Download Invasional meltdown 6 years later: important

Ecology Letters, (2006) 9: 912–919
doi: 10.1111/j.1461-0248.2006.00939.x
IDEA AND
PERSPECTIVE
Invasional meltdown 6 years later: important
phenomenon, unfortunate metaphor, or both?
Daniel Simberloff*
Department of Ecology and
Evolutionary Biology, University
of Tennessee, Knoxville, TN
37996, USA
*Correspondence: E-mail:
[email protected]
Abstract
Cases in which introduced species facilitate one another’s establishment, spread, and
impacts are increasingly noted, and several experimental studies have provided strong
evidence of a population-level impact. However, a full Ôinvasional meltdownÕ, in which
interspecific facilitation leads to an accelerating increase in the number of introduced
species and their impact, has yet to be conclusively demonstrated. The great majority of
suggested instances of Ôinvasional meltdownÕ remain simply plausible scenarios of longterm consequences based on short-term observations of facilitatory interactions between
individuals of two species. There is a particular dearth of proven instances in which two
invasive species each enhance the impact and/or probability of establishment and spread
of the other. By contrast, in many authenticated cases, at least one partner is aided. The
metaphor of meltdown focused attention on facilitation in invasion and has probably
helped inspire recent studies. As have other metaphors from invasion biology and other
sciences, ÔmeltdownÕ has struck a responsive chord with writers for the lay public; some
have stretched it well beyond its meaning as understood by invasion biologists. There is
no evidence that this hyperbole has impeded scientific understanding or caused loss of
scientific credibility.
Keywords
Facilitation, invasion, meltdown, metaphor, mycorrhizae, pollination.
Ecology Letters (2006) 9: 912–919
INTRODUCTION
In 1999, Simberloff & Von Holle (1999) introduced the
term Ôinvasional meltdownÕ to describe the process by which
non-indigenous species facilitate one another’s invasion in
various ways, increasing the likelihood of survival and/or
magnitude of impact and potentially leading to an accelerating increase in number of introduced species and their
impact. Among many examples, they described introduced
animals modifying habitat (say, by trampling) to favour
introduced over native plants, introduced plants modifying
habitat (for instance, by fixing nitrogen or changing fire
regime) to favour other introduced plants over natives, and
introduced ants tending introduced scale insects. Soon
thereafter, Richardson et al. (2000) wrote a similar review,
pointing out that pollinator, dispersal and especially
mycorrhizal mutualisms with other introduced species often
aid the survival and spread of nonindigenous plants. This
was not a new idea. Howarth (1985) noted several likely
2006 The Author, Journal compilation 2006 Blackwell Publishing Ltd
mutualisms between pairs of introduced insect species,
while Crosby (1986) depicted the repeated invasion of other
areas by Eurasian species as resulting from synergisms and
mutualisms abounding among the plants and animals
Europeans carried with them. However, the reviews in
1999 and 2000 were the first systematic attempts to suggest
a ubiquitous invasion phenomenon, and this fact plus the
meltdown metaphor attracted great attention, not only
among invasion and conservation biologists, but also in the
popular press as well.
This paper clarifies the concept of invasional meltdown,
including the relationship of facilitatory interactions to
meltdown, and it explores the evidence uncovered for
meltdown since the reviews by Simberloff & Von Holle
(1999) and Richardson et al. (2000). It also addresses the
utility of the metaphor in understanding the nature and
scope of the phenomenon.
