Patterns of biological extinction
Introduction
When I give a talk about extinction, there are usually two questions I am asked:
• Isn’t extinction a natural process?
• What makes us think we’re so smart that we can manage species to prevent their
extinction?
I’ll deal with the first question in a moment, but let’s ponder the second one for now.
The data presented in the first set of notes for this course shows pretty convincingly that
human beings have an enormous impact on the earth and its inhabitants, an impact so large
that we can’t avoid having an impact on patterns and rates of species extinctions. Thus, we
may or may not be smart enough to manage well, but one thing’s for sure. We will have
an impact. We cannot choose not to have an impact. We can only choose what kind of
impact we’ll have. This observation reinforces a point you’ll hear me make again and again
this semester: conservation biologists have to make decisions in the face of incomplete data.
We don’t have any choice about whether or not to make a decision, and we don’t have any
choice about whether we’re going to manage. We can only make the decisions and choose to
manage in a way that we hope will produce the outcome we want (compare [18]).1 In some
cases that may amount to “letting nature take its course.” In others it will involve more
active, hands-on managing.
Take the case of the Greater Sage-Grouse for example. After spending the next couple
of weeks looking over the information presented in Manier et al. [10] you may feel as if you
need more data in order to decide whether a listing as endangered or threatened is justified.
It might feel as if there’s too much uncertainty associated with the questions I asked to
provide an answer now. Now suppose you answer the question “Do the data suggest that
the Greater Sage Grouse will go extinct if current trends continue?” by writing “The data
currently available do not allow me to answer that question. We should continue to monitor
1
Or we can choose not to (consciously) manage and hope that things turn out the way we want them
to — but that’s still a choice.
c 2001–2015 Kent E. Holsinger
populations for another 10 years before making a decision.” There’s no question that you’d
have more data and be more likely to answer the question correctly. But suppose that the
populations really are declining by about 5% per year. Then in 10 years, they will be about
40% smaller than they are now. Writing “I need more data before I can make a decision” is
equivalent to writing “If populations are declining now, they’re declining slowly enough we
can wait until we gather more data to take any actions that would reverse the decline.”
In short, the implication of the question “What makes us think that we’re smart enough
to manage species and prevent their extinction?” is that we’re not smart enough to do
anything, so we should keep our fingers out of the pot. The problem is that our fingers are
in the pot, and there’s no way to take them out as long as we’re on the planet. Not only do
we have to make decisions now, even before we have all of the facts we’d like to have, but we
also have to accept that there’s no way for us not to have an impact. Supporting 7 billion
people now and 9 billion (or more) by the middle of this century means that we will have
an impact. The question isn’t whether or not to have an impact. It’s what kind of impact
to have. Given that, if we’re concerned about loss of individual species or of biodiversity
more broadly, we have no choice but to do something about it. Doing nothing is equivalent
to saying that we don’t care about loss of species or biodiversity.
Extinction as a natural process
So what about the first question? Isn’t extinction a natural process? There are two components to my answer:
1. Contemporary rates of extinction are vastly greater than they are typical of the geological past, and they are projected to get worse.
2. The pattern of extinction is non-random, and the species that survive may not be those
that make the world an appealing place to live.
To start, let’s take a look at what the Millenium Ecosystem Assessment has to say about
extinction rates (Figure 1). Notice the scale on the vertical axis. It’s a logarithmic scale. So
our best guess about the current extinction rate is that it’s about 100 times higher than the
background extinction rate in the fossil record, and the projected future extinction rate 100010,000 times greater than the background extinction rate. Numbers like that are why you
often hear it said that we are living through the sixth mass extinction in the history of life.
Now those numbers, but as conservation biologists, it’s important that we understand where
they come from. We have to know the assumptions underlying the calculations in order to
judge how reliable they are. It’s our responsibility to present the scientific evidence fairly,
2
Figure 1: Past, present, and projected extinction rates [1].
and before we tell the public or policymakers about the Millenium Ecosystem Assessment’s
conclusions, we need to make sure the evidence on which they based their conclusions is
solid. Let’s start by reviewing a little of what’s know about extinction in the fossil record.
Background extinction
It is the fate of most living things eventually to go extinct. The species diversity now is
almost certainly greater than it ever has been in the past, but paleontologists tell us that
more than 99% of the species that have ever lived are now extinct.
