Conservation Biology

1
Conservation Biology
2
59 Conservation Biology

• 59.1 What Is Conservation Biology?
• 59.2 How Do Biologists Predict Changes in
  Biodiversity?
• 59.3 What Factors Threaten Species Persistence?
• 59.4 What Strategies Do Biologists Use to Protect
  Biodiversity?

3
59.1 What Is Conservation Biology?

• Conservation biology is an applied science,
  devoted to protecting and managing Earths
  biodiversity.
• It draws heavily on principles of ecology,
  ethology, and evolutionary biology.

4
59.1 What Is Conservation Biology?

• Early on, there were tensions between people
  whose main goal was to conserve natural resources
  for their economic benefits and people who
  believed that nature has intrinsic value.
• Today conservation biologists study the full
  range of goods and services provided by
  ecosystems, including aesthetic and psychological
  benefits.

5
59.1 What Is Conservation Biology?

• Understanding the global ecosystem and the
  effects of humans on that system is now
  understood to be essential to the long-term
  well-being of Homo sapiens.
• Conservation biology is an applied disciplineit
  involves the practical application of knowledge
  to solve problems.

6
59.1 What Is Conservation Biology?

• Conservation biology is guided by three basic
  principles
• Evolution is the process that unites all of
  biology
• The ecological world is dynamic
• Humans are part of ecosystems

7
59.1 What Is Conservation Biology?

• Biodiversity has multiple meanings
• The degree of genetic variation within a species,
  which allows organisms to adapt to environmental
  change
• Species richness in a particular community
• Ecosystem diversity the complex interactions
  within and between ecosystems

8
59.1 What Is Conservation Biology?

• Extinction is a constant theme in the history of
  life most of the species that have lived on
  Earth over the ages are extinct today.
• But the rate of extinctions occurring today
  rivals those of the five great mass extinctions
  of lifes history.

9
59.1 What Is Conservation Biology?

• Those mass extinctions were the result of
  cataclysmic natural disturbances modern
  extinctions can be attributed to effects of the
  human population.
• Humans have a tremendous capacity to alter
  ecosystems and cause extinctions.

10
59.1 What Is Conservation Biology?

• When humans first arrived in North America 14,000
  years ago, they encountered a diverse megafauna.
• Most became extinct within a few thousand years.
  Overhunting is thought to be the principle cause.
• Similar extinctions of megafauna coincided with
  human arrival in Australia and Hawaii.

11
Figure 59.1 Extinct North American Megafauna
12
59.1 What Is Conservation Biology?

• Over the past 400 years, increasing
  industrialization and urbanization have
  accelerated extinction rates astronomically.
• The renowned evolutionary biologist Edward O.
  Wilson estimates that Earth is losing about
  30,000 species per year, putting us in the midst
  of a sixth mass extinction.

13
59.1 What Is Conservation Biology?

• Conservation biologists are concerned about
  biodiversity loss for many reasons
• Species are necessary for the functioning of
  ecosystems. When species are lost, communities
  and ecosystems may change or be lost, along with
  the goods and services we depend on

14
59.1 What Is Conservation Biology?

• We depend on other species for food, fiber, and
  medicines
• In the United States more than a quarter of all
  medical prescriptions contain or are based on
  plant products
• A variety of microbial species provide services
  such as fermentation

15
59.1 What Is Conservation Biology?

• We derive enormous psychological benefit and
  aesthetic pleasure from watching and interacting
  with other species
• Aesthetic benefits can actually confer economic
  value on biodiversity, e.g., trees on a
  residential lot may increase the value beyond the
  value of the lumber in the tree

16
59.1 What Is Conservation Biology?

• Extinctions deprive us of the opportunity for
  scientific study and understanding ecological
  interactions
• Living in ways that cause extinctions raises many
  ethical concerns all species are judged to have
  intrinsic value

17
59.2 How Do Biologists Predict Changes in
Biodiversity?

• Predicting how many and which species will go
  extinct is difficult
• 1. We dont know how many species live on Earth
  today. Many species that may go extinct have not
  been named or described.
• Estimates of the number of species yet to be
  discovered range from about 2 million to more
  than 50 million.

18
59.2 How Do Biologists Predict Changes in
Biodiversity?