The inspiration for our use of the term ÔmeltdownÕ
(Simberloff & Von Holle 1999) was the idea that, by aiding
Idea and Perspective
one another, introduced species could produce an autocatalytic process that would accelerate the replacement of
native communities, perhaps to the point of no return. We
explicitly suggested a range of degrees of facilitation that
could aid the process. Weakest would simply be facilitation
pure and simple: one species aiding another. This is an
interspecific interaction that does not itself constitute
meltdown (a community-level process) but that may
contribute to it. The helping species could be unaffected
or could be harmed, but insignificantly, while the recipient
of the aid is greatly helped, so the net effect is increased
invasion. Stronger would be mutual facilitation, in which
each species aids the other. This is still a population process
and would still not, by itself, constitute meltdown. Meltdown is a community-level phenomenon in which the net
effect of facilitations would lead to an increasing rate of
establishment of introduced species and/or an accelerating
impact. Another goal of Simberloff & Von Holle (1999), in
addition to simply remarking on the frequency and variety
of such facilitatory interactions, was to determine their
importance relative to that in which one introduced species
hindered another from establishing or generating an impact
– such a comparison would provide a clue about the
likelihood that meltdown would occur. The rub was that
almost no published examples of facilitation demonstrated
population impacts. The great majority depicted individuals
of one species helping individuals of another (and sometimes vice-versa), and a plausible suggestion that such aid
could have a population consequence. However, published
accounts of introduced species harming one another also
rarely demonstrated a population impact.
Similarly, Richardson et al. (2000) found scant convincing
evidence for crucial population impact of a facilitating
pollination relationship between introduced plants and
animals but voiced concern that increasing introduced
honeybee (Apis mellifera) populations would enhance spread
of introduced weeds. Subsequently, Barthell et al. (2001)
examined visitors to introduced yellow star thistle (Centaurea
solstitialis) in California and also the impact of bagging
inflorescences to prevent pollination by honeybees, the
most common visitor. Although they could not prove
honeybees exacerbated spread of the thistle, such an impact
seemed likely because of greatly depressed seed set at certain
sites when honeybees were excluded. There was no evidence
in this instance that the plant significantly facilitated the bee.
By contrast, Morales & Aizen (2002) found that honeybees
in a Patagonian temperate forest exploited almost exclusively flowers of a subset of herbaceous exotic plants and
suggested that these plants might be facilitating the bee
invasion. A review of impacts of introduced bees by
Goulson (2003) concluded that increased spread of certain
invasive plants is likely but found that, as yet, no long-term
studies of weed dynamics clinch the case.
Invasional meltdown 6 years later 913
Richardson et al. (2000) cited dispersal of seeds of a
European pine (normally wind-dispersed) in South Africa by
the North American squirrel Sciurus carolinensis as critical to
the success of the pine, although even in this case there is no
evidence of population benefit to the squirrel. The only
instances Richardson et al. (2000) adduced in which survival
and/or spread of introduced plants required introduced
mycorrhizal fungi were pines introduced to the southern
hemisphere. In these cases, the specificity of the ectomycorrhizae implies that fungal survival depends on the
presence of the introduced pine. Dı́ez (2005) later showed
that survival and spread of Australian eucalypts on the
Iberian Pensinsula was probably due to the unwitting
simultaneous introduction of Australian mycorrhizal fungi,
of which one species is now spreading into native shrubland
without the eucalypts. Among introduced nitrogen-fixers,
Richardson et al. (2000) could point to no species of legume
or actinorhizal plant whose success could be shown to rest
on an introduced symbiont, although in certain instances the
origin of a symbiont was unknown.
RECENT RESEARCH AND THE PERVASIVENESS
OF MELTDOWN
In spite of many suggested examples of invasional meltdown
since the two early reviews, there are still very few instances
in which a true population impact is demonstrated. However,
several recent studies, mostly experimental, imply population
impact of at least one partner in facilitation interactions
among introduced species, often with quite strong evidence.
As an example of the strongest version of invasional
meltdown, mutual facilitation of both interactants, O’Dowd
et al. (2003) and Abbott (2004) depict dramatically increased
populations of the long-present introduced yellow crazy ant
(Anoplolepis gracilipes) on Christmas Island upon introduction
of a scale insect and outbreaks of a native scale. The ants are
presumed to protect the scales from predators and parasites,
increasing their populations and, in turn, feed on the
honeydew the scales produce. The ants devastate populations of the native red crab (Gecarcoidea natalis), leading to
massive growth of ground cover plants, seeds and seedlings
of which had been removed by crabs. The honeydew fosters
growth of a sooty mould, which causes canopy dieback of
large trees. Many aspects of this complex set of interactions
have been confirmed experimentally. Similar mutualisms
between introduced ants and introduced homopterans have
been noted previously (e.g., Helms & Vinson 2002), and the
partners need not even have a coevolutionary history (cf.