3
Some take comfort that we seem to have recovered from the great extinctions of the past
• Ordovician (440mya) — 50% of animal families
• Devonian (360mya) — 30% of animal families
• Permian (250mya) — 50% of animal families, including 95% of marine species
• Triassic (210mya) — 35% of animal families
• Cretaceous (65mya) — 60% of animal species
We’re still here, these people would argue, and the world doesn’t look so bad. Until a few
years ago, I argued that the fossil record showed that recovery from these extinctions took a
long time.2 But reanalysis of the record of Phanerozoic diversity for marine invertebrates [8,
9] suggests that the apparent delay in recovery could be an artifact of the incompleteness
of the fossil record. The recovery of biodiversity may, in fact, be geologically instantaneous,
i.e., on the order of 5 million years or less.
So the optimist in me says that we are very unlikely to destroy life on this planet and that
the diversity of life will recover fairly quickly once we are gone. The pessimist in me points
out that I don’t want to live in a world that has substantially less diversity than the one
I live in now, and that even if life’s diversity recovers instantaneously from the perspective
of deep geological time, it’s not likely to happen on a time scale of any interest to human
beings.3 To me, that means we want to do what we can to reduce the rate of extinction,
ideally to a rate 10-100 times lower than it is now.4
The causes of extinction
The causes of extinction are (mostly) fairly obvious, and my first set of notes touched on one
of the largest ones all ready — habitat destruction and conversion. But let’s spend a little
time reminding ourselves what the causes of extinction are, because knowing those causes
helps us better to understand their consequences.
2
Ordovician — 25 million years; Devonian — 30 million years; Permian/Triassic — 100 million years; Cretaceous — 20 million years
3
“Humans” didn’t even exist 5 million years ago, if by “human” we mean a member of the genus Homo.
4
Notice, how I expressed this conclusion. I come to this conclusion because “I don’t wnat to live in a
world that has substantially less diversity than the one I live in now.” That’s a personal preference, a value
judgment about the kind of world I want to live in, not an empirical, scientific assessment about the way
the world is or will be. This is another point we’ll return to frequently in the course, but I don’t want to
belabor it right now.
4
• Overexploitation
Whales
– A record three year cruise in the 19th century killed fewer than one hundred
whales.
– In 1933 almost 30,000 whales were killed, yielding 2.5 million barrels of whale oil.
– By 1967 60,000 were killed, but they yielded only 1.5 million barrels of oil —
because the larger species, e.g., Blues and Fins, had been hunted virtually to
extinction.
– Analyses of genetic diversity suggest pre-exploitation population sizes 6-20 times
greater than current estimates [17].5
Atlantic cod
Wildlife trade, orchids and succulents
Predator control — Until 1952 the Bald Eagle had a price on its head. The grey wolf
was eliminated in the United States except in Alaska and a small population in northern
Minnesota by federal wildlife agents.6
• Development — urbanization, agriculture, & mining
Cape flora
– 9,000 species of native vascular plants; 1,200 threatened; 36 are extinct
– More than 60% of the area previously occupied by the Cape flora has been replaced
by farms, plantations, roads, dams, towns, etc.
– More than 80% of the $4 million earned in the cut-flower industry is extracted
directly from the wild
Dams: Pacific northwest salmon fishery has declined dramatically in the last 50 years.
Many runs are now listed (or proposed for listing) under the Endangered Species Act.
5
This estimate is very sensitive to estimates of the rate of nucleotide substitution in mitochondria. Roman
and Palumbi use estimates of 1.5-2.0 ×10−8 per base pair per year. If the rate were higher, say, 1.5-2.0 ×10−7
per base pair per year, the pre-exploration population sizes would be estimated at 0.6-2 times greater than
current estimates.
6
Read Aldo Leopold’s “Thinking like a mountain” (http://www.eco-action.org/dt/thinking.html) for a
moving account of his experience on a wolf hunt in New Mexico when he worked for the U.S. Forest Service.
5
Country/Region
% of old growth forest
USA
15
Washington & Oregon
13
Canada
52
British Columbia
40
Europe
Western Europe
1
Scotland
1
Sweden
1
Finland
2
Norway
3
Oceania
New Zealand
25
Australia
5–21
Asia
China
1
Table 1: Proportion of old-growth forest remaining in selected temperate forest countries
(from Table 11.2-6 [20]).
Nearly 30% of the ca. 1400 historical populations are extinct (14% of populations from
coastal regions, 55% from interior regions) [3].
Deforestation (Table 1) is just one aspect of habitat conversion. Loss of grasslands and
savannas may be equally extreme.
