BIODIVERSITY by PROF.V.C.SONI

BIODIVERSITY

PROF.V.C.SONI

How does a species become extinct? Here’s one scenario. Imagine a species whose habitat constantly shrinks, squeezing populations into ever-smaller areas with ever-diminishing resources. As a population dwindles, the loss of even a single member to hunting, disease, or natural disaster brings the population closer to the brink. And then one day, that population is simply gone. Imagine a relentless succession of such occurrences—one population after another passing quietly out of existence, until none remains. This is the series of events unfolding as tigers slide toward extinction. A hundred years ago, scientists estimate, about 100,000 tigers (Panthera tigris) could be found in the wild. Now that number has plummeted to around 3,200. Three of the world’s nine tiger subspecies have disappeared entirely, and one has not been seen for the past 25 years. Tigers now occupy just 7% of their original range, and even that remaining sliver is decreasing. In Indonesia and Malaysia, tropical forests that are home to two tiger subspecies are being replaced by plantations for palm oil, paper, and rubber. In Russia, logging in temperate forests is destroying the habitat of the Siberian tiger. People moving into rural areas of South Asia are encroaching on the habitat of the Bengal tiger. And throughout their range, tigers are at risk from poachers eager to sell the big cats’ bones and internal organs, considered potent ingredients in some traditional Asian medicines. In this final chapter, you will learn about one of the major ecological challenges of our time—the rapid loss of biodiversity that is a result of our dominance over the environment. As you learn about the fight to save our biological heritage, you will see that conservation biology touches all levels of ecology, from a single tiger to the forest it roams.

The Loss of Biodiversity

The decline of tiger populations is just one example of the worldwide loss of biodiversity. Why do we care about losing species, especially ones that are less charismatic than the magnificent tiger? One reason is what Harvard biologist E. O. Wilson calls biophilia, our sense of connection to nature and to other forms of life. And many people share a moral belief that other species have an inherent right to life. But as you learned in Module 37.23, our dependence on vital ecosystem services also gives us practical reasons for preserving biodiversity. Biodiversity encompasses more than individual species— it includes ecosystem diversity, species diversity, and genetic diversity. Let’s examine each level of diversity to see what we stand to lose if the decline is not stopped.

Ecosystem Diversity

The world’s natural ecosystems are rapidly disappearing. Nearly half of Earth’s forests are gone, and

thousands more square kilometers disappear every year. Grassland ecosystems in North America where millions of bison roamed as recently as the 19th century have overwhelmingly been lost to agriculture and development.  A wide variety of other ecosystems are found in this rapidly vanishing treasure trove of biodiversity. Only about a quarter of the original area remains in its natural state.

Aquatic ecosystems are also threatened. For example, an estimated 20% of the world’s coral reefs, ecosystems known for their species richness and productivity have been destroyed by human activities, and 15% are in danger of collapse within the next two decades. The deteriorating state of freshwater ecosystems is particularly worrisome. Tens of thousands of species live in lakes and rivers, and these ecosystems supply food and water for many terrestrial species, as well—including us.

Loss of biodiversity includes the loss of ecosystems, species, and genes

As natural ecosystems are lost, so are essential services. Water purification is one of the services provided free of charge by healthy ecosystems. As water moves slowly through forests, streams, and wetlands, pollutants and sediments are filtered out. Whether taken from surface waters such as lakes or subsurface sources (groundwater), the drinking water supplied by public water systems typically has passed through this natural filtration process. In some places, including New York City, no further filtration is required, although the water is chlorinated to kill microorganisms. As farm fields and housing developments replaced the naturally diverse ecosystems in watershed, the land’s ability to purify water deteriorated. The additional pollution from agricultural runoff and sewage reduced

water quality to the point where the city had to take action. Officials considered spending $8 billion to build a filtration plant, which would cost a further $1 million per day to operate. They decided to invest in lower cost ecosystem services instead. Actions included more tightly restricting land use in the watershed, purchasing land to preserve natural ecosystems, and helping landowners better manage their land to protect the watershed. As a result of these measures, the quality of naturally

filtered water supplied to New York City remains high.

Species Diversity

When ecosystems are lost, populations of the species that make up their biological communities are also lost. A species may disappear from a local ecosystem but remain in others; for example, a population of tigers may be lost from one region of India while other populations survive elsewhere. Ecologists refer to the loss of a single population of a species as extirpation. Although extirpation and declining population sizes are strong signals that a species is in trouble, it may still be possible to save it.