Simberloff & Von Holle 1999). Ness & Bronstein (2004)
find great variety in the nature of interactions between
introduced ants and native and introduced homopteran
trophobionts. There is rarely definitive experimental proof
that the mutualism produces increased populations of both
2006 The Author, Journal compilation 2006 Blackwell Publishing Ltd
914 D. Simberloff
partners on a regional scale, but the circumstantial case for
such an effect is often strong.
On offshore islands of southeastern France, introduced
rats and rabbits are the main seed dispersers for two
introduced species of Carpobrotus, while no such dispersal
was detected on the adjacent mainland (Bourgeois et al.
2005). Experiments showed that digestion by these two
mammal species increased germination probability and
speed. Furthermore, the succulents provide an energy- and
water-rich food source to the rats and rabbits during the
summer dry season. This study does not prove a population
impact on any of the actors, but the data strongly suggest
that at least the plants are more widespread than they would
be without these animals.
Several recent reports document a weaker version of
meltdown, in which one invader facilitates population
survival and/or growth of one or more other invaders
without itself receiving an evident benefit. For instance,
Wonham et al. (2005) studied impacts of an abundant Asian
snail, Batillaria attramentaria, on other species in a Washington state bay. Its shells are used as habitat almost exclusively
by an Atlantic slipper shell, an Asian anemone, and two
native hermit crabs. In addition, experiments showed that a
snail plus the eelgrass Zostera japonica, both introduced,
increase when Batillaria is present. Their data strongly
suggest an increase at least in spread, and possibly in initial
probability of survival, for several of the invaders, but
Wonham et al. (2005) concede that they lack sufficient
population and historical data to prove the role of Batillaria
in fostering invasion by the other species. In any event, no
impact on Batillaria is evident.
Grosholz (2005) demonstrated experimentally in Bodega
Harbor, California, that the decline of two previously
dominant native clams and their replacement by an
introduced clam that had been present in low numbers for
at least five decades resulted from the introduction, c. 1993,
of the European green crab (Carcinus maenas), which preys
voraciously on the native species. Experiments demonstrated that native clams in densities comparable to those before
the crab invasion strongly compete with the invader, while
at post-invasion densities there is no competitive effect.
There is, however, no evidence that the crab benefits from
the presence of the non-indigenous clam.
In Queensland, Australia, Floerl et al. (2004) found that
the hull-fouling introduced bryozoan Watersipora subtorquata
tolerates several antifouling biocides. They demonstrated
experimentally that colonies of this species serve as nontoxic refugia for other, less-tolerant fouling organisms to
settle on, thus facilitating transport of other species at
greater abundance and frequency. They report no reciprocal
effect favouring this bryozoan.
Adams et al. (2003) provided strong experimental evidence that introduced fishes facilitate invasion by bullfrogs
2006 The Author, Journal compilation 2006 Blackwell Publishing Ltd
Idea and Perspective
(Rana catesbeiana) in western North America by preying on
native macroinvertebrates (such as dragonfly nymphs) that
otherwise preclude establishment or severely limit numbers
of frogs. There is, however, no evidence that the frogs
facilitate the fishes.
Constible et al. (2005) collected hair clumps of introduced
bison (Bison bison) on Santa Catalina Island, California. The
hair contained more than 10 times as many seeds of
introduced plants as of native species. Experiments showed
that seeds were mostly viable, and germination was high.
They did not seek evidence that the bison determine
population size and spread of the introduced plants, but
they caution that this interaction may jeopardize ongoing
native plant restoration. In any event, there is no evidence
that the presence of the introduced plants is critical to the
bison.
