Copyrighted Material

V.1
Causes and Consequences of
Species Extinctions
Navjot S. Sodhi, Barry W. Brook,
and Corey J. A. coextinction. Extinction of one species
triggers the loss of another
species.
Bradshaw extinction debt. This refers to the
extinction of species or
OUTLINE populations long after habitat
1. Introduction alteration.
2. Extinction drivers extinction vortex. As populations
3. Extinction vulnerability decline, an insidious mutual
4. Consequences of extinctions reinforcement occurs among
5. Conclusions biotic and abiotic processes
driving population size downward
The five largest mass die-offs in which to extinction.
50–95% of species were eliminated extirpation. This refers to extinction
occurred during the Ordovician [490– of a population rather than of an
443 million years ago (mya)], Devonian entire species.
(417–354 mya), Permian (299–250 mya), invasive species. These are
Triassic (251–200 mya), and Cretaceous nonindigenous species introduced
(146–64 mya) periods. Most recently, to areas outside of their natural
human actions especially over the past range that have become
two centuries have precipitated a global established and have spread.
extinction crisis or the ‘‘sixth great megafauna. This refers to large-
extinction wave’’ comparable to the bodied (>44 kg) animals,
previous five. Increasing human commonly (but not exclusively)
populations over the last 50,000 years used to refer to the large mammal
or so have left measurable negative biota of the Pleistocene.
footprints on biodiversity. minimum viable population. This is the
number of individuals in a
GLOSSARY population required to have a
specified probability of
Allee effects. These factors cause a persistence over a given period of
reduction in the growth rate of time.
small populations as they decline
(e.g., via reduced survival or
reproductive success).
 Copyrighted Material
515 Conservation Biology
1. INTRODUCTION known amphibian, bird, and mammal
species, respectively (by far the best-
In the Americas, charismatic large-
studied of all animal groups), are
bodied animals (megafauna) such as
currently listed by the IUCN as
saber-toothed cats (Smilodon spp.),
under threat.
mammoths (Mammuthus spp.), and
Just how many species are being
giant ground sloths (Megalonyx
lost each year is also hotly debated.
jeffersonii) vanished following Various estimates range from a few
human arrival some 11,000–13,000 thousand to more than 100,000
years ago. Similar losses occurred in species being extinguished every
Australia 45,000 years ago, and in year, most without ever having been
many oceanic islands within a few scientifically described. The large
hundred years of the arrival of uncertainty comes mainly through
humans. Classic examples of the loss the application of various species–
of island endemics include the dodo area relationships that vary
(Raphus cucullatus) from Mauritius, substantially among communities
moas (e.g., Dinornis maximus) from and habitats. Despite substantial
New Zealand, and elephantbirds prediction error, it is nevertheless
(Aepyornis maximus) from certain that human actions are
Madagascar. Megafaunal collapse causing the structure and function of
during the late Pleistocene can natural systems to unravel. The past
largely be traced to a variety of five great extinctions shared some
negative human impacts, such as important commonalities: (1) they
overharvesting, biological invasions, caused a catastrophic loss of global
and habitat transformation. biodiversity; (2) they unfolded
The rate and extent of human- rapidly (at least in the context of
mediated extinctions are debated, but evolutionary and geological time);
there is general agreement that (3) taxonomically, their impact was
extinction rates have soared over the not random (that is, whole groups of
past few hundred years, largely as a related species were lost while other
result of accelerated habitat related groups remained largely
destruction following European unaffected); and (4) the survivors
colonialism and the subsequent were often not previously dominant
global expansion of the human evolutionary groups. All four of
population during the twentieth these features are relevant to the
century. Humans are implicated current biodiversity crisis. This sixth
directly or indirectly in the 100- to great extinction is likely to be most
10,000-fold increase in the ‘‘natural’’ catastrophic in tropical regions given
or ‘‘background’’ extinction rate that the high species diversity there
normally occurs as a consequence of (more than two-thirds of all species)
gradual environmental change, and the large, expanding human
newly established competitive populations that threaten most
interactions (by evolution or species there as well.
invasion), and occasional chance The major ‘‘systematic drivers’’
calamities such as fire, storms, or of modern species loss are changes
disease. The current and future in land use (habitat loss degradation
extinction rates are estimated using a and fragmentation), overexploitation,
variety of measures such as species– invasive species, disease, climate
area models and changes in the change (global warming) connected
World Conservation Union’s (IUCN) to increasing concentration of
threat categories over time. Based on atmospheric carbon dioxide, and
the global assessment of all known increases in nitrogen deposition.
