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In Chapter 5, we noted that the world has experienced five major extinctions during the past 500 million years. Many scientists have suggested that we may currently be in the midst of a sixth mass extinction event. In the most recent assessment made in 2014, scientists estimate that the world is currently experiencing approximately 1,000 species extinctions per year. This sixth mass extinction is unique because it is happening over a relatively short period of time and is the first mass extinction to occur since humans have been present on Earth.

In this module, we will examine the declines in biodiversity of Earth at various levels of complexity including genetic diversity, species diversity, and ecosystem function. In each case, we will examine the roles that humans have played in the decline of biodiversity.

Learning Objectives

After reading this module you should be able to

At the lowest level of complexity, environmental scientists are concerned about conserving genetic diversity. Populations with low genetic diversity are not well suited to surviving environmental change and they are prone to inbreeding depression, as we discussed in Chapter 6. Inbreeding depression by parents that each carry a harmful recessive mutation causes some of their offspring to receive two copies of the harmful mutation and, as a result, causes the offspring to have a poor chance of survival and later reproduction. High genetic diversity ensures that a wider range of genotypes is present, which reduces the probability that an offspring will receive the same harmful mutation from both parents. In addition, high genetic diversity improves the probability of surviving future change in the environment. This happens because high genetic diversity produces a wide range of phenotypes that survive and reproduce under different environmental conditions.

Some declines in genetic diversity have natural causes. Cheetahs, for example, possess very low genetic diversity. Researchers have determined that this condition is the result of a population bottleneck that occurred approximately 10,000 years ago (see FIGURE 15.10). Other declines in genetic diversity have human causes. For example, we discussed in Chapter 5 that the Florida panther once roamed throughout the southeastern United States (FIGURE 59.1). Because of hunting and habitat destruction, the population of the Florida panther shrank to only a small group in south Florida and this led to inbreeding. This inbreeding caused a number of harmful defects that caused the population to decline even further. After scientists released 8 panthers from Texas into Florida to add genetic diversity, the Florida panther population increased from 20 to nearly 100 individuals.

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Although declining genetic variation of plants and animals in the wild is of great concern to scientists, there are also major concerns about declining genetic variation in the domesticated species of crops and livestock on which humans depend. The United Nations notes that the majority of livestock species comes from seven species of mammals (donkeys, buffalo, cattle, goats, horses, pigs, and sheep) and four species of birds (chickens, ducks, geese, and turkeys). In different parts of the world, these species have been bred by humans for a variety of characteristics including adaptations that allow them to survive local climates. For example, humans have bred for a tremendous diversity of traits in cattle, as illustrated in FIGURE 59.2. This wide variety of adaptations, which is produced by a great deal of genetic variation, could be used for adapting to changing environmental conditions in the future or resisting new diseases. Unfortunately, livestock producers have concentrated their efforts on the breeds that are most productive and much of this genetic variation is being lost. In Europe, for example, half of the breeds of livestock that existed in 1900 are now extinct. Of those that remain, 43 percent are currently at serious risk of extinction. Of the 200 breeds of domesticated animals that have been evaluated in North America, 80 percent of these breeds are either declining or are already facing extinction.

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A similar story exists for crop plants. A century ago, most of the crops that humans consumed were composed of hundreds or thousands of unique genetic varieties. Each variety grew well under specific environmental conditions and was usually resistant to local pests. In addition, each variety often had its own unique flavor. As we saw in Chapter 11, the green revolution in agriculture focused on techniques that increased productivity. Farmers planted fewer varieties, concentrating on those with higher yields. Fertilizers and irrigation helped humans control many of the abiotic conditions, allowing fewer but higher-yielding varieties to be grown across large regions of the world. For example, at the turn of the twentieth century, farmers grew approximately 8,000 varieties of apples. Today, that number has been reduced to about 100, and considerably fewer are available in your local grocery store.

Planting only a few varieties leaves us open to crop loss if the abiotic or biotic environment changes. For example, in the 1970s, a fungus spread through cornfields of the southern United States and killed half the crop. Although the fungus was uncommon, the high-yielding variety of corn that most farmers planted turned out to be susceptible to it. Following this crisis, scientists modified this high-yielding corn by adding a gene from a variety that is resistant to the fungus. Had the resistant variety not been preserved, this gene would not have been available.

The nations of the world have recognized the problem of declining seed diversity and have responded by storing seed varieties in specially designed warehouses to preserve genetic diversity. In fact, there are currently more than 1,400 such storage facilities around the world. However, many of these facilities are at risk from war and natural disasters. In the past decade, nations and philanthropists have funded an international storage facility known as the Svalbard Global Seed Vault (FIGURE 59.3). This facility consists of a tunnel built into the side of a frozen mountain on an island in the Arctic region of northern Norway. It was designed to resist a wide range of possible calamities, including natural disasters and global warming. Should the environment change in future years, either in terms of abiotic conditions or because of emergent diseases, the seed bank will be available to help scientists address the challenge. The Svalbard facility opened in 2008 with a capacity of 14.5 million seed varieties. As of 2013, more than 700,000 seed samples had been sent to Svalbard for long-term storage.