• Invasive exotics
Zebra mussel
Chestnut blight
Wilcove, Rothstein, Dubow, Phillips, and Losos [23] surveyed recovery plans for species
listed under the United States Endangered Species Act and categorized the threats they
identified into one of five categories: habitat degradation/loss, alien species, pollution, overexploitation, and disease. Table 2 shows the percentage of listed species for which each of
these five factors was mentioned as a cause contributing to endangerment.7
7
The numbers add to more than 100% because each species may face threats in more than one category.
6
Cause
Habitat degradation
Alien species
Pollution
Overexploitation
Disease
All
Vertebrates
(n = 1880) (n = 494)
85%
92%
49%
47%
24%
46%
17%
27%
3%
11%
Invertebrates
(n = 331)
87%
27%
45%
23%
0%
Plants
(n = 1055)
81%
57%
7%
10%
1%
Table 2: Causes of endangerment mentioned in species recovery plans [23].
Conservation status
Secure/apparently secure
Vulnerable
Imperiled
Critically imperiled
Presumed/possibly extinct
Percent of taxa
67%
16%
8%
7%
1%
Table 3: Percentage of species in taxonomically well-characterized taxa falling into different
endangerment categories in the United States.
In short, many species are going extinct now to reasons related to human activities. We
are responsible for the elevated rates of extinction, and back to my earlier point. We are
having an impact, and it’s up to us to choose what kind of impact we want to have. Of
course so far, I haven’t told you anything about how many species are going extinct.
Rates of extinction
Rates of extinction are very difficult to estimate, because we don’t even know within an order
of magnitude how many species there are. Nonetheless, we can be quite sure that a large
proportion of taxa are threatened with extinction. A little over 15 years ago, The Nature
Conservancy [11] assessed the status of 20,892 species in groups that are taxonomically
well-characterized in the United States (freshwater mussels, crayfishes, vertebrates, vascular
plants, tiger beetles, butterflies/butterflies, and dragonflies/damselflies; Table 3).
These statistics don’t tell us how many extinctions are happening. And depending on
whether you’re an optimist or a pessimist you can take this as good news or as bad news.
7
Two out of three species in the United States are apparently secure. That’s the good news.
One out of three species in the United States is vulnerable or imperiled. That’s the bad
news.
But how do these numbers compare with what might have characterized the biota of
the U.S. prior to the arrival of humans? Do they, in fact, tell us that extinction rates are
elevated, or are they simply characteristic of what we would have found at any time in the
geological past? Have a large fraction of species been on the edge of extinction during most of
geological history? Two different approaches have been employed to try to compare current
rates of extinction with those inferred from the fossil record.
Species-area relationships
Statistics on the rates of extinction have often been inferred from a species-area relationship
and projections of habitat destruction.
S = CAz
If we’re only interested in the proportion of species remaining after some portion of the
habitat has been destroyed
CA0z
S0
=
S
CAz!
z
A0
=
A
(1)
(2)
z is generally between 0.15 and 0.35. So for example, if we’re interested in projecting the
fraction of species that will be lost from tropical rainforests as a result of deforestation, we
can make the following back of the envelope calculation.
If current rates of tropical deforestation continue for another 30 years half of the remaining
rain forest will be gone. If we then prevent all further deforestation, between
0.50.35 = 0.78
and
0.50.15 = 0.90
of the species presently there will remain, i.e., 10%–20% will go extinct. More sophisticated
approaches use an exponential decay model, recognizing that the predictions of the above approach apply only after the remaining forest fragments have reach an immigration-extinction
8
equilibrium. Notice, however, that these calculations only reflect the effects of lost habitat,
not increased exposure to disease, competition with invasive exotics, overxploitation, or
habitat degradation.
He and Hubbell [5] pointed out that the way I’ve just described these species-area calculations leads to an overestimate of the number of speices likely to be lost, but the difference
between these calculations and those He and Hubbell present is not great. We’ll go into the
details when I describe the critiques a little later.
Rates from known extinctions
It may come as a surprise to you to learn that we can also get reasonable estimates of current
extinction rates from examining documented extinctions in groups that are well-studied.8 In
the United States alone, for example, 45 vertebrates (over half of which are birds), 347
invertebrates, and 147 plants are either presumed or possibly extinct [11]. By calculating
the fraction of known species that have gone extinct in historical times, we get a direct
estimate of extinction rates. The calculations that follow are based on the data and the
approach described in [12, 19].