species, some 31, 12, and 20% of Mechanisms for prehistoric (caused
 Copyrighted Material
Species Extinctions
516
by humans >200 years ago) operate synergistically with other
extinctions are likely to have been drivers such as habitat loss. For
similar: overhunting, introduced example, roads and trails created to
predators and diseases, and habitat allow logging operations to penetrate
destruction when early people first into virgin forests make previously
arrived in virgin landscapes. remote areas more accessible to
human hunters, who can, in turn,
cause the decline and eventual
2. EXTINCTION DRIVERS
extirpation of forest species. It is
Some events can instantly eliminate estimated that overexploitation is a
all individuals of a particular species, major threat to at least one-third of
such as an asteroid strike, a massive threatened birds and amphibians,
volcanic eruption, or even a rapid with wildlife currently extracted
loss of large areas of unique and from tropical forests at
critical habitat because of approximately six times the
deforestation. But ultimately, any sustainable rate. In other words, the
phenomena that can cause mortality quantity, and most likely the
rates to exceed reproductive diversity, of human prey— both
replacement over a sustained period fisheries and ‘‘bush’’ (wild) meat—
can cause a species to become are rapidly diminishing.
extinct. Such forces may act Megafauna—those species
independently or synergistically, and weighing in the tens to hundreds of
it may be difficult to identify a single kilograms—are among the most
cause of a particular species vulnerable to overexploitation. In
extinction event. For instance, general, a species’ generation time
habitat loss may cause some (interval from birth to reproductive
extinctions directly by removing all age) is a function of body mass
individuals, but it can also be (allometry), so larger, longerlived,
indirectly responsible for an and slower-reproducing animal
extinction by facilitating the populations are generally unable to
establishment of an invasive species compensate for high rates of
or disease agent, improving access to harvesting. Because slow-breeding
human hunters, or altering large animals, such as apes,
biophysical conditions. As a result, carnivores (e.g., the lion, Panthera
any process that causes a population leo), and African elephants
to dwindle may ultimately (Loxodonta africana), are particularly
predispose that population to vulnerable to hunting, the potential
extinction. for population recovery in these
Evidence to date suggests that animals over short time scales is low.
deforestation is currently, and is As an example supporting this
projected to continue to be, the prime generality, there is evidence that 12
direct and indirect cause of reported large vertebrate species have been
extirpations. For example, it is extirpated from Vietnam, primarily
predicted that up to 21% of because of excessive hunting, within
Southeast Asian forest species will the past 40 years. The Steller’s sea
be lost by 2100 because of past and cow (Hydrodamalis gigas), an aquatic
ongoing deforestation. Similar herbivorous mammal that inhabited
projections exist for biotas in other the Asian coast of the Bering Sea, is
regions. the quintessential example of the
Overexploitation is also an rapid demise of a species as a result
important driver of extinctions of overexploitation. Discovered in
among vertebrates and tends to
 Copyrighted Material
517 Conservation Biology
1741, it became extinct by 1768 arrived soon after. Since then, avian
because of overhunting by sailors, malaria (in conjunction with other
seal hunters, and fur traders. This threats) has been responsible for the
species was hunted for food, its skin decline and extinction of some 60
for making boats, and its species of endemic forest birds on
subcutaneous fat for use in oil lamps. the Hawaiian Islands. Having
The ecosystem and biological evolved in the absence of the
community changes precipitated by disease, Hawaiian bird species were
invasive species represent another generally unable to cope with the
leading cause of biodiversity loss. Of debilitating effects of the novel
170 extinct species for which causes parasite. However, more than 100
have been identified reliably, years after the establishment of the
invasive species contributed directly disease, some native thrushes
to the demise of 91 (54%). In (Myadestes spp.) are now showing
particular, the rates of extinctions resistance to the disease. Sadly,
occurring on islands have been many of the remaining species,
greatly elevated by the introduction especially forest birds in the family
of novel predators. Several Drepanididae, are still vulnerable
ecological and life-history attributes and are now restricted to altitudes
of island species, such as their where temperatures are below the
naturally constrained geographic thermal tolerance limits of the
range, small population sizes, and mosquito vector. Global warming is
particular traits (e.g., lack of flight in predicted to increase the altitudinal
birds or lack of thorns in plants) distribution of the mosquito, thus
make island biotas vulnerable to spelling doom for diseasesusceptible
predation from invading species. For birds as mosquito-free habitats
example, the introduction of the disappear. The most feasible method
brown tree snake (Boiga irregularis) of reducing transmission of malaria
shortly after World War II wreaked is to reduce or eliminate vector
havoc on the biodiversity of the mosquito populations through
island of Guam in the South Pacific. chemical treatments and the
In all likelihood, tree snakes were elimination of larval habitats.