• 2. We do not know where species live, especially
  ones that are small, reclusive, and rare to start
  with.
• 3. It is difficult to determine when a species
  actually becomes extinct.
• Pygmy tarsiers were discovered on Sulewesi in
  2008, 85 years after the last sighting.

19
59.2 How Do Biologists Predict Changes in
Biodiversity?

• The status of rare, reclusive species with
  poorly known life histories is difficult to
  determine, as has been the case with the
  ivory-billed woodpecker.
• Sightings of the bird in 2004 have not been
  followed by clear evidence of its existence.

20
Figure 59.2 Is It Really Extinct?
21
59.2 How Do Biologists Predict Changes in
Biodiversity?

• 4. We rarely know all the connections among
  species.
• Extinction of Blackburns sphinx moths on some
  Hawaiian islands may lead to extinction of the
  alula plant because the moth was apparently its
  only pollinator.
• The ecological associations of species are not
  always documented before they go extinct.

22
Chapter Openers Extinction and Back/One
Extinction May Lead to Another
23
59.2 How Do Biologists Predict Changes in
Biodiversity?

• In spite of these difficulties, probable rates of
  extinction resulting from human activities can be
  estimated using the speciesarea relationship and
  the theory of island biogeography.
• Species richness decreases with decreasing
  habitat patch size. On average, a 90 percent loss
  in habitat will result in a 50 percent loss of
  species.

24
59.2 How Do Biologists Predict Changes in
Biodiversity?

• The current rate of loss of tropical evergreen
  forest (the most species-rich biome) is about 2
  percent per year.
• If this rate of loss continues, at least 1
  million species will be lost from this biome in
  this century.

25
Figure 59.3 Deforestation Rates in Tropical
Forests
26
59.2 How Do Biologists Predict Changes in
Biodiversity?

• Biologists use computer models to estimate the
  probability that a population will go extinct.
• Species in imminent danger of extinction in all
  or most of their range are labeled endangered.
• Threatened species are likely to become
  endangered in the near future.

27
59.2 How Do Biologists Predict Changes in
Biodiversity?

• Species whose populations suddenly shrink rapidly
  are at high risk. Small population size can lead
  to genetic drift and loss of genetic variation.
• Species with highly specialized food or habitat
  requirements are at more risk of extinction than
  generalists.

28
59.2 How Do Biologists Predict Changes in
Biodiversity?

• Small populations can easily be wiped out by
  local natural disasters, such as fire.
• The Cozumel thrasher, known only on the island of
  Cozumel, may have been wiped out by a series of
  hurricanes, beginning with Hurricane Gilbert in
  1988.

29
59.3 What Factors Threaten Species Persistence?

• Human activities that threaten survival of other
  species include the following
• Habitat destruction
• Introduction of exotic species
• Overexploitation
• Climate change

30
59.3 What Factors Threaten Species Persistence?

• Habitat loss is the most important cause of
  endangerment in the world.
• Many habitats are degraded by pollution,
  especially in freshwater.
• Toxic substances released by human activities
  have negative effects on the behavior,
  reproduction, and development of species.

31
Figure 59.4 Proportions of U.S. Species Extinct
or Threatened
32
59.3 What Factors Threaten Species Persistence?

• Habitat loss also occurs through outright
  removalphysical destruction.
• Habitat loss also affects nearby habitats that
  are not destroyed.
• As habitat is progressively lost, remaining
  habitat patches get smaller and more isolated
  habitat is increasingly fragmented.

33
59.3 What Factors Threaten Species Persistence?

• Small habitat patches cannot maintain populations
  of species that require large areas.
• Small patches can support only small populations
  (greater risk of extinction).
• Edge effects As patch size decreases, it has
  more edge. Factors originating outside the
  patch can have more influence.

34
Figure 59.5 Edge Effects
35
59.3 What Factors Threaten Species Persistence?

• Forest fragmentation in midwestern North America
  has increased populations of brown-headed
  cowbirds.
• These brood parasites lay their eggs in nests of
  other species the cowbird hatchling is raised by
  the other species, to the detriment of their own
  hatchings.

36
59.3 What Factors Threaten Species Persistence?

• Cowbirds historically followed bison and other
  mammals, and laid eggs in nests of grassland bird
  species.
• Forest fragmentation has allowed the cowbirds to
  lay eggs in forest birds nests that are at the
  edges.
• Fragmentation favors proliferation of cowbirds at
  the expense of other bird species.

