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V.1
Causes and Consequences
of Species Extinctions
Navjot S. Sodhi, Barry W. Brook,
and Corey J. A. Bradshaw
OUTLINE
1. Introduction
2. Extinction drivers
3. Extinction vulnerability
4. Consequences of extinctions
5. Conclusions
The five largest mass die-offs in which 50– 95% of species
were eliminated occurred during the Ordovician [490–443
million years ago (mya)], Devonian (417–354 mya), Permian
(299–250 mya), Triassic (251–200 mya), and Cretaceous
(146–64 mya) periods. Most recently, human actions espe-
cially over the past two centuries have precipitated a global
extinction crisis or the ‘‘sixth great extinction wave’’ com-
parable to the previous five. Increasing human populations
over the last 50,000 years or so have left measurable
negative footprints on biodiversity.
GLOSSARY
Allee effects. These factors cause a reduction in the
growth rate of small populations as they decline
(e.g., via reduced survival or reproductive success).
coextinction. Extinction of one species triggers the loss
of another species.
extinction debt. This refers to the extinction of species
or populations long after habitat alteration.
extinction vortex. As populations decline, an insidious
mutual reinforcement occurs among biotic and
abiotic processes driving population size downward
to extinction.
extirpation. This refers to extinction of a population
rather than of an entire species.
invasive species. These are nonindigenous species in-
troduced to areas outside of their natural range that
have become established and have spread.
megafauna. This refers to large-bodied (>44 kg) ani-
mals, commonly (but not exclusively) used to refer
to the large mammal biota of the Pleistocene.
minimum viable population. This is the number of in-
dividuals in a population required to have a speci-
fied probability of persistence over a given period of
time.
1. INTRODUCTION
In the Americas, charismatic large-bodied animals
(megafauna) such as saber-toothed cats (Smilodon
spp.), mammoths (Mammuthus spp.), and giant ground
sloths (Megalonyx jeffersonii) vanished following hu-
man arrival some 11,000–13,000 years ago. Similar
losses occurred in Australia 45,000 years ago, and in
many oceanic islands within a few hundred years of the
arrival of humans. Classic examples of the loss of is-
land endemics include the dodo (Raphus cucullatus)
from Mauritius, moas (e.g., Dinornis maximus) from
New Zealand, and elephantbirds (Aepyornis maximus)
from Madagascar. Megafaunal collapse during the late
Pleistocene can largely be traced to a variety of negative
human impacts, such as overharvesting, biological in-
vasions, and habitat transformation.
The rate and extent of human-mediated extinctions
are debated, but there is general agreement that ex-
tinction rates have soared over the past few hundred
years, largely as a result of accelerated habitat de-
struction following European colonialism and the sub-
sequent global expansion of the human population
during the twentieth century. Humans are implicated
directly or indirectly in the 100- to 10,000-fold in-
crease in the ‘‘natural’’ or ‘‘background’’ extinction
rate that normally occurs as a consequence of gradual
environmental change, newly established competitive
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515 Species Extinctions
interactions (by evolution or invasion), and occasional
chance calamities such as fire, storms, or disease. The
current and future extinction rates are estimated using
a variety of measures such as species–area models and
changes in the World Conservation Union’s (IUCN)
threat categories over time. Based on the global as-
sessment of all known species, some 31, 12, and 20%
of known amphibian, bird, and mammal species, re-
spectively (by far the best-studied of all animal groups),
are currently listed by the IUCN as under threat.
Just how many species are being lost each year is
also hotly debated. Various estimates range from a few
thousand to more than 100,000 species being ex-
tinguished every year, most without ever having been
scientifically described. The large uncertainty comes
mainly through the application of various species–area
relationships that vary substantially among communi-
ties and habitats. Despite substantial prediction error,
it is nevertheless certain that human actions are causing
the structure and function of natural systems to un-
ravel. The past five great extinctions shared some im-
portant commonalities: (1) they caused a catastrophic
loss of global biodiversity; (2) they unfolded rapidly (at
least in the context of evolutionary and geological
time); (3) taxonomically, their impact was not random
(that is, whole groups of related species were lost while
other related groups remained largely unaffected); and
(4) the survivors were often not previously dominant
evolutionary groups. All four of these features are rel-
evant to the current biodiversity crisis. This sixth great
extinction is likely to be most catastrophic in tropical
regions given the high species diversity there (more
than two-thirds of all species) and the large, expanding
human populations that threaten most species there as
well.
