Outline and chapter notes to accompany chapter 15 BIODIVERSITY AND THREATENED HABITATS Jan., 2001 A. BIODIVERSITY RESULTS FROM ECOLOGICAL AND EVOLUTIONARY PROCESSES. FACTORS INFLUENCING THE DISTRIBUTION OF BIODIVERSITY Biodiversity is the number and variety of biological species. Speciation increases biodiversity; extinction reduces biodiversity. About 1,500,000 species are currently known to science; over half of these are insects. 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. 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. 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. One reason for preserving biodiversity is that 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, we must preserve them all in order to make sure that the ones of possible use to us can survive. 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. A third reason for preserving biodiversity is to preserve entire ecosystems. The stability of ecosystems is related to their complexity. 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. A fourth reason for preserving biodiversity is that 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 ENTIRE HABITATS ARE THREATENED 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. 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. VALUING HABITAT 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; ecotourism is another. D. POLLUTION THREATENS MUCH OF LIFE ON EARTH DETECTING, MEASURING, AND PREVENTING POLLUTION To pollute originally meant "to make dirty" or to defile. Nowadays, pollution can be defined as the presence of something in unwanted amounts at the time and place of its occurrence. From this definition, it is clear that pollution is assessed by measuring quantities. Small amounts of unwanted material may be acceptable or even undetectable. Many substances are biodegradable, and become problematical only when quantities exceed the capacity of the ecosystem to break them down into usable materials. People may disagree about the substances that are unwanted or the quantities that are problematical. There is generally more agreement when human health is impaired or when living organisms are killed. Pollution can arise from household activities (e.g., food wastes, sewage), from industrial processes, from agriculture, or from vehicles. Careless disposal of wastes in inappropriate places is one type of problem whose remedy is relatively simple. Toxicology is the study of poisons and their effects on organisms. AIR POLLUTION Air pollution includes oxides of carbon, nitrogen, and sulfur released by nearly all forms of combustion, plus certain additional gases released by some industrial processes. Indoor air pollution also includes second-hand cigarette smoke. An important first step is awareness of the ways in which we are polluting. In many cases, reducing pollution brings other benefits such as saving money and fuel by carpooling or recycling. Many consumers now prefer to buy "earth-friendly" products. Frogs and certain other species can serve as sentinel species-- like the canaries carried by underground miners, they serve as sensitive indicators of unsafe conditions. ACID RAIN Oxides of sulfur are oxidized in the atmosphere to sulfur trioxide, which dissolves in rain water to form sulfuric acid. Sulfur occurs as an impurity in coal and many mineral ores. Oxides of nitrogen are released by motor vehicles; these eventually become nitric acid. Acid rain (and acid snow) erodes marble (buildings and statues) and metals (including automobiles). Acidification of lakes also kills many fish. Acid rain releases toxic metals from pipes, causing lead poisoning and other toxic effects. One large problem with acid rain is that it travels great distances. Much of the acid rain that falls in New England or upstate New York originates in states further west, while much of the acid rain in Sweden originates in Germany. Under political systems as they now exist, people (or legislatures) in New England or Sweden have great difficulty influencing corrective legislation in Illinois, Indiana, or Germany. E. POLLUTED HABITATS CAN BE RESTORED. BIOREMEDIATION OF OIL SPILLS Biodegradation is the natural biological decomposition of organic materials. Bioremediation is the enhancement or manipulation of biodegradation by humans. Bioremediation is sometimes used to help clean up oil and chemical spills. Oil and other nonpolar chemicals do not mix well with water, since water is strongly polar. If they are heavier than water, they sink to the bottom and kill many bottom-dwelling organisms. If they are less dense, they float on the surface. Oily compounds can be directly toxic to organisms. More often, oily compounds can coat respiratory surfaces and kill organisms by interfering with gas exchange. Oily compounds can also coat the fur of mammals and the feathers of birds, interfering with buoyancy and also with insulation. Some oil-degrading microorganisms have been used to help clean up oil spills: The oil must be tested to see what compounds it contains, and what kinds of microorganisms are capable of degrading those compounds. The proper species of bacteria or fungi must be introduced if not already present. Growth of the biodegrading microorganisms must often be enhanced by supplying a limiting nutrient such as nitrogen or phosphorus (nutrient enrichment). BIOREMEDIATION OF WASTEWATER Wastewater is water that has already been used, for whatever purpose. Biodegradable material in wastewater can be treated by bacteria, as in septic tanks (Fig. 14.15). Wastewater lagoons are ecosystems that allow solids to settle and various algae and bacteria to grow under aerobic conditions and kill off more harmful bacteria that could cause disease. Three-stage water treatment is used on a municipal scale: Primary treatment removes solids and floating particles skimming, filtering, and sedimentation (settling). Secondary treatment uses aerobic organisms such as zoogloea to cause certain suspended particles to cling together into larger aggregates that settle more easily. Sludge from secondary water treatment may be disinfected with chlorine or spread on fields. Tertiary treatment uses activated carbon filters, electrical precipitation, denitrification, and various other processes to make the water fit for drinking once again. Because it is costly, tertiary treatment is not used everywhere. Indicator bacteria are those which indicate that contamination by human wastes is present, and that harmful pathogens have been removed if the indicator species are no longer present. Escherichia coli is the most commonly used indicator species. Marshlands and other natural wetlands can sometimes be used as wastewater treatment systems. TREATMENT OF DRINKING WATER Drinking water is typically purified in the following steps: 1. filtration and sedimentation (allowing particles to settle) 2. flocculation with alum (a process that removes many bacteria) 3. passing over beds of sand or diatomaceous earth (Fig. 14.17) (a process which adsorbs many microorganisms) 4. chlorination, to kill any remaining microorganisms 5. fluoridation, a public health measure which reduces tooth decay COSTS AND BENEFITS Cost-benefit analysis is often used to assess proposed anti-pollution measures. One problem is that the costs (e.g., of new equipment, or of disposal in another location) can easily be measured, while the benefits (in terms of lives saved, diseases prevented, or health improvement generally) are harder to quantify and subject to much greater uncertainty. Even when benefits can be measured, they are often in terms that do not translate readily into dollar amounts: improvements in human health, greener forests, cleaner recreational areas, etc. Measuring environmental quality in a way that can be compared to costs is a new problem that societies must learn to deal with. ---------------------------------- Jan., 2001