Are invasive species a major cause of extinctions?-Gurevitch & Padilla-Trends in Ecology & Evolution

Are invasive species a major cause of extinctions?-Gurevitch & Padilla-Trends in Ecology & Evolution
Lee Poulsen (Thu, 16 Sep 2004 11:39:09 PDT)

This isn't going to come out as readable as the original PDF file
(especially the tables), but it is an interesting journal article that
just appeared showing that it is as yet unknown and unproven whether
invasive species actually cause the extinction of native species.

--Lee Poulsen
Pasadena area, California, USDA Zone 9-10
=============================================

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Trends in Ecology & Evolution
Volume 19, Issue 9, September 2004, Pages 470-474
Copyright © 2004 Elsevier Ltd All rights reserved.

Are invasive species a major cause of extinctions?
Jessica Gurevitch and Dianna K. Padilla

Department of Ecology and Evolution, Stony Brook University, Stony
Brook, NY 11794-5245, USA

The link between species invasions and the extinction of natives is
widely accepted by scientists as well as conservationists, but
available data supporting invasion as a cause of extinctions are, in
many cases, anecdotal, speculative and based upon limited observation.
We pose the question, are aliens generally responsible for widespread
extinctions? Our goal is to prompt a more critical synthesis and
evaluation of the available data, and to suggest ways to take a more
scientific, evidence-based approach to understanding the impact of
invasive species on extinctions. Greater clarity in our understanding
of these patterns will help us to focus on the most effective ways to
reduce or mitigate extinction threats from invasive species.

Article Outline
1. What do we know about invasive species and extinctions?
2. Assessing the contribution of alien species to native declines
2.1. Generalizing from available information on threats
3. Species imperiled by aliens in the USA
4. Global data from the IUCN red list
5. All invasions are not created equal
6. Conclusions
Acknowledgements
References

Ecologists, conservation biologists and managers widely believe that
invasions by non-native species are a leading cause of recent species
extinctions 1 and 2. The introduction and spread of non-native species
has become a global ecological and conservation crisis as invasive
organisms are increasingly altering terrestrial and aquatic communities
worldwide. The loss of biodiversity and species extinction are,
likewise, major ongoing crises. Native species declines often occur
simultaneously and in the same place as invasion by non-native species,
leading many conservationists and researchers to believe that invasions
and extinctions are closely linked.

We suggest that there are several problems with the seemingly
inextricable link between species invasions and the extinction of
natives. To date, there has been insufficient critical evaluation of
overall global patterns of the extent to which invasion is implicated
in extinctions, or the conditions and circumstances under which
invasions are most likely to lead to extinctions. We aim to prompt a
more critical evaluation of these data, and to suggest ways to take a
more scientific, evidence-based approach to understanding the role of
invasive species in extinctions.

1. What do we know about invasive species and extinctions?

Existing data on causes of extinctions and threats are, in many cases,
anecdotal, speculative, or based upon limited field observation.
Although it is clear that obtaining quantitative and experimental data
are impossible under many circumstances, the problem remains that
correlation is too often assumed to imply causation. For example,
severe habitat alteration (e.g. deforestation), decline or extinction
of native plants, and the proliferation of exotic plant species
commonly co-occur. Are non-native plants causing the decline of
natives, or are the decline of the natives and the proliferation of the
exotics both a result of habitat alteration? It is important to
distinguish between these alternatives: is removing exotics essential
to prevent the extinction of endemic natives, or is it largely a waste
of managers’ time and effort? Multiple threats can also act
synergistically to cause declines or extinctions. However, if invasives
are not a primary cause of extinction or major contributors to declines
of species (locally or globally) but are instead merely correlated with
other problems, the resources and efforts devoted to removing exotics
might be better focused on more effective means to preserve threatened
species.

The overarching category ‘threatened by aliens’ might also be
misleading, for two reasons: we must distinguish the relative
importance of different functional groups in causing extinctions, and
also examine whether broad groups of invasives, or merely particular
species, are largely responsible. Based upon theory and observational
data, alien predators and pathogens have been predicted to be far more
likely than exotic competitors to cause the extinction of native
species [3].

