, probably in increased grazing by livestock, or both. Natural soil ballast (Saltonstall 2002). Similarly, reed fire had precluded them, and when fire was canarygrass (Phalaris arundinacea), native to suppressed, livestock served the same role North America but previously uncommon, (Simberloff 2008). By contrast, Virginia pine became highly invasive in wetland habitats (Pinus virginiana) in the eastern United States continues

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132 CONSERVATION BIOLOGY FOR ALL

Box 7.1 (Continued)

invaded serpentine grasslands when ﬁres were emy of Sciences of the United States of America, 99, suppressed and long‐time grazing practices 2445–2449. were restricted (Thiet and Boerner 2007). Simberloff, D. (2008). Invasion biologists and the biofuels boom: Cassandras or colleagues? Weed Science, 56, REFERENCES 867–872. Thiet, R. K and Boerner, R. E. J. (2007). Spatial patterns Lavergne, S. and Molofsky, J. (2007). Increased genetic of ectomycorrhizal fungal inoculum in arbuscular my- variation and evolutionary potential drive the success of corrhizal barrens communities: implications for an invasive grass. Proceedings of the National Academy controlling invasion by Pinus virginiana. Mycorrhiza, 17, of Sciences of the United States of America, 104, 507–517. 3883–3888. Valéry, L., Bouchard, V., and Lefeuvre, J.‐C. (2004). Saltonstall, K. (2002). Cryptic invasion by a non‐native Impact of the invasive native species Elymus athericus genotype of the common reed, Phragmites australis, on carbon pools in a salt marsh. Wetlands, into North America. Proceedings of the National Acad- 24, 268–276.

Box 7.2 Invasive species in New Zealand Daniel Simberloff

Many islands have been particularly afflicted by changes nutrient cycling by mixing organic and introduced species, even large islands such as mineral layers of the soil. Of 120 introduced those comprising New Zealand (Allen and Lee bird species, 34 are established. To some extent 2006). New Zealand had no native mammals, they probably compete with native birds and except for three bat species but now has 30 prey on native invertebrates, but their impact is introduced mammals. Among these, several poorly studied and certainly not nearly as are highly detrimental to local fauna and/or severe as that of introduced mammals. flora. The Australian brushtail possum European brown trout (Salmo trutta) are (Trichosurus vulpecula; Box 7.2 Figure) now widely established and have caused the local numbers in the millions and destroys extirpation of a number of fish species. broadleaved native trees, eating bird eggs and Among the estimated 2200 established chicks as well. Pacific and Norway rats are also introduced invertebrate species in devastating omnivores that particularly plague native birds. Introduced carnivores—the stoat (Mustela erminea), weasel (M. nivalis), ferret (M. furo), and hedgehog (Erinaceus europaeus) —are all widespread and prey on various combinations of native birds, insects, skinks, geckos, and an endemic reptile (Sphenodon punctatus). Many ungulates have been introduced, of which European red deer (Cervus elaphus) is most numerous. Trampling and grazing by ungulates has greatly damaged native vegetation in some areas. Feral pigs (Sus scrofa) are now widespread in forest and scrub habitats, and their rooting causes erosion, Box 7.2 Figure Brushtail possum. Photograph by Rod Morris. reduces populations of some plant species, and continues

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Box 7.2 (Continued)

New Zealand, German wasps (Vespula plant species, and even these have been germanica) and common wasps (V. vulgaris) outcompeted by introduced nitrogen‐fixers have probably had the most impact, especially such as gorse (Ulex europaeus), Scotch broom by monopolizing the honeydew produced by (Cytisus scoparius), and tree lupine (Lupinus native scale insects that had supported several arboreus). As in other areas (see above), in native bird species, including the kaka (Nestor parts of New Zealand these nitrogen‐fixers meridionalis), the tui (Prosthemadera have, by fertilizing the soil, favored certain novaeseelandiae), and the bellbird (Anthornis native species over others and have induced an melanura). invasional meltdown by allowing other About 2100 species of introduced plants are introduced plant species to establish. now established in New Zealand, Given the enormous number of introduced outnumbering native species. Several tree species invading New Zealand and the many species introduced about a century ago are sorts of impacts these have generated, it is not now beginning to spread widely, the lag surprising that New Zealand enacted the first caused by the fact that trees have long life comprehensive national strategy to address the cycles. Most of the introduced plants in New entire issue of biological invasions, the Zealand, including trees, invade largely or Biosecurity Act of 1993. wholly when there is some sort of disturbance, such as land‐clearing or forestry. However, once established, introduced plants have in REFERENCE some instances prevented a return to the original state after disturbance stopped. New Allen, R. B. and Lee, W. G., eds (2006). Biological invasions Zealand also has relatively few nitrogen‐fixing in New Zealand. Springer, Berlin, Germany.

gardens with human assistance but cannot estab- 7.1 Invasive species impacts fi lishinlessmodi ed habitats. The fraction of in- 7.1.1 Ecosystem modification troduced species that establish and spread is a matter under active research, but for some organ- The greatest impacts of invasive species entail isms it can be high. For example, half of the modifying entire ecosystems, because such mod- freshwater fish,mammal,andbirdspeciesintro- ifications are likely to affect most of the originally duced from Europe to North America or vice- resident species. Most obviously, the physical versa have established populations, and of structure of the habitat can be changed. For in- these, more than half became invasive (Jeschke stance, in Tierra del Fuego, introduction of a few and Strayer 2005). North American beavers (Castor canadensis)in Invasive species can produce a bewildering 1946 has led to a population now over 50 000, array of impacts, and impacts often depend on and in many areas they have converted forests of context; the same introduced species can have southern beech (Nothofagus spp.) to grass- and minimal effects on native species and ecosys- sedge-dominated meadows (Lizarralde et al. tems in one region but can be devastating 2004). In the Florida Everglades, introduced Aus- somewhere else. Further, the same species can tralian paperbark (Melaleuca quinquenervia) trees affect natives in several different ways simul- have effected the opposite change, from grass- taneously. However, a good way to begin to and sedge-dominated prairies to nearly mono- understand the scope of the threat posed by specific paperbark forests (Schmitz et al. 1997). biological invasions is to classify the main In parts of Hawaii, Asian and American man- types of impacts. grove species have replaced beach communities