The major ‘‘systematic drivers’’ of modern species
loss are changes in land use (habitat loss degradation
and fragmentation), overexploitation, invasive species,
disease, climate change (global warming) connected
to increasing concentration of atmospheric carbon di-
oxide, and increases in nitrogen deposition. Mechan-
isms for prehistoric (caused by humans >200 years
ago) extinctions are likely to have been similar: over-
hunting, introduced predators and diseases, and habi-
tat destruction when early people first arrived in virgin
landscapes.
2. EXTINCTION DRIVERS
Some events can instantly eliminate all individuals of
a particular species, such as an asteroid strike, a mas-
sive volcanic eruption, or even a rapid loss of large
areas of unique and critical habitat because of defor-
estation. But ultimately, any phenomena that can cause
mortality rates to exceed reproductive replacement
over a sustained period can cause a species to become
extinct. Such forces may act independently or syner-
gistically, and it may be difficult to identify a single
cause of a particular species extinction event. For in-
stance, habitat loss may cause some extinctions directly
by removing all individuals, but it can also be indirectly
responsible for an extinction by facilitating the estab-
lishment of an invasive species or disease agent, im-
proving access to human hunters, or altering biophys-
ical conditions. As a result, any process that causes a
population to dwindle may ultimately predispose that
population to extinction.
Evidence to date suggests that deforestation is cur-
rently, and is projected to continue to be, the prime
direct and indirect cause of reported extirpations. For
example, it is predicted that up to 21% of Southeast
Asian forest species will be lost by 2100 because of past
and ongoing deforestation. Similar projections exist for
biotas in other regions.
Overexploitation is also an important driver of ex-
tinctions among vertebrates and tends to operate syn-
ergistically with other drivers such as habitat loss. For
example, roads and trails created to allow logging op-
erations to penetrate into virgin forests make previ-
ously remote areas more accessible to human hunters,
who can, in turn, cause the decline and eventual ex-
tirpation of forest species. It is estimated that overex-
ploitation is a major threat to at least one-third of
threatened birds and amphibians, with wildlife cur-
rently extracted from tropical forests at approximately
six times the sustainable rate. In other words, the
quantity, and most likely the diversity, of human prey—
both fisheries and ‘‘bush’’ (wild) meat—are rapidly
diminishing.
Megafauna—those species weighing in the tens to
hundreds of kilograms—are among the most vulnera-
ble to overexploitation. In general, a species’ genera-
tion time (interval from birth to reproductive age) is a
function of body mass (allometry), so larger, longer-
lived, and slower-reproducing animal populations are
generally unable to compensate for high rates of har-
vesting. Because slow-breeding large animals, such as
apes, carnivores (e.g., the lion, Panthera leo), and Af-
rican elephants (Loxodonta africana), are particularly
vulnerable to hunting, the potential for population
recovery in these animals over short time scales is low.
As an example supporting this generality, there is evi-
dence that 12 large vertebrate species have been ex-
tirpated from Vietnam, primarily because of excessive
hunting, within the past 40 years. The Steller’s sea cow
(Hydrodamalis gigas), an aquatic herbivorous mam-
mal that inhabited the Asian coast of the Bering Sea,
is the quintessential example of the rapid demise of a
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516 Conservation Biology
species as a result of overexploitation. Discovered in
1741, it became extinct by 1768 because of overhunt-
ing by sailors, seal hunters, and fur traders. This species
was hunted for food, its skin for making boats, and its
subcutaneous fat for use in oil lamps.
