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OCR for page 58
Colloquium
The evolutionary impact of invasive species
H. A. Mooney* and E. E. Cleland
Department of Biological Sciences, Stanford University, Stanford, CA 94305-5020
Since the Age of Exploration began, there has been a drastic
breaching of biogeographic barriers that previously had isolated
the continental biotas for millions of years. We explore the
nature of these recent biotic exchanges and their consequences
on evolutionary processes. The direct evidence of evolutionary
consequences of the biotic rearrangements is of variable quality,
but the results of trajectories are becoming clear as the number
of studies increases. There are examples of invasive species
altering the evolutionary pathway of native species by compet-
itive exclusion, niche displacement, hybridization, introgression,
predation, and ultimately extinction. Invaders themselves
evolve in response to their interactions with natives, as well as
in response to the new abiotic environment. Flexibility in be-
havior, and mutualistic interactions, can aid in the success of
invaders in their new environment.
The Nature of the Problem. Those of us alive today are witness-
ing the consequences a number of truly grand, but un-
planned, biological experiments. They are the result of the
activities of a massive human population that is still growing and
increasing its impact on the Earth. Because there are no controls
on these experiments, as such, we must look to biological
patterns through time for perspective on the consequences of the
mixing of biotas. This is a challenge because environments of the
past also changed, sometimes abruptly.
These historical fluctuations in climate and biota of the past
have led some to say that nothing new is happening that has not
already happened before. The response to this proposition is yes,
but the rate of change in the composition of the atmosphere today
exceeds anything of the past, as will the consequent rate of
climate change. This is also true to a large degree in the extent
of migration of species among continents. Before the Age of
Exploration, dispersal of organisms across these great biogeo-
graphic barriers was a low-probability event; however, today this
is routine. In this paper we briefly summarize the consequences
of the massive movement of organisms across these barriers in
terms of the course of future evolution.
We start this essay with two quotes providing perspectives on
the problem. One is from the pioneering work of Charles Elton
(1), who stated, "We must make no mistake: we are seeing one
of the great historical convulsions in the world's fauna and
flora." Elton certainly had no doubts of the magnitude of the
invasive species issue. More recently, Geerat Vermeij (2) re-
marked specifically about the evolutionary consequences of this
convulsion, ". . . if newcomers arrive from far away as the result
of large-scale alterations in geography or climate, the change in
selective regime and the evolutionary responses to this change
could be dramatic." We examine here some of the evidence for
this potentially dramatic scenario.
The Changing Evolutionary Landscape. It is commonly acknowl-
edged that the abiotic environment is being greatly altered
because of massive land-use alteration and emerging climate
change (3, 4~. However, an equally drastic alteration is occurring
in the composition of biotic communities. The kinds of physical
5446-5451 1 PNAS 1 May 8, 2001 1 vol. 98 1 no. 10
and biotic environments that exist now are quite different from
those that have existed in recent geological times.
International commerce has facilitated the movement of
species; this is true globally and across taxonomic groups.
Ironically, this has increased species richness in many places
where new species are introduced. The actual numbers of
individuals and species being transported across biogeographical
barriers every day is presumably enormous. However, only a
small fraction of those transported species become established,
and of these generally only about 1% become pests (5~. Over
time however, these additions have become substantial. There
are now as many alien established plant species in New Zealand
as there are native species. Many countries have 20% or more
alien species in their floras (6~. There are few geographic
generalities to these trends; the strongest is that islands, in
particular, have been the recipients of the largest proportional
numbers of invaders. Biotic homogenization within continents is
equally as striking as mixing among oceans. As one example,
Rahel (7) notes that in the United States pairs of states on
average now share 15 more species than they did before Euro-
pean settlement. The states of Arizona and Montana, which
previously had no fish species in common, now share 33 species
in their faunas.
