Biodiversity and Conservation Biology. 55. Key Concepts. Biodiversity is quantified at the level of allelic diversity, species diversity, and ecosystem diversity.
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Solutions to the biodiversity crisis include protecting key habitats, lowering human population growth and resource use, restoring ecosystems, mitigating climate change, and supporting sustainable development.
Biodiversity can be thought of as the tree of life, which describes the evolutionary relationships among all forms of life. The branches represent all of the lineages of organisms living today and the tips represent all of the species.
When biodiversity increases, branches and tips are added to the tree. When extinctions occur, tips and perhaps branches are removed.
Recent efforts to document species diversity have used a new technique called bar coding: the use of a well-characterized gene sequence to identify distinct species, using the phylogenetic species concept.
Taxonomic diversity, an additional aspect of species diversity, is important because some lineages on the tree of life are extremely species rich while other lineages are extremely species poor.
Biodiversity can be recognized and quantified on several distinct levels, but it is also dynamic.
Mutations that create new alleles increase genetic diversity; natural selection, genetic drift, and gene flow may eliminate certain alleles or change their frequency, leading to an increase or decrease in overall genetic diversity.
Speciation increases species diversity; extinction decreases it.
New ecosystems may form as a result of changes in abiotic conditions; disturbances, on the other hand, can destroy ecosystems.
Researchers estimated the number of insect species living in the canopy of a single tropical tree. They identified over 900 species of beetles alone.
From these data, they estimated that the world total of arthropod species exceeds 30 million species.
More recently, a taxon-specific survey of marine mollusks identified 2738 species, supporting the hypothesis that the 93,000 known mollusk species may represent just a third to a half of the actual total.
Although extinction is natural, the rates of extinction are increasing.
Today, species are vanishing faster than at virtually any other time in Earth’s history. Modern rates of extinction are 100 to 1000 times greater than the average, or “background,” rate recorded in the fossil record over the past 550 million years.
Directly or indirectly, recent extinctions are being caused by human population growth.
Most biologists agree that the sixth mass extinction in the history of multicellular life is now under way.
Human activities also result in habitat fragmentation—the breakup of large, contiguous areas of natural habitat into small, isolated pieces.
Habitat fragmentation concerns biologists for several reasons:
Habitat fragmentation can reduce habitats to a size that is too small to support some species.
Fragmentation reduces the ability of individuals to disperse from one habitat to another. Small, isolated populations may be more vulnerable to catastrophes, and can suffer from inbreeding depression and loss of alleles due to genetic drift.
A long-term experiment in a tropical wet forest is documenting the decline in habitat quality caused by fragmentation.
Plots of different sizes were shown to have a rapid loss of species diversity, especially from the smaller fragments; and astartling drop in biomass, or the total amount of fixed carbon, in the study plots located near the edges of logged fragments.
This experiment demonstrates that when habitats are fragmented, the quality and quantityof habitat decline drastically.
The best information on current extinction rates comes from studies on birds.
Recent analyses suggest that birds have been going extinct at a rate 100 times the background rate, or at a rate of one species per year rather than one species per 100 years. This rate is also expected to increase.
Similar trends are occurring in other well-studied groups.
Biologists can use habitat-loss projections to estimate rates of extinction based on well-documented species–area relationships.
Over many different habitat types, the relationship is consistently described by the function S = cAz, where S is the number of species, c is a constant that is high in species-rich areas and low in species-poor areas, A is habitat area, and z is the slope of the line on a log-log plot of species number versus area.
Thus, z describes how rapidly species numbers change with area. Typically, z is about 0.25. When z is higher, more species are predicted to be lost.
Recreation based on visiting wild places, or ecotourism, is a major industry internationally and is growing rapidly.
Vegetation on steep slopes and wetlands in low-lying areas dramatically reduces flood damage and the danger posed by mudslides.
In addition, the benefits of biodiversity extend beyond the direct use of diverse genes and species by humans to include ecosystem services―processes that increase the quality of the abiotic environment.
The Gap Analysis Program (GAP) identifies gaps between geographic areas that are particularly rich in biodiversity and areas that are actually managed for the preservation of biodiversity.
One recent GAP analysis combined data sets on the distribution of mammals, birds, amphibians, and freshwater turtles with a map of world protected areas. The analysis revealed that the ranges of many species occur completely outside any protected areas.
Most GAP analyses suggest that the 11.5 percent of Earth’s surface area that is now being managed for biodiversity will not be enough to conserve many species.
Given the magnitude of the current extinction rate, the continued growth in human population size and resource use, and the difficulty of setting up enough preserved areas in the world’s conservation hotspots, conservation biologists are advocating a multipronged strategy.
Preserving land will not be enough. At least four other strategies need to be an important part of a lasting solution to the biodiversity crisis.
In almost every case, the underlying causes of the biodiversity crisis are socioeconomic factors that encourage the short-term overexploitation of land and other resources and discourage long-term sustainability.
Sustainability is the managed use of resources at a rate no faster than the rate at which they are replaced.
The challenge is to create ways for humans to live off the resources that are being produced continuously on Earth, rather than mining resources that have been stored for centuries or millennia.
In many areas of the world, ecosystems are already heavily degraded or lost.
Thousands of large-scale and small-scale ecosystem restoration and reforestation projects are now occurring around the globe.
In 2004, Wangari Maathai won the Nobel Peace Prize to honor her work founding the Green Belt Movement—a reforestation organization that has now sponsored the planting of over 1 billion trees, beginning in Kenya and expanding internationally.
One of the most successful ecological restoration efforts focused on the seasonally dry forest at the Area de Conservación Guanacaste in northwestern Costa Rica.
The primary task was to stop human-caused fires.
Their efforts since the mid-1980s have succeeded in transforming a vast swath of marginally productive ranching land into an increasingly popular ecotourist destination and a water source for neighboring farms and ranches.
In response to threats from development, a group of ranchers banded together to form an association they called the Malpai Borderlands group. To preserve biodiversity and their livelihood, the ranchers:
Protected 42,000 acres of their land with conservation easements.
Set up cooperative “grassbanks,” available to ranchers whose own grazing areas are suffering from short-term drought.
Reintroduced fire to the area via prescribed burns, which removes encroaching woody shrubs and encourages the growth of native grasses.
Today, the Malpai Borderlands group is considered a model of innovative action by private individuals, in cooperation with governmental agencies and nongovernmental organizations.
The project benefits local people as well as biodiversity.
It can be done. Your generation is facing the most serious global environmental crisis in the history of our species. The decisions you make, ranging from how many children you have to how much you drive a car, will have far-reaching consequences.