Outline and chapter notes to accompany chapter 18 BIODIVERSITY AND THREATENED HABITATS Jan., 2014 A. BIODIVERSITY AND ITS IMPORTANCE. MEASURING BIODIVERSITY: HOW MUCH IS THERE? Biodiversity is the number and variety of biological species. About 1,500,000 species are currently known to science; over half of these are insects. Up to 10 million species are estimated to exist. Habitats are places and environmental conditions in which species live. In any given habitat, species of smaller organisms are considerably more numerous than species of larger organisms, partly because of reduced energy requirements, and partly because of the smaller scale of usable habitats. FACTORS INFLUENCING THE DISTRIBUTION OF BIODIVERSITY Biodiversity results from ecological and evolutionary processes. Speciation increases biodiversity; extinction reduces biodiversity. Species that live together and interact with one another form a community. Each species has its own role in the community, also called its niche. A community plus its physical surroundings is called an ecosystem. The ecosystem of the entire planet is called the biosphere. Biodiversity is a measure of ecosystem complexity. Biodiversity is greatest in tropical ecosystems such as rainforests and coral reefs; biodiversity is smallest in harsh, polar regions. Possible explanations follow the energy-stability-area (ESA) theory: a. Energy: Each biological population needs a certain amount of energy to maintain a population size capable of reproducing itself, so more populations (and thus more species) can live in places with more abundant energy, and fewer populations in places where energy is scarce. If other things are equal, a greater mass of living things (greater biomass) will grow in areas that combine warmth and humidity. b. Stability: High-latitude climates show much more seasonal variation, and species adapted to living in all the seasons of a temperate or polar climate are thereby adapted to ranging widely because climate varies much less from place to place than from season to season. Tropical species, however, can specialize much more to stable habitats that persist season after season but may vary more from place to place. Thus, tropical species can find more specialized yet persistent niches, while high-latitude species must be relatively more generalized in their ecological requirements. c. Area: The relation between land area and biodiversity often follows the equation S = C A^z [ C times A to the Z power] where A stands for area and S stands for the number of species. WHY SHOULD WE CARE? INTERDEPENDENCE OF HUMANS AND BIODIVERSITY Hunting and gathering peoples must be familiar with all the many visible species in their habitat if they are to find food, find medicines, and avoid important hazards. Agriculture caused people to pay more careful attention to a few dozen species only, although the need for medicines continued to make it necessary for them to know most of the species around them. REASONS FOR PRESERVING BIODIVERSITY: 1. Many benefits (medicines, foods, dyes, etc.) may be obtainable from species that we don't know very well. Since we don't know which species these are, and since they are all highly inter- dependent, we must preserve them all in order to make sure that the ones of possible use to us can survive. 2. A second reason for preserving biodiversity lies in the preservation of genes for heritable traits that our domestic species once possessed but have since lost. Traits that are now desirable (or traits that become desirable) often exist in the greater genetic diversity of related wild species and can be bred therefrom. 3. Preserving biodiversity also preserves entire ecosystems. The stability of ecosystems is related to theircomplexity. Species in an ecosystem are interdependent, so changes in one species can affect the whole ecosystem. If one important "keystone" species is allowed to become extinct, many other species might also become extinct if their habitats or niches are adversely affected. 4. The health of our entire planet depends on the composition of our atmosphere, which depends heavily upon the photosynthetic activities of plant species in tropical rainforests. B. EXTINCTION REDUCES BIODIVERSITY TYPES OF EXTINCTION: Species no longer alive are considered extinct. A lineage is a sequence of species in ancestor-to-descendent sequence. True extinction is the dying out of an entire lineage with no descendents. Pseudoextinction is the evolutionary transformation of a species into a different species, so that descendent species outlive the original species. ANALYZING PATTERNS OF EXTINCTION: Has extinction occurred at random in the past? Was the risk of extinction equal across various time periods, and were all species equally at risk? Departures from randomness are of two kinds: 1. Circumstances in which the chances of extinction were very low, such as in various "living fossils", species that have avoided extinction and also changed very little for many millions of years. 