Issues In Ecology Number 5 Spring 2000 and as a result the parrots abandon the beech forests escaped into the northwest Mediterranean,and its new duri he native biota of the GalapagosIs threatened by goats and donkeys.nt ket large stretches of the seafloor,threatening nearshore cause of their grazing but because they trample the breed- marine communities.Evolution can also change potentia ing sites of tortoises and land iguanas.They also destroy impacts in subtler ways.A parasitic wasp imported to the the forest cover in the highlands,thereby affecting the U.S.to control the alfalfa weevil was originally ineffec islands'water cycle invasive plants have diverse means tive againstantherint,the Egyptian alfalfa weevil. are The e wasps lay their eggs in weevil la providing their probably the most comn .For young w sour 。of food d.Dissections of larval Egyp tian weevils showed that 35 to 40 percent of the wasp's mat over native plants in coastal California and removes scarce eggs were destroyed by the immune response of the lar water that the natives would otherwise use. vae.Fifteen years later,however,only 5 percent of the When a species interferes with or harms another eggs were being lost to these weevil defenses. in the competition for resources.ecologists call it inter ference cor mpetition,and the tactic has been well den Community-and Ecosystem-Level Effects strated e species For mple invasive introduced ant specie the red fir ant,and the big-headed ant-all devastate large frac invasive plants that replace natives.For example.th tions of native ant communities by aggression.Reports Australian paperbark or melaleuca tree,which until recently of interference competition among plants through their was increasing its range in south Florida by more than 20 production of toxins often spark controversy.although hectares per day.replaces cypress,sawgrass,and other Quackgrass,a persistent invader in agriculture,may well native species.It now covers about 160.000 hectares produce such pa often in densest tands that clude veg can also eliminate natives etation ive animals mating with them,a particular danger when the native uses huge amounts of water,and intensifies the fire re species is rare.For example,hybridization with the intro- gime.A vine-like perennial shrub from South America duced North American mallard threatens the existence Chromolaena odorata or siam weed.is not only an ag. -at least as distinct species-of both the New Zealand gressive invader in both Asia and Africa,suppressing re gray duck and the Hawaiian duck.Hybridization between generation of primary forest trees,but also provides feed s species and a one an niches that sustain oth er pests. Anothe highl invasive neotropical shrub.Lantana camara,serves as hab can cordgrass,carried in shipping ballast to southern En- tat for the normally stream-dwelling tsetse fly in East Af gland,hybridized occasionally with the native cordgrass rica.increasina the incidence of sleepina sickness in both there.These hybrid individuals were sterile,but one even- wild and domesticated animals,as well as in humans(Fig.1) tually underwent a genetic change and produced a fer- many invasive species wreak havoc on ecosys tile,highly invasive species ofo rdgrass.Hybridization tems by fostering more frequent or intense fires.to which species even the hybrids succeed,simply becau key not adapted. The pape erbark tre crossbreeding reduces the e in Florida,as do numerous invasive grasse number of new offspring added to the speciesown popu worldwide.In general,grasses produce a great deal o lation.Females of the European mink,already gravely flammable standing dead material,they can dry out rap- threatened by habitat deterioration,hybridize with males idly,and many resprout quickly after fires (Fig.4). of introduced North American mink.Embryos are invari- An invasion of Hawaii Volcanoes National Park ably aborted.but the wastage of eggs exacerbates the by a small tree.Myrica faya. native to the canary is because after introduction to a new inva supplies of t range.For example,a tropical seaweed,Caulerpa taxifolla nutrient in the nitrogen-poor volcanic soils at a rate 90 evolved tolerance for colder temperatures while it was fold greater than native plants.Many other non-native growing at the aquarium of the Stuttgart Zoo and other plants in Hawaii are able to enter only sites with relatively public and private aquaria in Europe.Since then it has fertile soils,so Myrica paves the way for further inva9 Issues in Ecology Number 5 Spring 2000 and as a result the parrots abandon the beech forests during this season. The native biota of the Galapagos Is￾lands is threatened by goats and donkeys, not only be￾cause of their grazing but because they trample the breed￾ing sites of tortoises and land iguanas. They also destroy the forest cover in the highlands, thereby affecting the islands’ water cycle. Invasive plants have diverse means