37
59.3 What Factors Threaten Species Persistence?

• In a tropical evergreen forest in Brazil,
  research was conducted before logging began. Land
  owners agreed to leave forested patches of
  different sizes.
• Researchers counted species in the future
  patches before logging began then again after
  logging.

38
Figure 59.6 Species Losses in Brazilian Forest
Fragments
39
59.3 What Factors Threaten Species Persistence?

• Monkeys that travel over large areas were the
  first species eliminated. Next were army ants,
  then the birds that follow army ant swarms.

40
59.3 What Factors Threaten Species Persistence?

• When species are lost from small habitat patches,
  recolonization is unlikely.
• Dispersing individuals are not likely to find
  isolated habitat patches.
• Species may be able to persist in small habitat
  patches if there are corridors connecting patches.

41
59.3 What Factors Threaten Species Persistence?

• In the Brazilian forest study, completely
  isolated patches lost species more rapidly than
  did patches that were connected to unfragmented
  forest by corridors.

42
59.3 What Factors Threaten Species Persistence?

• Recognition of the importance of corridors has
  led to new conservation initiatives.
• A CanadaUnited States nonprofit organization
  aims for sustainable management of the mountain
  ecosystem extending from Yellowstone National
  Park to the Yukon Territory, the largest intact
  ecosystem of its kind.

43
59.3 What Factors Threaten Species Persistence?

• Although habitat loss is the main threat to
  species, overexploitation still threatens many.
• Elephants and rhinoceroses are killed for their
  tusks and horns.
• The body parts of tigers are used in traditional
  medicines poaching tigers can bring high profits.

44
Figure 59.7 Endangered by Exploitation (A)
45
59.3 What Factors Threaten Species Persistence?

• There is also massive international trade in
  exotic pets and aquarium fishes, ornamental
  plants, and tropical forest hardwoods.
• The Banggai cardinalfish is on the brink of
  extinction entirely due to demand by saltwater
  aquarium enthusiasts.

46
Figure 59.7 Endangered by Exploitation (B)
47
59.3 What Factors Threaten Species Persistence?

• Humans move many species to regions outside their
  original range, both intentionally and
  accidentally.
• Some exotic species become invasive. They
  reproduce rapidly, spread widely, and have
  negative effects on the native species of the
  region.

48
59.3 What Factors Threaten Species Persistence?

• Introduced species may not encounter their
  natural enemies and can reach very high
  population densities.
• The native species in an invaders new range may
  not have evolved specific defenses against these
  new competitors or antagonists.

49
59.3 What Factors Threaten Species Persistence?

• Species are moved by many means.
• The brown tree snake arrived on Guam in air cargo
  in the 1940s.
• It has now reached very high densities and caused
  the extinction of 15 bird species, including
  three found only on Guam.

50
Figure 59.8 An Agent of Extinction
51
59.3 What Factors Threaten Species Persistence?

• Colonizing Europeans deliberately introduced many
  species.
• In Australia, rabbits, foxes, dogs, and cats have
  led to extermination of half of small to
  medium-sized native marsupials over the last 100
  years.
• Some species that are introduced to control an
  invasive become invasive themselves, such as the
  cane toad.

52
59.3 What Factors Threaten Species Persistence?

• Invasive species can include pathogens.
• In Hawaii, avian malaria has eliminated native
  bird species living below 1,500 m elevation.
• Before Europeans arrived, there were no
  mosquitoes on the islands (no vector for
  malaria).
• The range of the mosquitoes may move up the
  mountains with climate warming.

53
59.3 What Factors Threaten Species Persistence?

• Global warming will increase average temperatures
  by 25C by the end of this century.
• Species will have to shift ranges to remain in
  the same temperature regimes, or evolve new
  adaptations within a single century.
• Some habitats, such as alpine tundra, may be
  completely eliminated.

54
59.3 What Factors Threaten Species Persistence?

• In order to understand and predict future shifts,
  biologists study how species ranges shifted
  during the last 8,000 years of post-glacial
  warming.
• Trees with lightweight seeds such as pines were
  able to expand their ranges northward however,
  earthworms eliminated by glaciers have expanded
  northward only very slowly.

55
Figure 59.9 Some Species Have Expanded Their
Range
56
59.3 What Factors Threaten Species Persistence?

• Global warming may result in entirely new
  climates.
• A warming of 2C in tropical low elevations will
  result in a climate warmer than anywhere in the
  humid tropics today.
• Warmer nights have already been documented at
  several sites. Trees use more energy reserves,
  and growth rates have slowed.