Several other convincing reports, often of experimental
studies, describe interactions in which one invader facilitates
the invasion of another species, while incurring some loss at
the individual and perhaps population level, but not of
sufficient magnitude to prevent its own continued thriving.
For example, Levin et al. (2002) experimentally studied
replacement of native kelps in southern Maine by the nonindigenous green alga Codium fragile ssp. tomentosoides. They
demonstrated that Codium does not inhibit growth or
survival of kelp, but that it colonizes gaps in kelp beds and,
once established, inhibits kelp recruitment. A second nonindigenous species, the encrusting bryozoan Membranipora
membranacea, grows epiphytically on kelp and reduces growth
and survival, leading to gaps in kelp beds. When Codium is
absent, kelp recolonizes the gaps, but when Codium is
present, it monopolizes the gaps and prevents kelp
recolonization. The invasion of the bryozoan is therefore
crucial to that of the green alga. Codium does not aid
Membranipora, however. In fact, because the bryozoan rarely
colonizes Codium, increase in Codium leads to long-term
decrease in the bryozoan. Membranipora would always be
present, however, on kelps at exposed sites that Codium
cannot colonize.
Populations of the recently introduced Asian goby
Tridentiger bifasciatus have increased in the San Francisco
Estuary while those of several native fish species have
declined. Mattern & Brown (2005) show that this goby preys
heavily on two introduced benthic invertebrates little used
by native fishes and suggest that this predation may explain
the goby increase. Although the population consequences to
both the goby and the prey have not been demonstrated,
this invasion may be one in which the benefit to a predator
is critical and the loss to prey species has little population
consequence.
Ricciardi & MacIsaac (2000) summarizing the flood of
introduced species in the Great Lakes, point to instances of
suspected facilitation among invaders and raise the spectre
Idea and Perspective
of the Great Lakes entering an invasional meltdown phase.
As one of a few examples, they note that the Ponto-Caspian
hydroid Cordylophora caspia feeds on zebra mussel (Dreissena
polymorpha) larvae and uses zebra mussel shells as a substrate.
Its population size in Lake Michigan expanded dramatically
when zebra mussel beds formed. Ricciardi (2001) elaborated
on this theme, finding for the Great Lakes, as did Simberloff
& Von Holle (1999) for the literature as a whole, that
reported positive interactions exceeded reported purely
negative ones and noting that meltdown, in the sense of
acceleration, is at least consistent with the increasing rate of
newly established introduced species in the Great Lakes.
However, as in most studies cited in the earlier reviews, the
population consequences of the reported interactions were
rarely if ever tested. A number of other publications on
Great Lakes invaders speculate that they may be associated
with meltdown (e.g. Horvath et al. 2001; Dick et al. 2002)
without presenting experimental or other evidence of
population impact.
The net result is that, 6 years after the formal addition of
meltdown to the lexicon of invasion phenomena, cases of
meltdown are increasingly cited, though it is uncertain that
any full-blown case of acceleration of number of introduced
species has been demonstrated. In several cases (e.g. on
Christmas Island), a greatly increased impact of introduced
species seems certain. Few cases of mutual facilitation are
documented at the population level even though it is
reasonable to infer their existence from observations and
short-term experiments. The paucity of cases with conclusive evidence of a population impact is not unique to
invasional meltdown, or even to interactions between
introduced species and natives. For the great majority of
coexisting native species suspected of consequential interaction, definitive proof at a population level is lacking. With
respect to determining the frequency of meltdown, this
situation is exacerbated by a persistent unwarranted
de-emphasis of facilitation and mutualistic interactions in
general in comparison with competitive and predatory ones
(Boucher et al. 1982; Bruno et al. 2003). For meltdown,
researchers are additionally often hindered by the fact that
experiments are impossible because it would be unethical
and/or illegal to perform certain sorts of manipulations with
introduced species that are or may become invasive.