Even within functional groups, a few species appear to have caused a
disproportionate share of incipient and actual extinctions. A few
widespread rat species, feral pigs (as in Hawaii, Box 1), several
predatory snakes (particularly on islands), possibly annual
Mediterranean grasses and several other plants, a few microbial
pathogens and a finite list of other invaders might be responsible for
most of the extinction risk posed by aliens. Alien plants might be more
likely to cause displacement and community change rather than causing
species extinctions. This is the case, for example, for Psidium
cattleianum in rainforests in Madagascar, where its presence has
altered diversity patterns in communities that were disturbed as long
as 150 years ago, but its presence has not resulted in the loss of
native plant species [4]. If a few cosmopolitan invaders are causing
the extinction of many native endemics, we should focus on those
particular invasive species, rather than on alien species in general,
to mitigate extinction risk. The better we understand both patterns and
mechanisms causing declines, the better we can focus our efforts on the
most effective ways to reduce or mitigate threats.

----
Box 1

Leading alien agents of extinctions?

High profile invaders are often implicated in species extinction.
However, when the data are more thoroughly examined, their role as the
direct causes of extinction can be drawn into question in some cases.
Although extinction might be coincident with the appearance of
invaders, it can be difficult to disentangle the relative impacts of
the invader and other stressors. The Nile perch (Lates niloticus)
introduced into Lake Victoria during the 1960s is frequently implicated
in the extirpation of native cichlids from the lake 11, 12 and 13, but
the decline in cichlids probably started during the 1920s with the
development of railroads, erosion, and shoreline destruction [12].
Urbanization during the 1970s increased eutrophication and decreased
lake transparency from 8 to 1.5 m 12 and 13. With increased nutrient
loading, anoxic events resulting in fish kills are now common.
Increased nutrients appear to favor another invader, the water hyacinth
(Eichhornia crassipes), which has also been implicated in fish declines
by altering nursery areas for juvenile fishes [11]. But removal of both
water hyacinth and the Nile perch, even if possible, would not solve
the problems created by altered land use and nutrient pollution.

Zebra mussels (Dreissena polymorpha) are currently considered to be the
major threat to North American freshwater unionid bivalves [14]. Of a
historic 281 species, 19 are known to be extinct, 21 are thought to be
extinct, 77 are endangered, 43 are threatened, and 72 are of special
concern. Zebra mussels require hard substrates for attachment. In lake
bottoms hard substrates tend to be rare, and the shells of native
bivalves offer the most abundant substrates for zebra mussel
settlement. Overgrowth by zebra mussels can make it difficult for
unionids to burrow and move through sediment, can increase drag and the
likelihood of dislodgment by water motion, can occlude the openings in
unionid valves, prevent opening for respiration, feeding and
reproduction, and zebra mussels may directly compete with unionids for
food [15]. Nevertheless the role of zebra mussels in unionid declines
in North America is unclear. Unionid declines began long before zebra
mussels were introduced during the mid-1980s [16] and, to date, no
species have gone extinct as a result of the introduction of zebra
mussels. Pre-introduction declines were caused by habitat destruction
and deterioration resulting from water diversion, erosion, an increase
in eutrophication (which causes periods of anoxia), pesticides, loss of
host fish for parasitic unionid larvae, historic harvesting for the
button industry and harvesting for the pearl industry 15 and 16.
----

There are several well known cases in which invasions are strongly
linked to extinctions. In some of these, invaders have been a major
cause of the decline and loss of species. For example, the predatory
brown tree snake Boiga irregularis was introduced into Guam during the
early 1950s and has since been linked, both directly and indirectly, to
the extinction of the native vertebrate species [2]. However, in other
cases, the picture is less clear (Box 1).

Several recent papers have questioned the link between invasion and
loss of diversity 3, 5, 6 and 7. These studies do not directly address
whether invasions are causing the extinction of native species but
focus on the total number of species before and after invasion.
Although this approach is intriguing, total numbers do not tell the
whole story. Most ecologists would not, for example, regard the
establishment of five new widespread alien species in a region as
‘biotic compensation’ for the extinction of five endemics.

