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Biological Conservation 78 (1996) 3-9
Copyright © 1996 Elsevier Science Limited
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A N A G E N D A FOR I N V A S I O N BIOLOGY
Geerat J. Vermeij
Department of Geology and Centerfor Population Biology, University of California. Davis, CA 95616, USA
Abstract
Here I advocate a comparative and systematic approach
in which invasion (the extension of species ranges to
areas not previously occupied by that species) is studied
from the perspective of individual species as well as of
the regions and biotas that export and receive invaders.
In order to go beyond the particulars of invasion, it is
important to ask: (1) how invaders differ from noninvaders in the arrival, establishment, and integration
phases of invasion; (2) how donor regions or communities that have produced many successful invaders differ
from those in which few resident species have been able
to extend their ranges; (3) how recipient ecosystems with
many successfully established invaders differ from those
in which few species have taken hold," and (4) how invasion affects evolution not only of the invader itself, but of
species in the recipient community with which the invader
interacts. Copyright © 1996 Elsevier Science Limited
Keywords: invasion, biotic
introduced species, dispersal.
interchange,
extinction,
INTRODUCTION
Few biological phenomena match species invasion in
grabbing headlines and engaging the attention of the
public. Zebra mussels Dreissena polymorpha from
Europe interfere with shipping and with electricitygenerating plants around the American Great Lakes,
ctenophores from the western Atlantic threaten fisheries in the Black Sea; rabbits Oryctolagus cuniculus
from Europe and cactuses from North America destroy
the Australian bush; Asian chestnut blight brings the
American chestnut to the brink of extinction and forever alters forest structure in the eastern United States;
the Floridian predatory snail Euglandina rosea devastates the land-snail fauna of the Society Islands but not
the African land snail Achatina fulica, for whose control the introduction was intended; South American fire
ants devour native ants and affect the gardening practices of home-owners in the southern United States;
and rinderpest introduced with cattle from Asia causes
mass mortality among African ungulates, including the
extinction of one species. A European visitor to New
Zealand feels at home as he walks through pastures of
European grasses and weeds and listens to European
blackbirds, finches, and larks singing overhead. Invasions and introductions of species by human agency
have had profound economic and cultural consequences.
Scientists have responded to such crises largely in a
piecemeal fashion. Particular invasions and invaders
have been studied in great detail, and a few preliminary
attempts at inductive generalization have been made
(Elton, 1958; Ehrlich, 1986; Brown, 1989; Pimm, 1989;
Lodge, 1993); but on the whole, the particulars of individual cases have obscured broader patterns, and
doubts have been raised about the existence of general
patterns or about the wisdom of looking for them
(Ehrlich, 1986). Invasion biology has had surprisingly
little guidance from, or made contributions to, the larger
disciplines of ecology, biogeography, and evolutionary
biology. Because of the emphasis on human-introduced
species, the importance of invasion throughout the history
of life has generally not been appreciated, and few
researchers have taken advantage of a systematic
approach to the subject.
Important and illuminating as past efforts have been,
I believe the time has come to emphasize a more systematic approach, one in which basic questions about
the process, participants, and outcomes of invasion are
asked in a broad comparative context. My aim here is
to outline an agenda for invasion biology. This may
strike many readers as an overly ambitious or even
arrogant enterprise, but I believe that a framework of
answerable scientific questions will benefit future
research as well as the development of policies on the
transport and introduction of species. I shall argue that
invasion must be probed at each of several spatial and
temporal scales, that the stages of invasion from arrival
to integration must be recognized as distinct and
treated with different questions, and that a comparative
and systematic rather than an anecdotal approach is
needed in order to achieve objectivity and perspective.
D E F I N I T I O N S , SCALES AND S T A G E S
Correspondence to: G. J. Vermeij
Tel.: 916 752 2234; Fax: 916 752 0951;
e-mail: [email protected]
Because clear thought is contingent on the precise and
consistent use of terms, I begin with a few key definitions.
4
G.J. Vermeij
By invasion I mean the geographical expansion of a
species into an area not previously occupied by that species.
Invasions may occur as the result of climatic and tectonic
changes as well as through introduction by humans.