Extinction means that all populations of a species have disappeared, an irreversible situation. How rapidly are species being lost? Because biologists are uncertain of the total number of species that exist, it is difficult to determine the actual rate of species loss. Some scientists estimate that current extinction rates are around 100 times greater than the natural rate of extinction. The International Union for Conservation of Nature (IUCN) is a global environmental network that keeps track of the status of species worldwide. The 2009 IUCN assessment of five major groups of animals. Notice the large proportions of amphibians and freshwater fishes that are considered threatened further indications of the declining health of freshwater ecosystems. Because of the network of community interactions among populations of different species within an ecosystem, the loss of one species can have a negative impact on the overall species richness of the ecosystem. Keystone species illustrate

this effect. Other species modify their habitat in ways that encourage species diversity. In prairie

ecosystems, for instance, plant and arthropod diversity is greatest near prairie dog burrows, where the soil has been altered by the animal’s digging . Abandoned burrows provide homes for cottontail rabbits, burrowing owls, and other animals. Thus, extirpation of prairie dogs results in lower species diversity in prairie communities. In the United States, the Endangered Species Act protects species and the ecosystems on which they depend. Many other nations have also enacted laws to protect

biodiversity, and an international agreement protects some 33,000 species of wild animals and plants from trade that would threaten their survival. Species loss also has practical consequences

for human well-being. Many drugs have been developed from substances found in the natural world, including penicillin, aspirin, antimalarial agents, and anticancer drugs. Dozens more potentially useful chemicals from a variety of organisms are currently being investigated. For example, researchers are testing possible new antibiotics produced by microbial symbionts of marine sponges; painkillers extracted from a species of poison dart frog; and anti-HIV and anticancer drugs derived from compounds in from rain forest plants.

Genetic Diversity

The genetic diversity within and between populations of a species is the raw material that makes microevolution and adaptation to the environment possible—a hedge against future environmental changes . If local populations are lost and the total number of individuals of a species declines, so, too, do the genetic resources for that species. Severe reduction in genetic variation threatens the survival of a species. The enormous genetic diversity of all the organisms on Earth has great potential benefit for people, too. As you learned in Module 17.13, breeding programs have narrowed the genetic diversity of crop plants to a handful of varieties, leaving them vulnerable to pathogens. For example, researchers are currently scrambling to stop the spread of a deadly new strain of wheat stem rust, a fungal pathogen that has devastated harvests in Africa and central Asia. Resistance genes found in the wild relatives of wheat (Figure 38.1D) may hold the key to the world’s future food supply. Many researchers and biotechnology leaders are enthusiastic about the possibilities that “bioprospecting” for potentially useful genes in other organisms holds for the development of new medicines, industrial chemicals, and other products. Now that you have some insight into the nature and value of biodiversity, let’s examine in more detail the causes for its decline.

What are two reasons to be concerned about the impact of the biodiversity crisis on human welfare?

Habitat loss, invasive species, over harvesting, pollution, and climate change are major threats to biodiversity

The human population has been growing exponentially for more than 100 years. We have supported this growth by using increasingly effective technologies to capture or produce food, to extract resources from the environment, and to build cities. In industrialised countries, we consume far more resources than are required to meet our basic requirements for food and shelter. Thus, it should not surprise you to learn that human activities are largely responsible for the current decline of biodiversity. In this section, we examine the major factors that threaten biodiversity.

Habitat Loss

Human alteration of habitats poses the single greatest threat to biodiversity throughout the biosphere. Agriculture, urban development, forestry, mining, and environmental pollution have brought about massive destruction and fragmentation of habitats. Deforestation continues at a blistering pace in tropical and coniferous forests . The amount of human-altered land surface is approaching 50%, and we use over half of all accessible surface fresh water. The natural course of most of the world’s major rivers has been changed. Worldwide, tens of thousands of dams constructed for flood control, hydroelectric power, drinking water, and irrigation have damaged river and wetland ecosystems. Some of the most productive aquatic habitats in estuaries and intertidal

wetlands have been overrun by commercial and residential development. The loss of marine habitat is severe, especially in coastal areas and coral reefs.

Invasive Species

Ranking second behind habitat loss as a threat to biodiversity are invasive species, which disrupt communities by competing with, preying on, or parasitizing native species. The lack of interspecific interactions that keep the newcomer populations in check is often a key factor in a non-native species becoming invasive . Meanwhile, a newly arrived species is an unfamiliar biotic factor in the

environment of native species. Natives are especially vulnerable when a new species poses an unprecedented threat. In the absence of an evolutionary history with predators, for example, animals may lack defense mechanisms or even a fundamental recognition of danger. The Pacific island of

Guam was home to 13 species of forest birds—but no native snakes—when brown tree snakes

arrived as stowaways on a cargo plane. With no competitors, predators, or parasites to hinder them, the snakes proliferated rapidly on a diet of unwary birds. Four of the native species of birds were extirpated, although they survive on nearby islands. Three species of birds that lived nowhere else but Guam are now extinct. As the populations of two other species of birds became perilously low, officials took the remaining individuals into protective custody; they now exist only in zoos. The brown tree snake also eliminated species of seabirds and lizards.