Mack (8) estimates that over the last 500 years, invasive species
have come to dominate 3% of the Earth's ice-free surface. Vast
land or waterscapes, in certain regions, are completely domi-
nated by alien species, such as the star thistle Centaurea solstitialis
in the rangelands of California, cheatgrass (Bromus tectorum) in
the intermountain regions of the western United States, and
water hyacinth (Eichornia crassipes) in many tropical lakes and
rivers.
The Rates of Exchange. As the volume of global trade increases,
one would expect the rate of establishment of alien species to
increase also; data support this prediction. Cohen and Carlton
(9) noted that the rate of invasion into San Francisco Bay has
increased from approximately one new invader per year in the
period of 1851-1960, to more than three new invaders per year
in the period of 1961-l99S. In the United States the numbers of
fish introductions, either from foreign sources or across water-
shed boundaries, has increased dramatically. In the period
between 1850 and 1900, 67 species were introduced, between
1901 and 1950, 140 species, and between 1951 and 1996, 488
species (ref. 10 and the web site referred to therein).
In addition to the greater number of species crossing borders
there is also a buildup in the invasive potential of those nonnative
species already established in a region, as immigration increases
their population sizes. "Introduced species" may stay at a fairly
low population size for years and then explode at some later
date- the so-called lag effect. This lag effect may simply be the
result of the normal increase in size and distribution of a
population. For instance, Bromus tectorum was introduced to
This paper was presented at the Nationai Academy of Sciences colloquium, "The Future of
Evolution," heic] March ~ 6-20, 2000, atthe Arnoid and Mabei Beckman Center in irvine, CA.
*To whom reprint requests should be acldressed. E-maii: hmooney~jasper.stanforci.edu.
v~Nw.pnas.org/cgi/doi/1 0.1 073/pnas.091093398
OCR for page 59
intermountain western North America around 1890, and re-
mained in localized populations for 20 years. This lag phase was
followed by 20 years of logistic range expansion; by 1930 B.
tectorum was dominant over 200,000 km2 (11~.
Crooks and Soule (12) note that in addition to the normal
population growth lag phase there are other mechanisms that
can keep newly introduced species at low levels for decades
before they become invasive. These include environmental
change, both biotic and abiotic, after establishment and genetic
changes to the founder populations that enable subsequent
spread. Evidence for the former cases is abundant but scarce for
the latter.
In summary, the biotic background for evolution has been
changing since the Age of Exploration, and at an ever-
accelerating pace because of accumulative effects of the num-
bers of species involved, the increased rate of exchange, and the
lag debts that communities have amassed.
Looking to the Past. There are examples from the past of sudden
mixing of biotas that were formerly isolated; one of these is fairly
recent and instructive. The biota from the Red Sea and the
~ a. . · ~
The Direct Evolutionary Consequences of Mixing
Evolutionary Adjustments of Invaders and of the Invaded. We turn to
contemporary studies to give us some indication of the evolu-
tionary impact of invasive species. Recent studies have shown
that invaders can rapidly adapt to the new environments in which
they find themselves. Huey et al. (18) demonstrated how an
introduction of a new fruit fly into the west coast of North
America resulted in the evolution, in only 20 years, of an
apparently adaptive cline related to wing size, throughout the
vast new latitudinal range extending from southern California to
British Columbia. The cline that developed in North American
female flies was similar to that found in the European native
populations. Interestingly, the developmental basis for the cline
of wing size was different in Europe than for the invader in North
America, although the functional result was the same, providing
additional evidence for the adaptive advantage of this set of
traits.
Drosphilia subobscura were introduced into North America in
1982; shortly thereafter Ayala et al. (19) described the invasion
as "a grand experiment in evolution." This was certainly an
accurate prediction given the results of Huey et al 10 years later
Mediterranean Pea were reconnected alter a separation OI ' . . . .