57
59.4 What Strategies Do Biologists Use to Protect
Biodiversity?

• Establishing protected areas is an important part
  of efforts to preserve biodiversity.
• Protected areas preserve habitat and prevent
  human exploitation.
• They can act as nurseries from which individuals
  can disperse, replenishing populations that might
  otherwise go extinct.

58
59.4 What Strategies Do Biologists Use to Protect
Biodiversity?

• Some areas are selected for protection based on
  species richness and number of endemic species.
• Biodiversity hotspots have been identified.
  They occupy 15.7 percent of Earths surface but
  have 77 percent of terrestrial vertebrate
  species.
• Most are also areas with high human populations
  and pressures.

59
Figure 59.10 Hotspots of Biodiversity
60
59.4 What Strategies Do Biologists Use to Protect
Biodiversity?

• After areas are identified, a conservation
  strategy must be developed.
• Conservation biologists make detailed analyses of
  species distributions, locations of special
  resources, and factors that threaten or support
  biodiversity in the region.

61
59.4 What Strategies Do Biologists Use to Protect
Biodiversity?

• To further pinpoint sites with threatened species
  that are found nowhere else, conservation
  biologists identified 595 centers of imminent
  extinction.
• Only one-third of the sites are legally
  protected. Most are surrounded by rapid human
  development and are in urgent need of protection.

62
Figure 59.11 Centers of Imminent Extinction
63
59.4 What Strategies Do Biologists Use to Protect
Biodiversity?

• Some degraded ecosystems can be restored.
• In restoration ecology, methods are being
  developed to restore degraded habitats to a more
  natural state.

64
59.4 What Strategies Do Biologists Use to Protect
Biodiversity?

• Many grasslands have rich soils, and have been
  converted to agriculture. Large herds of grazing
  mammals have been reduced to tiny remnants.
• It may be possible to reintroduce large mammals
  if their habitat can be restored.
• Many prairie restoration efforts are underway.

65
59.4 What Strategies Do Biologists Use to Protect
Biodiversity?

• One such project is in Montana. A 15,000 km2 will
  be returned to native prairie plants and animals.
• The region was never plowed, so native vegetation
  should recover rapidly.
• The ranchers who own the land are retiring and
  their children are not interested in continuing
  the ranches.

66
Figure 59.12 Restoring a North American Prairie
67
59.4 What Strategies Do Biologists Use to Protect
Biodiversity?

• Bison, elk, and wolves will be reintroduced.
• The restored prairie is expected to draw
  ecotourists, which will provide major economic
  benefits to the region.

68
59.4 What Strategies Do Biologists Use to Protect
Biodiversity?

• In the United States, some policies allow
  developers to destroy habitats, especially
  wetlands, with the idea that they can be
  recreated elsewhere.
• But creating new wetlands requires detailed
  ecological knowledge that is not always available.

69
59.4 What Strategies Do Biologists Use to Protect
Biodiversity?

• In Southern California, 90 percent of coastal
  wetlands have been destroyed. Some restoration
  attempts using a few easily planted species
  failed.
• An experiment at Tijuana Estuary has shown that
  plots planted with species-rich mixtures
  developed more complex vegetation structure
  (important for insects and birds) and accumulated
  nitrogen faster than species-poor plots.

70
Figure 59.13 A Wetlands Laboratory
71
Figure 59.14 Species Richness Enhances Wetland
Restoration
72
59.4 What Strategies Do Biologists Use to Protect
Biodiversity?

• Restoration often involves restoring disturbance
  patterns.
• Many species depend on disturbances such as fire,
  windstorms, grazing.
• For many years fire suppression was official
  policy in the Untied States Forest Service. But
  now controlled burning is used in many forest and
  grassland management programs.

73
59.4 What Strategies Do Biologists Use to Protect
Biodiversity?

• To use fire as a management tool, it is important
  to know the historical pattern of fires in an
  area.
• Annual growth rings of trees have fire scars and
  can reveal the frequency of low-intensity fires
  in the past.
• Controlled burning then mimics this frequency, to
  avoid a buildup of fuel that can lead to intense,
  tree-killing canopy fires.