The ecosystem and biological community changes
precipitated by invasive species represent another
leading cause of biodiversity loss. Of 170 extinct spe-
cies for which causes have been identified reliably,
invasive species contributed directly to the demise of
91 (54%). In particular, the rates of extinctions oc-
curring on islands have been greatly elevated by the
introduction of novel predators. Several ecological and
life-history attributes of island species, such as their
naturally constrained geographic range, small popula-
tion sizes, and particular traits (e.g., lack of flight in
birds or lack of thorns in plants) make island biotas
vulnerable to predation from invading species. For
example, the introduction of the brown tree snake
(Boiga irregularis) shortly after World War II wreaked
havoc on the biodiversity of the island of Guam in the
South Pacific. In all likelihood, tree snakes were di-
rectly responsible for the loss of 12 of 18 native bird
species, and they also reduced the populations of other
vertebrates such as flying foxes (Pteropus mariannus),
mainly because of the inability of the island’s native
species to recognize the novel predator as a threat.
Despite an annual expenditure of US$44.6 million for
the management of this problem, tree snakes on Guam
are still not under control, largely because of their
ability to penetrate artificial snake barriers such as
fences.
The mosquito Culex quinquefasciatus was inad-
vertently introduced to Hawaii in 1826, and the
disease-causing parasite (Plasmodium relictum) it car-
ries arrived soon after. Since then, avian malaria (in
conjunction with other threats) has been responsible
for the decline and extinction of some 60 species of
endemic forest birds on the Hawaiian Islands. Having
evolved in the absence of the disease, Hawaiian bird
species were generally unable to cope with the debili-
tating effects of the novel parasite. However, more
than 100 years after the establishment of the disease,
some native thrushes (Myadestes spp.) are now show-
ing resistance to the disease. Sadly, many of the re-
maining species, especially forest birds in the family
Drepanididae, are still vulnerable and are now re-
stricted to altitudes where temperatures are below the
thermal tolerance limits of the mosquito vector. Global
warming is predicted to increase the altitudinal distri-
bution of the mosquito, thus spelling doom for disease-
susceptible birds as mosquito-free habitats disappear.
The most feasible method of reducing transmission of
malaria is to reduce or eliminate vector mosquito
populations through chemical treatments and the elim-
ination of larval habitats.
Perhaps one of the most infamous examples of an
invasion catastrophe occurred in the world’s largest
freshwater lake—Lake Victoria in tropical East Africa.
Celebrated for its amazing collection of over 600
endemic haplochromine (i.e., formerly of the genus
Haplochromis) cichlid fishes (Family Cichlidae), the
Lake Victoria cichlid community is perhaps one of the
most rapid, extensive, and recent vertebrate radiations
known. There is also a rich community of endemic
noncichlid fish that inhabit the Lake. In addition to the
threats posed to this unique biota by a rapid rise in
fisheries exploitation, human density, deforestation,
and agriculture during the past century, without doubt
the most devastating effect was the introduction of the
predatory Nile perch (Lates niloticus) in the 1950s.
This voracious predator, which can grow to more than
2 m in length, was introduced from lakes Albert and
Turkana (Uganda and Kenya, respectively) to com-
pensate for depleting commercial fisheries in Lake
Victoria. Although the Nile perch population remained
relatively low for several decades after its introduc-
tion, an eventual population explosion in the 1980s
caused the devastating direct or indirect extinction of
200–400 cichlid species endemic to the Lake as well as
the extinction of several noncichlid fish species. Al-
though many other threats likely contributed to the
observed extinctions, including direct overexploitation
and eutrophication from agriculture and deforestation
leading to a change in the algal plankton community,
there are few other contemporary examples of such a
rapid and massive extinction event involving a single
group of closely related species.