. . , ~ ~ . . A ~ and only 20 Years after the be~1nnma of the Invasion event.
millions of years by the construction ot t ae Suez canal in Ashy v
~ ' . ~ ,. , There are other documented instances of an invading species
The pathway for movement between tnese water oodles nas
. . . adapting to its new environment. For example Johnston and
changed since 1869 because of the varying salinity of a lake in the ~ ~ . '
.. ~ . Selander 20) described the evolution of annarently adaptive
canal system; that IS, there nas not oeen totally tree exchange . . , ^^ A
. . . . ~ . ~ clones in hod r size and feather color in English sparrows that
Wlt lout carriers. . Monet. :~e ess over ~~t species ~` new genera . . ~ ~
^~ r ·~ I. ~ ~ ~ · ~ .¢ '~. ~ ~ were introduced Into North America in 1852 and that subse-
anct '~ new Amides nave moved into tne ~v~ealterranean Yea
~ ~ . auentlv established a tar e Cobra Cal ran e Further Cod
rom t le Alec Yea, yet t lere has on y neen one c ~ocumente 1 , . ~ ~ , ,
~ xtinct on 13 I. These inv asion have primarily been accommo and Overton (21) described the reduction in distance of dispers-
dated by niche displacements through competitive interactions ability for w1nd-dlspersed seeds of Invasive species onto islands
among the congeners (14~. Many of the native fish in the In Just a few generationsinsmallisolatedpopulations.similarly,
Mediterranean have maintained their preinvasion feeding habits Losos et al (22) demonstrated that within 10-14 years species of
but have been displaced in depth by the Red sea invaders, which lizards int:ro(iuced onto a series of island in the Caribbean
prefer the shallower, warmer, waters at the surface ¢15~. Two showed adaptive morphological adjustments.
nocturnally foraging fish (Sargocentron rubrum and Pmpheris There are also examples of relatively rapid, nonadaptive,
vanicolensis) have shown large population increases after invad- genetic change of Invaders as seen in house mice introduced into
ing the Mediterranean from the Red Sea. Night foraging is an Madeira; localized differentiation of chromosomal races is the
uncommon strategy among native Mediterranean fish (only one result of genetic drift in isolated valleys (23~. Similarly, genetic
feeds at night), hence these migrants were probably successful drift has been responsible for geographic genetic patterns found
because this novel behavior allowed them to exploit resources in the introduced Bufo marinas in Australia (24~.
that the native fauna had not yet used.
There have been a number of spectacular population explo-
sions of the Red Sea immigrants through time, most of which
have eventually become reduced in size (14~. An exception is
Rhopilema nomadica, a large Red Sea jellyfish that experiences
population explosions and crashes each summer off the coast of
Israel.
The Great American Interchange of biota, the result of the
isthmian land bridge that formed during the Late Pliocene,
provides further information on the consequences of the mixing
of previously isolated biota. However, the course of temporal
resolution of the information available does not make it possible
to say with certainty whether the losses of biota that occurred
subsequent to the bridge were due to competition with new
arrivals, although it appears likely (16~. The effects of the
interchange apparently were asymmetrical, with the immigrants
from the south "insinuating" into the northern biota, whereas
the northern immigrants to the south may have caused extinc-
tions and undergone subsequent evolutionary radiation (17~.
What we lack is detailed information on the impacts of the
exchanges of biota on time frames greater than centuries but less
than millions of years. In the century time frames we have
processes that are still in a state of flux at the community level
and ones that have been that have not been studied in detail. In
the geological time frame, the poor temporal resolution does not
permit us to clearly understand the mechanisms that have led to
what we see in the fossil record.
Mooney and Cleland
Evolution in Response to an Invader. There are also examples of
rapid evolution in native species in response to an introduced
species. Carrot and Dingle (25) indicate that populations of the
soapberry bug (Jadera hematoloma) have evolved differing beak
lengths in response to the introduction of new invasive hosts,
within only 50 years time. Singer et al. (26) have shown rapid
evolution in the feeding preferences of the Euphydras butterfly
for the invading herb, Plantago lanceolata.
