Chapter 8 Biodiversity and Extinction
Chapter 8 Outline:
8.1 Introduction to Biodiversity
8.3 Importance of Biodiversity
8.4 Threats to Biodiversity and Extinction
8.5 Changes in Biodiversity through Time and Mass Extinction Events
Learning Outcomes:
After studying this chapter, each student should be able to:
- 8.1 Describe biodiversity in terms of taxonomy and the various organisms on earth
- 8.2 Describe the different types and patterns of biodiversity
- 8.3 Identify benefits of biodiversity for humans and our planet
- 8.4 Explore the various threats to biodiversity, including climate chane, and explain extinction
- 8.5 Explore changes in biodiversity through time, including major extinction events
- 8.6 Explain ways to preserve biodiversity, such as the legislative framework for conservation and habitat restoration
- 8.7 Identify examples of nature based solutions to climate change
8.1 Introduction to Biodiversity
Earth is home to an impressive array of life forms. From single-celled organisms to creatures made trillions of cells, life has taken on many wonderful shapes and sizes and evolved countless strategies for survival.
Biologists have the task of naming and classifying organisms based on similarities in genetics and morphology (structure). This classification branch of biological science is known as taxonomy. Taxonomists group organisms into categories that range from very broad to very specific (Figure 1). The broadest category is the Domain and the most specific is species (notice the similarities between the words specific and species). Currently, many taxonomists recognize three domains of living things: Bacteria, Archaea, and Eukarya (the eukaryotes). The domain Eukarya is further divided into four kingdoms: Protista, Fungi, Plantae, and Animalia.
Figure 1. This illustration shows the taxonomic groups, in sequence, with examples. (Credit: This illustration by OpenStax is licensed under CC BY 4.0)
Domain – Bacteria
Bacteria include relatively simple and small prokaryotic, unicellular organisms. They are incredibly abundant and found in nearly every imaginable type of habitat, including the human body. Bacteria come in many different shapes, as shown in Figure 2. While many people view bacteria only as disease-causing organisms, most species are actually either benign or beneficial to humans.
Bacteria are well-known for their metabolic diversity. Metabolism is a general term describing the complex biochemistry that occurs inside of cells. Types of metabolism in bacteria include autotrophy and heterotrophy.
Figure 2. Many prokaryotes fall into three basic categories based on their shape: (a) cocci, or spherical; (b) bacilli, or rod-shaped; and (c) spirilla, or spiral-shaped. (credit a: modification of work by Janice Haney Carr, Dr. Richard Facklam, CDC; credit c: modification of work by Dr. David Cox, CDC; scale-bar data from Matt Russell). (Credit: This figure by OpenStax is licensed under CC BY 4.0)
Many species of bacteria are autotrophs, meaning they can create their own food source without having to eat other organisms. Most autotrophic bacteria do this by undergoing photosynthesis, a process that converts light energy into chemical energy (that can be utilized by cells). A well-known and ecologically important group of photosynthetic bacteria are the cyanobacteria. These used to be called blue-green algae, but this name is not appropriate because algae are not bacteria. Cyanobacteria play important roles in food webs of aquatic systems such as streams and lakes.
Other species of bacteria are heterotrophs, meaning they need to acquire their food by consuming organic matter or other organisms. This classification includes the bacteria that cause disease in humans, which are gaining nutrients and other resources from the human body. However, most heterotrophic bacteria are harmless to humans, and live in every imaginable habitat and on every living thing. Humans have hundreds of species of bacteria living on the skin which are fairly benign, while the bacteria in the large intestine actually help with digestion and produce needed vitamins. Heterotrophic bacteria play vital roles in ecosystems, especially soil-dwelling bacteria that convert atmospheric nitrogen into a form needed by plants, and decompose living matter, making nutrients available to plants.
Domain – Archaea
Like bacteria, organisms in domain Archaea are small, simple, prokaryotic and unicellular. But unlike bacteria, archaea have cell walls made of pseudopeptidoglycan and different ribosomal RNA structure. Superficially, they look a lot like bacteria, and many biologists confused them with bacteria until a few decades ago. But modern DNA and RNA analysis has revealed that archaea are so different genetically that they belong in their own kingdom.
Many archaea species are found in very inhospitable environments, such as areas of immense pressure (at the bottom of the ocean), salinity (in the Great Salt Lake), or high heat (geothermal springs). Organisms like these archaeans that can tolerate and even thrive in such conditions are known as extremophiles. The fact that a large percentage of archaeans are extremophiles (along with genetic evidence), suggests that they may be descendants of some of the most ancient life forms on Earth – life forms that originated on a young planet that was very inhospitable compared to today’s conditions.
Domain – Eukarya
This domain is probably the most familiar to us because it includes humans and other animals, along with plants, fungi, and protists. Unlike the other domains, Domain Eukarya contains multicellular organisms in addition to unicellular species. The domain is characterized by the presence of eukaryotic cells. For this domain, you will be introduced to its four kingdoms; the taxonomic group immediately below domain (see Figure 1 on page 1).