Immigrants are species or populations in an area not
occupied by their immediate ancestors. The words
emigrant and emigration should probably be avoided,
because they imply that the population or species as a
whole moved from one place to another. Such emigration can occur in principle, but I do not know of documented cases. Good candidates for emigrants are
species whose latitudinal limits shift northward or
southward as the climate changes. Usually, however,
such geographical shifts do not move entire populations. Only individuals near the limits of range move or
die. The word migration should be reserved for temporary movements of individuals or populations.
The term biotic interchange refers to the expansion
of species' ranges from one biota to another across a
present or former barrier. The barrier is crossed by
means of dispersal. I choose the word interchange
because, at least in principle, dispersal between two
biotas can proceed in either direction.
Invaders come from a donor biota or region, and
enter a recipient one. The species pool or donor biota
from which invaders are drawn can be characterized at
any of several spatial scales ranging from a local community to an entire biogeographical province. The
same applies to the species assemblage or the region
that is being invaded. Estimates of the relative importance of invasion (number of invaders divided by the
number of species in the donor or recipient assemblage)
depends critically on the spatial scale being used.
Rocky-shore assemblages of molluscs in New England,
for example, contain a very high proportion (about
80°/,,) of Pacific-derived invaders, whereas the cool-temperate northwestern Atlantic molluscan fauna as a whole
contains only about 33% such invaders (Vermeij, 1991b).
Similar contrasts exist for California grasslands as compared to the California flora as a whole (Baker, 1989).
Given the large differences that exist in the extent to
which particular habitats or communities in a region
have been invaded, most comparisons among donor
biotas or among recipient ones should probably be
done within carefully specified habitat and geographical limits. Thus, the differences in the ratio of introduced to total number of species observed among
countries by Heywood (1989) are done at a scale that
may often be too coarse. Even small countries typically
contain many different habitats as well as biogeographically distinct subregions, each of which may have a
unique invasion history and be differently susceptible
to invasion. I suspect that generalizations and patterns
will be more discernible if ecologically and biogeographically homogeneous assemblages are compared.
In particular, I think it is important to treat agricultural, aquacultural, urban, and other human-altered
communities differently and separately from communi-
ties that have been less overtly affected by humans.
The process of invasion can be divided somewhat
arbitrarily into three successive stages: arrival, establishment, and integration. By arrival I mean the dispersal of individuals to the recipient region. This may
occur naturally or with the aid of humans. Establishment implies that the new population can sustain itself
through local reproduction and recruitment, which thus
augments or replaces dispersal from the donor region
as a means for the invading population's persistence.
Integration occurs when, as the invading species forges
ecological links with other species in the recipient
region, evolution occurs, reflecting the changed selective regime in the recipient community. For each of
these stages, we can ask how species that succeed differ
from those that fail, and how success and failure are
influenced by prior circumstances in the recipient biota,
the donor biota, and the invading species itself.
The study of the arrival phase is the study of opportunity. One approach is to assess the dispersibility and
routes of transport of known invaders, but a more
informative line of attack is to ask what makes some
species in a given donor biota good dispersers while
others perform poorly in this respect. In other words,
what kind of selectivity exists among potential invaders
in a donor biota for the ability to place recruits or
propagules in a recipient region? The answer requires
that we: (1) identify a donor region (there may be more
than one); (2) identify modes and routes of transport;
and (3) assess the abilities of species in the donor region
to take advantage of these routes and means of transport. For cases of invasion unaided by humans, potential
means of transport include ocean currents, winds, hitchhiking on the bodies of other organisms, and active
locomotion (walking, swimming, or flying) across barriers that have disappeared or are no longer effective.
Human-aided invasion is made possible through the
accidental or deliberate transport of crops, game animals,
ballast water, soil, agricultural control agents, wool, and
living food organisms; and by the construction of floating or permanent islands such as ships, planes, buoys,
and oil derricks, which can serve as homes or stepping
stones for recruits. The emphasis in the study of the
arrival phase must therefore be placed on the attributes
of donor biotas and regions, potential dispersers, and
the routes, means, and conditions of transport.
The second stage of invasion is establishment. By
establishment I mean the persistence of an immigrant
population by means of local reproduction and recruitment. This may be accompanied by a continued
spreading of the population in the recipient region. In
practice it may often be difficult to infer whether an
invader has become established. Some species may
invade so frequently that they appear to maintain local
populations when in fact the populations are continually being recharged by dispersers from elsewhere.