Zimmerman (27) documents an interesting case of evolution
in response to an introduced crop species. At least five species
of host-specific moths (Hedylepta) have evolved since the intro-
duction of banana into Hawaii ~1,000 years ago. These species
were threatened at the time of Zimmerman's study by parasitic
wasps and flies introduced for agricultural pest control.
There is a large literature on the evolution of weeds in
response to human activities, including agricultural practices.
Harlan (28) noted that some weeds have evolved to be crop
mimics. Not only are they are similar in their phonological
development and morphological appearance to the crops with
which they have co-evolved, but also their seeds have evolved a
similar appearance so they are not sorted and discarded during
harvesting. For example, the lentil mimic (Vicia saliva) has
evolved a seed shape and color comparable to the lentil (Lens
culinaris). This trait is under control of a single gene. Similarly,
Echinochloa crus-galli has evolved mimics to rice, Oryza saliva,
which are very difficult to distinguish from the crop.
PNAS I May 8, 2001 | volt. 98 | no. 10 | 5447
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OCR for page 60
De Wet and Harlan (29) surmised that many plant weeds
might have evolved from natural pioneer species associated with
continuous disturbance by humans. Some weeds that have
developed in association with agriculture have become crop
mimics as described above. Weeds are also derived from hybrid-
ization and introgression with crops as happened with Johnson
grass (Sorghum halepense) and the cultivated Sorghum bicolor.
Weeds have also evolved from abandoned domesticated plants.
Thus there are many cases that have been documented of the
evolutionary response to the new environment that an invasive
species may encounter as well as cases of the adaptive response
of organisms to a new invader.
Hybridization and Introgression. In addition to direct evolutionary
responses of organisms involved in invasions there are also very
important indirect effects through changes in the genetic struc-
ture of invasive species in relation to the new organisms that they
encounter. These major effects are related to hybridization and
introgression. Rhymer and Simberloff (30) have recently sum-
marized our knowledge in this area. There are many examples
extending over many different taxonomic groups, a few of which
are noted below. These authors conclude that in the case of
invasive species hybridization with native species can cause a loss
in fitness in the latter and even a threat of extinction. McMillan
and Wilcove (31) have documented that of 3 of 24 species listed
as Endangered in the United States and that subsequently went
extinct, 3 were the result of hybridization with alien species.
Birds Mallard ducks (Anas platyrhynchos) that have been
introduced into various regions of the world have had large
genetic effects. They have hybridized and reduced populations of
the New Zealand gray duck (Anas superciliosa superciliosa), the
Hawaiian duck (Anas wyvilliana), and the Florida mottled duck
(Anas fulvigula fulvigula) (30~.
Mammals Sitka deer (Cervus nippon) were introduced into
Great Britain from Japan over a hundred years ago. They have
hybridized with the native reed deer (Cervus elephaus) al-
though they are different in body size. It appears that the
genetic integrity of the native red deer is threatened in some
regions (32~.
Fish- There are a number of cases of hybridization and
subsequent introgression in fish, primarily game fish where there
are massive introductions of foreign stock. These include trout
in western and eastern United States as well as in Europe (33,
34~. It has been shown, however, that even small introductions
of nonnative species can have large impacts on the genetics of
native species through hybridization and introgression, as was
found for native pupfish in Texas (33~.
Plants Abbott (35) notes that of 2,834 species listed in the
New Flora of the British Isles 1,264 are aliens. There are 70
recognized hybrids between native and alien species and 21
between aliens. About half of these hybrids show some degree of
fertility.
There are many examples of the large populations of invading
species swamping small populations of native species by hybrid-
ization, but in certain cases small populations of an invader can
threaten native species that have much larger populations. This
is the case with the invading Spartina alterniflora into the San
Francisco Bay. It hybridizes with the native Spartina foliosa. The
invader has a higher pollen output, and greater male fitness, than
the native species and the hybrids and it occupy lower intertidal
habitats. In time introgression will threaten the native species
(36~. Conversely, small populations of rare species can be
threatened by hybridization in a number of ways (37), including
infertility of the hybrids.