134 CONSERVATION BIOLOGY FOR ALL of herbs and small shrubs with tall mangrove Introduced species can change entire forests (Allen 1998). ecosystems by changing the fire regime (see Introduced plant species can modify an entire Chapter 9). The invasion of the Florida Ever- ecosystem by overgrowing and shading out native glades by Australian paperbark trees, noted species. South American water hyacinth (Eichhor- above, is largely due to the fact that paperbark nia crassipes) now covers parts of Lake Victoria in catches fire easily and produces hotter fires than Africa (Matthews and Brand 2004a), many lakes the grasses and sedges it replaces. The opposite and rivers in the southeastern United States transformation, from forest to grassland, can (Schardt 1997), and various waterbodies in Asia also be effected by a changed fire regime. In and Australia (Matthews and Brand 2004b), often Hawaii, African molassesgrass (Melinis minuti- smothering native submersed vegetation. Vast flora) and tropical American tufted beardgrass quantities of rotting water hyacinth, and conse- (Schizachyrium condensatum) have replaced na- quent drops in dissolved oxygen, can also affect tive-dominated woodland by virtue of increased many aquatic animal species. Similar overgrowth fire frequency and extent (D’Antonio and Vitou- occurs in the Mediterranean Sea, where Caulerpa sek 1992). taxifolia (Figure 7.1), an alga from the tropical Introduced plants can change entire ecosys- southwest Pacific Ocean, replaces seagrass mea- tems by modifying water or nutrient regimes. At dows over thousands of hectares, greatly changing Eagle Borax Spring in California, Mediterranean the animal community (Meinesz 1999). salt cedars (Tamarix spp.) dried up a large marsh A new species of cordgrass (Spartina anglica) (McDaniel et al. 2005), while in Israel, Australian arose in England in the late nineteenth century by eucalyptus trees were deliberately introduced to hybridization between a native cordgrass and an drain swamps (Calder 2002). By fertilizing nitro- introduced North American species. The new spe- gen-poor sites, introduced nitrogen-fixing plants cies invaded tidal mudflats and, trapping much can favor other exotic species over natives. On the more sediment, increased elevation and converted geologically young, nitrogen-poor volcanic is- mudflats to badly drained, dense salt marshes with land of Hawaii, firetree (Morella faya), a nitro- different animal species (Thompson 1991). The hy- gen-fixing shrub from the Azores, creates brid species was later introduced to New Zealand conditions that favor other introduced species and the state of Washington with similar impacts. that previously could not thrive in the low-nutrient

Figure 7.1 Caulerpa taxifolia. Photograph by Alex Meinesz.

INVASIVE SPECIES 135 soil and disfavor native plants that had evolved to 7.1.3 Aggression and its analogs tolerate such soil (Vitousek 1986). The red imported fire ant (Solenopsis invicta) from Pathogens that eliminate a previously domi- southern South America has spread through the nant plant can impact an entire ecosystem. In southeastern United States and more recently has the first half of the twentieth century, Asian chest- invaded California. It attacks other ant species it nut blight (Cryphonectria parasitica) ripped encounters, and in disturbed habitats (which through eastern North America, effectively elim- comprise much of the Southeast) this aggression inating American chestnut (Castanea dentata), a has caused great declines in populations of native tree that had been common from Georgia ant species (Tschinkel 2006). The Argentine ant through parts of Canada and comprised at least (Linepithema humile), also native to South Ameri- 30% of the canopy trees in many forests (William- ca, similarly depresses populations of native ant son 1996). This loss in turn led to substantial species in the United States by attacking them structural changes in the forest, and it probably (Holway and Suarez 2004). The Old World greatly affected nutrient cycling, because chest- zebra mussel (Dreissena polymorpha; Figure 7.2), nut wood, high in tannin, decomposes slowly, spreading throughout much of North America, while the leaves decompose very rapidly (Ellison threatens the very existence of a number of native et al. 2005). Chestnut was largely replaced by oaks freshwater bivalve species, primarily by settling (Quercus spp.), which produce a recalcitrant litter. on them in great number and suturing their Because this invasion occurred so long ago, few valves together with byssal threads, so that they of its effects were studied at the time, but it is suffocate or starve (Ricciardi et al. 1998). Al- known that at least seven moth species host-spe- though plants do not attack, they have an analo- cific to chestnut went extinct (Opler 1978). Such gous ability to inhibit other species, by producing pathogens are also threats to forest industries or sequestering chemicals. For example, the Afri- founded on introduced species as well as natives, can crystalline ice plant (Mesembryanthemum crys- as witness the vast plantations in Chile of North tallinum) sequesters salt, and when leaves fall and American Monterrey pine (Pinus radiata) now decompose, the salt remains in the soil, rendering threatened by recently arrived Phytophthora pini- it inhospitable to native plants in California that folia (Durán et al. 2008). cannot tolerate such high salt concentrations (Vivrette and Muller 1977). Diffuse knapweed (Centaurea diffusa) from Eurasia and spotted 7.1.2 Resource competition knapweed (C. stoebe) from Europe are both major invaders of rangelands in the American In Great Britain, the introduced North American West. One reason they dominate native range gray squirrel (Sciurus carolinensis)foragesfor plants in the United States is that they produce nuts more efficiently than the native red squirrel (Sciurus vulgaris), leading to the decline of the latter species (Williamson 1996). The same North American gray squirrel species has recently invaded the Piedmont in Italy and is spreading, leading to concern that the red squirrel will also decline on the mainland of Europe as it has in Britain (Bertolino et al. 2008). The house gecko (Hemidactylus frenatus)fromSouth- east Asia and parts of Africa has invaded many Pacific islands, lowering insect populations that serveasfoodfornativelizards,whosepopula- tions have declined in some areas (Petren and Case 1996). Figure 7.2 Zebra mussel. Photograph by Tony Ricciardi.

