The planet is populated with a vast richness of living organism—so vast that we have only cataloged a minority of the species that exist. Unfortunately, we are altering ecosystems and ecosystem process to an extent that we are accelerating the extinction of species, creating a biodiversity crisis. Conservation biology, a recently conceived subdiscipline of biology, is dedicated to countering the biodiversity crisis.
The Biodiversity Crisis: An Overview
Extinction is a natural phenomenon, but it is the current rate of extinction that underlies the biodiversity crisis. The high rate of ecosystem degradation is being causes by one species—Homo sapiens.
A. Numerous examples indicate that estimates of extinction rates are on track
Extinction rates are usually expressed as the number or percentage of species expected to become extinct in an area in a unit of time. Estimates are difficult at best. Since birds are among the most studied animals, the extinction rates for less well-known, nonavian taxa are sometimes based on the rate of loss of bird species. Most often, extinction rates are estimated from the concept of species-area relations in which the number of species in an area is directly related to the size of the area.
This rule predicts that, on average, about 50% of the total number of species will be lost in an area where 90% of the habitat is lost. The absence of clear documentation of the rate of extinction has led some to argue that there is no reason to worry at this time. Population ensus data, however, indicate that extinction of known organisms is occurring at an alarming rate..
• 11% of the 9040 known species of bird are endangered
• 680 of the ca. 20,000 known plant species are in danger of becoming extinct by the year 2000
• Approximately 20% of the known species of freshwater fish have become extinct or
have become threatened during recorded history.
In order to know for certain that a given species is extinct, we must know its exact distribution and habits. However, we do not have a complete catalogue of biodiversity and knowledge of the geographic distribution and ecological roles of Earth’s species, thus, our understanding remains incomplete.
B. The major threats to biodiversity are habitat destruction, over-exploitation, and
competition by exotic species The most significant threat to biodiversity is human alteration of habitat. Human activities which disrupt entire systems include development, logging, war, and oil spills
• 73% of the IUCN’s designations of extinct, endangered, vulnerable, and rare species are
related to destruction of natural habitats.
• Marine biodiversity also is threatened by human activity. About 93 % of the coral reefs
(reefs are estimated to support about 1/3 of the known species of fish) have been
damaged, if the current rate of destruction continues, 40% to 50 % of the reefs could be
lost within 30 to 40 years.
Overexploitation of wildlife by humans is another source of threat.
• Species threatened by excessive commercial harvest or sport hunting include whales,
American bison, Galapagos tortoises, and numerous fishes.
• The often illegal trade of rare animals and animal products also jeopardizes many
species. The introduction of exotic species can cause a variety of problems. Although most transplanted species fail to survive, there are notable exceptions.
• The introduction of Nile perch into Lake Victoria in east Africa has resulted in the loss
of 200 of 300 species of chichlids
• Fire ants, which were accidentally introduced into the southern United States from
Brazil in 1918, have continued to spread northward
• Displacement by introduced species is considered at least partially responsible for 68%
of the IUCN’s listings of extinct, endangered, vulnerable, and rare species.
C. Biodiversity is vital to human welfare
We care about the loss of biodiversity?
Answers to this question-
• Biophilia, the human sense of connection to nature and other forms of life, is centered around aesthetics and ethics.
• Biodiversity is a crucial natural resource and threatened species could provide crops, fibers, and medicines.
• 25% of the prescriptions dispensed from pharmacies in the United States contain substances derived from plants.
In the 1970s, alkaloids isolated from the rose periwinkle of Madagascar were found to inhibit cancer cell growth and result in remission of childhood leukemia and Hodgkin’s disease.
• The loss of species results in the loss of genes and all the genetic potential.
• Humans are dependent on ecosystems and other species. By allowing the extinction of species and degradation of habitats to continue, we are taking a risk with our own species survival. In an effort to influence policy-making, ecologists and economists have estimated the cost of replacing ecosystem “services” as a measure of the services’ value at US$33 trillion.
D. Change in ecological and evolutionary time is the focus of conservation biology Globally, an area half the size of the U.S. has been protected as natural areas .
Three concepts form the roots of conservation biology
1. Preservation is the practice of setting aside select areas to remain natural and undeveloped.
2. Resource conservation is a management scheme aimed at balancing “multiple uses” of natural resources (e.g., agricultural, industrial, preservation, recreation)
3. Evolutionary/ecological view recognizes that natural systems are the result of millions of years of evolution and that ecosystems processes are critical for the maintenance and proper function of the biosphere.
The goal of conservation biology is preserve individuals species and to sustain ecosystems, where natural selection can continue to function and to maintain the genetic variability upon which it acts.
• Conservation biology follows the ecological tenets of nonequilibrium discussed in
• Consideration of human presence is vital in conservation biology because no ecosystems are unaffected by humans. Conservation biology seeks to foster human activities that sustain ecosystems and reduce the current rate of environmental degradation.
The Geographic Distribution of Biodiversity
Biodiversity is not evenly distributed and there are recognizable patterns of distribution, including clines (gradual variation), hot spots, concentrations of diversity, and ranges of migratory species.
A. A gradual variation in biodiversity correlates with geographical gradients
Biogeographers have long recognized the existence of clines in species diversity in the form of major geographical gradients.