It is important to note that absence of proof does not
mean the phenomenon does not occur, and many cases in
the invasion literature pointing to ecosystem impacts are
suggestive. For example, in central New Jersey hardwood
forests, populations of introduced European earthworm
species were elevated under introduced Berberis thunbergii and
Microstegium vimineum relative to under native vegetation, and
nitrate was increased (Kourtev et al. 1999). Although
Kourtev et al. (1999) are uncertain of causal relationships,
they suggest that changes in soil properties associated with
Invasional meltdown 6 years later 915
the plants and possibly the worms may increase invasion
rates of other weedy introduced species. Aplet (1990)
reported a similar increase in Hawaii of introduced
earthworm populations under trees of introduced Myrica
faya, a nitrogen-fixer, and increased burial rates of nitrogenrich litter. Vitousek & Walker (1989) had already
demonstrated the increased nitrogen availability to other
organisms under M. faya and had speculated that this effect
would lead to invasion by previously nitrogen-limited plants.
The worms may exacerbate this impact (Aplet 1990) and
may contribute to increased rooting by introduced feral pigs,
which favours several other introduced plants (Aplet et al.
1991). Although the meltdown aspects of these joint
invasions by worms and plants have yet to be confirmed,
the similarity in the cases and the known nutrient limitations
for many introduced plants suggest meltdown is a strong
possibility.
The continuing absence of empirical evidence of population impacts in most pairwise species interactions,
including those of introduced species, means it is not yet
possible to answer one question Simberloff & Von Holle
(1999) posed: are facilitatory interactions among introduced
species more frequent or important than hindering ones.
However, more important than an answer to this question is
the recognition among biologists that interactions contributing to meltdown are sometimes important, that they
arise in many different kinds of ecosystems and engage
species of many taxa, and they are worth seeking and
studying.
THE ROLE OF METAPHOR IN INVASION BIOLOGY,
WITH SPECIAL REFERENCE TO MELTDOWN
Metaphors are rife in all sciences as aids in understanding
complicated concepts (Boyd 1993; Mayer 1993). Metaphors
are also more or less inexact – they carry baggage associated
with their referents, and the baggage of an entrenched
metaphor may, in the end, hinder rather than aid understanding a concept or dealing with real-world problems
associated with it (Mayer 1993). Larson (2005); cf. Subramaniam 2001) bemoans the prevalence of military and
pejorative metaphors in both popular and scientific writing
about biological invasions, arguing that they are inaccurate,
cause loss of scientific credibility, and interfere with
effective conservation planning. The basic problem, in his
view, is that they stem from a conception of non-indigenous
species as inherently evil; here he echoes other authors (e.g.
Sagoff 1999, 2005; Subramaniam 2001), who have joined
with a few ecologists (e.g. Rosenzweig 2001; Slobodkin
2001; Brown & Sax 2004; Gurevitch & Padilla 2004) in a
rearguard action to convince biologists and the lay public
that the ecological threat from introduced species is
overblown.
2006 The Author, Journal compilation 2006 Blackwell Publishing Ltd
916 D. Simberloff
Although the term ÔmeltdownÕ is only tangentially
military, it is certainly pejorative. ÔMeltdownÕ first appeared
in 1965 with reference to nuclear reactors and, in the wake
of the Three Mile Island disaster, became increasingly widely
used, even metaphorically, to describe processes irreversibly
deteriorating, apparently at an accelerating rate – children’s
temper tantrums, escalating internet crashes, the 2005
University of Tennessee football team, and the like. Gabriel
et al. (1993) applied the term to the process by which a small
population accumulates deleterious mutations at an increasing rate, ultimately leading to extinction.
Not only is invasional meltdown among phenomena now
routinely considered in various explorations by ecologists,
conservationist biologists, and invasion biologists (e.g.
