Behavioral and Trait Shifts. In addition to the evolution of traits to adapt to new environments and to new invaders there are cases of behavioral shifts in the invaders themselves or in response to invaders. Holway and Suarez (40) give examples of shifts in behavior of populations of invading species from that found in their native ranges. Two ant species originating from Argentina (the fire ant Solenopsis invicta and the Argentine ant Linepirtma humile) both exhibit these shifts. It is not known whether these shifts are founder effects or adaptive. These authors make the case that behavior should be more fully incorporated into research as we build an understanding of the invasion process.
The introduction of brown trout into the streams of New Zealand started in the mid-1800s. They have driven to extinction some local populations of native fish and, in addition, they have evidently resulted in changed behavior of native mayfly nymphs and, to a certain extent, crayfish (
41).
In addition to behavioral shifts, either in response to an invader or in response to the new biotic community that an invader encounters, shifts in traits have been observed in an invader in a new environment. Blossey and Notzold (42) note that in populations of invasive species, the individuals are often larger in their new territory than in their native land. They compared plants from populations from the United States and as well as those from Europe where they are subject to natural predation in their native habitat. They attributed the size differences to the consequences of natural selection for greater competitive capacity after release from herbivore attack and the need to produce defensive compounds. Although this particular explanation has been challenged (43), others have noted similar cases of this phenomenon in comparing invading plants from Australia into California (44) and comparing invasions from South Africa into Australia and vice versa (45).
Invasive ants may also benefit from release from native pathogen populations, leading to larger colony size that confers greater exploitative competitive capacity, as discussed in Holway (46) and Human and Gordon (47) (see below). Colonies of invasive Argentine ants are larger in areas where they invade than they are in their native habitat.
Niche Displacement. Gray squirrels (Sciurus carolinensis) from North America have displaced the native red squirrel (Sciurus vulgaris) throughout most of the deciduous and mixed woodlands of Britain. This displacement apparently has resulted from food competition between these species, with gray squirrels favored by high quantities of oaks in the canopy. Recent decline of hazelnuts over oaks has evidently contributed to the demise of the red squirrel (48).
There has been a detailed study of the interaction between a California native mudsnail,
Cerithidea californica, and an invasive mudsnail,
Ilyanassa obsoleta, from the American Atlantic. Populations of
Ilyanassa have locally displaced
Cerithidea from the open tidal flats, restricting its distribution to the upper intertidal area.
Cerithidea's former functional role has been taken over by
Ilyanassa (
49).
Douglas et al. (50) have described the apparent niche shift in the native fish Meda fulgida when they co-occur with the introduced red shiner (Cyprinella lutrensis).
Competitive Exclusion. Some invasive species completely eliminate native species through competitive exclusion. The invasive fire ant (Solenopsis invicta), for example, has had a devastating effect on the arthropod biota that it encounters. In a detailed study in Texas, it was found that this fire ant reduced native ant diversity by 70% and the total number of native ant individuals by 90%, apparently by competitive exclusion. Similarly, overall non-ant arthropod diversity was reduced by 30% and the numbers of individuals by 70% (51). It should be noted, however, that while the fire ants excluded some native species from the invaded areas, the natives persisted in nearby uninvaded areas, such that no extinctions were observed.
The Argentine ant (
Linepithema humile) is a widely distributed invasive species that displaces native ants throughout its introduced range. It does so by being a better competitor for food resources than the native species (
46,
47).
There are accumulating studies examining the mechanisms of competitive displacement of native species by invaders. As examples, superior competition for food resources has resulted in the replacement of the native gecko, Lepidodactylus lugubris, by the invading Hemidactylus frenatus, throughout the Pacific (52). A higher resource-use efficiency of the available food resources has been implicated in the competitive superiority of the introduced snail Batillaria attramentaria over the native mud snail Cerithidea californica in the salt marshes and mud flats of northern California (53). Studies have also shown that behavioral differences in aggression and predation between a native and an invading amphipod explain competitive displacement (54). Competition for space by the invading mussel Mytilus galloprovincialis from southern Europe has displaced native mussels in California and South Africa (55).
Studies of such new interactions, brought about by invaders, are particularly revealing on the nature of competition because in "stable" ecosystems, with a long history of competition among its members, the resulting evolution of niche displacement makes it more difficult to observe the direct competitive process.
Mutualisms. In any ecosystem there is a web of interaction among the biotic components of differing specificities. Mutualisms, the tightest of such interactions, would seem to be a barrier to the success of a single player of a partnership becoming an invasive species. There is some evidence for this in the fact that nonmycorrhizal (i.e., do not depend on mutualistic root fungi) plant taxa, such as the Brassicaceae and the Chenopodiaceae, are particularly successful weeds. However, quite often the tightness of mutualisms is not as great as supposed and other species in the new habitat can play the required role for the invader (e.g., pollination). There are also examples of the arrival of one nonnative species, and the subsequent arrival of a co-evolved facilitator, thereby increasing the success of each in its new environment. This has happened with Pinus spp. and their mutualistic mycorrhizal fungi in the Southern Hemisphere; Richardson et al. (56) describe these as well as other examples.