136 CONSERVATION BIOLOGY FOR ALL root exudates that are toxic to native plants (Call- nested on the ground and were highly susceptible away and Ridenour 2004). An invasive intro- to the invaders. Introduced rats, for example, duced plant can also dominate a native species have caused the extinction of at least 37 species by interfering with a necessary symbiont of the and subspecies of island birds throughout the native. For instance, many plants have estab- world (Atkinson 1985). The brown tree snake lished mutualistic relationships with arbuscular (Boiga irregularis; Figure 7.3), introduced to mycorrhizal fungi, in which the fungal hyphae Guam from New Guinea in cargo after World penetrate the cells of the plants’ roots and aid War II, has caused the extinction or local extirpa- the plants to capture soil nutrients. Garlic mus- tion of nine of the twelve native forest bird spe- tard (Alliaria petiolata) from Europe, Asia, and cies on Guam and two of the eleven native lizard North Africa is a highly invasive species in the species (Lockwood et al. 2007). For these native ground cover of many North American wood- species, an arboreal habitat was no defense lands and floodplains. Root exudates of garlic against a tree-climbing predator. Another famous mustard, which does not have mycorrhizal as- introduced predator that has wreaked havoc with sociates, are toxic to arbuscular mycorrhizal native species is the Nile perch (Lates niloticus), fungi found in North American soils (Callaway deliberately introduced to Lake Victoria in the et al. 2008). 1950s in the hope that a fishery would be established to provide food and jobs to local commu- 7.1.4 Predation nities (Pringle 2005). Lake Victoria is home of one of the great evolutionary species radiations, the One of the most dramatic and frequently seen hundreds of species of cichlid fishes. About half impacts of introduced species is predation on of them are now extinct because of predation by native species. Probably the most famous cases the perch, and several others are maintained only are of mammalian predators such as the ship rat by captive rearing (Lockwood et al. 2007). (Rattus rattus), Norway rat (R. norvegicus), Pacific Many predators have been deliberately intro- rat (R. exulans), small Indian mongoose (Herpestes duced for “biological control” of previously in- auropunctatus), and stoat (Mustela erminea) intro- troduced species (see below), and a number of duced to islands that formerly lacked such spe- these have succeeded in keeping populations of cies. In many instances, native bird species, not the target species at greatly reduced levels. For having evolved adaptations to such predators, instance, introduction of the Australian vedalia

Figure 7.3 Brown tree snake. Photograph by Gad Perry.

INVASIVE SPECIES 137 ladybeetle (Rodolia cardinalis) in 1889 controlled American cassava mealybug (Phenacoccus mani- Australian cottony-cushion scale (Icerya purchasi) hoti), invading extensive cassava-growing parts on citrus in California (Caltagirone and Doutt of Africa, often destroys more than half the crop 1989). However, some predators introduced for yield (Norgaard 1988), while in the United biological control have attacked non-target spe- States, the Russian wheat aphid (Diuraphis cies to the extent of causing extinctions. One of noxia) caused US$600 million damage in just the worst such disasters was the introduction of three years (Office of Technology Assessment the rosy wolf snail (Euglandina rosea), native to 1993). In forests of the eastern United States, the Central America and Florida, to many Pacific European gypsy moth (Lymantria dispar) caused islands to control the previously introduced a similar amount of damage in only one year giant African snail (Achatina fulica). The predator (Office of Technology Assessment 1993). In high not only failed to control the targeted prey (which elevation forests of the southern Appalachian grows to be too large for the rosy wolf snail to Mountains, the Asian balsam woolly adelgid attack it) but caused the extinction of over 50 (Adelges piceae) has effectively eliminated the species of native land snails (Cowie 2002). The previously dominant Fraser fir tree (Rabenold small Indian mongoose, implicated as the sole et al. 1998), while throughout the eastern United cause or a contributing cause in the extinction of States the hemlock woolly adelgid (A. tsugae)is several island species of birds, mammals, and killing most hemlock trees, which often formed frogs, was deliberately introduced to all these distinct moist, cool habitats amidst other tree islands as a biological control agent for intro- species (Ellison et al. 2005). duced rats (Hays and Conant 2006). The mosqui- Plant-eating insects have been successful in tofish (Gambusia affinis) from Mexico and Central many biological control projects for terrestrial America has been introduced to Europe, Asia, and aquatic weeds. For instance, in Africa’s Africa, Australia, and many islands for mosquito Lake Victoria, a massive invasion of water hya- control. Its record on this score is mixed, and cinth was brought under control by introduction there is often evidence that it is no better than of two South American weevils, Neochetina eich- native predators at controlling mosquitoes. How- horniae and N. bruchi (Matthews and Brand ever, it preys on native invertebrates and small 2004a); these have also been introduced to attack fishes and in Australia is implicated in extinction water hyacinth in tropical Asia (Matthews and of several fish species (Pyke 2008). Brand 2004b). The South American alligator- weed flea beetle (Agasicles hygrophila) has mini- 7.1.5 Herbivory mized the invasion of its South American host plant (Alternanthera philoxeroides) in Florida (Cen- Introduced herbivores can devastate the flora of ter et al. 1997) and contributed greatly to its areas lacking similar native species, especially on control in slow-moving water bodies in Asia islands. Goats (Capra aegagrus hircus) introduced (Matthews and Brand 2004b). A particularly fa- to the island of St. Helena in 1513 are believed to mous case was the introduction of the South have eliminated at least half of 100 endemic American cactus moth (Cactoblastis cactorum)to plant species before botanists had a chance to Australia, where it brought a massive invasion of record them (Cronk 1989). European rabbits (Or- prickly pear cactus (Opuntia spp.) under control yctolagus cuniculus) introduced to islands world- (Zimmermann et al. 2001). In probably the first wide have devastated many plant populations, successful weed biological control project, a Bra- often by bark-stripping and thus killing shrubs zilian cochineal bug (Dactylopius ceylonicus) vir- and seedling and sapling trees. Rabbits also often tually eliminated the smooth prickly pear cause extensive erosion once vegetation has been (Opuntia vulgaris) from India (Doutt 1964). In destroyed (Thompson and King 1994). Damage 1913, the same insect was introduced to South to forests and crop plants by introduced herbi- Africa and effectively eliminated the same plant vores is often staggering. For instance, the South (Doutt 1964).

138 CONSERVATION BIOLOGY FOR ALL

However, occasionally, biological control intro- 7.1.6 Pathogens and parasites ductions of herbivorous insects have devastated Many introduced plant pathogens have modified non-target native species. The same cactus moth entire ecosystems by virtually eliminating domi- introduced to Australia was introduced to control nant plants. The chestnut blight was discussed pest prickly pear on the island of Nevis in the West above. A viral disease of ungulates, rinderpest, Indies. From there, it island-hopped through the introduced to southern Africa from Arabia or West Indies and reached Florida, then spread India in cattle in the 1890s, attacked many native further north and west. In Florida, it already ungulates, with mortality in some species reach- threatens the very existence of the native sema- ing 90%. The geographic range of some ungulate phore cactus (O. corallicola), and there is great species in Africa is still affected by rinderpest. concern that this invasion, should it reach the Because ungulates often play key roles in vegeta- American Southwest and Mexico, would not tion structure and dynamics, rinderpest impacts only threaten other native Opuntia species but affected entire ecosystems (Plowright 1982). also affect economically important markets for Of course, many introduced diseases have af- ornamental and edible Opuntia (Zimmermann fected particular native species or groups of them et al. 2001). The Eurasian weevil (Rhinocyllus con- without modifying an entire ecosystem. For in- icus), introduced to Canada and the United stance, avian malaria, caused by Plasmodium re- States to control introduced pest thistles, attacks lictum capristranoae, introduced with Asian birds severalnativethistlesaswell(Loudaet al. 1997), and vectored by previously introduced mosqui- and this herbivory has led to the listing of the toes, contributed to the extinction of several na- native Suisun thistle (Cirsium hygrophilum var. tive Hawaiian birds and helps restrict many of hygrophilum) on the U.S. Endangered Species the remaining species to upper elevations, where List (US Department of the Interior 1997). In mosquitoes are absent or infrequent (Woodworth each of these cases of herbivorous biological et al. 2005). In Europe, crayfish plague (Aphano- control agents threatening natives, the intro- myces astaci), introduced with the North Ameri- ducedherbivorewasabletomaintainhighnum- can red signal crayfish (Pacifastacus lenusculus; bers on alternative host plants (such as the Figure 7.4 and Plate 7) and also vectored by targeted hosts), so decline of the native did not the subsequently introduced Lousiana crayfish cause herbivore populations to decline.