Human-mediated climate change represents a po-
tentially disastrous sleeping giant in terms of future
biodiversity losses. Climate warming can affect species
in five principal ways: (1) alterations of species densi-
ties (including altered community composition and
structure); (2) range shifts, either poleward or upward
in elevation; (3) behavioral changes, such as the phe-
nology (seasonal timing of life cycle events) of migra-
tion, breeding, and flowering; (4) changes in mor-
phology, such as body size; and (5) reduction in genetic
diversity that leads to inbreeding depression. A related
threat for island and coastal biotas is the predicted loss
of habitat via inundation by rising sea levels. Although
large fluctuations in climate have occurred regularly
throughout Earth’s history, the implications of an-
thropogenic global warming for contemporary biodi-
versity are particularly pessimistic because of the rate
of change and previous heavy modification of land-
scapes by humans. Good empirical evidence for some
of these effects is rare, and speculations abound, but
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517 Species Extinctions
there are already many local or regional examples
and model-based predictions that support the view
that rapid climate change, acting in concert with other
drivers of species loss and habitat degradation, will be
one of the most pressing conservation issues global
biodiversity faces over the coming centuries.
One glimpse of a possible future crisis comes
from the highland forests of Monterverde (Costa Rica),
where 40% (20 of 50) of frog and toad species dis-
appeared following synchronous population crashes in
1987, with most crashes linked to a rapid progressive
warming and drying of the local climate. The locally
endemic golden toad (Bufo periglenes) was one of the
high-profile casualties in this area. It has been sug-
gested that climate warming resulted in a retreat of the
clouds and a drying of the mountain habitats, making
amphibians more susceptible to fungal and parasite
outbreaks. Indeed, the pathogenic chytrid fungus Ba-
trachochytrium dendrobatidis, which grows on am-
phibian skin and increases mortality rates, has been
implicated in the loss of harlequin frogs (Atelopus spp.)
in Central and South America and reductions in
other amphibian populations elsewhere. It is hypoth-
esized that warm and dry conditions may stress am-
phibians and make them more vulnerable to the fungal
infection.
Irrespective of the reason for a population’s decline
from a large to small population size, unusual (and
often random and detrimental) events assume promi-
nence at low abundances. For instance, although
competition among individuals is reduced at low den-
sities and can induce a population rebound, a coun-
tervailing phenomenon known as the ‘‘Allee effect’’ can
act to draw populations toward extinction by (for in-
stance) disrupting behavioral patterns that depend on
numbers (e.g., herd defense against predators) or by
genetic threats such as inbreeding depression. Small
populations, dominated by chance events and Allee
effects, are often considered to have dipped below their
‘‘minimum viable population’’ size. Thus, once a major
population decline has occurred (from habitat loss,
overexploitation, or in response to many other possible
stressors), an ‘‘extinction vortex’’ of positive feed-
back loops can doom species to extinction, even if the
original threats have been alleviated. Further, many
species may take decades to perish following habitat
degradation. Although some species may withstand
the initial shock of land clearing, factors such as the
lack of food resources, breeding sites, and dispersers
may make populations unviable, and they eventually
succumb to extinction. This phenomenon evokes the
concept of ‘‘living-dead’’ species, or those ‘‘committed
to extinction.’’ The eventual loss of such species is
referred to as the ‘‘extinction debt’’ caused by past
habitat loss. For example, even if net deforestation
rates can be reduced or even halted, the extinction debt
of remnant and secondary forest patches will see the
extinction of countless remaining species over this
interval.
3. EXTINCTION VULNERABILITY
Certain life-history, behavioral, morphological, and
physiological characteristics appear to make some spe-
cies more susceptible than others to the extinction
drivers described above. In general, large-sized species
with a restricted distribution that demonstrate habitat
specialization tend to be at greater risk of extinction
from human agency than others within their respective
taxa (e.g., Javan rhinoceros, Rhinoceros sondaicus),
especially to processes such as rapid habitat loss.
Because of their high habitat specificity and/or low
population densities, rare species may be more prone to
extinction than common species. The size of a species’
range is also a major determinant of its extinction
proneness. Small ranges may make species more vul-
nerable to stochastic perturbations, even if local abun-
dance is high; for example, proportionally more
passerines (perching birds) with relatively small geo-
graphic ranges in the Americas are at risk of extinction
than their more widely distributed counterparts. Such
trends are worrisome because those species with
shrinking ranges as a result of adverse human activities
become particularly vulnerable to other drivers such as
climate change. Habitat loss also reduces the patch
sizes necessary for species requiring large home ranges,
making them vulnerable to extinction from a loss of
subpopulation connectedness, reduced dispersal ca-
pacity, and the ensuing lower population viability.