The study of the establishment phase entails comparisons among arrivals as well as analyses of conditions
An agenda for invasion biology
in the recipient communities. We can ask how many of
the arriving species actually become established; how
successfully established species differ from other arrivals
in fecundity, environmental tolerance, competitive ability,
vulnerability to predation and disease, and flexibility;
and how communities in which a given species becomes
established differ from those in which that same species
fails to become self-sustaining. Aspects of the recipient
biota that are likely to be important include the presence of potential hosts, competitors, predators, and
parasites, the climate as compared to that of the donor
region, and the biological history of the biota in the
region being invaded. Other less commonly considered
factors may be equally important. For example, are
communities with low primary productivity more likely
to resist invasion than are those with high productivity
(Brown, 1989)? If low productivity or restricted access
to resources implies slow growth rates and limited
fecundity, establishment may be difficult to achieve.
More important, environments where the rate of production is low may be generally inappropriate for the
many invading species whose success hinges on high
fecundity and high per-capita growth rates, characteristics that require easy access to plentiful resources.
By distinguishing between arrival and establishment,
we acknowledge that the factors limiting populations
change during the course of an invasion. During the
arrival phase, the size of the invading population may
depend largely on the supply of recruits, as well as on the
effectiveness of species native to the recipient biota to
prevent establishment. Local recruitment of the invading
populations implies that the incumbent effect, or resistance of the recipient biota to invasion, has broken down.
Another important implication of this distinction is
that the attributes making species good dispersers may
not be the same as those that make species good at
establishing a beachhead in the recipient biota. In fact,
the species most apt to become established may be
effective dispersers as well as defensively, competitively,
and reproductively superior species.
The final stage of invasion can be described as integration, a process in which the species in the recipient
biota and the invader respond to each other ecologically and evolutionarily. Many invaders affect abundances of species in the recipient community more or
less profoundly, and in so doing modify the agencies of
selection on those species. Moreover, evidence from
marine as well as terrestrial invasions implies that
invaders quickly establish interactions with new hosts
or parasites, which may impose new population controls and selective regimes on the invaders themselves.
In the light of these outcomes, conservation biologists
must ask whether any of the ecological changes
wrought by invaders are reversible, and whether the
elimination of invading species necessarily brings the
recipient biota back to the conditions prevailing before
invasion. It is also important to ask to what extent the
establishment and integration of one invader affect
5
invasibility by later arrivals (see Drake, 1991). Finally,
does integration of invaders result in the local or global
extinction of native species?
In the remaining sections of this paper, I wish to
return to some of these questions in detail and suggest
avenues of attack that at the same time set the field of
invasion biology in the broader context it deserves.
Throughout, my emphasis will be on the comparative
and systematic approach, which builds on the detailed
piecemeal studies of individual cases of invasion.
Which attributes enable species to spread from a donor
to a recipient region?
Stated in explicitly comparative terms, how do species
that invade differ from those in the donor region that
do not? Does the answer depend on the route or means
of transport, or is invasiveness predictable from a
knowledge of species and populations alone?
Past efforts to characterize invasibility have typically
concentrated on the attributes of weedy (or opportunistic) species (see e.g. Lewontin, 1965). Such studies usually did not compare weeds with non-weeds, or even
weeds with other species whose characteristics appear
similar but whose populations have not spread. It may
well be that invaders as a group do not differ consistently from other species in the donor biota, but without careful comparative studies we cannot make any
pronouncements on the subject.
One potentially promising approach derives from
historical biogeography. For groups with a good fossil
record and whose ancestor-descendant relationships
are well understood, it is possible to identify species
with contrasting histories. During the last Few million
years, some species and higher-level clades have
expanded their ranges through dispersal, whereas others have undergone range contractions, often fragmenting into several divergent relict populations. What
circumstances and attributes enable some species to
expand and other co-occurring species to contract in range?