Figure 7.4 North American red signal crayﬁsh (right) and a native European crayﬁsh (Astacus astacus). Photograph by David Holdich.

INVASIVE SPECIES 139

(Procambarus clarkii), has devastated native Euro- Parasites and pathogens have also been used pean crayfish populations (Goodell et al. 2000). successfully in biological control projects against The European fish parasite Myxosoma cerebralis, introduced target hosts. For instance, the South which causes whirling disease in salmonid fishes, American cassava mealybug in Africa, discussed infected North American rainbow trout (Oncor- above, has been partly controlled by an introduced hynchus mykiss) that had been previously intro- South American parasitic wasp, Epidinocarsis lopezi duced to Europe and were moved freely among (Norgaard 1988), while the European yellow clo- European sites after World War II. Subsequently, ver aphid (Therioaphis trifolii), a pest of both clover infected frozen rainbow trout were shipped to and alfalfa, is controlled in California by three North America, and the parasite somehow got introduced parasitic wasps, Praon palitans, Trioxys into a trout hatchery in Pennsylvania, from utilis,andAphelinus semiflavus (Van Den Bosch which infected rainbow trout were shipped to et al. 1964). The New World myxoma virus, intro- many western states. In large areas of the West, duced to mainland Europe (where the European most rainbow trout contracted the disease and rabbit is native) and Great Britain and Australia sport fisheries utterly collapsed (Bergersen and (where the rabbit is introduced), initially caused Anderson 1997). Introduced plant parasites can devastating mortality (over 90%). However, the greatly damage agriculture. For example, parasit- initially virulent viral strains evolved to be more ic witchweed (Striga asiatica) from Africa reached benign, while in Great Britain and Australia, the southeastern United States after World War rabbits evolved to be more resistant to the virus. II, probably arriving on military equipment. It Mortality has thus decreased in each successive inflicts great losses on crops that are grasses (in- epidemic (Bartrip 2008). cluding corn) and has been the target of a lengthy, expensive eradication campaign (Eplee 2001). 7.1.7 Hybridization Introduction of vectors can also spread not only introduced pathogens (e.g. the mosquitoes vector- If introduced species are sufficiently closely ing avian malaria in Hawaii) but also native ones. related to native species, they may be able to For example, the native trematode Cyathocotyle mate and exchange genes with them, and a suffi- bushiensis, an often deadly parasite of ducks, has cient amount of genetic exchange (introgression) reached new regions along the St. Lawrence River can so change the genetic constitution of the na- recently as its introduced intermediate host, the tive population that we consider the original spe- Eurasian faucet snail (Bithynia tentaculata), has in- cies to have disappeared—a sort of genetic vaded (Sauer et al. 2007). Introduced parasites or extinction. This process is especially to be feared pathogens and vectors can interact in complicated when the invading species so outnumbers the ways to devastate a native host species. Chinese native that a native individual is far more likely grass carp (Ctenopharyngodon idella) infected with to encounter the introduced species than a native the Asian tapeworm Bothriocephalus acheilognathi as a prospective mate. The last gasp of a fish were introduced to Arkansas in 1968 to control native to Texas, Gambusia amistadensis, entailed introduced aquatic plants and spread to the Mis- the species being hybridized to extinction sissippi River. There the tapeworm infected native through interbreeding with introduced mosquito fishes, including a popular bait fish, the red shiner fish G. amistadensis (Hubbs and Jensen 1984), (Notropis lutrensis). Fishermen or bait dealers then while several fishes currently on the United States carried infected red shiners to the Colorado River, Endangered Species List are threatened at least from which by 1984 they had reached a Utah partly by hybridization with introduced rainbow tributary, the Virgin River. In the Virgin River, trout. The North American mallard (Anas platyr- the tapeworm infected and killed many woundfin hynchos), widely introduced as a game bird, inter- (Plagopterus argentissimus), a native minnow al- breeds extensively with many congeneric species ready threatened by dams and water diversion and threatens the very existence of the endemic projects (Moyle 1993). New Zealand grey duck (A. superciliosa superciliosa)