Extinction occurs when the last member of a species dies. These major extinction events are characterized as a loss of at least 75 percent of all species within a period of 2 million years. Scientists estimate that as a result of these multiple mass extinctions and many minor extinctions, nearly 99 percent of the 4 billion species that have existed on Earth have gone extinct. However, because each of these mass extinction events has been followed by high rates of speciation that produced new species, we still have millions of species on Earth.

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One way to assess the current extinction rate is by comparing the rate of extinction for groups of organisms, such as mammals, for which we have an excellent fossil record. Using this fossil record, we can compare the rate of species extinctions during the past 500 years to previous 500-year intervals. When we do this, we find that the rate of extinction during the past 500 years is higher than in previous 500-year periods. Indeed, the United Nations Convention on Biological Diversity estimates that the rate of extinction has been 1,000 times higher during the past 50 years than at any other time in human history and rivals the rates observed during the mass extinction event that eliminated the dinosaurs 65 million years ago.

To understand the current loss of species around the world, we can look at how particular groups of species are declining. When considering the status of a species, we use one of five categories defined by the International Union for Conservation of Nature (IUCN). Data-deficient species have no reliable data to assess their status; they may be increasing, decreasing, or stable. Species for which we have reliable data are placed in one of four categories. Extinct species are those that were known to exist as recently as the year 1500 but no longer exist today. The IUCN defines threatened species as those that have a high risk of extinction in the future and near-threatened species are very likely to become threatened in the future. Least concern species are widespread and abundant. These categories provide a mechanism for comparing the status of different groups of species.

Evaluating the status of different plant and animal groups presents several challenges. Many species fall under the category of data-deficient. At the same time, we are still discovering many new species, particularly in remote areas of the world. Since the number of species known to science constantly increases, it is not possible to evaluate every species and our estimates of what fraction of species are declining will constantly change. Finally, the work is expensive. Making an assessment for even one group of species, such as birds or mammals, requires thousands of scientists and millions of dollars.

Of the estimated 10 million species that currently live on Earth, ranging from bacteria to whales, only about 50,000 have been assessed to determine whether their populations are increasing, stable, or declining. Across all groups of organisms that have been assessed, nearly one-third are threatened with extinction. Given that some of the best data are for birds, mammals, and amphibians, we will examine these groups in more detail. FIGURE 59.4 shows the data for those species that are not yet extinct.

Since the year 1500, nearly 10,000 bird species have existed and 130 have become extinct. Today, 22 percent are threatened or near-threatened. Among the 800 species of birds living in the United States, nearly one-third are experiencing declining populations. These include 40 percent of bird species that live in grasslands and 30 percent of bird species that live in arid regions. Multiple threats, including reduced habitat and rising sea levels, have caused a growing concern for all species of birds that live on coastlines or on islands.

A similar pattern exists for mammals. Of the nearly 5,500 species of mammals known to have existed after 1500, 77 are extinct. Among the approximately 4,600 species for which there are reliable data, 25 percent are threatened and 32 percent are either threatened or near-threatened. This means that more than 1,400 species of mammals may be at risk of extinction.

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Amphibians are experiencing the greatest global declines. Of the more than 6,300 species of amphibians, 34 species are extinct. However, a recent assessment of amphibian populations suggests that the number of extinctions may accelerate in the coming decades. Among the approximately 4,700 species for which reliable data exist, 49 percent are either threatened or near-threatened. This means that nearly 2,300 species of amphibians are declining around the world.

Many other groups of organisms are also experiencing large declines, but complete assessments have not yet been conducted because of the time and money required for each assessment. However, from the sample of species that have been assessed in each group, we see an emerging picture that is far from positive. For example, from this sample, approximately one-third of all reptiles, fish, and invertebrates are threatened with extinction. Similarly, one-fourth of plant species are threatened. These results suggest that when the assessments are complete, the news will most likely not be good.

Given that we rely on a relatively small number of the millions of species on Earth for our essential needs, why should we care about the millions of other species that live in various ecosystems? To understand the value of ecosystems, we can consider both intrinsic values and instrumental values.

Many people believe that ecosystems have intrinsic value—that is, that ecosystems are valuable independent of any benefit to humans. These beliefs may grow out of religious or philosophical convictions. People who believe that ecosystems are inherently valuable may argue that we have a moral obligation to preserve them. They may equate the obligation of protecting ecosystems with our responsibility toward people or animals that might need our help to survive. People who argue that ecosystems are valuable independent of any benefit to humans generally believe that environmental policy and the protection of ecosystems should be driven by this intrinsic value.

An ecosystem may also have instrumental value, meaning that it has worth as an instrument or tool that can be used to accomplish a goal. Instrumental values, which include the value of items such as crops, lumber, and pharmaceutical drugs, can be thought of in terms of how much economic benefit a species bestows. As noted in Chapter 1, we often refer to these instrumental values as ecosystem services. When calculating the instrumental value of various ecosystem services, we can consider five categories: provisions, regulating services, support systems, resilience, and cultural services.