Overharvesting

The third major threat to biodiversity is overexploitation of wildlife by harvesting at rates that exceed the ability of populations to rebound. Such overharvesting has threatened some rare trees that produce valuable wood, such as mahogany and rosewood. Animal species whose numbers have been drastically reduced by excessive commercial harvest, poaching, or sport hunting include tigers, whales, rhinoceroses, Galápagos tortoises, and numerous fishes. In parts of Africa, Asia, and South America, wild animals are heavily hunted for food, and the African term “bushmeat” is now used to refer generally to such meat. As once-impenetrable forests are opened to exploitation, the commercial bushmeat trade has become one of the greatest threats to primates, including gorillas,

chimpanzees, and many species of monkeys, as well as other mammals and birds . No longer hunted only for local use, large quantities of bushmeat are sold at urban markets  or exported worldwide, including to the United States. Aquatic species are suffering overexploitation, too. Many edible marine fish and seafood species are in a precarious state . Worldwide, fishing fleets are working farther offshore and harvesting fish from greater depths in order to obtain hauls comparable

to those of previous decades.

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Habitat loss, invasive species, overharvesting, pollution, and climate change are major threats to biodiversity

 Pollution

Pollutants released by human activities can have local, regional, and global effects. Some pollutants, such as oil spills, contaminate local areas. Recall that the global water cycle can transport pollutants— for instance, pesticides used on land—from terrestrial to aquatic ecosystems hundreds of miles away. Pollutants that are emitted into the atmosphere, such as nitrogen oxides from the burning of fossil fuels, may be carried aloft for many of miles before falling to earth in the form of acid precipitation. Ozone depletion in the upper atmosphere is another example of the global impact of pollution.

ozone layer protects Earth from the harmful ultraviolet rays in sunlight. Beginning in the mid-1970s, scientists realized that the ozone layer was gradually thinning. The consequences of ozone depletion for life on Earth could be quite severe, not only increasing skin cancers, but harming crops and natural communities, especially the phytoplankton that are responsible for a large proportion of Earth’s primary production. International agreements to phase out the production of chemicals implicated in ozone destruction have been effective in slowing the rate of ozone depletion. Even so, complete ozone recovery is probably decades away. In addition to being transported to areas far from where they originate, many toxins produced by industrial wastes or applied as pesticides become concentrated as they pass through the food chain. This concentration, or biological magnification, occurs because the biomass at any given trophic level is produced from a much larger toxin-containing biomass ingested from the level below. Thus, top-level predators are usually the organisms most severely damaged by toxic compounds in the environment. In the Great Lakes food chain the concentration of industrial chemicals called PCBs increased at each successive trophic level. The PCB concentration measured in the eggs of herring gulls, top-level consumers, was almost 5,000 times higher than that measured in phytoplankton. Many other synthetic chemicals that cannot be degraded by microorganisms also become concentrated through biological magnification, including DDT and mercury. Mercury, a by-product of plastic production and coal-fired power plants, enters the food chain after being converted to highly toxic methylmercury by benthic bacteria. Since people are top-level predators, too, eating fish from contaminated waters can be dangerous. Recently, scientists have recognized a new type of aquatic pollutant: plastic particles that are small enough to be eaten by zooplankton. Many body washes and facial cleansers include

plastic “microbeads” to boost scrubbing power. Too small to be captured by wastewater treatment plants, these microparticles enter the watershed and eventually wash out to sea. Larger particles called preproduction pellets or “nurdles,” used in making plastic products, are also common marine pollutants. Nurdles may be broken down to microbead size in the ocean. Toxins such as PCBs and DDT adhere to these plastic spheres. Thus, toxins may be concentrated first on microparticles and then again by biological magnification.

Global Climate Change

According to many scientists, the changes in global climate that are occurring as a result of global

warming are likely to become a leading cause of biodiversity loss. In the next four modules, you’ll learn about some of the causes and consequences of global climate change.