140 CONSERVATION BIOLOGY FOR ALL and the Hawaiian duck (A. wyvilliana), as well as, before the European mink males. The European perhaps, the yellowbilled duck (A. undulata)and mink females subsequently abort the hybrid em- the Cape shoveller (A. smithii)inAfrica(Rhymer bryos, so no genes can be exchanged between the and Simberloff 1996, Matthews and Brand 2004a). species, but these females cannot breed again European populations of the white-headed duck during the same season, a severe handicap to a (Oxyura leucocephala)restrictedtoSpain,are small, threatened population (Maran and Hentto- threatened by hybridization and introgression nen 1995). with North American ruddy ducks (O. jamaicensis) (Muñoz-Fuentes et al. 2007). The latter had been 7.1.8 Chain reactions introduced years earlier to Great Britain simply as an ornamental; they subsequently crossed the Some impacts of introduced species on natives Channel, spread through France, and reached entail concatenated chains of various interactions: Spain. species A affecting species B, then species B af- Availability and increasing sophistication of fecting species C, species C affecting species D, molecular genetic techniques has led to the rec- and so forth. The spread of the Asian parasitic ognition that hybridization and introgression tapeworm from Arkansas ultimately to infect the between introduced and native species is far woundfin minnow (Plagopterus argentissimus)in more common than had been realized. Such Utah is an example. However, chains can be even hybridization can even lead to a new species. more complex, almost certainly unforeseeable. In the cordgrass (Spartina) case discussed An example involves the devastation of Europe- above, occasional hybrids were initially sterile, an rabbit populations in Britain by New World until a chromosomal mutation (doubling of myxoma virus, described above. Caterpillars of chromosome number) in one of them produced the native large blue butterfly(Maculina arion)in a fertile new polyploid species, which became Great Britain required development in under- highly invasive (Thompson 1991). A similar case ground nests of the native ant Myrmica sabuleti. involves Oxford ragwort (Senecio squalidus), a The ant avoids nesting in overgrown areas, which hybrid of two species from Italy, introduced for centuries had not been problematic because of to the Oxford Botanical Garden ca. 1690. S. squa- grazing and cultivation. However, changing land lidus escaped, first spread through Oxford, and use patterns and decreased grazing led to a situa- then during the Industrial Revolution through tion in which rabbits were the main species main- much of Great Britain along railroad lines, taining suitable habitat for the ant. When the producing sterile hybrids with several native virus devastated rabbit populations, ant popula- British species of Senecio. A chromosomal muta- tions declined to the extent that the large blue tion (doubling of chromosome number) of a butterfly was extirpated from Great Britain (Rat- hybrid between S. squalidus and S. vulgaris cliffe 1979). In another striking chain reaction, (groundsel) produced the new polyploid species landlocked kokanee salmon (Oncorhynchus S. cambrensis (Welsh groundsel) (Ashton and nerka), were introduced to Flathead Lake, Mon- Abbott 1992). tana in 1916, replacing most native cutthroat It is possible for hybridization to threaten a trout (O. clarki) and becoming the main sport species even when no genetic exchange occurs. fish. The kokanee were so successful that they Many populations of the European mink (Mustela spread far from the lake, and their spawning lutreola) are gravely threatened by habitat de- populations became so large that they attracted struction. North America mink (M. vison), widely large populations of bald eagles (Haliaeetus leuco- introduced in Europe to foster a potential fur- cephalus), grizzly bears (Ursus arctos horribilis), bearing industry, have escaped and established and other predators. Between 1968 and 1975, many populations. In some sites, many female opossum shrimp (Mysis relicta), native to large European mink hybridize with male American deep lakes elsewhere in North America and in mink, which become sexually mature and active Sweden, were introduced to three lakes in the

INVASIVE SPECIES 141 upper portion of the Flathead catchment in order the introduced snail and the introduced trema- to increase production of kokanee; the shrimp tode combine to produce more mortality in ducks drifted downstream into Flathead Lake by 1981 than either would likely have accomplished and caused a sharp, drastic decline in populations alone. This is but one of myriad instances of melt- of cladocerans and copepods they preyed on. down. However, the kokanee also fed on these prey, Sometimes introduced animals either pollinate and kokanee populations fell rapidly, in turn introduced plants or disperse their seeds. For causing a precipitous decline in local bald eagle instance, figs (Ficus spp.) introduced to Florida and grizzly bear numbers (Spencer et al. 1991; had until ca. 20 years ago remained where they Figure 7.5). were planted, the species unable to spread because the host-specific fig wasps that pollinate fi 7.1.9 Invasional meltdown the gs in their native ranges were absent, so the figs could not produce seeds. That situation An increasing number of studies of invasion changed abruptly upon the arrival of the fig- effects have pointed to a phenomenon called wasps of three of the fig species, which now “invasional meltdown” in which two or more produce seeds. One of them, F. microcarpa, has introduced species interact in such a way that become an invasive weed, its seeds dispersed by the probability of survival and/or the impact of birds and ants (Kauffman et al. 1991). On the at least one of them is enhanced (Simberloff and island of La Réunion, the red-whiskered bulbul Von Holle 1999). In the above example of an (Pycnonotus jocosus), introduced from Asia via introduced faucet snail (Bithynia tentaculata), vec- Mauritius, disperses seeds of several invasive toring a native trematode parasite of ducks and introduced plants, including Rubus alceifolius, thereby expanding the trematode’s range, a re- Cordia interruptus, and Ligustrum robustrum, cent twist is the arrival of a European trematode which have become far more problematic since (Leyogonimus polyoon). Bithynia also vectors this the arrival of the bulbul (Baret et al. 2006). The species, which has turned out also to be lethal to Asian common myna (Acridotheres tristis) was in- ducks (Cole and Friend 1999). So in this instance, troduced to the Hawaiian islands as a biological

phytop- lankton copepod McDonald Creek

cladoceran kokanee opossum shrimp salmon

Flathead Lake lake trout

Figure 7.5 Shrimp stocking, salmon collapse, and eagle displacement. Reprinted from Spencer et al. (1991) © American Institute of Biological Sciences.

142 CONSERVATION BIOLOGY FOR ALL control for pasture insects but has ended up dis- component of the vegetation in the vicinity of the persing one of the worst weeds, New World Lan- town of Darwin, but the water buffalo, opening tana camara, throughout the lowlands and even up the flood plains, created perfect germination into some native forests (Davis et al. 1993). Also in sites of Mimosa seedlings, and in many areas na- Hawaii, introduced pigs selectively eat and there- tive sedgelands became virtual monocultures of by disperse several invasive introduced plant M. pigra. The mimosa in turn aided the water species, and by rooting and defecating they also buffalo by protecting them from aerial hunters spread populations of several introduced inverte- (Simberloff and Von Holle 1999). brates, while themselves fattening up on intro- Aquatic plants and animals can also facilitate duced, protein-rich European earthworms one another. In North America, the introduced (Stone 1985). zebra mussel filters prodigious amounts of Habitat modification by introduced plants can water, and the resulting increase in water clarity lead to a meltdown process with expanded and/ favors certain plants, including the highly inva- or accelerated impacts. As noted above, the nitro- sive Eurasian watermilfoil (Myriophyllum spica- gen-fixing Morella faya (firetree) from the Azores tum). The milfoil then aids the mussel by has invaded nitrogen-deficient volcanic regions providing a settling surface and facilitates the of the Hawaiian Islands. Because there are no movement of the mussel to new water bodies native nitrogen-fixing plants, firetree is essential- when fragments of the plant are inadvertently ly fertilizing large areas. Many introduced plants transported on boat propellers or in water (Sim- established elsewhere in Hawaii had been unable berloff and Von Holle 1999). to colonize these previously nutrient-deficient Some instances of invasional meltdown arise areas, but their invasion is now facilitated by the when one introduced species is later reunited activities of firetree (Vitousek 1986). In addition, with a coevolved species through the subsequent firetree fosters increased populations of intro- introduction of the latter. The fig species and their duced earthworms, and the worms increase the pollinating fig wasps in Florida are an example; rate of nitrogen burial from firetree litter, thus the coevolved mutualism between the wasps and enhancing the effect of firetree on the nitrogen the figs is critical to the impact of the fig invasion. cycle (Aplet 1990). Finally, introduced pigs and However, meltdown need not be between coe- an introduced songbird (the Japanese white-eye, volved species. The water buffalo from Asia and Zosterops japonicus) disperse the seeds of the fire- Mimosa pigra from Central America could not tree (Stone and Taylor 1984, Woodward et al. have coevolved, nor could the Asian myna and 1990). In short, all these introduced species create the New World Lantana camara in Hawaii. a complex juggernaut of species whose joint interactions are leading to the replacement of 7.1.10 Multiple effects native vegetation. Large, congregating ungulates can interact Many introduced species have multiple direct with introduced plants, pathogens, and even and indirect effects on native species, harming other animals in dramatic cases of invasional some and favoring others at the same time. For meltdown. For instance, Eurasian hooved live- example, the round goby (Neogobius melanosto- stock devastated native tussock grasses in North mus), an Old World fish that arrived in ballast American prairie regions but favored Eurasian water, is widely recognized in the North Ameri- turfgrasses that had coevolved with such animals can Great Lakes as a harmful invader, feeding on and that now dominate large areas (Crosby 1986). native invertebrates and eggs and larvae of sev- In northeastern Australia, the Asian water buffalo eral native fishes. It also competes for food and (Bubalus bubalis), introduced as a beast of burden space with other native fish species. However, the and for meat, damaged native plant communities round goby also feeds on the harmful zebra mus- and eroded stream banks. The Central American sel and related quagga mussel (Dreissena bugen- shrub Mimosa pigra had been an innocuous minor sis), although the impact on their populations is

