Biodiversity and Conservation Biology

1. Biodiversity and Conservation Biology 55

2. Key Concepts • Biodiversity is quantified at the level of allelic diversity, species diversity, and ecosystem diversity. • According to recent analyses, the sixth mass extinction in the history of life is occurring due to habitat loss, overexploitation, and global climate change. • Humans depend on biodiversity for the products that wild species provide and for ecosystem services that protect the quality of the abiotic environment.

3. Key Concepts • 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.

4. Introduction • The species and resources on our planet are limited, but the human population is growing unchecked. • Biologists fear that this surge in human population will drastically affect biodiversity.

5. What Is Biodiversity? • 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.

6. Measuring and Analyzing Biodiversity • To get a complete understanding of the diversity of life, biologists recognize and analyze biodiversity at the genetic, species, and ecosystem level. • Genetic diversity is the total genetic information contained within all individuals of a species, measured as the number and relative frequency of all alleles present in a species. • Species diversity is the variety of life-forms on Earth, measured as the number and relative frequency of species in a particular region.

7. Measuring and Analyzing Biodiversity • 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.

11. Measuring and Analyzing Biodiversity • Ecosystem diversity is the variety of biotic communities in a region, along with abiotic components such as soil, water, and nutrients. • Attempts to measure ecosystem diversity focus on capturing the array of biotic communities and the variation of physical conditions in a region. • Biodiversity is dynamic—it has been changing since life on Earth began.

12. Change through Time • 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.

13. How Many Species Are Living Today? • Approximately 1.5 million species have been cataloged to date, but this represents only a tiny fraction of the number actually present. • Two general approaches have been used to estimate the total number of species: • Surveys of species-rich groups at small sites (taxon-specific surveys). • Surveys of all the species present in a particular region (all-taxa surveys).

14. Taxon-Specific Surveys • 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.

16. All-Taxa Surveys • The first effort to find and catalog all of the species present in a large area, the Great Smoky Mountains National Park, is now under way. • The survey started in 1999 and will be finished in 2015. • To date, over 890 new species, and over 6300 species that had never before been found in the park, have been discovered.

17. Where Is Biodiversity Highest? • In most taxonomic groups, species richness is highest in the tropics and declines toward the poles. • The tropical rain forests are particularly species rich. Even though they represent just 7 percent of Earth’s land area, they are thought to contain at least 50 percent of all species present.

18. Hotspots of Biodiversity and Endemism • Biodiversity hotspots are regions that are much more species rich than others. • In addition, some regions of the world have a high proportion of endemicspecies—species that are found in a particular area and nowhere else.

22. Conservation Hotspots • Biologists are studying the geographic distribution of biodiversity as a way of focusing conservation efforts. • Conservation hotspots are regions that contain at least 1500 endemic plant species and from which at least 70 percent of the traditional or primary vegetation has been lost. • These conservation hotspots are areas that are in most urgent need of conservation action and where efforts to preserve habitat would have the highest return on investment.

24. Threats to Biodiversity • 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.

25. Changes in the Nature of the Problem • Most extinctions that have occurred over the past 1000 years took place on islands as the result of overhunting or introduction of exoticspecies—nonnative competitors, diseases, or predators. • Fossil evidence on islands in the South Pacific suggests that about 2000 bird species were wiped out as people colonized this area around A.D. 1200.

27. Changes in the Nature of the Problem • Starting in the twentieth century, though, two patterns in species loss began to change: • Endangeredspecies, species that are almost certain to go extinct without effective conservation programs, are now more likely to live on continents than on islands. • Habitat destruction has replaced overhunting and introduction of exotic species as the primary threat to such species worldwide.

28. Changes in the Nature of the Problem • Recent analysis of causes of endangerment for 488 endangered species native to Canada show several patterns: • Habitat loss is the single most important factor in the decline of these species. • Virtually all of the endangered species are affected by more than one factor. • Overharvesting is the dominant problem for marine species, while pollution plays a large role for freshwater species. • Factors beyond human control can also be important.

30. Changes in the Nature of the Problem • Although invasive species are a major problem in some regions, most analyses completed to date are broadly consistent with the Canadian data. • Overhunting has also emerged recently as a dire threat to many mammal populations in Africa and southeast Asia.

32. Habitat Destruction • Humans cause habitat destruction in many ways, from logging and burning forests, to grazing livestock, filling in wetlands, and building housing developments. • On a global scale, one of the most important types of habitat destruction is deforestation. As many as 3 million hectares (ha) have been deforested each year in the Amazon in the 1990s. • The global rate of deforestation has now slowed compared with these rates, but as of 2005, 7.3 million ha/year are still being lost.

34. Habitat Destruction • The total area of wet tropical forest dropped by 2.36 percent during 2000–2005. • If this rate of tropical deforestation continues, over 28 percent of the wet tropical forest that exists today will be gone in your lifetime, including almost half of the Brazilian Amazon. • Forest loss in South America and Africa is particularly important because it is occurring in biodiversity hotspots.

38. Habitat Fragmentation • 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.

39. Habitat Fragmentation • Fragmentation creates large amounts of “edge” habitat. These fragmented habitats can suffer a rapid loss of species diversity and a startling drop in biomass. • When habitats are fragmented, the quality and quantity of habitat decline drastically.

40. Habitat Fragmentation • 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.

45. Habitat Fragmentation Web Activity: Habitat Fragmentation

46. Problems in Small Populations • The small, isolated populations that make up a metapopulation are more likely than large populations to be wiped out because: • Stochastic—chance—catastrophic events such as storms, disease outbreaks, or fires exterminate small populations more readily than large populations. • Small populations suffer from inbreeding depression and random loss of alleles due to genetic drift. • Fitness declines in small, isolated populations can be documented by experimentally increasing gene flow; introducing new alleles may counteract the effects of genetic drift and inbreeding.

48. Climate Change • Climate change might change extinction rates: • Loss of coral reefs due to high-temperature-induced “bleaching.” • Loss of habitat for species native to arctic and alpine tundras. • Trees and other slowly dispersing species are unable to track changes in climate. • Ocean acidification—due to increased carbon dioxide concentrations—inhibit the ability of marine animals to make calcium carbonate skeletons.

49. Can Biologists Predict Future Extinction Rates? • To estimate current extinction rates and predict how they might change in the near future, biologists use two approaches: • Direct counts of extinct species. • Using species–area relationships to predict the consequences of habitat destruction.

50. Estimates Based on Direct Counts • 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.