In its broadest form, this question can be rephrased
as Follows. What factors prevent populations from
spreading beyond their geographical limits? One answer
might be that physiological tolerance is evolutionarily
conservative. If so, many populations would have
ranges whose limits are set by physical circumstances
that prevent reproduction or survival. It is striking, For
example, that very few tropical marine invertebrate lineages have given rise to species or populations capable
of withstanding cold waters at higher latitudes. On the
other hand, species may be prevented from extending
their range by the presence of competitors, predators,
or disease organisms, or the absence of critical host,
food, or symbiotic species. The high frequency of host
switching by parasites (Thompson, 1994) implies that
some of these biological barriers may not be difficult to
breach, but the whole subject requires that we move
beyond the anecdotal stage in which it currently
resides.
6
G.J. Vermeij
One characteristic that invaders might have in common is flexibility or what some researchers have called
plasticity. Individuals in novel situations are able to
adjust physiologically or morphologically in an apparently adaptive manner. This idea is attractive, but it
has never been rigorously evaluated. An obvious test
would involve experiments in which a representative
range of invading species and non-invading species
from a given donor community is exposed to a common novel environment, such as a new host for parasites, a new set of competitors or predators, or a
different climate.
Another fruitful avenue is to rank species from a
given donor biota according to the frequency of success
with which that species has invaded a given set of
recipient regions to which it was introduced. Simberloff
and Boecklen (1991) pointed out that most bird species
that had become successfully established on one
Hawaiian island also became established on all the
other islands to which they had been introduced.
Species that have been introduced but have not become
self-sustaining could be compared with those that have
a high rate of success.
Are there attributes of donor biotas or regions that make
the latter good sources for invaders?
This question can be phrased in explicitly comparative
terms. How do donor regions that export many species
differ from those from which few species spread?
A promising approach to this question is to take
advantage of the fact that, among two or more biotas
that are exchanging species, one typically exports many
more species than do the others. Traditionally, this
asymmetry has been ascribed to differences in the competitive or predational environment in the donor biota.
According to this view, species are more apt to spread
if they have evolved in a biota in which persistence of
populations requires a high degree of specialization to
competition, predation, and disease (see e.g. Darwin,
1872). The idea has never been properly evaluated in
part because other potential explanations have not
been seriously considered. For example, routes may
favor movement in one direction over that in the other
between two biotas. The predominance of weeds of
Mediterranean origin in many of the world's grasslands
(Baker, 1989) could be the consequence of exceptional
opportunity, in this case related to routes of commerce,
rather than of the unusual reproductive, defensive, or
competitive vigor of species native to the lands of
southern Europe, western Asia, or North Africa.
Grassland species from North or South America and
Australia might have been less likely to be shipped
back to the Mediterranean countries, instead of being
less vigorous in transit or in establishing sustainable
populations in the presence of Mediterranean natives.
Another explanation that should be ruled out before
others are considered is the null hypothesis that the
asymmetry arises solely because of differences in the
donor biotas' species pools. Invasion from one biota to
another may be rare because the first biota has few
species to offer as potential invaders. Analyses of biotic
interchange generally imply that the null hypothesis
does not provide a full explanation for the observed
levels of asymmetry (Vermeij, 1991a). Typically, the
asymmetry is much more marked than would be
expected on the basis of differences in diversity between
two biotas that are exchanging species.
It is, of course, also possible that asymmetry of invasion reflects differences in the recipient rather than in
the donor biotas. This possibility sets the stage for the
next question.
Are there attributes of recipient biotas or regions that make
the latter particularly susceptible to invasion?
Again, this question can be posed in comparative
terms. How do assemblages rich in immigrants differ
from those in which few or no foreign species have
become established?
The difficulties that arise in comparisons among
donor regions apply with equal force in comparisons
among recipient ones. There may be few immigrants
because barriers to dispersal from elsewhere are so
effective that opportunity for invasion has been limited.
On the other hand, recipient biotas may be difficult to
invade because their component species singly or collectively can resist newcomers competitively. Related to
the latter explanation is the hypothesis that recipient
biotas become invasible after diversity has been
reduced through extinction or overexploitation (Vermeij, 1991a). Quantitative studies of biotic interchange
in marine as well as terrestrial communities from the
last 20 million years strongly support the hypothesis
that prior extinction makes recipient communities
highly susceptible to invasion (Vermeij, 1991a).
An experimental approach to this problem is potentially fruitful but has, as far as I know, not yet been
explored. Replicate recipient communities differing in
species composition (number of species, presence of
predators, intensity of competition, incidence of parasitism, and presence of potential hosts) could be
exposed to a given mix of invading species. Those
donor communities that are poor in species or that are
missing key incumbents should be most easily and most
extensively invaded.