Case studies of particular instances of exotic species as major
contributors to extinction on the one hand, and counts of numbers of
species before and after invasions on the other, each offer insights
into whether invasions are a major cause of extinction. However, can we
say anything more general about whether the invasion of exotic species
is a major cause of the disappearance of natives? Are there some
conditions under which, or some kinds of species or systems in which,
this is more likely to occur? To answer such questions, it is necessary
to synthesize quantitatively the results of many case studies,
preferably of the highest-quality data that can be obtained.

2. Assessing the contribution of alien species to native declines

Until recently, it has not been possible to quantify or assess general
patterns of threats to endangered and threatened species (and causes of
extinction), because data on threats and causes of extinction were
limited and scattered. Several recent major efforts to collect and
compile such data and to categorize threats and causes of extinction
are beginning to make this information available and searchable
electronically. Two of the most important sources are Wilcove et al.
[1] for the USA, and the International Union for Conservation of
Natural Resources (IUCN) Red List [8] for species threatened worldwide.
We examined these two sources to evaluate some of the widespread
assumptions about invasion and extinction.

2.1. Generalizing from available information on threats

These and other similar data sets are invaluable assets in efforts to
begin to better understand the nature of the causes of decline for
threatened or extinct species. All currently available data have
inherent limitations: most of the information is based upon unpublished
observation and impressions, and is highly variable in quality,
depending upon the observers and the system and taxon in question. Most
imperiled species face more than one threat, and it is difficult to
disentangle proximate and ultimate causes of decline or interactions
between different threats and to evaluate their relative importance.
Exotic species might be a primary cause for decline, a contributing
factor for a species already in serious trouble, the final nail in the
coffin or merely the bouquet at the funeral.

Although there are limitations to the data available on causes of
extinction threats, assembling these large databases is a difficult
task. We believe that these data compilations, even given their
limitations, are more valuable and offer more information in assessing
overarching patterns than does attempting to generalize from individual
case studies, because the compilations present the opportunity to
obtain a more comprehensive picture of the nature of threats. Although
it is a daunting task, we must begin to identify general patterns of
the role and importance of invasions in extinctions as best as we can,
to attempt to understand the nature of global threats to biodiversity
and prioritize our responses to these threats.

3. Species imperiled by aliens in the USA

Wilcove et al. were the first to quantify data on threats to imperiled
species based upon a range of sources, including published sources,
government lists and interviews with specialists familiar with
particular species and regions [1]. The paper is widely cited and is
the primary source in the literature for the belief that invasive
species are a direct and leading cause of extinction. The authors
reported that habitat loss was the greatest threat to imperiled species
within the USA (threatening 85% of the species classified as
imperiled), followed by alien species (threatening ~50% of species).
Wilcove et al. [1] examined 2490 USA species categorized as threatened,
of which 1880 species had specific threat data. We reanalyzed the
Wilcove et al. [1] data for the 930 species that they categorized as
being imperiled by aliens (i.e. the half of their dataset for which
species were listed as being affected by aliens). We classified each
threatened species according to all of the particular classes of threat
(Table 1, plus a single class for otherwise unclassified or unknown
threats) listed as affecting it.

----
Table 1.

Numbers of species affected by different threats believed to be
responsible for causing population declinesa

Causes of decline
All species [930]
Plants [602]
Birds [68]

Direct human habitat destruction and fragmentation, including logging,
road building and diversion of water
497
233
48

Exploitation (hunting, fishing and collecting) and poisoning and/or
trapping
90
19
11

Fire and changes in fire regime
102
92
1

Pollution (herbicides, pesticides, oil spills, etc.)
32
4
5

Invasive alien predators and herbivores
131
73
39

Alien plants: competition and indirect habitat effects
431
410
19

Competition with exotic animals (excluding feral and domestic animals)b
67
0
14

Feral pigs (herbivory, predation, competition and/or habitat effects)
268
257
8

Grazing and/or trampling by domestic and feral cattle, goats, sheep,
horses and burros
327
295
13