Small populations on islands are particularly vulnerable to
extinction by hybridization because they are often less genetically
divergent than mainland species and have weak crossing barriers
as well as unspecialized pollinators. Levin et al. (37) describe a
5448 1 www.pnas.org/cgi/doi/10.1073/pnas.091093398
number of cases of extinction by hybridization on islands,
including the endemic shrub Cercocarpus traskoei with the
widespread Cercocarpus betuloides and the endangered Lotus
scoparius traskiae with the Lotus argophyllus ornithopus. They
specifically note that introductions may threaten rare species on
islands and give a number of examples from around the world,
including threats to the rare Arbutus canariensis and Senecio
teneriffae on the Canary Islands, Gossypium tomentosum on the
Hawaiian Islands, and Pinguicula vulgaris and Linaria vulgaris in
the British Islands. They posit that the threat of extinction of rare
species by hybridization is very high and that habitat disruption
and invasive species are increasing this threat to the degree that
conservation programs should strive to isolate rare species from
cross-compatible congeners.
The Origin of New Taxa Through Hybridization and Introgression.
While hybridization with invaders can be a threat to species
integrity, it can also be a source of new variation and the origin
of new species. Spartina alterniflora from the east coast of North
America was introduced into Southampton in shipping ballast in
the early 19th Century. It subsequently hybridized with the local
Spartina maritima, producing a sterile hybrid. The hybrid in turn
underwent chromosome doubling to produce the new fertile
species, Spartina anglica. Spartina anglica has become very
aggressive and occupies large areas of the coastline of the British
Isles while at the same time the original invader, Spartina
alterniflora, and the native Spartina maritima have maintained
limited distributions. The new polyploid evidently has charac-
teristics that enable it to occupy bare tidal flats that were not
available to the parents (38~. This event was apparently seren-
dipitous and has not been replicated artificially (39~.
In addition to the Spartina anglica there are other cases of
alloploids that have originated from hybridization of native and
invasive species. These include species of Tragopogon in North
America and Senecio in Great Britain (35~.
There are also examples of introgressive hybrids between
native and weedy species becoming stabilized to form new taxa.
The introduced Helianthus annuus hybridized with native Heli-
anthus debilia. The hybrids adapted to the new conditions it
encountered to form the subspecies Helianthus annuus texanus.
Abbott (35) cites six such cases of origins of new taxa.
The Indirect Evolutionary Consequences of Mixing
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 dif-
Mooney ancl C~eland
OCR for page 61
ferences 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 inva-
sive mudsnail, Ilyanassa obsolete, 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 fi~lgida 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 intro-
duced 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 behav-
ioral 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
Mooney and Cleland
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 nonmy-
corrhizal (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 rela-
tionships 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 dis-
perse the seeds of certain protects in South Africa. These native
ants have been displaced by Argentine ants that are not suc-
cessful 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 interac-
tions 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 contin-
uum of impacts, with the main concern and statistical informa-
tion available on the total global extinction of a species whereas,
of course, local extinctions and population reductions are im-
portant 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 abun-
PNAS | May 8, 2001 | vol. 98 | no. 10 | 5449
OCR for page 62
dance and has caused habitat displacement and reduced colo-
nization 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 envi-
ronment 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 Silica. 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 move-
ment 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 main-
tained. 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
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Concluding Remarks
In the course of this review we have discussed the mechanisms
by which invasive species evolve in response to their new biotic
and abiotic environments, and how invasive species have altered
the evolutionary trajectory of native species with which they
interact. While it is not surprising that an invasive species would
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The biota of the Earth is undergoing a dramatic transforma-
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PNAS 1 May 8, 2001 1 vol. 98 1 no. 10 1 5451
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native species