How often, and under what circumstances, does invasion
resolt in the extinction of native species in the recipient
region?
The evidence so far points to the conclusion that
invaders often cause extinction on oceanic islands and
in lakes, but rarely in the sea or on large land masses.
Probably much more commonly, invaders restrict the
ecological range of native species. Wilson (1961) proposed that, as invaders colonize terrestrial communities
adjacent to shores, native species are progressively
restricted to forests in the interior, especially in mon-
An agenda for invasion biology
tane habitats. Does this pattern hold up under scrutiny,
and are there other generalities to be uncovered about
the way that native species are ecologically restricted by
invading species?
Here again, an experimental approach with replicate
donor communities would seem to be promising. In this
instance, recipient communities would differ in total
numbers of individual organisms or in absolute size as
well as in composition.
Are there rules governing the order of invasion of species
and the way in which invaders are ecologically and
evolutionarily integrated into recipient communities?
Diamond (1975) used a comparative approach to
suggest that bird communities in New Guinea and surrounding islands differ in predictable ways according to
the number of co-occurring species; and that enrichment and impoverishment (the addition or subtraction
of species) occur with relatively precise assembly rules
rather than by random addition or loss of species. In a
similar vein, studies of bird introductions to Hawaii
and Tahiti have suggested that only those species
differing sufficiently from species already present are
likely to become established (Moulton & Pimm, 1986a,
b; Lockwood et al., 1993). The view of community
organization encouraged by such studies is that species
number and composition are relatively deterministic
aspects of communities, and that the success or failure
of invaders to establish themselves and to integrate into
communities can be predicted a priori if enough is
known about the invaders and the recipient biota. This
interpretation is enforced by Wilson's (1961) taxoncycle hypothesis, according to which terrestrial invaders
arrive in relatively disturbed lowland areas and progressively displace natives to more inland environments. The main arguments for such an interpretation
have come from studies of ants and birds.
This deterministic view of communities and invasions
is by no means universally accepted, even by those who
work with birds. Wiens (1989), for example, sees little
evidence for deterministic assembly rules, and Simberloff
and Boecklen (1991) have offered alternative plausible
explanations for the observed pattern of bird invasion
in Hawaii. These authors are generally convinced that
invasion depends more on the characteristics of individual invaders than on the attributes of recipient
communities (see also Simberloff, 1981, 1986).
Those who work on invasions in the sea or on terrestrial invasions by plants and low-energy organisms generally see the world as indeterministic. Introduced
insects quickly adapted to new hosts, even within periods as short as 10 years (Strong et al., 1977; Singer et
al., 1993; Thompson, 1994). Similarly, introduced hosts
rapidly acquire new parasites and predators. Large
differences in diversity among communities (Vermeij,
1978, 1989, 1991b; Ricklefs & Latham, 1992, 1993;
Cornell & Hawkins, 1993) imply that recipient communities of plants, insects, or marine invertebrates are
more forgiving of newcomers than are communities of
high-energy birds, mammals, or perhaps ants. Community
composition of low-energy organisms may generally be
less predictable, less orderly, and less integrated than in
high-energy forms.
Invasion biology has much to offer ecologists and
evolutionary biologists in probing the integrity and
orderliness of recipient biotas. For example, we can ask
whether invaders tend to usurp the ecological roles of
native forms or instead to use resources and fashion
ways of life not previously exploited in the recipient
community. Brown (1989) believes that most invading
warm-blooded animals take ecological roles not occupied by native species; but on islands, in lakes, and
among land plants, such unique roles of invaders may
be less common.
What role does invasion play in long-term evolution?
Invasion implies change. Not only is it often triggered
by change - - the elimination of a barrier to dispersal,
the establishment of means by which dispersal can
occur, for example - - but in modifications in the recipient biota. Invasions provide opportunities and challenges. The invaders as well as the native species in the
recipient communities affected by invaders must cope
with and respond to the changing selective regime, or
suffer the consequences of restriction and even extinction. In the absence of invasions, communities and the
species and interactions comprising them may stagnate,
especially if the economic base of energy and nutrients
remains fixed. In other words, adaptation and accommodation by organisms to each other may quickly
reach a stalemate in the absence of immigration or an
economic stimulus (Vermeij, 1987, 1995). If invasions
occur on a small scale as the result of minor shifts in
the spatial limits of species, the resulting evolutionary
realignments may be modest; but 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. These possibilities remain highly speculative
at present, but they point to the potentially crucial role
that invasions and invaders have played in stimulating
evolution. Given the large number of species now being
transported by humans, this aspect of invasion biology
cannot be ignored.