Blackburn et al. 2005; Parker et al. 2006), but it has entered
the lay literature as public concern with invasions has rapidly
increased. Recent popular invasion books (Baskin 2002;
Burdick 2005) feature meltdown, and the high visibility of
certain cases (Christmas Island, the Great Lakes, San
Francisco Bay, Bodega Harbor, CA, USA), with specific
note of meltdown by scientists studying them, has led to
increasing notice in newspapers, magazines, and even
political speeches. Inevitably, some popular articles get it
wrong. A particular tendency is to use the term meltdown to
mean simply many invading species causing widespread
damage at a site, or a dramatic, sudden change in an
ecosystem owing to invasions (e.g. Bohan 2005; Miller 2005;
Rauber 2005).
Unsurprisingly, loose usage in the popular press has led to
a backlash. For instance, Andy Cohen, a researcher on
invasive species in San Francisco Bay, says of invasional
meltdown, ÔI think it was a term coined to gather attention
to a phenomenon that no doubt occurs sometimes. It may
be a little exploitative termÕ, (Bohan 2005). He feels that it
may be overly dramatic, but that it has succeeded in calling
attention to possible instances of the phenomenon (A.
Cohen, personal communication, 19 April 2006). Ted
Grosholz, author of the study on meltdown in Bodega
Harbor (Grosholz 2005), concedes that Ôthe phrase Ôinvasional meltdownÕ may also be an unfortunate choice of
words,…but it has become established in the fieldÕ (Bohan
2005). He sees it as useful in drawing attention to invasions
and believes any possible problems so far have to do with
how the press has used the term (E. Grosholz, personal
communications, 18 April 2006). He is correct that the term
is now established, surely, but it is unclear that the net
impact of the phrase is unfortunate.
This returns us to the matter of metaphor, especially
metaphor in invasion biology. That martial, pejorative
metaphors abound in the popular press and, to a lesser
extent, the scientific literature on invasions does not prove
that invasion biologists are xenophobes who see all
introduced species as bad and native species as good
2006 The Author, Journal compilation 2006 Blackwell Publishing Ltd
Idea and Perspective
(Simberloff 2003). This conception of invasion biology is
essentially a version of the strong program of social
construction of the science (Bloor 1976), an example of
an approach by a small minority of sociologists who
construe developments in the sciences as reflecting social
factors and the psychology of its practitioners rather than
advances in understanding the workings of the universe (cf.
Brown 2001). The contention that invasion biologists
generally hold the view that introduced species are
generically bad is belied by a wealth academic publications
and government and international activities that explicitly
acknowledge the great benefits conferred by some introduced species and the fact that the problems are caused by a
minority of them (Simberloff 2003). The argument that
introduced species are not so awful (Sagoff 1999, 2005;
Rosenzweig 2001; Slobodkin 2001; Brown & Sax 2004) rests
partly on the related, and equally false, charge that invasion
biologists are not accounting for the benefits of some
introduced species and the apparent harmlessness of most.
It is just as misguided (Simberloff 2003, 2005).
It is true that martial metaphors occur in the invasion
biology literature, just as they do in the medical literature as
it relates to public health, and in both fields such metaphors
become more vivid and pervasive when the popular press
reports on these subjects. There is little mystery in invasion
biology and public health about why such metaphors come
to mind – the analogy in each case is patent. There is
combat against an invader, there are substantial losses if the
invader establishes a foothold and spreads, some of the
methods used to combat invaders (in public health as in
biological invasions) are very close to those used in warfare:
chemicals, hand-to-hand combat, Maginot lines. There is
also no mystery why these metaphors become more
dramatic and dominant in the lay literature: its purveyors
are selling newspapers, magazines, books, films, etc. As in so
many instances in which a more or less complicated and
sometimes abstract scientific notion is being explained,
metaphors with familiar referents are a useful tool for
engaging non-specialists (Lakoff & Johnson 1999).