directly responsible for the loss of 12 Perhaps one of the most infamous
of 18 native bird species, and they examples of an invasion catastrophe
also reduced the populations of other occurred in the world’s largest
vertebrates such as flying foxes freshwater lake—Lake Victoria in
(Pteropus mariannus), mainly tropical East Africa. Celebrated for
because of the inability of the its amazing collection of over 600
island’s native species to recognize endemic haplochromine (i.e.,
the novel predator as a threat. formerly of the genus Haplochromis)
Despite an annual expenditure of cichlid fishes (Family Cichlidae), the
US$44.6 million for the management Lake Victoria cichlid community is
of this problem, tree snakes on Guam perhaps one of the most rapid,
are still not under control, largely extensive, and recent vertebrate
because of their ability to penetrate radiations known. There is also a rich
artificial snake barriers such as community of endemic noncichlid
fences. fish that inhabit the Lake. In addition
The mosquito Culex to the threats posed to this unique
quinquefasciatus was inadvertently biota by a rapid rise in fisheries
introduced to Hawaii in 1826, and exploitation, human density,
the disease-causing parasite deforestation, and agriculture during
(Plasmodium relictum) it carries the past century, without doubt the
most devastating effect was the
 Copyrighted Material
Species Extinctions
518
introduction of the predatory Nile warming for contemporary
perch (Lates niloticus) in the 1950s. biodiversity are particularly
This voracious predator, which can pessimistic because of the rate of
grow to more than 2 m in length, was change and previous heavy
introduced from lakes Albert and modification of landscapes by
Turkana (Uganda and Kenya, humans. Good empirical evidence
respectively) to compensate for for some of these effects is rare, and
depleting commercial fisheries in speculations abound, but there are
Lake Victoria. Although the Nile already many local or regional
perch population remained relatively examples and model-based
low for several decades after its predictions that support the view that
introduction, an eventual population rapid climate change, acting in
explosion in the 1980s caused the concert with other drivers of species
devastating direct or indirect loss and habitat degradation, will be
extinction of 200–400 cichlid species one of the most pressing
endemic to the Lake as well as the conservation issues global
extinction of several noncichlid fish biodiversity faces over the coming
species. Although many other threats centuries.
likely contributed to the observed One glimpse of a possible future
extinctions, including direct crisis comes from the highland
overexploitation and eutrophication forests of Monterverde (Costa Rica),
from agriculture and deforestation where 40% (20 of 50) of frog and
leading to a change in the algal toad species disappeared following
plankton community, there are few synchronous population crashes in
other contemporary examples of 1987, with most crashes linked to a
such a rapid and massive extinction rapid progressive warming and
event involving a single group of drying of the local climate. The
closely related species. locally endemic golden toad (Bufo
Human-mediated climate change periglenes) was one of the high-
represents a potentially disastrous profile casualties in this area. It has
sleeping giant in terms of future been suggested that climate warming
biodiversity losses. Climate warming resulted in a retreat of the clouds and
can affect species in five principal a drying of the mountain habitats,
ways: (1) alterations of species making amphibians more susceptible
densities (including altered to fungal and parasite outbreaks.
community composition and Indeed, the pathogenic chytrid
structure); (2) range shifts, either fungus Batrachochytrium
poleward or upward in elevation; (3) dendrobatidis, which grows on
behavioral changes, such as the amphibian skin and increases
phenology (seasonal timing of life mortality rates, has been implicated
cycle events) of migration, breeding, in the loss of harlequin frogs
and flowering; (4) changes in (Atelopus spp.) in Central and South
morphology, such as body size; and America and reductions in other
(5) reduction in genetic diversity that amphibian populations elsewhere. It
leads to inbreeding depression. A is hypothesized that warm and dry
related threat for island and coastal conditions may stress amphibians
biotas is the predicted loss of habitat and make them more vulnerable to
via inundation by rising sea levels. the fungal infection.