of competing with natives. Usurping light and water are probably the most common tactics. For example, the suc￾culent highway ice plant, Carpobrotus edulis, both forms a mat over native plants in coastal California and removes scarce water that the natives would otherwise use. When a species interferes with or harms another in the competition for resources, ecologists call it inter￾ference competition, and the tactic has been well demon￾strated in invasive species. For example, several widely introduced ant species — the red fire ant, the Argentine ant, and the big-headed ant — all devastate large frac￾tions of native ant communities by aggression. Reports of interference competition among plants through their production of toxins often spark controversy, although Quackgrass, a persistent invader in agriculture, may well produce such phytotoxins. Invasive species can also eliminate natives by mating with them, a particular danger when the native species is rare. For example, hybridization with the intro￾duced North American mallard threatens the existence — at least as distinct species — of both the New Zealand gray duck and the Hawaiian duck. Hybridization between a non-indigenous species and a native one can even pro￾duce a new invasive species. For instance, North Ameri￾can cordgrass, carried in shipping ballast to southern En￾gland, hybridized occasionally with the native cordgrass there. These hybrid individuals were sterile, but one even￾tually underwent a genetic change and produced a fer￾tile, highly invasive species of cordgrass. Hybridization can threaten a native species even when the hybrids do not succeed, simply because crossbreeding reduces the number of new offspring added to the species’ own popu￾lation. Females of the European mink, already gravely threatened by habitat deterioration, hybridize with males of introduced North American mink. Embryos are invari￾ably aborted, but the wastage of eggs exacerbates the decline of the native species. Species can evolve after introduction to a new range. For example, a tropical seaweed, Caulerpa taxifolia, evolved tolerance for colder temperatures while it was growing at the aquarium of the Stuttgart Zoo and other public and private aquaria in Europe. Since then it has escaped into the northwest Mediterranean, and its new tolerance of winter temperatures has permitted it to blan￾ket large stretches of the seafloor, threatening nearshore marine communities. Evolution can also change potential impacts in subtler ways. A parasitic wasp imported to the U.S. to control the alfalfa weevil was originally ineffec￾tive against another insect, the Egyptian alfalfa weevil. The wasps lay their eggs in weevil larvae, providing their young with a source of food. Dissections of larval Egyp￾tian weevils showed that 35 to 40 percent of the wasp’s eggs were destroyed by the immune response of the lar￾vae. Fifteen years later, however, only 5 percent of the eggs were being lost to these weevil defenses. Community- and Ecosystem-Level Effects The biggest ecological threat posed by invasive species is the disruption of entire ecosystems, often by invasive plants that replace natives. For example, the Australian paperbark or melaleuca tree, which until recently was increasing its range in south Florida by more than 20 hectares per day, replaces cypress, sawgrass, and other native species. It now covers about 160,000 hectares, often in dense stands that exclude virtually all other veg￾etation. It provides poor habitat for many native animals, uses huge amounts of water, and intensifies the fire re￾gime. A vine-like perennial shrub from South America, Chromolaena odorata or Siam weed, is not only an ag￾gressive invader in both Asia and Africa, suppressing re￾generation of primary forest trees, but also provides feed￾ing niches that can sustain other pests. Another highly invasive neotropical shrub, Lantana camara, serves as habi￾tat for the normally stream-dwelling tsetse fly in East Af￾rica, increasing the incidence of sleeping sickness in both wild and domesticated animals, as well as in humans (Fig. 1). Many invasive species wreak havoc on ecosys￾tems by fostering more frequent or intense fires, to which key native species are not adapted. The paperbark tree has this effect in Florida, as do numerous invasive grasses worldwide. In general, grasses produce a great deal of flammable standing dead material, they can dry out rap￾idly, and many resprout quickly after fires (Fig. 4). An invasion of Hawaii Volcanoes National Park by a small tree, Myrica faya, native to the Canary Is￾lands, is transforming an entire ecosystem because the invader is able to fix nitrogen and increase supplies of this nutrient in the nitrogen-poor volcanic soils at a rate 90- fold greater than native plants. Many other non-native plants in Hawaii are able to enter only sites with relatively fertile soils, so Myrica paves the way for further inva-