2. Mass extinctions, in which large numbers of species have suffered extinction within a short time. Various explanations have been suggested for these mass extinctions: a. The mass extinction at the end of the Cretaceous period brought about the extinction of both dinosaurs and ammonoids. One theory attributes this extinction to the collision of the Earth with an asteroid. Other possible explanations include climate changes, tectonic changes, or ecological changes brought about by the diversification of flowering plants (angiosperms) and the insects associated with them. b. An even more devastating mass extinction occurred at the end of the Permian period. This "great dying" has been attributed by paleontologists to the movement of tectonic plates resulting in the disappearance of shallow inland seas. c. The extinctions of the Pleistocene and Recent epochs seem to coincide with the appearance of humans on each continent or large island, and seem to affect mostly the species that people were likely to hunt, while many other species of no concern to humans remained unaffected. SPECIES THREATENED WITH EXTINCTION TODAY Endangered species are those threatened with extinction. Small population sizes make a species more subject to: - random, abrupt changes in gene frequencies (genetic drift); also - inbreeding depression from loss of genetic diversity These factors often hasten extinction once the population falls below a certain minimum size. Several government agencies and wildlife protection organizations keep lists of species that are considered endangered. Disappearance of habitat or of a key food species can lead to the extinction of niches and thus of many species. C. SOME THREATENED HABITATS AND ECOSYSTEMS Threatened "hot spots" are those where threatened habitats put many thousands of species at risk simultaneously. Similar ecosystems on different continents are grouped together into biomes. CORAL REEFS have the highest biodiversity of all marine ecosystems. Increasing carbon dioxide levels bring the dual threat of increased temperature ans also increased acidity; "bleaching" of coral reefs results when photosynthetic algae die TROPICAL RAINFOREST DESTRUCTION Tropical rainforests are a biome with extremely high diversity of habitats and of animal and plant species. Tall trees form a continuous canopy, beneath which lies a vast understory. Tree-dwelling animals can live all their lives in the canopy, passing from tree to tree without ever descending. The trees may be as tall as a 17-story building or taller; the tallest of trees protrude above the canopy and are called emergents. Many plants grow as epiphytes, perched high on these trees, deriving support but not nutrients from them. The diversity of small-scale habitats for animals allows numerous species to subdivide ecological niches to a very fine scale. Many species form complex interactions in which they are interdependent with one or more other species. Many species of figs are dependent on wasps to pollinate their flowers and on orangutans and other species of large animals to disseminate their seeds. Destruction of their habitat threatens orangutans with extinction, and this, in turn, threatens the extinction of many species of figs, wasps, and other species. In any clearing made by humans or by natural processes, certain "pioneer species" will become established, only to be replaced in stages by more shade-tolerant species and by species whose existence depends on the presence of previous colonizers. This process of succession continues until a climax community is reached, a process that may take centuries to complete in some places. Tropical deforestation: people are destroying rainforests at an alarming rate of about 410 square kilometers per day, for a variety of reasons. Already, an estimated 40% of the world's rainforests have been destroyed. Since rainforests create their own rain clouds, destruction of rainforests result in drastic climate changes. Rainforests can protect our atmosphere against a global increase in atmospheric carbon dioxide, so their destruction makes the problem worse. DESERTIFICATION Prevailing wind patterns often create climatic zones differing in rainfall and consequently in vegetation. Slash-and-burn agriculture and overgrazing both contribute to the advance of desert regions in certain areas of the world, including the southern margin of the Sahara desert in Africa, the Mojave desert in California, and the Great Indian Desert along the India-Pakistan border region. The result is a loss of croplands and rangelands, a shrinking of water supplies, and a decrease in rainfall that brings about further desertification. Desertification can only be reversed at a very high cost, beyond the means of many nations. D. VALUING HABITAT (WHY SAVE HABITATS?) Habitat may be valued for its intrinsic value (as an end in itself) or for its instrumental value (as a means to some other end). Tropical rainforests and many other habitats are important for the global ecosystem as a whole, but the economic self-interests of many tropical countries appear to lie (at least in the short run) in a type of economic development which often encourages processes that clear rainforest tracts for agriculture, mining, or human habitation. The interests of rainforest peoples, governments in control of rainforest areas, and citizens of the planet more generally can be made to coincide only by promoting sustainable uses of forest resources. Sustainable rainforest agriculture is one possibility; well-managed ecotourism is another. ---------------------------------- Jan., 2014 PERMISSION IS HEREBY GRANTED to instructors who have adopted the book BIOLOGY TODAY for classroom use to download, modify, and use these notes as needed to aid them in in their teaching. Students of such instructors may likewise use and modify these notes as study aids.