Provisions

Goods produced by ecosystems that humans can use directly are called provisions. Examples include lumber, food crops, medicinal plants, natural rubber, and furs. Of the top 150 prescription drugs sold in the United States, about 70 percent come from natural sources. For example, Taxol, a potent anticancer drug, was originally discovered in the bark of the Pacific yew (Taxus brevifolia), a rare tree that grows in forests of the Pacific Northwest (FIGURE 59.5). Once approved by the FDA, the synthetic version of this single drug has had annual sales of over $1.5 billion. There is no way to estimate the potential value of natural pharmaceuticals that have yet to be discovered, but currently more than 800 natural chemicals have been identified as having potential uses to improve human health. Therefore, our best strategy may be to preserve as much biodiversity as we can to improve our chances of finding the next critical drug.

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Regulating Services

Natural ecosystems help to regulate environmental conditions. For example, humans currently add about 8 gigatons of carbon to the atmosphere annually (1 gigaton = 1 trillion kilograms), but only about 4 gigatons of carbon remain there. The rest is removed by natural ecosystems, such as tropical rainforests and oceans, which provide us with more time to address climate change than we would otherwise have (FIGURE 59.6). As we have already seen, ecosystems also are important in regulating nutrient and hydrologic cycles.

Support Systems

Natural ecosystems provide numerous support services that would be extremely costly for humans to generate. One example is pollination of food crops (FIGURE 59.7). The American Institute of Biological Sciences estimates that crop pollination in the United States by native species of bees and other insects, hummingbirds, and bats is worth roughly $3.1 billion in added food production. In addition to providing habitat for animals that pollinate crops, ecosystems provide natural pest control services because they provide habitat for predators that prey on agricultural pests. Although organic farmers, who rarely use synthetic pesticides, gain the most from these pest controls, conventional agriculture benefits as well.

Healthy ecosystems also filter harmful pathogens and chemicals from water, leaving humans with water that requires relatively little treatment prior to drinking. Without these water-filtering services, humans would have to build many new water treatment facilities that use expensive filtration technologies. New York City, for example, draws its water from naturally clean reservoirs in the Catskill Mountains. But residential development and tourism in the area has threatened to increase contamination of the reservoirs with silt and chemicals. Building a filtration plant adequate to address these problems would cost $6 billion to $8 billion. For this reason, New York City and the U.S. Environmental Protection Agency have been working to protect sensitive regions of the Catskills.

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Resilience

We have already seen that resilience ensures an ecosystem will continue to exist in its current state, which means it can continue to provide benefits to humans. Resilience depends greatly on species diversity. For example, several different species may perform similar functions in an ecosystem but differ in their susceptibility to disturbance. If a pollutant kills one plant species that contains nitrogen-fixing bacteria, but does not kill all plant species that contain nitrogen-fixing bacteria, the ecosystem can still continue to fix nitrogen (FIGURE 59.8).

Cultural Services

Ecosystems provide cultural or aesthetic benefits to many people. The awe-inspiring beauty of nature has instrumental value because it provides an aesthetic benefit for which people are willing to pay (FIGURE 59.9). Similarly, scientific funding agencies may award grants to scientists for research that explores biodiversity with no promise of any economic gain. Nevertheless, the research itself has instrumental value because the scientists and others benefit from these activities by gaining knowledge. While intellectual gain and aesthetic satisfaction may be difficult to quantify, they can be considered cultural services that have instrumental value.

The Monetary Value of Ecosystem Services

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Most economists believe that the instrumental values of an ecosystem can be assigned monetary values, and they are beginning to incorporate these values into their calculations of the economic costs and benefits of various human activities. However, assigning a dollar value is easier for some categories of ecosystem services than for others. In 1997, a team of scientists and economists attempted to estimate the total value of ecosystem services to the human economy. They considered replacement value—the cost to replace the services provided by natural ecosystems. They also looked at other factors, such as how property values were affected by location relative to these services—for example, oceanfront housing. Finally, they considered how much time or money people were willing to spend to use these services—for example, whether they were willing to pay a fee to visit a national park. Using this method, researchers estimated that ecosystem services were worth over $30 trillion per year, or more than the entire global economy at that time.

The Decline of Ecosystem Services

Because species help determine the services that ecosystems can provide, we would expect declines in species diversity to be associated with declines in ecosystem function. In the Millennium Ecosystem Assessment conducted in 2005, the most recent assessment conducted, scientists from around the world examined the current state of 24 ecosystem functions, including food production, pollination, water purification, and the cycling of nutrients such as nitrogen and phosphorus. Of these 24 different ecosystem functions, 15 were found to be declining or used at a rate that cannot be sustained. If we want to improve ecosystem functions, we need to improve the fate of the species and ecosystems that provide these services.