INVASIVE SPECIES 143 not known. It also now is by far the main food has yet to be explained. In other instances, a source for the threatened endemic Lake Erie change in the physical or biotic environment can water snake (Nerodia sipedon insularum), constitut- account for a sudden explosion of a formerly ing over 92% of all prey consumed. Further, restricted introduced species. The spread of Bra- snakes that feed on the goby grow faster and zilian pepper in Florida after a century of harm- achieve large size, which may well decrease pre- less presence was caused by hydrological dation on the snake and increase population size changes—draining farmland, various flood con- (King et al. 2006). On balance, almost all observers trol projects, and lowering of the water table for would rather not have the round goby in this agricultural and human use. As described earlier, region, but it is well to bear in mind the complex- the sudden invasion by long-present figs in south ity of its impacts. Florida was spurred by the arrival of pollinating fig wasps. In some instances, demography of a species dictates that it cannot build up population 7.2 Lag times sizes rapidly even if the environment is suitable; trees, for example, have long life cycles and many Introduced species may be innocuous in their do not begin reproducing for a decade or more. new homes for decades or even centuries before As genetic analysis has recently rapidly ex- abruptly increasing in numbers and range to gen- panded with the advent of various molecular erate major impacts. The case of the hybrid cord- tools, it appears that some, and perhaps many, grass Spartina anglica, discussed above, is an sudden expansions after a lag phase occur be- excellent example. The introduced progenitor, cause of the introduction of new genotypes to a North American S. alterniflora, had been present previously established but restricted population. in Great Britain at least since the early nineteenth The brown anole population in Florida was aug- century and had even hybridized with the native mented in the twentieth century by the arrival of S. maritima occasionally, but the hybrids were all individuals from different parts of the native sterile until one underwent a chromosomal mu- range, so that the population in Florida now has tation ca. 1891, producing a highly invasive weed far more genetic diversity than is found in any (Thompson 1991). Brazilian pepper (Schinus tere- native population. It is possible that the rapid binthifolius) had been present in Florida since the range expansion of this introduction results mid-nineteenth century as isolated individual from introductions to new sites combined with trees, but it became invasive only when it began the advent of new genotypes better adapted to to spread rapidly ca. 1940 (Ewel 1986). Giant reed the array of environmental conditions found in (Arundo donax) was first introduced from the Florida (Kolbe et al. 2004). The northward range Mediterranean region to southern California in expansion of European green crab (Carcinus mae- the early nineteenth century as a roofing material nas) along the Atlantic coast of North America and for erosion control, and it remained restricted was produced by the introduction of new, cold- in range and unproblematic until the mid-twenti- tolerant genotypes into the established popula- eth century, when it spread widely, becoming a tion (Roman 2006). fire hazard, damaging wetlands, and changing An improved understanding of lag times is im- entire ecosystems (Dudley 2000). The Caribbean portant in understanding how best to manage brown anole lizard (Anolis sagrei) first appeared biological invasions (Boggs et al. 2006). It is not in Florida in the nineteenth century, but it was feasible to attempt active management (see next restricted to extreme south Florida until the section) of all introduced species—there are sim- 1940s, when its range began an expansion that ply too many. Typically in each site we focus on accelerated in the 1970s, ultimately to cover most those that are already invasive or that we suspect of Florida (Kolbe et al. 2004). will become invasive from observations else- Many such invasion lags remain mysterious. where. However, if some currently innocuous es- For instance, the delay for giant reed in California tablished introduced species are simply biological