• 100 documented extinctions of birds and mammals worldwide in the last century out
of ≈ 14, 000 total. That’s a rate of 7 × 10−3 yr−1 .
• The average life span of bird and mammal species in the fossil record is about 1 ×
106 years. This is equivalent to an extinction rate of about 1 × 10−6 y−1 .9
• So the recent historical rate of vertebrate extinctions is a little over 7,000 times greater
than the background rate of extinction.
The rates calculated by these very different approaches are reasonably comparable, given
the great uncertainties involved. They suggest that contemporary rates of extinction are
100 to 1,000 (and possibley 10,000) times higher than at any time in the last 65 million
years. The figures in Table 4 show how many documented extinctions have occurred in the
last 400 years, along with an estimate of the corresponding median lifetime of taxa in each
group (derived from [19]).
8
Régnier et al. [16] point out that by excluding invertebrates from the analyses below, we focus on only
about 800 of the 130,000 extinctions likely to have happened in the recent past.
9
De Vos et al. [2] argue that the background extinction rate has been substantially overestimated. Their
analysis suggests that the background extinction rate is closer to 0.1 × 10−6 y−1 .
9
Animals
Plants
Molluscs
Crustaceans
Insects
Vertebrates
Fishes
Amphibians
Reptiles
Birds
Mammals
Total
Gymnosperms
Dicotyledons
Monocotyledons
Palms
Total
Extinct Threatened
Extant Time to 50% extinction
191
354
105
60,000
4
126
4 × 103
—
6
61
873
10
—
4
229
2,212 4.7 × 10
600
29
452 2.4 × 104
900
3
2
59
3 × 10
3,000
23
167
6 × 103
2,000
3
116
1,029 9.5 × 10
350
3
59
505 4.5 × 10
250
15,000
485
3,565 1.4 × 106
2
242
758
—
5
120
17,474 1.9 × 10
1,000
462
4,421 5.2 × 104
1,700
4
925
2820
70
584
22,137 2.4 × 105
1,100
Table 4: Extinction in major taxa since 1600.
Critiques of these estimates
Critics of estimates based on species-area relationships have pointed out that > 95% of the
eastern forests in North America were cut while only 4 bird species went extinct. How can
we account for this apparent discrepancy [14]?
• The region was not simultaneously deforested. As agriculture moved westward, forest
reclaimed abandoned fields. Forests always covered > 50% of the land area.
0.50.25 = 0.84
16% or 25-26 out of ≈160 forest species should have gone extinct.
• May not have been enough time for extinction. Species-area relationship is an equilibrium expectation.
Committed to extinction versus actually extinct
Halley and Iwasasa [4] provide mathematical formulas that can be combined with
species-area relationships to determine the magnitude of any extinction debt. They
10
show
S(t) =
S0
0
1 + tS
τN
,
where τ is the generation time, S0 is the initial number of species, N is the total
number of individuals in the community, and S(t) is the number of species in an
isolated community, i.e., one with no immigration or speciation.
If we were to apply this relationship to birds of eastern North America τ N would be
on the order of several million or more, so that the denominator of this equation would
be nearly 1, meaning that even though some species would be committed to extinction
as a result of habitat loss, we wouldn’t expect to see any actual extinctions in fewer
than several centuries.
• Only 28 of the 160 species are restricted to eastern forests. The rest are widely distributed elsewhere and would have persisted there. They could have later moved back
into eastern forests.
(0.50.25 )(28) = 23.5
,
i.e., 23-24 of the 28 species should have persisted. Only 4-5 should have gone extinct,
which is about what we saw.
This is He and Hubbel’s [5] point: Expected extinctions should be calculated from the
endemic area relationship, not the species area relationship (Figure 2).
• We lost few bird species from eastern North American forests because we had few
endemics to lose.
Population deletions
We tend to focus on extinction as a species-level phenomenon, but doing so substantially
understates the impact we are having. So long as a single population of a species is extant
the species is not extinct, even if 99.9% of its populations have been eliminated. Species may
be eliminated from most of their former range and persist only in small refuge areas. Nearly
two decades ago, population declines in amphibians began to receive world-wide attention.
Hobbs and Mooney [6] cite several similar examples:10
• Yellow-legged frog (Rana muscosa) — 1989 survey found in only 1 of 27 sites where it
had been found 10 years before.
10
And don’t forget the example of Pacific northwest salmon populations cited earlier [3].
11
Figure 2: The difference between a species area curve (SAR) and an endemic area curve
(EAR). Using the EAR consistently predicts a smaller, but still large, number of extinctions.