Hybridization with alien species
22
5
0

Diseases (including alien and native species)
33
3
23

Parasites (physiological and behavioral)
3
0
2

Other or unknown
169
134
8
a Reclassification of data on all of the cases in which species were
categorized as being imperiled by aliens by Wilcove et al. ([1],
http://www.natureserve.org/). Categories are nonexclusive and so numbers
do not sum to total species numbers.
b We believe that domestic cattle should be categorized separately from
alien invasive species, even though they are non-native in most areas
in which they affect native species. Unlike invasive aliens, the
population sizes and distribution of cattle are usually controlled by
humans. Thus, cattle are not invasive in any of the usual meanings of
the word, although they might have large effects on native populations,
communities and ecosystems.
----

We found that these threatened species are, on average, faced with 2.5
specific types of threat (of those listed on Table 1) per species.
Species in two of the largest threatened groups, plants and birds, are
both typically affected by multiple threats (2.6 threats per species
for plants and 2.8 for birds). Hawaiian endemics constitute a large
proportion of the imperiled USA birds (43%) and plants (39%) threatened
by alien species. By looking at the nature of the combined threats to
these species, we can make several inferences (Box 2). If feral pigs,
goats and alien plants are causing the declines of many native Hawaiian
plants, are the alien plants the primary, or even a contributory, cause
of the decline in natives, or are they coincidental to the disturbance
caused by feral pigs and goats? It is not possible to distinguish
between these alternatives definitively with these data. However, if
competition with alien plant species was a primary cause for population
declines in native plants, one would expect >2–4% of species to be
affected by invasive plants alone.

----
Box 2

Threats by aliens to USA species (see [1] for details and definitions)

There are 602 USA plant species affected by alien species (of 1055
threatened plants, http://www.natureserve.org/). However, competitive
displacement by aliens is rarely described as posing the only threat to
a native plant. Of these 602 species, 20% are threatened by both
habitat loss and by exotic plant species. Almost 40% (231 species) of
all plants listed as imperiled by alien species in the USA are Hawaiian
endemics that are imperiled by the ‘vicious triumvirate’ of feral pigs,
goats and alien plants (and often cattle). This is a particular problem
because these threatened plants occur in areas that are protected, or
are otherwise not currently subject to habitat loss or direct habitat
destruction by humans.

But how much of a threat do the alien plants pose themselves? Of all
the USA plants considered imperiled and affected by aliens, only 4% are
affected only by alien plant species but not by cattle, pigs, goats, or
other alien herbivores, or by direct habitat damage (i.e. 2.3% of the
total imperiled USA plants; http://www.natureserve.org/). Alien plants
affect native plants in combination with habitat damage by humans, or
by pigs, goats and cattle. For example, Xylosma crenatum (=X. crenata,
Flacourtiaceae, no known common name) is an endangered (<20 extant
individuals) montane tree that is endemic to Kauai in the Hawaiian
Islands. Its decline has been attributed to competition from invasive
exotic plants (http://www.natureserve.org/). However, major threats to
the survival of this species also include habitat loss and degradation
caused by domestic livestock and feral pigs, and intrinsically low
reproductive rates [8].

For birds, population declines for 68 of the 98 imperiled species in
the USA have been attributed to aliens (Table 1, main text;
http://www.natureserve.org/). Direct effects of habitat destruction by
humans, and hunting or collecting are also major threats ([1],
http://www.natureserve.org/). Alien predators presented the greatest
threat of all alien groups, threatening 57% of the bird species
affected by aliens (of a total of 68 species), followed by alien
pathogens (all in Hawaii, representing 34% of the birds affected by
aliens of all kinds); feral pigs and goats and domestic cattle also
play a destructive role, as do other exotic animals acting as
competitors (alone or in combination affecting 47% of the birds
affected by aliens; see also Table 1, main text). Alien plants also
appear to affect birds, threatening 24% of the bird species affected by
aliens, but most of these are affected by both alien plants and direct
habitat damage by humans (<6% are affected by alien plants but not
habitat destruction). Here, again, the evidence for damage owing to
plants is correlative, and it is unknown whether the alien plants
played a definitive causal role in the decline of any bird species. For
bird species that are affected by multiple agents, direct habitat
destruction by humans in addition to alien predators affected 38% of
the birds affected by aliens. Another 19% of these threatened birds are
affected by alien predators but not by direct habitat destruction by
humans, suggesting that alien predators play a more direct role in bird
declines than do alien plants in the decline of either plants or birds.
----