74
Figure 59.15 Mimicking Natural Disturbance
Patterns
75
59.4 What Strategies Do Biologists Use to Protect
Biodiversity?

• The Convention on International Trade in
  Endangered Species (CITES) is an international
  treaty to prohibit trade in endangered species,
  or any parts of these speciese.g., whale meat,
  rhinoceros horn, parrots, orchids, etc.
• For some items, such as elephant ivory, the
  demand remains strong, so poaching is common.

76
59.4 What Strategies Do Biologists Use to Protect
Biodiversity?

• Some areas of Africa have so many elephants that
  they are culled, and countries would like to sell
  the ivory to fund conservation efforts.
• Sources of elephant tusks can now be pinpointed
  by matching DNA extracted from tusks with
  geographically based frequencies of 16 DNA
  markers.

77
59.4 What Strategies Do Biologists Use to Protect
Biodiversity?

• Legal sales of ivory from Namibia, Botswana,
  Zimbabwe, and South Africa were sanctioned
  beginning in 2008.
• The sale generated 15 million for conservation
  of elephants, and was monitored by CITES. But
  there is still concern that legal ivory will
  mingle with illegal ivory in international
  markets and encourage illegal elephant slaughter.

78
59.4 What Strategies Do Biologists Use to Protect
Biodiversity?

• The international online marketplace eBay banned
  ivory sales effective January 2009.
• An investigation had revealed that two-thirds of
  online sales of protected wildlife products take
  place on eBay.
• Thus, eBays actions should help dry up markets
  and protect elephants from poaching.

79
59.4 What Strategies Do Biologists Use to Protect
Biodiversity?

• Controlling invasions of exotic species is an
  important part of conservation biology.
• The best option is to prevent introductions in
  the first place, but given the huge volume of
  global trade, this is daunting.

80
59.4 What Strategies Do Biologists Use to Protect
Biodiversity?

• Transoceanic transport of invasive species in
  ship ballast water could be eliminated by
  deoxygenating ballast water before it is pumped
  out.
• This kills most organisms, and extends the life
  of ballast tanks, providing an economic benefit
  to shippers.

81
59.4 What Strategies Do Biologists Use to Protect
Biodiversity?

• Regulation of importation and sale of exotic
  species
• The American horticultural industry has a
  voluntary code of conductthe invasive potential
  of a plant must be thoroughly researched before
  an introduction.
• Stocks of invasives will be phased out, and
  gardeners will be encouraged to use noninvasive
  species.

82
59.4 What Strategies Do Biologists Use to Protect
Biodiversity?

• A decision tree has been developed, based on
  traits that characterize invasive plant species,
  to help determine whether a species should be
  allowed into North America.
• These protocols can greatly reduce the risk of
  introducing invasives.

83
Figure 59.16 A Decision Tree (Part 1)
84
Figure 59.16 A Decision Tree (Part 2)
85
59.4 What Strategies Do Biologists Use to Protect
Biodiversity?

• Many studies have demonstrated the market value
  of protecting biodiversity.
• The field of ecological economics provides tools
  for these assessments.
• They show that biodiversity conservation is
  compelling not only from an ecological or ethical
  perspective, but also from an economic
  perspective.

86
59.4 What Strategies Do Biologists Use to Protect
Biodiversity?

• Ecotourism is a major source of income for many
  developing countries.
• It contributes to conservation of biodiversity
  and economic well-being of local communities.
• Example Wild dogs of Africa have been declining
  due to many factors they are found today in only
  14 countries.

87
59.4 What Strategies Do Biologists Use to Protect
Biodiversity?

• The endangered status of wild dogs has piqued
  interest among tourists who hope to see them
• There are 400 in South Africas Kruger National
  Park

88
Figure 59.17 A Sight for Travelers Eyes
89
59.4 What Strategies Do Biologists Use to Protect
Biodiversity?

• Investigators are now working with other parks,
  lodges, and ranchers to re-establish wild dog
  populations.

90
59.4 What Strategies Do Biologists Use to Protect
Biodiversity?

• Studies of the species-rich area called the
  fynbos, in Western Cape Province, South Africa,
  have caculated the economic benefits of this
  natural ecosystem.
• Two thirds of the 8,500 plant species in this
  region are endemics
• The shrubby vegetation survives droughts, poor
  soils, and frequent fires

91
Figure 59.18 Biodiversity Maintains Ecosystem
Functioning (A)
92
59.4 What Strategies Do Biologists Use to Protect
Biodiversity?