With the mixing of biota and thus new interaction potentials there is the great possibility of new kinds of mutualistic relationships evolving. Richardson
et al. (
56) note several such cases, including the dispersal of North American and European pine seeds, which are normally wind-dispersed, being dispersed into new areas by cockatoos and European pines being dispersed in South Africa by alien American squirrels. Simberloff and Von Holle (
57) also note cases of one invading species facilitating the success of another, including a bird of Asian origin being the prime disperser of a shrub from the Canary Islands, all in their new Hawaiian home.
There are also instances of an invasive species disrupting mutualistic relationships (58). Native seed-harvesting ants disperse the seeds of certain proteas in South Africa. These native ants have been displaced by Argentine ants that are not successful in dispersing the Protea seeds to suitable germination microsites, thus potentially leading to the extinction of rare and endemic Protea species.
Finally, there are striking examples of host shifts as species are mixed through invasions and a parasite of one infects the other which is less able to cope with the parasite, as is happening with the parasitic mite Varroa jacobsoni, which evolved as a brood parasite of the Asian hive bee, Apis cerana, but which has now also switched host to the western honeybee, Apis mellifera, with disastrous results (59).
Extinctions. Invasive species not only alter competitive interactions and reduce native populations within a community but they can also lead to extinctions. Overall they are considered the second greatest threat to imperiled species in the United States (60). Carlton et al. (55) make the useful distinction among extinction events as local, regional, or global extinctions. They also recognize functional extinctions where individuals of a species are so reduced in numbers that they no longer play a major role in ecosystem processes. Thus there is a large continuum of impacts, with the main concern and statistical information available on the total global extinction of a species whereas, of course, local extinctions and population reductions are important in ecosystem functional considerations as noted by Carlton et al.
The literature abounds with examples of invasive species driving local native species to extinction, primarily on islands, and especially involving predators. Rodda
et al. (
61) detail the particularly dramatic case of the impact of the invasive brown tree snake (
Boiga irregularis) on the biota of Guam, which has caused a major conservation crisis through negative effects on birds, reptiles, and mammals. In a review of the impacts of introduced species on reptiles on islands Case and Bolger (
62) note that, "Although competition has led to changes in abundance and has caused habitat displacement and reduced colonization success, extinctions of established reptile populations usually occur only as a result of predation." They do note the large number of examples of the latter that have occurred as a result of predation by rats, feral cats, and mongooses.
It has been well documented that of all ecosystems lakes and streams have been most modified by invasive species, mainly because of the persistent efforts of humans to stock with game fish. Many of the introductions into these bodies result in species enrichment rather than extirpation (63). However, one of the most spectacular example of species extinctions in lakes comes from the introduction of the Nile perch into Lake Victoria, resulting in the loss of hundreds of species of cichlid fish (64). Ricciardi and Rasmussen (65) call attention to the fact that the freshwater fauna of temperate North America has extinction rates matching that of tropical forests, in part because of invasive species. Ricciardi et al. (66), for example, note a global pattern, that within 4-8 years after invasion by zebra mussel (Dreissena polymorpha) local native mussel populations are extirpated. Over 60 endemic mussel species of the Mississippi River Basin are threatened with global extinction by the effects of zebra mussel and environmental degradation.
Although the introduction of an organism into a new environment always provides risks and surprises as to the impact it will have on other organisms, it is particularly disconcerting when organisms that are introduced to control the activities of an unwanted invader instead do collateral damage to other species, even driving them to extinction. This is apparently the case with the introduction of the rosy wolf snail, Euglandina rosea, which was imported into Hawaii in 1958 to control the giant African snail, Achatina fulica. Unfortunately, Euglandina did not restrict its predatory activity to the African snail but also attacked rare native Hawaiian snails (67), apparently driving some to extinction. Between 1977 and 1987 E. rosea pushed the endemic tree snails of the island of Moorea to extinction (68). There is another extinction crisis in the making with the movement of Cactoblastis cactorum from its point of introduction for the control of Opuntia in the Caribbean, to a trajectory that will bring it to a center of diversity of Opuntia in Mexico (H. G. Zimmermann, personal communication).
There have been attempts to give us some sense of the ultimate result of the mixing of the biota of world. Brown (69) has calculated, based on species-area relationships, the worst-case scenario for the impact of free exchange of biotic material across former biogeographic barriers. This was done assuming the Earth's land surface was contained into one supercontinent but that the current climates and geological features were maintained. With these assumptions there would be massive decrease in species, amounting to 65.7% for land mammals, 47.6% for land birds, 35% for butterflies, and 70.5% for angiosperms. McKinney (70) has made similar calculations for the ocean and concludes that there would be a reduction of about 58% in the current diversity. McKinney points out, however, that for the theory to be fulfilled there would have to be unfiltered faunal exchange around the world and the lack of physical variability. McKinney notes that it is because these conditions are not fulfilled that we have not seen extinctions in relation to the Suez Canal exchanges.
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