Although large fluctuations in Irrespective of the reason for a
climate have occurred regularly population’s decline from a large to
throughout Earth’s history, the small population size, unusual (and
implications of anthropogenic global
 Copyrighted Material
519 Conservation Biology
often random and detrimental) events species more susceptible than others
assume prominence at low to the extinction drivers described
abundances. For instance, although above. In general, large-sized species
competition among individuals is with a restricted distribution that
reduced at low densities and can demonstrate habitat specialization
induce a population rebound, a tend to be at greater risk of
countervailing phenomenon known extinction from human agency than
as the ‘‘Allee effect’’ can act to draw others within their respective taxa
populations toward extinction by (for (e.g., Javan rhinoceros, Rhinoceros
instance) disrupting behavioral sondaicus), especially to processes
patterns that depend on numbers such as rapid habitat loss.
(e.g., herd defense against predators) Because of their high habitat
or by genetic threats such as specificity and/or low population
inbreeding depression. Small densities, rare species may be more
populations, dominated by chance prone to extinction than common
events and Allee effects, are often species. The size of a species’ range
considered to have dipped below is also a major determinant of its
their ‘‘minimum viable population’’ extinction proneness. Small ranges
size. Thus, once a major population may make species more vulnerable
decline has occurred (from habitat to stochastic perturbations, even if
loss, overexploitation, or in response local abundance is high; for example,
to many other possible stressors), an proportionally more passerines
‘‘extinction vortex’’ of positive (perching birds) with relatively small
feedback loops can doom species to geographic ranges in the Americas
extinction, even if the original threats are at risk of extinction than their
have been alleviated. Further, many more widely distributed counterparts.
species may take decades to perish Such trends are worrisome because
following habitat degradation. those species with shrinking ranges
Although some species may as a result of adverse human
withstand the initial shock of land activities become particularly
clearing, factors such as the lack of vulnerable to other drivers such as
food resources, breeding sites, and climate change. Habitat loss also
dispersers may make populations reduces the patch sizes necessary for
unviable, and they eventually species requiring large home ranges,
succumb to extinction. This making them vulnerable to extinction
phenomenon evokes the concept of from a loss of subpopulation
‘‘living-dead’’ species, or those connectedness, reduced dispersal
‘‘committed to extinction.’’ The capacity, and the ensuing lower
eventual loss of such species is population viability.
referred to as the ‘‘extinction debt’’ Larger-bodied vertebrates are
caused by past habitat loss. For considered to be more extinction-
example, even if net deforestation prone than smaller-bodied ones when
rates can be reduced or even halted, the threatening process unfolds
the extinction debt of remnant and rapidly or intensely. Indeed,
secondary forest patches will see the threatened mammals are an order of
extinction of countless remaining magnitude heavier than
species over this interval. nonthreatened ones. A common
explanation for this trend is that body
3. EXTINCTION VULNERABILITY size is inversely correlated with
population size, making large-bodied
Certain life-history, behavioral, animals less abundant and more
morphological, and physiological vulnerable to chronic environmental
characteristics appear to make some perturbations (while being buffered
 Copyrighted Material
Species Extinctions
520
against short-term environmental taxa are acutely vulnerable to
fluctuations). The extinction extinction following deforestation
proneness of large-bodied animals to and forest fragmentation. Possible
human activities is further enhanced mechanisms include reductions in
because of other correlated traits, breeding and feeding sites, increased
such as their requirement of large predation, elevated soil erosion and
area, greater food intake, high habitat nutrient loss, dispersal limitation,
specificity, and lower reproductive enhanced edge effects, and other
rate. stressors. Conversely, non-forest-
Large species can also be more dependent species or those that
vulnerable to human persecution prefer open habitats are often better
such as hunting, whereas smaller able to persist in disturbed
species are generally more landscapes and may even be favored
vulnerable to habitat loss. It is by having fewer competitors or
important, however, to be cautious expanded ranges following
when constructing generalized rules deforestation. It is important to be
regarding the role of body size in the aware that in relatively stable
extinction process. Because they systems, evolution engenders the
have a slower reproductive rate, speciation of taxa that occupy all
larger parrots are more vulnerable to available niches so both specialist
overexploitation than smaller and generalist species can coexist. As
finches, despite fewer numbers of the a result, the rapid pace of habitat and
former being captured for the pet climate change renders specialization
trade. However, some smaller a modern ‘‘curse’’ in evolutionary
species (e.g., white-eyes, Zosterops terms.