(From [15])
12
• Owens pupfish (Cyprinodon radiosus) — Originally widespread in Owens Valley. Once
thought extinct. A single population was rediscovered in 1956.
• Twenty-six species of west Australian mammals occur only as remnant populations
occupying less than 20% of their original range. 42% of mammals originally found in
the wheatbelt region are no longer found there, although only 14% of those are extinct
in the entire state.
• Half of the bird species in the agricultural part of western Australia have declined in
distribution and abundance since 1900.
Climate change and extinction
And, of course, in all of this, I’ve ignored the elephant in the room: climate change. Mark
Urban recently synthesized a wide variety of data sets and estimated the worldwide extinction
risk associated with climate change ([21]: Figure 3). If we limit the increase in global average
surface temperature to 2◦ C, as is the current policy target, then then extinction risk is about
5.2%. If we continue on with business as usual, meaning something more than a 4◦ C rise,
the extinction risk will be nearly 16%. Interestingly, the estimated risks from climate change
were lowest in Europe and North America and highest in South America, Australia, and
New Zealand.
Biological consequences of human-caused extinctions
We have now seen that rate at which human-caused extinctions are occurring is much higher
than the rate at which extinction occurred before we became so dominant.11 What are the
immediate consequences of such extinctions? Geerat Vermeij [22] suggested three questions
to be answered to understand the implications of extinction for human welfare.
1. Which kinds of species are susceptible to extinction, and which are not?
2. How will the extinction of species affect the communities in which these species lived
and alter the evolutionary environment of surviving life forms?
3. Can new species evolve on a human-dominated planet to replace the species that have
disappeared and, if so, what will they look like?
11
We’ll return to this topic again when we talk about landscape change in a couple of months. There is
substantial evidence that human occupation of the Americas and of Australia led to many extinctions among
large mammals.
13
Figure 3: Extinction risk due to climate change as a function of the increase from preindustrial temperature (from [21]).
14
Group
Proportion threatened1
Freshwater mussels
69%
Crayfishes
51%
Stoneflies
43%
Freshwater fishes
37%
Amphibians
36%
Flowering plants
33%
Gymnosperms
24%
Pteridophytes
22%
Tiger beetles
19%
Butterflies
19%
Reptiles
18%
Odanates
18%
Mammals
16%
Birds
14%
1
Presumed/possibly extinct, critically imperiled, imperiled, or vulnerable
Table 5: The taxonomic distribution of endangerment in the United States.
Selectivity of extinctions
Vanished species are unlikely to be a random subset of the biota.
• Karr (Barro Colorado) — ground-dwelling birds suffered higher rate of extinction than
canopy dwellers [7].
• The proportion of species threatened differs dramatically among taxonomic groups in
the United States (Table 5; [11]).
Consequences of extinctions
If these speculations are correct, not only are human-caused extinctions occurring at a rate
greater than at any time in the last 65 million years, but the species that are going extinct
are predominantly those adapted to special conditions of life in localized habitats.
The result is a more homogeneous biotic environment, one in which many of the same
plants and animals are found worldwide — English sparrows, starlings, dandelions, wild oats.
Most of the grasses you see on the hills of California are native to the Mediterranean.
Native Californian grasslands are largely confined to serpentine outcrops. Nearly all of the
15
vegetation in lowland areas of Hawaii is introduced from other areas in the tropics. Of the
roughly 1,700 plants native to Hawaii, almost half are introduced (see [13] for a more careful
analysis of this issue). Gordon Orians suggested in a Teale Lecture a few years ago that we
should refer to the current age as the Homogocene.
Whether it is a good thing for the character of our natural world to be changed in this
way is something we must decide on non-biological grounds. Life will continue on this planet
whatever we decide, but what we decide will have an enormous impact on what kind of life
does survive.
References
[1] Millenium Ecosystem Assessment. Ecosystems and Human Well-being: Synthesis. Island
Press, Washington, DC, 2005.
[2] Jurriaan M De Vos, Lucas N Joppa, John L Gittleman, Patrick R Stephens, and Stuart L
Pimm. Estimating the normal background rate of species extinction. Conservation
Biology, 29(2):452–462, 2015.
[3] Richard G Gustafson, Robin S Waples, James M Myers, Laurie A Weitkamp, Gregory J
Bryant, Orlay W Johnson, and Jeffrey J Hard. Pacific Salmon Extinctions: Quantifying
Lost and Remaining Diversity. Conservation Biology, 21(4):1009–1020, 2007.