• The number of terrestrial bird species in North and Central America increases steadily
from the Arctic to the tropics.
• The number of marine benthic species increases with depth.Four hypotheses have been developed to explain clines, and more generally, the factors that influence patterns of diversity in all communities.
1. Energy availability. Holds that because solar radiation is greatest in the tropics, theresource base is greatest there.
2. Habitat heterogeneity. Holds that tropical regions experience more local disturbances that contribute to greater environmental patchiness; the greater patchiness allows a greater diversity of plants species to form a resource base for diverse communities of animals.
3. Niche specialization. Holds that the stability and predictability of tropical climate may
allow organisms to specialize on a narrower range of resources; smaller niches would
reduce competition and contribute to greater species diversity.
4. Population interactions. Holds that diversity is self-propagating because population
interactions coevolve, and the resulting predator-prey and symbiotic interactions in a
diverse community prevent any populations from becoming dominant.
Many ecologist believe that a complex combination of factors is responsible for clines.
B. Biodiversity hot spots have high concentrations of endemic species
Biodiversity hot spot = Relatively small areas with exceptional concentrations of species
Endemic species = A species found nowhere else
Biogeographers have identified 18 vascular plant hot spots
• These hot spots contain about 20% of the known vascular plant species and 7% of all
land vertebrate species.
• Six of the 18 hot spots have lost close to 90% of their original habitats to human
development; as a result, the biodiversity hot spots also are hot spots of extinction.
Islands are hot spots of bird extinction.
• 30% of all bird species are endemic.
• Approximately 90% of the 104 species of birds lost in the last 400 years were endemic
on islands
• Today, all of the areas where over 10% of the bird species are threatened with
extinction are islands (e.g., Hawaii, Philippines, New Zealand).
• It seems likely that most of the non-avian threatened species also are endemic on islands. In the U.S., most of the endangered species are found in the areas with the most endemic species: Hawaii, southern California, southern Appalachians, southeastern coastal states. Most of these species are threatened because of loss of habitat due to human population growth and agriculture. Studies of biodiversity and recent extinctions show that many threatened, endangered, and potentially endangered species are concentrated in biodiversity hot spots.
• This pattern suggests that with appropriate measures, many species could be saved in relatively small areas.
• The biodiversity crisis is a global problem and focus on hot spots should not detract form efforts to preserve biodiversity in other areas.
C. Migratory species present special problems in conservation
The preservation of migratory species is complicated by a life history that involves residence in multiple jurisdictions.
• Monarch butterflies, for example, migrate from Canada to Mexico. Human intrusion is making the migration an “endangered phenomenon.”
• Similar situations exist for migratory songbirds, sea turtles, and marine mammals. Successful conservation efforts for such species require international cooperation and the careful preservation of habitat in both parts of the species’ range.
III. Conservation at the Population and Species Level
Much of the attention of the biodiversity crisis has focused on species.
Endangered species are species that are in danger of extinction in all or a significant portion of its range
Threatened species are species that are likely to become endangered in the foreseeable future throughout all or a significant portion of its range.
A. Sustaining genetic diversity and the environmental arena for evolution is an ultimate goal
Species are only one component of earth’s biodiversity. Other components include:
• Genetic variability within populations of species
• Myriad biotic and abiotic factors that provide the arena for evolution
Modern conservation science attempts to concentrate more on sustaining ecosystem processes and the evolutionary lineages that species represent than on conserving individual species. Alteration of ecosystems by human activities already makes it impractical to conserve all the genetic diversity within most species.
Conservation biology has focused on understanding the dynamics of small populations, diagnosing declines, assessing the factors responsible for a population’s decline, and determining how to revise declines and sustain small, often fragmented populations.
• Currently, conservation efforts lag far behind the rate of decline and loss of species.
• Many species are at critically low numbers and the strategy has mainly been to reverse the trend.
• Conservation biologists also use some features of crisis management and apply some
untested hypotheses and concepts.
B. The dynamics of subdivided populations apply to problems caused by habitat fragmentation
Degradation of habitats often leads reduction in the area of suitable habitat as well as to fragmentation of the remaining area.
• Some ecologists have likened fragmentation to islands surrounded by areas of human
activity.
• The island model may be overly simplistic, and concepts developed by studying subdivided populations may prove to be more useful to conservation efforts.
Metapopulation = A subdivided population or a network of subpopulations of a species
• Vary greatly, depending on size, quality, spatial arrangement, and persistence of habitat patches
The subpopulations of a metpopulation are separated into habitat patches that vary in quality.
• Patches with abundant, high-quality resources tend to have persistent subpopulations that produce more offspring.
• Low-quality patches may be populated only when new individuals reared in high-quality patches disperse to them.
• Dispersal is essential to maintaining genetic variability within subpopulations; a subpopulation that is cut off from others may eventually become genetically extinct.
Human activity may impact population structure.
• Fragmentation may result in the conversion a population to a metapopulation with
reduced genetic variability.
• Human encroachment on a metapopulation (e.g., loss or reduction of habitat patches;
restriction of dispersal) may decrease the number, size, and or genetic variability of subpopulations.
Zoology Learning Point 