Larger-bodied vertebrates are considered to be more
extinction-prone than smaller-bodied ones when the
threatening process unfolds rapidly or intensely. In-
deed, threatened mammals are an order of magnitude
heavier than nonthreatened ones. A common expla-
nation for this trend is that body size is inversely cor-
related with population size, making large-bodied an-
imals less abundant and more vulnerable to chronic
environmental perturbations (while being buffered
against short-term environmental fluctuations). The
extinction proneness of large-bodied animals to human
activities is further enhanced because of other corre-
lated traits, such as their requirement of large area,
greater food intake, high habitat specificity, and lower
reproductive rate.
Large species can also be more vulnerable to human
persecution such as hunting, whereas smaller species
are generally more vulnerable to habitat loss. It is im-
portant, however, to be cautious when constructing
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518 Conservation Biology
generalized rules regarding the role of body size in the
extinction process. Because they have a slower repro-
ductive rate, larger parrots are more vulnerable to
overexploitation than smaller finches, despite fewer
numbers of the former being captured for the pet trade.
However, some smaller species (e.g., white-eyes, Zos-
terops spp.) with small population sizes are also vul-
nerable to extinction because of heavy harvest rates for
the pet trade, suggesting that only when the threatening
processes are approximately equivalent will the larger
of two species being compared demonstrate a higher
risk of extinction. In addition to body size, other
morphological characteristics affect extinction prone-
ness. For instance, large investment in secondary sexual
characteristics may render highly dimorphic species
less adaptable in a changing environment or more at-
tractive to specimen or pet-trade collectors.
When an environment is altered abruptly or sys-
tematically at a rate above normal background change,
or beyond the capacity of adaptation via natural se-
lection, specialist species with narrow ecological niches
often bear the brunt of progressively unfavorable
conditions such habitat loss and degradation. For in-
stance, highly specialized forest-dependent taxa are
acutely vulnerable to extinction following deforesta-
tion and forest fragmentation. Possible mechanisms
include reductions in breeding and feeding sites, in-
creased predation, elevated soil erosion and nutrient
loss, dispersal limitation, enhanced edge effects, and
other stressors. Conversely, non-forest-dependent spe-
cies or those that prefer open habitats are often better
able to persist in disturbed landscapes and may even be
favored by having fewer competitors or expanded
ranges following deforestation. It is important to be
aware that in relatively stable systems, evolution en-
genders the speciation of taxa that occupy all available
niches so both specialist and generalist species can co-
exist. As a result, the rapid pace of habitat and climate
change renders specialization a modern ‘‘curse’’ in
evolutionary terms.
Foraging specialization is one mechanism that can
compromise a species’ ability to persist in altered
habitats. Many studies have shown that frugivorous
and insectivorous birds are more extinction-prone than
other avian feeding guilds, with the lack of year-round
access to fruiting plants in fragmented forests being the
culprit for the former. A number of hypotheses have
been proposed to explain the disappearance of insec-
tivorous birds from deforested or fragmented areas.
First, deforestation may impoverish the insect fauna
and reduce selected insectivore microhabitats (e.g.,
dead leaves). Second, insectivores may be poor dis-
persers and have near-ground nesting habits, the latter
trait making them more vulnerable to nest predators
penetrating smaller forest fragments. Absence of some
insectivorous bird species from small fragments may
not be related to food scarcity; rather, it may result
from their poorer dispersal abilities. The ability to
disperse in birds and insects depends on morphological
characteristics such as wing loading, and physiological
restrictions such as intolerance to sunlight when mov-
ing within the nonforested matrix landscape separat-
ing fragments. As a result, poor dispersal ability may
make certain species vulnerable to extinction because
they cannot readily supplement sink habitats (habi-
tats in which populations cannot replace themselves),
supporting otherwise unviable subpopulations, or
colonize new areas. Because of poor dispersal abil-
ity, patchy distributions, and generally low popula-
tion densities, the genetic diversity of species in
fragmented landscapes may be difficult to maintain,
with the resulting inbreeding depression further re-
ducing population size toward extinction. However,
clear and quantitative demonstrations of the role of
life-history traits in the extinction process of biotas are
still rare.