[4] John M Halley and Yoh Iwasa. Neutral theory as a predictor of avifaunal extinctions
after habitat loss. Proceedings of the National Academy of Sciences of the United States
of America, 108(6):2316–21, February 2011.
[5] Fangliang He and Stephen P Hubbell. Species-area relationships always overestimate
extinction rates from habitat loss. Nature, 473(7347):368–71, May 2011.
[6] R J Hobbs and H A Mooney. Broadening the extinction debate: population deletions
and additions in California and western Australia. Conservation Biology, 12:271–283,
1998.
[7] J R Karr. Avian extinction on Barro Colorado Island, Panama: a reasssessment. American Naturalist, 119:220–239, 1982.
[8] Richard A Kerr. PALEOBIOLOGY: Revised Numbers Quicken the Pace of Rebound
From Mass Extinctions. Science, 311(5763):931a–, 2006.
16
[9] Peter J Lu, Motohiro Yogo, and Charles R Marshall. Phanerozoic marine biodiversity
dynamics in light of the incompleteness of the fossil record. Proceedings of the National
Academy of Sciences, 103(8):2736–2739, 2006.
[10] D J Manier, D J A Wood, Z H Bowen, R M Donovan, M J Holloran, L M Juliusson,
K S Mayne, S J Oyler-McCance, F R Quamen, D J Saher, and A J Titolo. Summary
of science, activities, programs, and policies that influence the rangewide conservation
of Greater Sage-Grouse∼(Centrocercus urophasianus). U.S. Geological Survey, Reston,
VA, 2013.
[11] L L Master, B A Stein, L S Kutner, and G A Hammerson. Vanishing assets: conservation
status of U.S. species. In Bruce A Stein, Lynn S Kutner, and Jonathan S Adams, editors,
Precious Heritage: The Status of Biodiversity in the United States, pages 93–118. Oxford
University Press, New York, NY, 2000.
[12] Robert M May, John H Lawton, and Nigel E Stork. Assessing extinction rates. In John H
Lawton and Robert M May, editors, Extinction Rates, pages 1–24. Oxford University
Press, New York, NY, 1995.
[13] M L McKinney and J L Lockwood. Biotic homogenizations: a few winners replacing
many losers in the next mass extinction. Trends in Ecology & Evolution, 14(11):450–453,
1999.
[14] S J Pimm and R A Askins. Forest losses predict bird extinctions in eastern North
America. Proceedings of the National Academy of Sciences USA, 92:9343–9347, 1995.
[15] Carsten Rahbek and Robert K Colwell. Species loss revisited. Nature, 473(7347):288–
289, 2011.
[16] Claire Régnier, Guillaume Achaz, Amaury Lambert, Robert H Cowie, Philippe Bouchet,
and Benoı̂t Fontaine. Mass extinction in poorly known taxa. Proceedings of the National
Academy of Sciences, 112(25):7761–7766, 2015.
[17] Joe Roman and S R Palumbi. Whales before whaling in the North Atlantic. Science,
301:508–510, 2003.
[18] E W Sanderson, M Jaiteh, M A Levy, K H Redford, A V Wannebo, and G Woolmer.
The Human Footprint and the Last of the Wild. BioScience, 52(10):891–904, 2002.
[19] Fraser D M Smith, Robert M May, Robin Pellew, Timothy H Johnson, and Kerry S
Walter. Estimating extinction rates. Nature, 364:494–496, 1993.
17
[20] UNEP. Global Biodiversity Assessment. Cambridge University Press, Cambridge, 1995.
[21] Mark C Urban.
Accelerating extinction risk from climate change.
348(6234):571–573, 2015.
Science,
[22] G Vermeij. The biology of human-caused extinction. In Bryan G Norton, editor, The
Preservation of Species: The Value of Biological Diversity, pages 28–49. Princeton University Press, Princeton, NJ, 1986.
[23] D S Wilcove, D Rothstein, J Dubow, A Phillips, and E Losos. Quantifying threats to
imperiled species in the United States. BioScience, 48(8):607–616, 1998.
Creative Commons License
These notes are licensed under the Creative Commons
Attribution-NonCommercial-ShareAlike License. To view a copy of this license, visit
http://creativecommons.org/licenses/by-nc-sa/3.0/ or send a letter to Creative Commons,
559 Nathan Abbott Way, Stanford, California 94305, USA.
18