spp.) with small population sizes are Foraging specialization is one
also vulnerable to extinction because mechanism that can compromise a
of heavy harvest rates for the pet species’ ability to persist in altered
trade, suggesting that only when the habitats. Many studies have shown
threatening processes are that frugivorous and insectivorous
approximately equivalent will the birds are more extinction-prone than
larger of two species being compared other avian feeding guilds, with the
demonstrate a higher risk of lack of year-round access to fruiting
extinction. In addition to body size, plants in fragmented forests being
other morphological characteristics the culprit for the former. A number
affect extinction proneness. For of hypotheses have been proposed to
instance, large investment in explain the disappearance of
secondary sexual characteristics may insectivorous birds from deforested
render highly dimorphic species less or fragmented areas. First,
adaptable in a changing environment deforestation may impoverish the
or more attractive to specimen or insect fauna and reduce selected
pet-trade collectors. insectivore microhabitats (e.g., dead
When an environment is altered leaves). Second, insectivores may be
abruptly or systematically at a rate poor dispersers and have near-
above normal background change, or ground nesting habits, the latter trait
beyond the capacity of adaptation via making them more vulnerable to nest
natural selection, specialist species predators penetrating smaller forest
with narrow ecological niches often fragments. Absence of some
bear the brunt of progressively insectivorous bird species from small
unfavorable conditions such habitat fragments may not be related to food
loss and degradation. For instance, scarcity; rather, it may result from
highly specialized forest-dependent their poorer dispersal abilities. The
 Copyrighted Material
521 Conservation Biology
ability to disperse in birds and Although this ‘‘mesopredator-
insects depends on morphological release’’ hypothesis has been
characteristics such as wing loading, applied largely to mammals (e.g.,
and physiological restrictions such as Australian dingoes, Canis lupus,
intolerance to sunlight when moving suppressing foxes and cats; coyotes
within the nonforested matrix in California controlling cat
landscape separating fragments. As a abundance), the loss of large
result, poor dispersal ability may predatory birds such as the harpy
make certain species vulnerable to eagle (Harpia harpyja) may have
extinction because they cannot similar ecosystem effects. Similar
readily supplement sink habitats mesopredator release has been
(habitats in which populations cannot demonstrated for the first time in
replace themselves), supporting the marine environment, where the
otherwise unviable subpopulations, overexploitation of large pelagic
or colonize new areas. Because of sharks resulted in an increase in
poor dispersal ability, patchy rays and skates that eventually
distributions, and generally low suppressed commercially important
population densities, the genetic scallop populations. Likewise, does
diversity of species in fragmented the disappearance of a competitor
landscapes may be difficult to result in the niche expansion and
maintain, with the resulting higher densities of subordinate
inbreeding depression further species? This phenomenon has been
reducing population size toward observed between unrelated taxa—
extinction. However, clear and the extinction of insectivorous birds
quantitative demonstrations of the from scrub forests of West Indian
role of life-history traits in the islands correlated with the
extinction process of biotas are still subsequent higher biomass of
rare. competing Anolis lizards.
Conservation biologists have
4. CONSEQUENCES OF EXTINCTIONS traditionally focused on the study of
the independent declines,
The extinction of certain species extirpations, or extinctions of
such as large predators and individual species while paying
pollinators may have more relatively less attention to the
devastating ecological possible cascading effects of species
consequences than the extinction of coextinctions (e.g., hosts and their
others. Ironically, avian parasites). However, it is likely that
vulnerability to predation is often many coextinctions between
exacerbated when certain large interdependent taxa have occurred,
predatory species become rarer in but most have gone unnoticed in
tropical communities. For example, these relatively understudied
although large cats such as jaguars systems. For example, an extinct
(Panthera onca) do not prey on feather louse (Columbicola extinctus)
small birds directly, they exert a was discovered in 1937, 23 years
limiting force on smaller predators after likely coextinction with its host
such as medium-sized and small passenger pigeon (Ectopistes
mammals (mesopredators), which migratorius). Ecological processes
become more abundant with the disrupted by extinction or species
former species’ decline. The decline may also lead to cascading
corollary is that abundant and catastrophic coextinctions.
mesopredators inflict an Frugivorous animals and fruiting
aboveaverage predation rate on the plants on which they depend have a
eggs and nestlings of small birds.