4. CONSEQUENCES OF EXTINCTIONS
The extinction of certain species such as large preda-
tors and pollinators may have more devastating eco-
logical consequences than the extinction of others.
Ironically, avian vulnerability to predation is often
exacerbated when certain large predatory species be-
come rarer in tropical communities. For example, al-
though large cats such as jaguars (Panthera onca)do
not prey on small birds directly, they exert a limiting
force on smaller predators such as medium-sized and
small mammals (mesopredators), which become more
abundant with the former species’ decline. The cor-
ollary is that abundant mesopredators inflict an above-
average predation rate on the eggs and nestlings of
small birds. Although this ‘‘mesopredator-release’’
hypothesis has been applied largely to mammals (e.g.,
Australian dingoes, Canis lupus, suppressing foxes and
cats; coyotes in California controlling cat abundance),
the loss of large predatory birds such as the harpy eagle
(Harpia harpyja) may have similar ecosystem effects.
Similar mesopredator release has been demonstrated
for the first time in the marine environment, where the
overexploitation of large pelagic sharks resulted in an
increase in rays and skates that eventually suppressed
commercially important scallop populations. Likewise,
does the disappearance of a competitor result in the
niche expansion and higher densities of subordinate
species? This phenomenon has been observed between
unrelated taxa—the extinction of insectivorous birds
from scrub forests of West Indian islands correlated
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519 Species Extinctions
with the subsequent higher biomass of competing
Anolis lizards.
Conservation biologists have traditionally focused
on the study of the independent declines, extirpations,
or extinctions of individual species while paying rela-
tively less attention to the possible cascading effects of
species coextinctions (e.g., hosts and their parasites).
However, it is likely that many coextinctions between
interdependent taxa have occurred, but most have gone
unnoticed in these relatively understudied systems. For
example, an extinct feather louse (Columbicola ex-
tinctus) was discovered in 1937, 23 years after likely
coextinction with its host passenger pigeon (Ectopistes
migratorius). Ecological processes disrupted by ex-
tinction or species decline may also lead to cascading
and catastrophic coextinctions. Frugivorous animals
and fruiting plants on which they depend have a key
interaction linking plant reproduction and dispersal
with animal nutrition. Thus, the two interdependent
taxa are placed in jeopardy by habitat degradation.
Many trees produce large, lipid-rich fruits adapted for
animal dispersal, so the demise of avian frugivores may
have serious consequences for forest regeneration, even
if the initial drivers of habitat loss and degradation are
annulled.
Essential ecosystem functions provided by forest
invertebrates are also highly susceptible when species
are lost after habitat loss and degradation. Acting as
keystone species in Southeast Asian rainforests, figs
rely on tiny (1–2 mm) species-specific wasps for their
pollination. Some fig wasps may have limited dispersal
ability, suggesting that forest disturbance can reduce
wasp densities and, by proxy, the figs that they polli-
nate. Similarly, dung beetles are essential components
of ecosystem function because they contribute heavily
to nutrient-recycling processes, seed dispersal, and the
reduction of disease risk associated with dung accu-
mulation. In Venezuela, heavier dung beetles were
more extinction-prone than lighter species on artifi-
cially created forested islands, which predicts particu-
larly dire ecosystem functional loss given the former
group’s greater capacity to dispose of dung.