Alien predators and competitors threaten many of the imperiled bird
species (Box 2). However, although 28% of bird species were listed as
being negatively affected by alien plants (presumably owing to
alterations in habitat or food resources), only four bird species were
listed as being affected by alien plants species but not by the direct
effects of humans on their habitats. Consequently, until we have better
data, it is difficult to know whether the alien plants are responding
opportunistically to the same habitat damage that is negatively
affecting the birds, or if the effects of the alien plants on native
bird habitats are harming bird populations.

4. Global data from the IUCN red list

The IUCN Red List [8] includes documentation of 18 318 species that are
extinct, endangered, or threatened globally. (Not all countries and all
imperiled species are included, owing to insufficient data.) The IUCN
Red List is the most comprehensive database available for the
conservation status of threatened plant and animal species globally. In
contrast to the USA data, we examined the entire database, not just
those species threatened by aliens. We used the threat categories
specified by that database.

Threat data are not available for all species on the IUCN Red List;
most of those with such data list multiple threats. Of the species with
known threats, 33% (6069) include habitat loss and alteration as a
cause of decline or potential threat. Exploitation (e.g. hunting,
fishing, trapping and poisoning) is listed as a cause of decline for
7.6% of species. Only 6% of imperiled taxa list direct and indirect
effects of all kinds from naturalized alien species as contributing to
their decline (Table 2); that is, more than five times as many species
are categorized as being threatened or endangered by habitat loss than
by alien species.

----
Table 2.

Numbers of species affected by alien species and livestock from the
IUCN Red List (of 18 318 species total)a

Threat
No. of species affected

Alien species (naturalized)

Direct effects of all kinds, including effects of an unspecified nature
911

Competition from aliens
137

Effects on plants
100

Effects on animals
37

Herbivory and/or predation by aliens
161

On plants
48

On animals
113

Alien pathogens and parasites
58

Indirect effects (habitat alteration, etc.)
172

Affecting plants
108

Affecting animals
63

Livestock (domesticated)

Effects on plants
521

Effects on animals
92
a Calculated from data from [8].
----

Competition with invasive species is implicated as a threat to three
times as many plants as animals, whereas predation or herbivory by
invasives threatens about twice as many animal as plant species (Table
2). Strikingly, more than three and a half times as many plant species
have been affected by livestock grazing and trampling than by either
competition with or herbivory by nondomesticated, invasive aliens;
livestock threaten almost as many animal species as are threatened by
alien predators and many more than are affected by alien competitors
(Table 2). Many of the plant and animal species threatened by alien
species are also threatened by other factors, especially habitat
destruction, and some threatened groups (particularly freshwater fish)
tend to be affected by both competition and predation from aliens.

Little is known about the specific causes of extinction for most
species that are already extinct [8]. Of the 762 species globally
documented to have become extinct as a result of human activities in
the past few hundred years, <2% list alien species as a cause.

5. All invasions are not created equal

Much of the discussion of the threats to biodiversity posed by
invasions is couched in overarching terms, as though all invasive
species pose equal threats, and all invaded communities are equally
threatened. This is certainly not true.