 Copyrighted Material
Species Extinctions
522
key interaction linking plant cross-pollination. A study in Costa
reproduction and dispersal with Rica found that forest bees increased
animal nutrition. Thus, the two coffee yield by
interdependent taxa are placed in 20% in fields within 1 km of the
jeopardy by habitat degradation. forest edge. Between 2000 and 2003,
Many trees produce large, lipid-rich the pollination services provided by
fruits adapted for animal dispersal, forest bees were worth US$60,000 to
so the demise of avian frugivores a 1100-ha farm. A forest patch as
may have serious consequences for small as 20 ha located near farms can
forest regeneration, even if the initial increase coffee yield and thus bring
drivers of habitat loss and large economic benefits to the
degradation are annulled. farmers. Such findings illustrate the
Essential ecosystem functions imperative of preserving native
provided by forest invertebrates are forests near agroforestry systems to
also highly susceptible when species facilitate the travel by
are lost after habitat loss and forestdependent pollinating insects.
degradation. Acting as keystone
species in Southeast Asian
rainforests, figs rely on tiny (1–2 5. CONCLUSIONS
mm) species-specific wasps for their Although extinctions are a normal
pollination. Some fig wasps may part of evolution, human
have limited dispersal ability, modifications to the planet in the last
suggesting that forest disturbance few centuries, and perhaps even
can reduce wasp densities and, by millennia, have greatly accelerated
proxy, the figs that they pollinate. the rate at which extinctions occur.
Similarly, dung beetles are essential Habitat loss remains the main driver
components of ecosystem function of extinctions, but it may act
because they contribute heavily to synergistically with other drivers
nutrient-recycling processes, seed such as overharvesting and pollution,
dispersal, and the reduction of and, in the future, climate change.
disease risk associated with dung Large-bodied species, rare species,
accumulation. In Venezuela, heavier and habitat specialists are
dung beetles were more extinction- particularly prone to extinction as a
prone than lighter species on result of rapid human modifications
artificially created forested islands, of the planet. Extinctions can disrupt
which predicts particularly dire vital ecological processes such as
ecosystem functional loss given the pollination and seed dispersal,
former group’s greater capacity to leading to cascading losses,
dispose of dung. ecosystem collapse, and a higher
Almost all flowering plants in extinction rate overall.
tropical rainforests are pollinated by
animals, and an estimated one-third
of the human diet in tropical FURTHER READING
countries is derived from insect- Brook, Barry W., Navjot S. Sodhi, and
pollinated plants. Therefore, a Peter K. L. Ng. 2003. Catastrophic
decline of forestdwelling pollinators extinctions follow deforestation in
impedes plant reproduction not only Singapore. Nature 424: 420–423. This
in forests but also in neighboring is one of few papers reporting broad-
agricultural areas visited by these scale extinctions driven by tropical
deforestation.
species. Lowland coffee (Coffea
Clavero, Miguel, and Emili Garcia-
canephora) is an important tropical Berthou. 2005. Invasive species are a
cash crop, and it depends on bees for leading cause of animal extinctions.
 Copyrighted Material
523 Conservation Biology
Trends in Ecology and Evolution 20: services can affect pollination of
110. The article highlights that commercial crops.
invasive species represent one of the Rosser, Alison M., and Sue A. Manika.
primary threats to biodiversity. 2002. Overexploitation and species
Dirzo, Rudolfo, and Peter J. Raven. 2003. extinctions. Conservation Biology 16:
Global state of biodiversity and loss. 584–586. This work provides a
Annual Review of Environment and quantitative overview of the extent
Resources 28: 137–167. The article of threat faced by birds and
constitutes a major review of the mammals from direct exploitation by
state of the modern global people.
biodiversity and its associated losses. Sekercioglu, Cagan H., Gretchen C.
Fagan, William F., and E. E. Holmes. Daily, and Paul R. Ehrlich. 2004.
2006. Quantifying the extinction Ecosystem consequences of bird
vortex. Ecology Letters 9: 51–60. declines. Proceedings of the National
This is the only study yet to quantify Academy of Sciences U.S.A. 101:
the final phases of extinction in 18042–18047. This article provides a
vertebrates for which date of framework for assessing the loss of
extinction was known. ecosystem functions caused by avian
IUCN Red List of threatened species. declines.
Download from http:
//www.iucnredlist.org. This presents
an up-to-date classification 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 species could indirectly result in the
extinction of dependent species.
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 MerinoViteri, 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 Academy of Sciences
U.S.A. 34: 12579–12582. This work
shows how the loss of ecosystem