Almost all flowering plants in tropical rainforests
are pollinated by animals, and an estimated one-third
of the human diet in tropical countries is derived from
insect-pollinated plants. Therefore, a decline of forest-
dwelling pollinators impedes plant reproduction not
only in forests but also in neighboring agricultural
areas visited by these species. Lowland coffee (Coffea
canephora) is an important tropical cash crop, and it
depends on bees for cross-pollination. A study in Costa
Rica found that forest bees increased coffee yield by
20% in fields within 1 km of the forest edge. Between
2000 and 2003, the pollination services provided by
forest bees were worth US$60,000 to a 1100-ha farm.
A forest patch as small as 20 ha located near farms
can increase coffee yield and thus bring large eco-
nomic benefits to the farmers. Such findings illustrate
the imperative of preserving native forests near agro-
forestry systems to facilitate the travel by forest-
dependent pollinating insects.
5. CONCLUSIONS
Although extinctions are a normal part of evolution,
human modifications to the planet in the last few
centuries, and perhaps even millennia, have greatly
accelerated the rate at which extinctions occur. Habitat
loss remains the main driver of extinctions, but it may
act synergistically with other drivers such as over-
harvesting and pollution, and, in the future, climate
change. Large-bodied species, rare species, and habitat
specialists are particularly prone to extinction as a re-
sult of rapid human modifications of the planet. Ex-
tinctions can disrupt vital ecological processes such as
pollination and seed dispersal, leading to cascading
losses, ecosystem collapse, and a higher extinction rate
overall.
FURTHER READING
Brook, Barry W., Navjot S. Sodhi, and Peter K. L. Ng. 2003.
Catastrophic extinctions follow deforestation in Singa-
pore. Nature 424: 420–423. This is one of few papers
reporting broad-scale extinctions driven by tropical de-
forestation.
Clavero, Miguel, and Emili Garcia-Berthou. 2005. Invasive
species are a leading cause of animal extinctions. Trends in
Ecology and Evolution 20: 110. The article highlights that
invasive species represent one of the primary threats to
biodiversity.
Dirzo, Rudolfo, and Peter J. Raven. 2003. Global state of
biodiversity and loss. Annual Review of Environment and
Resources 28: 137–167. The article constitutes a major
review of the state of the modern global biodiversity and
its associated losses.
Fagan, William F., and E. E. Holmes. 2006. Quantifying the
extinction vortex. Ecology Letters 9: 51–60. This is the
only study yet to quantify the final phases of extinction in
vertebrates for which date of extinction was known.
IUCN Red List of threatened species. Download from http:
//www.iucnredlist.org. This presents an up-to-date clas-
sification of and reasons for a listed species’ conservation
status.
Koh, Lian P., Robert R. Dunn, Navjot S. Sodhi, Robert
K. Colwell, Heather C. Procter, and Vince S. Smith.
2004. Species co-extinctions and the biodiversity crisis.
Science 305: 1632–1634. This models how loss of a spe-
cies could indirectly result in the extinction of dependent
species.
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520 Conservation Biology
Pimm, Stuart L., and Peter Raven. 2000. Extinction by
numbers. Nature 403: 843–845. The article summarizes
the likely extent of biodiversity losses as a result of human
activities.
Pounds, J. Alan, Martin R. Bustamante, Luis A. Coloma,
Jamie A. Consuegra, Michael P. L. Fogden, Pru N. Foster,
Enrique La Marca, Karen L. Masters, Andres Merino-
Viteri, Robert Puschendorf, Santiago R. Ron, G. Arturo
Sanchez-Azofeifa, Christopher J. Still, and Bruce E.
Young. 2006. Widespread amphibian extinctions from
epidemic disease driven by global warming. Nature 439:
161–167. The article provides evidence on the role of
climate change in recent amphibian extinctions.
Ricketts, Taylor H., Gretchen C. Daily, Paul R. Ehrlich, and
C. D. Michener. 2004. Economic value of tropical forest
to coffee production. Proceedings of the National Acad-
emy of Sciences U.S.A. 34: 12579–12582. This work
shows how the loss of ecosystem services can affect pol-
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Rosser, Alison M., and Sue A. Manika. 2002. Over-
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