EXPERIMENTS, HISTORICAL RECONSTRUCTION
AND MODELING
It should be clear that invasion biology will profit from
a diversity of methodological approaches, and that no
single style of inquiry should be seen as superior to all
others. Experiments have the virtue of enabling the
investigator to vary aspects of species or communities
in a systematic manner, but they have the disadvantage
of typically being run on small spatial and temporal
scales. Moreover, there is a deep-seated tendency
8
G.J. Vermeij
among experimentalists to view potential outcomes as
mutually exclusive alternatives rather than as potentially complementary. I think most biological systems
are sufficiently complex that single explanations or
hypotheses will rarely suffice to account for the patterns we see. Experiments should be used not to
exclude explanations, but to rank them or to explore
their interdependence.
It has been fashionable in ecology to regard field
experiments as inherently superior to experiments carried out in the laboratory. Although field experiments
clearly have their place, I believe that experiments with
replicated microcosms are more informative as well as
ethically more acceptable, especially in studies of invasion. Invaders can have unforeseen and often destructive effects on recipient communities. The field of
invasion biology should therefore adopt the standard
that work with non-native species or populations be
carried out under strictly controlled laboratory conditions from which accidental release of individuals,
including gametes and dispersal stages, is impossible.
Experiments become impracticable at larger temporal and spatial scales. In such situations, the reconstructive techniques familiar to historians, including
paleobiologists, provide the only empirical means of
studying invasions. This comparative approach incorporates observations of species and communities before
and after invasion, as well as the identification of
invading species by means of cladistic methods in
which geographical shifts in range can be traced back
through time. Armed with such knowledge, we can
contrast recipient communities rich in invaders with
those that have received few immigrants, or compare
donor biotas that have exported many species with
those that have provided few invaders. For such longterm studies, groups and community types with a good
fossil record clearly show the greatest promise.
Although my emphasis throughout this essay has
been empirical, there is much room for theoretical
analyses as well. Almost all the questions I have posed
can serve as guides for models that incorporate principles of population diffusion, population dynamics,
community stability, probabilities of extinction, habitat
size, per-capita energetics, and the like. Models, however, are a means to an end, not an end in themselves.
If the science of invasion biology is to have an impact
on policies, it must have at its command empirical data
rather than suppositions and extrapolations.
The aim of invasion biology should be to seek generalizations and principles, but we must be prepared to
accept and to explain a diversity of responses and consequences. For example, whereas invasion by large
endothermic mammals may result in extinctions in
a recipient biota, plants and smaller animals may
not have this consequence. Prior impoverishment or
exploitation may provide better opportunities for subsequent invasion in many marine and terrestrial communities, but may have little effect in some lakes such
as the African rift lakes. Many of our conceptions of
invasion have been based on species introductions to
oceanic islands. There is currently very little understanding of long-term plant invasions, and we have
scarcely begun to compare taxonomic groups or community types for their ability to invade, susceptibility to
invasion, and propensity to export invaders. From
these considerations, it should be obvious that we must
continue to document individual cases of invasion.
Because invasion biology, like extinction biology,
exists in order to study a phenomenon that is justifiably
perceived to be important in the human-dominated
biosphere, there is a tendency to view the field as somewhat divorced from the larger enterprises of ecology
and evolutionary biology. The creation of conservation
biology as a discipline and as a field in which students
can take advanced degrees is contributing to this isolation. I believe that the phenomenon of invasion is of
fundamental importance, and that its study will contribute to an overall understanding of how species
interact with each other. As I see it, the success of invasion biology will depend on our insistence that it be
incorporated as an integral part of the larger science of
biology. |n the long run, our understanding of invasion
will be greatly enriched by broad principles, and it will
in turn contribute to the further development of these
ideas.
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An agenda for invasion biology
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