The particular complaint in invasion biology about
martial metaphors, and by extension, the meltdown
metaphor, is that the inevitable inaccuracies hinder understanding, cause a loss of scientific credibility, and retard
effective response (Larson 2005). Similar concerns arise
about many if not all metaphors in science. The keystone
species metaphor (Paine 1969), for instance, was widely
applied in ecology, eventually used in senses beyond its
original meaning (e.g. Bond 1993), claimed to have been so
warped and expanded that its meaning was obscure and it
was dangerous to apply it to management (Mills et al. 1993),
and the subject of a rescue attempt (Power et al. 1996).
It is not at all evident that the meltdown metaphor is
hindering understanding. Metaphors serve several useful
Idea and Perspective
purposes in advancing science, aside from their usefulness in
transmitting scientific ideas to the public. In early stages of
concept development, when the precise nature and domain
of an idea are still uncertain, they help scientists focus on the
problem and refine its definition (Black 1962). The keystone
metaphor drew attention to the important fact that loss of
species of low abundance could have major impacts on
entire communities. The meltdown metaphor appears to
have attracted greatly increased attention to facilitation and
the possibility of enhanced invasion impact (consider the
response to the identical phenomena, presented without the
metaphor, by Howarth in 1985), and the increased attention
has surely contributed to the interest of researchers whose
recent work is summarized above. As a theory develops, at
least for a time a metaphor may become an irreplaceable
part of the linguistic machinery used to address it – Boyd
(1993) terms such metaphors ÔconstitutiveÕ. Typically,
successful constitutive metaphors become the property of
the whole scientific community, and variations on them are
explored by many workers without the metaphor becoming
trite or being used catachrestically, as often happens with
catchy literary metaphors. Again in distinction to literary
metaphors, the task of explicating the metaphor falls to the
individuals presenting it and those using it in research; in
literature, explication is the role of critics, not the original
authors or others who borrow the metaphor (Black 1962;
Boyd 1993). Finally, constitutive metaphors are invitations
to future research, including research into the degree of
analogy between the developing concept and the referent of
the metaphor (Boyd 1993). This was the goal of Power et al.
(1996) in their refinement of the keystone species concept –
they outlined a number of promising research directions. It
is exactly in this sense that Simberloff & Von Holle (1999)
presented the meltdown metaphor, along with a large group
of sample cases of facilitation of various sorts.
CONCLUDING REMARKS AND FUTURE
DIRECTIONS
It is difficult to measure scientific credibility. I am unaware
that invasion biology has lost credibility, in spite of this
metaphor, the martial metaphors, and the few critics whose
work I cite above. The problems associated with invasions
have become so evident that the science continues to
increase in prominence and activity, despite their writings.
An assessment of who loses credibility can only be
performed by historians of science a number of years
hence. It is also too early to determine if the meltdown
metaphor is retarding effective response to the phenomenon itself. However, response of any kind was unlikely
without recognition of the problem in the first place, and to
the extent that the meltdown metaphor has both fostered
recognition and led to research elucidating the mechanism
Invasional meltdown 6 years later 917
of the phenomenon in several instances, it has surely aided
response. Exploration of meltdown already enlightens
management in several cases discussed above.
An increase in the catalogue of well-studied cases will
undoubtedly prove useful in this regard. It will also permit a
more definitive to response to one of the questions
Simberloff & Von Holle (1999) first posed, what are the
relative frequencies of facilitative and detrimental interactions among invaders. It is disappointing that, in spite of
frequent allusion to invasional meltdown, there have been
few instances (mostly among those cases listed above) in
which researchers have gathered the basic natural historic
and other biological data that would be needed to
understand the frequency of various types of meltdown
and to assess their prevalence in nature. This is part of the
larger problem that ecology is fundamentally an idiographic
science, and general laws are likely to obtain only at very
large scales, so we will need a large catalogue of case studies
to generate the understanding required to deal with many
environmental problems (Simberloff 2004). Finally, there
has been little attempt to predict in advance when
mutualism will occur. This is part of the larger search for
the Holy Grail of invasion biology – predicting impacts of
invasions. As prediction capability improves, regulatory and
management procedures will inevitably be informed. Almost
certainly, such predictive ability will increase only apace with
a growing number of empirically studied cases.