Rapid warming is changing the global climate

The scientific debate about global warming is over. The vast majority of scientists now agree that rising concentrations of greenhouse gases in the atmosphere . such as carbon dioxide (CO2), ethane

(CH4), and nitrous oxide (N2O), are changing global climate patterns. This was the overarching  conclusion of the assessment report released by the Intergovernmental Panel on Climate Change (IPCC) in 2007. Thousands of scientists and policymakers from more than 100 countries participated

in producing the report, which is based on data published in hundreds of scientific papers. The signature effect of increasing greenhouse gases is the steady increase in the average global temperature, which has risen 0.8°C (1.5°F) over the last 100 years, with 0.6°C of that increase occurring over the last three decades. Further increases of 2–4.5°C (3.6–8.1°F) are likely by the end of the 21st century, depending on the rate of future greenhouse gas emissions. Ocean temperatures are also rising, in deeper layers as well as at the surface. But the temperature increases are not distributed evenly around the globe. Warming is greater over land than sea, and the largest increases are in the northernmost regions of the Northern Hemisphere. In parts of Alaska and Canada, for example, the temperature has risen 1.4°C (2.5°F) just since 1961. Some of the consequences of the global warming trend are already clear from rising temperatures, unusual

precipitation patterns, and melting ice.

Rapid warming is changing the global climate

● The high-latitude biomes of the Northern Hemisphere, tundra and taiga, and the polar ice biomes will be most affected. Those biomes are experiencing the greatest temperature change. Also, the organisms that live there are adapted to cold weather and a short growing season, so their survival is on the line. _

Many of the world’s glaciers are receding rapidly, including mountain glaciers in the Himalayas, the Alps, the Andes, and the western United States. Glacier National Park in northwest Montana will need a new name by 2030, when its glaciers are projected to disappear entirely. For example, almost all of the Grinnell Glacier is now a meltwater . The permafrost that characterizes the tundra biome is also melting. Permanent Arctic sea ice is shrinking; each summer brings increased melting and thinner ice. The massive ice sheets of Greenland and Antarctica are thinning and collapsing. If this melting trend accelerates, rising sea levels would cause catastrophic flooding of coastal areas worldwide. Warm weather is beginning earlier each year. Cold days and nights and frosts have become less frequent; hot days and nights have become more frequent. Deadly heat waves are increasing in frequency and duration. Precipitation patterns are changing, bringing longer and more intense drought to some areas. In other regions, a greater proportion of the total precipitation is falling in torrential downpours that cause flooding. Hurricane intensity is increasing, fueled by higher sea surface temperatures. Many of these changes will have a profound impact on biodiversity.

causes of rising greenhouse gas emissions.

Human activities are responsible for rising concentrations of

greenhouse gases

Without its blanket of natural greenhouse gases such as CO2 and water vapor to trap heat, Earth would be too cold to support most life. However, increasing the insulation that the blanket provides is making the planet uncomfortably warm, and that increase is occurring rapidly. For 650,000 years, the atmospheric concentration of CO2 did not exceed 300 parts per million (ppm); the preindustrial concentration was 280 ppm. Today, atmospheric CO2 is approximately 385 ppm. The levels of nitrous oxide (N2O) and methane (CH4), which also trap heat in the atmosphere, have increased dramatically, too. CO2 and N2O are released when fossil fuels—oil, coal, and natural gas—are burned. N2O is also released when nitrogen fertilizers are used in agriculture. Livestock and landfills are among the factors responsible for increases of atmospheric CH4. The consensus of scientists, as reported by the IPCC, is that rising concentrations of greenhouse gases—and thus, global warming—are the result of human activities. Let’s take a closer look at CO2, the dominant greenhouse gas. The atmospheric CO2 is a major reservoir for carbon. (CH4 is also part of that reservoir.) CO2 is removed from the atmosphere by the process of photosynthesis and stored in organic molecules such as carbohydrates . Thus, biomass, the organic molecules in an ecosystem, is a biotic carbon reservoir. The carbon-containing molecules in living organisms may be used in the process of cellular respiration, which releases carbon in the form of CO2. Nonliving biomass may be