Kingdom Animalia includes the animals and are comprised of multicellular, heterotrophic organisms. This kingdom includes humans and other primates, arthropods (such as insects, spiders, and crabs), fish, amphibians, reptiles, and many other types of small and large animals.
Kingdom Plantae includes the plants and are multicellular, autotrophic organisms. Except for a few species that are parasitic, plants are usually green with the chlorophyll pigment needed for photosynthesis. Plants make their own sugars using carbon dioxide, water and light energy.
Kingdom Fungi includes all type of fungi, which include many multicellular forms such as mushrooms and unicellular yeast (Figure 3). Members of this kingdom are heterotrophic, drawing nutrients from living things or decaying organic matter. Fungi are sometimes mistaken for plants because many species of fungi grow in the ground, but they lack chlorophyll and cannot photosynthesize. Some fungi take the form of molds and mildews, which are commonly seen on moist, organic rich surfaces (such as rotting food). Many species of yeast are important to humans, especially baker’s and brewer’s yeast, important in bread making and alcoholic beverage production. Yeast produce CO2 gas and alcohol as byproducts of their metabolism – this gas makes bread rise and the alcohol is used to produce alcoholic beverages.
Figure 3. The (a) familiar mushroom is only one type of fungus. The brightly colored fruiting bodies of this (b) coral fungus are displayed. This (c) electron micrograph shows the spore-bearing structures of Aspergillus, a type of toxic fungi found mostly in soil and plants. (Credit a: modification of work by Chris Wee; Credit b: modification of work by Cory Zanker; Credit c: modification of work by Janice Haney Carr, Robert Simmons, CDC; scale-bar data from Matt Russell). This work by OpenStax is licensed under CC BY 4.0)
Kingdom Protista includes mostly unicellular, eukaryotic organisms although some are colonial or occur as part of filaments (Figure 4). The protists include a highly varied assemblage of organisms, both in their structure and in their metabolism, with many being heterotrophic while others are photosynthetic autotrophs. Examples of protists include macroalgae such as kelps and seaweeds, microalgae such as diatoms and dinoflagellates, and disease-causing microbes such as Plasmodium, the parasite that causes malaria. Just one drop of pond water can contain a vast array of protist species.
Figure 4. Protists range from the microscopic, single-celled (a) Acanthocystis turfacea and the (b) ciliate Tetrahymena thermophila to the enormous, multicellular (c) kelps (Chromalveolata) that extend for hundreds of feet in underwater “forests.” (Credit a: modification of work by Yuiuji Tsukii; Credit b: modification of work by Richard Robinson, Public Library of Science; Credit c: modification of work by Kip Evans, NOAA; scale-bar data from Matt Russell). Attribution: This work by OpenStax is licensed under CC BY 4.0)
By gaining an understanding of the breath of biological diversity, as shown in Figure 5, one is better equipped to study the role of biodiversity in the biosphere (where all life occurs) and in human economics, health, politics, and culture. Studying various organisms can be beneficial for our biological understanding and even human survival.
Figure 5. The diversity of life on Earth is the result of evolution, a continuous process that is still occurring. (Credits: “wolf”: modification of work by Gary Kramer, USFWS; Credit: “coral”: modification of work by William Harrigan, NOAA; Credit “river”: modification of work by Vojtěch Dostál; Credit “protozoa”: modification of work by Sharon Franklin, Stephen Ausmus, USDA ARS; Credit “fish” modification of work by Christian Mehlführer; Credit “mushroom”, “bee”: modification of work by Cory Zanker; Credit “tree”: modification of work by Joseph Kranak)
8.2. Types of Biodiversity
Biodiversity is a broad term for biological variety, and it can be measured at a number of organizational levels. Traditionally, ecologists have measured biodiversity by taking into account both the number of species and the number of individuals of each species (known as relative abundance). However, biologists are using different measures of biodiversity, including genetic diversity and ecosystem diversity, to help focus efforts beyond particular species to the unique genes present among species, and the unique habitats that support so many different species of all kingdoms (Figure 6).
Figure 6. This tropical lowland rainforest in Madagascar is an example of a high biodiversity habitat. This particular location is protected within a national forest, yet only 10 percent of the original coastal lowland forest remains, and research suggests half the original biodiversity has been lost. (Credit: Frank Vassen)
As mentioned previously, biodiversity can be described in three different ways, species diversity, genetic diversity, and ecosystem diversity, as explored in more detail below:
Species Diversity – As evolution through natural selection occurs, organisms evolve and develop new species. Thus, speciation increases biodiversity. A common measure of biodiversity is species diversity, which is just the number of species in a location or on the earth. The American Ornithologists’ Union, for example, lists 2078 species of birds in North and Central America. This is one measure of the bird biodiversity on the continent. More sophisticated measures of diversity take into account the relative abundances of species. For example, a forest with 10 equally common species of trees is more diverse than a forest that has 10 species of trees with just one of those species making up 95 percent of the trees.