• Two-thirds of Western Capes water comes from the
  fynbos
• Endemic plants are harvested for dried flowers
  and thatching. Rooibos is a fynbos shrub used for
  herbal tea
• Hundreds of thousand of ecotourists come each
  year to see the fynbos

93
59.4 What Strategies Do Biologists Use to Protect
Biodiversity?

• Recently, exotic trees and shrubs have invaded
  the fynbos, displacing native vegetation,
  increasing fire intensity, and transpiring a lot
  of water, reducing stream flows and water
  supplies.
• Stream flows have been decreased by half.

94
Figure 59.18 Biodiversity Maintains Ecosystem
Functioning (B)
95
59.4 What Strategies Do Biologists Use to Protect
Biodiversity?

• Removal of the invasives will be very costly
  (140 to 830 per hectare).
• But replacing the services of the fynbos would be
  vastly more expensive.
• A sewage purification plant would cost 135
  million to build and 2.6 million per year to
  maintain.
• Desalination of seawater would cost four times as
  much.

96
59.4 What Strategies Do Biologists Use to Protect
Biodiversity?

• Alternatives for water delivery would cost 1.8 to
  6.7 times more than the cost of maintaining
  natural vegetation in the watershed.
• Maintaining the fynbos vegetation would be less
  expensive and more labor-intensive (creating
  jobs) than technological methods that could
  substitute for its services.

97
59.4 What Strategies Do Biologists Use to Protect
Biodiversity?

• Protected areas alone cannot preserve
  biodiversity.
• Landscapes where people live and extract
  resources must also play a role.
• Using these lands in ways that sustain
  biodiversity is known as reconciliation ecology.

98
59.4 What Strategies Do Biologists Use to Protect
Biodiversity?

• Most ecosystem services are provided locally, and
  people are more likely to work to protect local
  interests than on national or global issues.
• The National Wildlife Federation has a program in
  which individuals can have their backyards
  certified as wildlife-friendly, by providing food
  plants for birds and butterflies, etc.

99
59.4 What Strategies Do Biologists Use to Protect
Biodiversity?

• Tucson, Arizone has initiated a project to make
  the city into an important habitat for many
  species of birds.

100
59.4 What Strategies Do Biologists Use to Protect
Biodiversity?

• The Turkey Point power plant in southern Florida
  dug a system of canals that cover 6,000 acres for
  cooling the water used in the plant.
• The canals have become home to red mangroves and
  American crocodiles, an endangered species. The
  company now hires biologists to monitor the
  crocodiles and ensure their success.

101
59.4 What Strategies Do Biologists Use to Protect
Biodiversity?

• Some of the worlds most endangered species are
  held in captivity while threats to their habitats
  are reduced.
• There is not enough space in zoos to maintain
  adequate populations, but captive breeding
  programs have played an important role during
  critical periods.
• These programs also raise public awareness.

102
59.4 What Strategies Do Biologists Use to Protect
Biodiversity?

• Captive propagation has helped to save the
  California condor.
• In 1983, only 22 wild condors remained.
  Biologists captured all the individuals and
  started a breeding program.
• The first captive-bred birds were released in
  1992.

103
Figure 59.19 A California Condor Soars
104
59.4 What Strategies Do Biologists Use to Protect
Biodiversity?

• By 2008, there were 327 condors in the world,
  half living in the wild.
• In 2003, a wild-born chick fledged in the wild
  for the first time in 20 years.
• Most threats to condor survival have been
  mitigated, including laws requiring hunters to
  use non-lead bullets.

105
59.4 What Strategies Do Biologists Use to Protect
Biodiversity?

• The Plimsoll line was a line painted on British
  ships in the 19th century to indicate the ship
  was overloaded. It was created by Samuel Plimsoll
  to reduce loss of ships and sailors from
  overloading.
• (Ships were overloaded to increase profits, even
  though it made them unstable and unseaworthy, and
  sink.)

106
59.4 What Strategies Do Biologists Use to Protect
Biodiversity?

• We can make an analogy to Earth today The loss
  of species suggests that the load of human
  activities has pushed the planet below its
  Plimsoll line.
• Science cannot determine an acceptable rate of
  loss. Ethical considerations must figure
  prominently in the decisions that society makes.