144 CONSERVATION BIOLOGY FOR ALL time bombs waiting to explode when the right For such lists to be effective, the risk analyses conditions prevail in the future, the existing ap- have to be accurate enough, and the lists suffi- proach clearly will not suffice. ciently large, that the great majority of species that would become invasive are actually identified as such and placed on black lists or kept off white lists. There are grave concerns that neither 7.3 What to do about invasive species criterion is met. For instance, the black list of the By far the best thing to do about invasive intro- Lacey Act is very short, and many animal species duced species is to keep them out in the first that have a high probability of becoming invasive place. If we fail to keep them out and they estab- if introduced are not on the list. The risk assess- lish populations, the next possibility is to attempt ment tools, on the other hand, all yield some — to find them quickly and perhaps to eradicate percentage of false negatives that is, species as- them. If they have already established and sessed as unlikely to cause harm, therefore eligi- begun to spread widely, we may still try to eradi- ble for a white list, when in fact they will become cate them, or we can instead try to keep their harmful. Much active research (e.g. Kolar and populations at sufficiently low levels that they Lodge 2002) is aimed at improving the accuracy — do not become problems. of risk analyses especially lowering the rate of false negatives while not inflating the rate of false positives (species judged likely to become inva- 7.3.1 Keeping them out sive when, in fact, they would not). For inadvertent introductions, one must first Introductions can be either planned (deliberate) identify pathways by which they occur (Ruiz or inadvertent, and preventing these two classes and Carlton 2003). For instance, many marine involves somewhat different procedures. In each organisms are inadvertently carried in ballast instance, prevention involves laws, risk analyses, water (this is probably how the zebra mussel and border control. For planned introductions, entered North America). Insects stow away on such as of ornamental plants or new sport fish ornamental plants or agricultural products. The or game species, the law would be either a “white Asian longhorned beetle (Anoplophora glabripen- list,” a “black list,” or some combination of the nis), a dangerous forest pest, hitchhiked to North two. A white list is a list of species approved for America in untreated wooden packing material introduction, presumably after some risk analysis from Asia, while snails have been transported in which consideration is given to the features of worldwide on paving stones and ceramics. The the species intended for introduction and the out- Asian tiger mosquito (Aedes albopictus) arrived in come in other regions where it has been intro- the United States in water transported in used duced. The most widely used risk analyses tires. Once these pathways have been identified, currently include versions of the Australian their use as conduits of introduction must be Weed Risk Assessment, which consists of a series restricted. For ballast water, for example, water of questions about species proposed for introduc- picked up as ballast in a port can be exchanged tion and an algorithm for combining the answers with water from the open ocean to lower the to those questions to give a score, for which there number of potential invaders being transported. is a threshold above which a species cannot be For insects and pathogens carried in wood, heat admitted (Pheloung et al. 1999). A black list is a and chemical treatment may be effective. For ag- list of species that cannot be admitted under any ricultural products, refrigeration, and/or fumiga- circumstances, and for which no further risk anal- tion are often used. The general problem is that ysis is needed. Examples of black lists include the each of these procedures entails a cost, and there United States Federal Noxious Weed list and a has historically been opposition to imposing such short list of animals forbidden for entry to the US costs on the grounds that they interfere with free under the Lacey Act. trade and make goods more expensive. Thus it

INVASIVE SPECIES 145 remains an uphill battle to devise and to imple- ular media and on the web, but they can yield ment regulations sufficiently stringent that they enormous benefits. For instance, the invasion of constrict these pathways. the Asian longhorned beetle to the Chicago re- Whatever the regulations in place for both de- gion was discovered by a citizen gathering fire- liberate and unplanned introductions, inspec- wood who recognized the beetle from news tions at ports of entry are where they come into reports and checked his identification on a state play, and here a variety of detection technologies agency website. This early warning and a quick, are available and improvements are expected. aggressive response by authorities led to success- Trained sniffer dogs are commonplace in ports ful regional extirpation of this insect after a five- in many countries, and various sorts of machinery, year campaign. Similarly, the invasion in Califor- including increasingly accurate X-ray equipment, nia of the alga Caulerpa taxifolia was discovered are widely in use (Baskin 2002). Although technol- probably within a year of its occurrence by a ogies have improved to aid a port inspector to diver who had seen publicity about the impact identify a potential invader once it has been of this species in the Mediterranean. This discov- detected, in many nations these are not employed ery led to successful eradication after a four-year because of expense or dearth of qualified staff. effort, and citizens have been alerted to watch for Also, improved detection and identification cap- this and other non-native algal species in both abilities are only half of the solution to barring the Mediterranean nations and California. introduction of new species either deliberately or Many introduced species have been successful- by accident (as for example, in dirt on shoes, or in ly eradicated, usually when they are found early untreated food). The other half consists of penalties but occasionally when they have already estab- sufficiently severe that people fear the conse- lished widespread populations. The keys to suc- quences if they are caught introducing species. cessful eradication have been as follows; Many nations nowadays have extensive publicity (i) Sufficient resources must be available to see at ports of entry, on planes and ships, and some- the project through to completion; the expense of times even in popular media, that combine educa- finding and removing the last few individuals tional material about the many harmful activities may exceed that of quickly ridding a site of the of invasive species and warnings about penalties majority of the population; (ii) Clear lines of au- for importing them. thority must exist so that an individual or agency can compel cooperation. Eradication is, by its 7.3.2 Monitoring and eradication nature, an all-or-none operation that can be sub- verted if a few individuals decide not to cooper- The key to eradicating an introduced species be- ate (for instance, by forbidding access to private fore it can spread widely is an early warning- property, or forbidding the use of a pesticide or rapid response system, and early warning re- herbicide); (iii) The biology of the target organism quires an ongoing monitoring program. Because must be studied well enough that a weak point in of the great expense of trained staff, few if any its life cycle is identified; and (iv) Should the nations adequately monitor consistently for all eradication succeed, there must be a reasonable sorts of invasions, although for specific habitats prospect that reinvasion will not occur fairly (e.g. waters in ports) or specific groups of species quickly. (e.g. fruit fly pests of agriculture) intensive ongo- In cases where these criteria have been met, ing monitoring exists in some areas. Probably the successful eradications are numerous. Many are most cost-effective way to improve monitoring is on islands, because they are often small and to enlist the citizenry to be on the lookout for because reinvasion is less likely, at least for unusual plants or animals and to know what isolated islands. Rats have been eradicated agency to contact should they see something from many islands worldwide; the largest to (see Figure 7.6 and Plate 8). Such efforts entail date is 113 km2. Recently, large, longstanding public education and wide dissemination in pop- populations of feral goats and pigs have been

© Oxford University Press 2010. All rights reserved. For permissions please email: [email protected] oh n Ehrlich: and Sodhi © 146 xodUiest rs 00 l ihsrsre.Frprisospes mi:[email protected] email: please permissions For reserved. rights All 2010. Press University Oxford OSRAINBOOYFRALL FOR BIOLOGY CONSERVATION osrainBooyfrAll for Biology Conservation http://ukcatalogue.oup.com/product/9780199554249.do .