decomposed by microorganisms or fungi that also release CO2. Overall, uptake of CO2 by photosynthesis roughly equals the release of CO2 by cellular respiration. CO2 is also exchanged between the atmosphere and the surface waters of the oceans. Fossil fuels consist of biomass that was buried under sediments without being completely decomposed . The burning of fossil fuels and wood, which is also an organic material, can be thought of as a rapid form of decomposition. While cellular respiration releases energy from organic molecules slowly and harnesses it to make ATP, combustion liberates the energy rapidly as heat and light. In both processes, the carbon atoms that make up the organic fuel are released in CO2. The CO2 flooding into the atmosphere from combustion of fossil fuels may be absorbed by photosynthetic organisms and incorporated into biomass. But deforestation has significantly decreased the number of CO2 molecules that can be accommodated by this pathway. CO2 may also be absorbed into the ocean. For decades, the oceans have been absorbing considerably more CO2 than they have released, and they will continue to do so, but the excess CO2 is beginning to affect ocean chemistry. When CO2 dissolves in water, it becomes carbonic acid. Recently, measurable decreases in ocean pH have raised concern among biologists. Organisms that construct shells or exoskeletons out of calcium carbonate (CaCO3), including corals and many plankton, are most likely to be affected as decreasing pH reduces the concentration of the carbonate ions . Greenhouse gas emissions are accelerating. From 2000 to 2005, global CO2 emissions increased four times faster than in the preceding 10-year span. At this rate, further climate change is inevitable

Global climate change affects biomes, ecosystems, communities, and populations

The distribution of terrestrial biomes, which is primarily determined by temperature and rainfall, is changing as a consequence of global warming. Melting permafrost is shifting the boundary of the tundra as shrubs and conifers are able to stretch their ranges into the previously frozen ground. Prolonged droughts will increasingly extend the boundaries of deserts. Great expanses of the Amazonian tropical rain forest will gradually become savanna as increased temperatures dry out the soil. The combined effects of climate change on components of forest ecosystems in western North America have spawned catastrophic wildfire seasons . In these mountainous regions, spring snowmelt releases water into streams that sustain forest moisture levels over the summer dry season. With the earlier arrival of spring, snowmelt begins earlier and dwindles away before the dry season ends. As a result, the fire season has been getting longer since the 1980s. In addition, drought conditions have made trees more vulnerable to insect and pathogen attack; vast numbers of dead trees add fuel to the flames. Fires burn longer, and the number of acres burned has increased dramatically. As dry conditions persist and snowpacks diminish, the problem will worsen. The earlier arrival of warm weather in the spring is disturbing ecological communities in other ways. In many animal and plant species, certain annual spring events are triggered by temperature increases. With temperatures rising earlier in the year, a variety of species, including some birds and frogs, have begun their breeding season earlier. Satellite images show earlier greening of the landscape, and flowering occurs sooner. For other species, day length is the environmental cue that spring has arrived. Because global climate change affects temperature but not day length, interactions between species may become out of sync. For example, plants may bloom before pollinators have emerged, or eggs may hatch before a dependable food source for the young is available. Because the magnitude of seasonal shifts increases from the tropics to the poles, migratory birds may also experience timing mismatches. For instance, birds arriving in the Arctic to breed may find that the period of peak food availability has already passed. Warming oceans threaten tropical coral reef communities. When stressed by high temperatures, corals expel their symbiotic algae in a phenomenon called bleaching. Corals can recover if temperatures return to normal, but they cannot survive prolonged temperature increases. When corals die, the community is overrun by large algae, and species diversity plummets. The distributions of populations and species are also changing in

response to climate change. The distribution of a species may be determined by its adaptations to the abiotic conditions in its environment. With rising temperatures, the ranges of many species have already shifted toward the poles or to higher elevations. For example, researchers in Europe and the United States have reported that the ranges of more than two dozen species of butterflies have moved north by as much as 150 miles. Shifts in the ranges of many bird species have also been reported; the Inuit peoples living north of the Arctic Circle have sighted birds such as robins in the region for the first time. However, species that live on mountaintops or in polar regions have nowhere to go. Researchers in Costa Rica have reported the disappearance of 20 species of frogs and toads as warmer Pacific Ocean temperatures reduce the dry-season mists in their mountain habitats. In the Arctic, polar bears, which stalk their prey on ice and need to store up body fat for the warmer months, are showing signs of starvation as their hunting grounds melt away. Similarly, in the

Antarctic, the disappearance of sea ice is blamed for recent decreases in populations of Emperor and Adélie penguins. Global climate change has been a boon to some organisms, but so far the beneficiaries have been species that have a negative impact on humans. For example, in mountainous regions of Africa, Southeast Asia, and Central and South America, the ranges of mosquitoes that carry diseases such as malaria, yellow fever, and dengue are restricted to lower elevations by frost. With rising temperatures and fewer days of frost, these mosquitoes—and the diseases they carry—are appearing at higher elevations. In another example, longer summers in western North America have enabled bark beetles to complete their life cycle in one year instead of two, promoting beetle outbreaks that have destroyed millions of acres of conifers. Undesirable plants such as poison ivy and kudzu have also benefited from rising temperatures. Environmental change has always been a part of life; in fact, it is a key ingredient of evolutionary change.