Of all of the modern extinctions catalogued in the IUCN Red List [8],
most are species from terrestrial habitats (570 species), followed by
those from freshwater habitats (222), with fewest modern extinctions
occurring in marine habitats (21, mostly sea birds). More terrestrial
than aquatic species are also endangered or otherwise impacted by
aliens. For all species in the IUCN Red List, aliens directly affect
882 terrestrial species (of 15 504 species total, 5.7%), 59 of 3042
freshwater species (2.0%, mostly birds) and 87 of 737 marine species
(11.8%, mostly sea birds). For all marine species considered to be
critically endangered and impacted by aliens (one mammal and 14 birds),
other causal factors, in addition to alien species, are also listed. Of
the 21 total marine species listed to have gone extinct (four mammals,
11 birds, one fish, four molluscs, one alga), none are attributed to
invasive alien species; most were extinct before 1900 and before many
modern invasions. Marine species are considered to have very low risks
of extinction because the size of the oceans of the world creates large
continuous habitats, and because the open nature of marine habitats and
the life-history characteristics of many marine species result in
extensive dispersal potential to recolonize and repopulate depauperate
areas ([9], but see [10]).

We also considered the evidence for the prediction that introduced
predators would be more likely to cause extinctions than would
introduced competitors [3]. These predictions are only partially
supported by patterns that we found for the data sets that we looked
at. Neither the effects of competition nor predation were definitively
greater overall in either data base. For example, in the Red List data,
plants were threatened more by competition than were animals, whereas
for predation the results were opposite (Table 2). Domesticated
livestock and feral grazers (cattle, goats, pigs, etc., including the
physical disturbance that they cause) affected more plants than did
either competition or predation, and roughly as many animal species as
did predation (Table 1 and Table 2). We suggest that the contrasts
between the effects of aliens in aquatic versus terrestrial systems are
at least as important as the differences between alien competitors and
predators. Future data syntheses, as well as the development of theory,
must go beyond comparing the effects of alien predators and competitors
to consider a greater range of contrasts among functional types of
invader and systems invaded to reach a better understanding of these
patterns.

6. Conclusions

Invasive exotic species are causing dramatic changes in many ecological
systems worldwide, and there is no question that invasive species are
profoundly altering many communities and ecosystems. Seeing these
widespread changes, biologists, environmentalists and managers are
alarmed about invasions leading to large-scale declines and extinctions
of natives. This might prove to be a realistic concern. However, the
assumed importance of the invaders in causing widespread extinctions is
to date unproven, and is based upon limited observation and inference.
Evidence supporting a general and primary role for invasive aliens in
extinctions remains limited.

We must be as specific and as clear as possible about the nature of
threats to species at risk. If we determine that domestic livestock are
causing widespread plant extinctions, it is far more informative to
focus on the impact of domestic livestock than to say, more generally,
that aliens are causing these extinctions even when cattle are
non-native, because then we can deal with this threat directly rather
than diluting preservation efforts trying to combat all aliens. The
more specific our understanding is of the nature of the threats, the
more specifically we can address mitigation of those threats. The
generalization that alien species are playing a widespread role in
extinctions is, to date, too unspecific to be either accurate or
useful.

More work is needed to document and better understand the role of alien
species in pushing native species towards extinction, to evaluate their
impact relative to that of other factors, to determine in which systems
species are most likely to be endangered by aliens and to identify
which aliens are most likely to cause extinctions.

Acknowledgements

We thank Lynn Korner of Nature Serve (http://www.natureserve.org/) and
David Wilcove for making their data base available to us and for
providing additional data on threats to USA species, and wish to
express our appreciation to the people at the IUCN for putting together
and making worldwide threat data electronically freely accessible.
Three anonymous reviewers greatly improved the clarity of the paper,
for which we thank them. We gratefully acknowledge funding from the
U.S. Environmental Protection Agency (EPA R828900010 to J.G. and M.
Lerdau) and U.S. NOAA/Sea Grant (R/CE-18 to D.K.P., D. Schneider, and
C. Rehmann; and Award NA16RG1044, Project R/ES-54 to D.K.P. and T.
Klinger).

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mussel (Bivalvia: Unionoidea) resources in the United States,
Malacologia 41 (1999), pp. 461–474.

Trends in Ecology & Evolution
Volume 19, Issue 9, September 2004, Pages 470-474