Figure 7.6 Maryland’s aquatic invasive species. Poster courtesy of the Maryland Department of Natural Resources. 1

INVASIVE SPECIES 147 eradicated from Santiago Island (585 km2)inthe effectively controlled by mechanical means Galapagos (Cruz et al. 2005). The giant African alone—cutting and pulling roots—so long as suffi- snail has been successfully eradicated from sites cient labor is available (Matthews and Brand 2004a). in both Queensland and Florida (Simberloff Sometimes chemical control alone can keep a pest at 2003). Even plants with soil seed banks have low numbers. The Indian house crow (Corvus splen- been eradicated, such as sand bur (Cenchrus echi- dens), is an aggressive pest in Africa, attacking na- natus) from 400 ha Laysan Island (Flint and Re- tive birds, competing with them for food, preying hkemper 2002). When agriculture or public on local wildlife, stripping fruit trees, and even dive- health are issues, extensive and expensive eradi- bombing people and sometimes stealing food from cation campaigns have been undertaken and young children. It can be controlled by a poison, haveoftenbeensuccessful,crownedbytheglob- Starlicide, so long as the public does not object al eradication of smallpox. The African mosquito (Matthews and Brand 2004a). Many invasive plants (Anopheles gambiae), vector of malaria, was era- have been kept at acceptable levels by herbicides. dicated from a large area in northeastern Brazil For instance, in Florida, water hyacinth was drasti- (Davis and Garcia 1989), and various species cally reduced and subsequently managed by use of of flies have been eradicated from many large the herbicide 2,4-D, combined with some mechani- regions, especially in the tropics (Klassen 2005). cal removal (Schardt 1997). For lantana in South The pasture weed Kochia scoparia was eradicated Africa, a combination of mechanical and chemical from a large area of Western Australia (Randall control keeps populations minimized in some areas 2001), and the witchweed eradication campaign (Matthews and Brand 2004a). A South African pub- in the southeastern United States mentioned lic works program, Working for Water, has had above is nearing success. These successes sug- great success using physical, mechanical, and chem- gest that, if conservation is made a high enough ical methods to clear thousands of hectares of land priority, large-scale eradications purely for con- of introduced plants that use prodigious amounts of servation purposes may be very feasible. water, such as mesquite (Prosopis spp.) and several A variety of methods have been used in these species of Acacia (Matthews and Brand 2004a). Sim- campaigns: males sterilized by X-rays for fruit- ilarly, in the Canadian province of Alberta, Norway flies, chemicals for Anopheles gambiae and for rats, rats have been kept at very low levels for many hunters and dogs for goats. Some campaigns that years by a combination of poisons and hunting by probably would have succeeded were stopped the provincial Alberta Rat Patrol (Bourne 2000). short of their goals not for want of technological However, long-term use of herbicides and pes- means but because of public objections to using ticides often leads to one or more problems. First is chemicals or to killing vertebrates. A notable ex- the evolution of resistance in the target species, so ample is the cessation, because of pressure from that increasing amounts of the chemical have to be animal-rights groups, of the well-planned cam- used even on a controlled population. This has paign to eradicate the gray squirrel before it happened recently with the use of the herbicide spreads in Italy (Bertolino and Genovese 2003). used to control Asian Hydrilla verticillata in Florida (Puri et al. 2007), and it is a common phenomenon 7.3.3 Maintenance management in insect pests of agriculture. A second, related problem is that chemicals are often costly, and If eradication is not an option, many available tech- they can be prohibitively expensive if used over nologies may limit populations of invasive species large areas. Whereas the market value of an agri- so that damage is minimized. There are three main cultural product may be perceived as large enough methods—mechanical or physical control, chemical to warrant such great expense, it may be difficult control, and biological control. Sometimes these to convince a government agency that it is worth methods can be combined, especially mechanical controlling an introduced species affecting conser- and chemical control. In South Africa, the invasive vation values that are not easily quantified. Final- Australian rooikrans tree (Acacia cyclops)canbe ly, chemicals often have non-target impacts,

148 CONSERVATION BIOLOGY FOR ALL including human health impacts. The decline of Summary raptor populations as DDT residues caused thin eggshells is a famous example (Lundholm 1997). • Invasive species cause myriad sorts of conserva- Many later-generation herbicides and pesticides tion problems, many of which are complicated, have few if any non-target impacts when used some of which are subtle, and some of which are properly, but expense may still be a major issue. not manifested until long after a species is intro- These problems with pesticides have led to duced. great interest in the use of classical biological con- • The best way to avoid such problems is to prevent trol—deliberate introduction of a natural enemy introductions in the first place or, failing that, to find (predator, parasite, or disease) of an introduced them quickly and eradicate them. pest. This is the philosophy of fighting fire with • However, many established introduced species fire. Although only a minority of well-planned can be managed by a variety of technologies so biological control projects actually end up that their populations remain restricted and their controlling the target pest, those that have suc- impacts are minimized. ceeded are often dramatically effective and con- ferred low-cost control in perpetuity. For instance, massive infestations of water hyacinth in the Sepik River catchment of New Guinea were well con- Suggested reading trolled by introduction of the two South American weevils that had been used for this purpose in Baskin, Y. (2002). A plague of rats and rubbervines. Island Lake Victoria, Neochetina eichhorniae and N. bruchi Press, Washington, DC. (Matthews and Brand 2004b). A recent success on Davis, M. A. (2009). Invasion biology. Oxford University the island of St. Helena is the control of a tropical Press, Oxford. American scale insect (Orthezia insignis) that had Elton, C. E. (1958). The ecology of invasions by animals and threatened the existence of the endemic gumwood plants. Methuen, London (reprinted by University of Chicago Press, 2000). tree (Commidendrum robustum). A predatory South Lockwood, J. L., Hoopes, M. F., and Marchetti, M. P. (2007). American lady beetle (Hyperaspis pantherina)now Invasion ecology. Blackwell, Malden, Massachusetts. keeps the scale insect population at low densities Van der Weijden, W., Leewis, R., and Bol, P. (2007). (Booth et al. 2001). Even when a biological control Biological globalisation. KNNV Publishing, Utrecht, the agent successfully controls a target pest at one site, Netherlands. it may fail to do so elsewhere. The same two weevils that control water hyacinth in New Guinea and Lake Victoria had minimal effects on the hya- Relevant websites cinth in Florida, even though they did manage to establish populations (Schardt 1997). • However, in addition to the fact that most World Conservation Union Invasive Species Specialist Group: http://www.issg.org/index.html. biological control projects have not panned out, • National Invasive Species Council of the United States: several biological control agents have attacked http://www.invasivespecies.gov. non-target species and even caused extinctions— • National Agriculture Library of the United States: the cases involving the cactus moth, rosy wolf snail, http://www.invasivespeciesinfo.gov. fi small Indian mongoose, mosquito sh, and thistle- • European Commission: http://www.europe‐aliens.org. eating weevil have been mentioned earlier. In general, problems of this sort have been associated with introduced biological control agents such as generalized predators that are not specialized to REFERENCES use the specific target host. However, even species that are restricted to a single genus of host, such as Allen, J. A. (1998). Mangroves as alien species: the case of thecactusmoth,cancreateproblems. Hawaii. Global Ecology and Biogeography Letters, 7,61–71.

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