ACKNOWLEDGEMENTS
Peter Kareiva originally suggested the term Ôinvasional
meltdownÕ in the course of intensive discussion of
facilitation phenomena at a working group of the National
Center for Ecological Analysis and Synthesis, sponsored by
NSF grant no. DEB-94-21535 to the Center, and by the
University of California at Santa Barbara and the State of
California. Nathan Sanders, Richard Mack, Betsy Von Holle,
and three referees provided insightful comments on early
drafts of this manuscript.
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COMMENTARY ON SIMBERLOFF (2006):
MELTDOWNS, SNOWBALLS AND POSITIVE
FEEDBACKS
Simberloff (2006) revisits the evidence for the hypothesis of
invasional meltdowns (Simberloff & Von Holle 1999) with a
narrative review of recent examples of facilitation between
invasive species, as well as a discussion of whether this – or
any – metaphor helps or hinders scientific progress. I argue
here that progress on evaluating invasional meltdowns will
be better served by focusing on positive feedbacks as an
ecological phenomenon rather than on the more restricted
issue of facilitation; that in addition to better primary studies
we need to employ better means to summarize and evaluate
those studies to answer the really interesting questions about
the generality of meltdowns; and, lastly, that while this
metaphor has been useful, scientists need to be held to a
higher standard than the general public in using metaphors
and concepts precisely.
The focus of both the original 1999 and the 2006 papers
was on facilitation among invasives, and acceleration of
invasion rates. Positive feedbacks are mentioned only in
passing in the 1999 paper, and not at all in the 2006 paper.
However, facilitation itself does not really lead to anything
in particular, and accelerating rates of invasion can have
many different causes. I argue that positive feedbacks are
the most interesting and important aspect of the hypothesis
of invasional meltdowns, and this is where the conceptual
and empirical focus should be. Different systems from
engineering to economics to ecology have positive feedbacks, but they have been greatly underemphasized in
community ecology relative to negative feedbacks (partic-
Commentary on Simberloff (2006) 919
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Wonham, M.J., O’Connor, M., & Harley, C.D.G. (2005). Positive
effects of a dominant invader on introduced and native mudflat
species. Mar. Ecol. Progr. Ser., 289, 109–116.
Editor, Nicholas Gotelli
Manuscript received 14 February 2006
First decision made 24 March 2006
Manuscript accepted 20 April 2006
ularly competition and predation). Negative feedbacks damp
down system level changes, leading to stability. Runaway
positive feedbacks in a system create ÔsnowballÕ effects in
which a phenomenon builds on itself in an accelerating
fashion, becoming unstoppable. Positive feedbacks are
therefore both of fundamental interest in ecology, and
potentially of enormous practical importance in conservation ecology. A ÔmeltdownÕ due to a positive feedback
implies that after a certain point is reached, ordinary
intercession is impossible, and a drastic state change is
inevitable – whether it occurs in a toddler in the
supermarket, a nuclear reactor, or an invaded ecological
community. The elements of inevitability and irreversibility
have important implications for conservation biology.
So, what is the evidence for invasional meltdowns in
nature? How frequent are they relative to all invasions? Do
facilitative interactions among invaders commonly alter
one another’s demographic parameters, leading to mutually
accelerating population increases of the invaders? Are
invasional meltdowns implicated in most cases of major
invasions, a substantial fraction, or very few? Do positive
feedbacks have, on average, large and persistent effects, or
minor effects? Do they occur evenly across taxa and
systems, or are they more prevalent for a few species or
only under certain circumstances? How important are
complex synergisms between positive feedbacks among
invasives and other effects such as climate change and
overharvesting, as proposed for the widespread system
changes in the Gulf of Maine (Harris & Tyrrell 2001), or
complex trophic cascades involving negative and positive
interactions between native and invasive species (Inderjit,
personal communication)?
2006 The Author, Journal compilation 2006 Blackwell Publishing Ltd