Genetic diversity is a second measure of biodiversity, and is represented by the variety of genes present within each population of organisms on our earth. Genetic diversity is therefore the raw material for evolutionary adaptation in a species. A species’ potential to adapt to changing environments or new diseases depends on this genetic diversity.
Ecosystem diversity is a third way to measure biodiversity – focusing on the number of different ecosystems in a geographical area, biome, or on the entire earth, such as the coral reefs (Figure 7a), kelp forests, deep ocean, and intertidal zones of the marine biome . The loss or degradation of an ecosystem may mean the loss of all species in that ecosystem and the unique biological and physical interactions that they represent. An example of a largely extinct ecosystem in North America is the prairie ecosystem, which has been largely converted from native species to farmland. (Figure 7b).
Figure 7. The variety of ecosystems on Earth—from a. coral reef to b. prairie enables a great diversity of species to exist. (Credit “coral reef”: modification of work by Jim Maragos, USFWS; Credit: “prairie”: modification of work by Jim Minnerath, USFWS)
The following video explores biodiversity in Panama, with an explanation of the various forms of biodiversity:
The Wild Classroom – Science Filmmaking Tips (2007, June 27) Biodiversity – from the wild classroom [Video – YouTube] https://youtu.be/vGxJArebKoc
Current Species Diversity
Despite considerable effort, knowledge of the species that inhabit the planet is limited. About 1.5 million species have been formally described in the scientific literature, and most of these are insects. Scientists generally agree, however, that many more species exist. The total number of species may be as few as 2 million to as high as 12 million (or more!). Recent estimates suggest that only 13% of eukaryotic species have been named. Given that the earth is losing species at an accelerating pace, some species may be lost before they are actually described and catalogued.
Naming, counting, and describing species is a complex process by which biologists determine an organism’s unique characteristics and whether that organism belongs to a described species, or could be a new species. This process allows biologists to find and recognize new species, and follow up discoveries with questions about its biology and to determine the value of the species to humans and to our understanding of ecosystems. Without a name and description, a species cannot be studied in depth and in a coordinated way by multiple scientists.
Patterns of Biodiversity – Biogeography
Biodiversity is not evenly distributed on the planet. Biogeography is the study of the distribution of the world’s species, both in the past and in the present. The work of biogeographers is critical to understanding our physical and biological environment, how the environment affects species, and how changes in the environment over time impacts the distribution of a species.
One observed pattern is that biodiversity typically increases as latitude declines, meaning biodiversity increases closer to the equator (Figure 8). A leading theory of why this occurs is that the greater age of the tropical ecosystems has allowed many years of uninterrupted evolution and speciation, while temperate regions lost species with each ice age. Another important factor may be that the tropics receive more sun and have greater precipitation (in species rich regions). The complexity of tropical ecosystems (e.g., many layers from treetops to ground) may also promote speciation by increasing the number of microhabitats, thus providing a much greater number of ecological niches for speciation to occur. Lastly, the tropics are more stable than temperate regions, which have pronounced climate changes (especially summer and winter temperatures) and changes in day length by season. This stability in the tropics might also promote speciation. These factors have led to high biodiversity and high numbers of endemic species in tropical regions. Endemic species are ones that are only found in one location. Due to their limited range, endemic species are especially vulnerable to extinction.
Figure 8. This map illustrates the number of amphibian species across the globe and shows the trend toward higher biodiversity at lower latitudes. A similar pattern is observed for most taxonomic groups. (Credit: Wikimedia Commons)
8.3 Importance of Biodiversity
Biodiversity is essential for the processes that support all life on earth, including humans. Without a wide range of animals, plants and microorganisms, we cannot have the healthy ecosystems that all living things rely on, including humans, to provide fresh air, food, essential habitats, and so much more. Loss of biodiversity has reverberating consequences on ecosystems because of the complex interrelationships between species. For example, the extinction of one species may cause the extinction of several others. Ecosystems are complex assemblages of organisms from all of the kingdoms, and their destruction or degradation by humans cannot be reversed easily, since the conditions of the intact ecosystem are hard to recreate artificially.
The following are two of the more important reasons for preserving biodiversity, from a human standpoint:
Human Health
Many medications are derived from natural sources, so maintaining biodiversity also means that possible sources of new medicines are maintained. For example, many plants produce compounds meant to protect the plant from insects and other herbivores. Some of these same compounds also work as human medicines.
Scientists working with modern pharmaceutical companies and other research organizations of course recognize the importance of these plant compounds. Examples of significant medicines derived from plants include aspirin, codeine, digoxin, atropine, and vincristine (Figure 9). Antibiotics, which are responsible for extraordinary improvements in health and lifespans, are compounds largely derived from naturally occurring fungi and bacteria. In recent years, animal venoms and poisons have stimulated research in regards to their medicinal potential. The FDA has approved five drugs based on animal toxins to treat diseases such as hypertension, chronic pain, and diabetes.
Figure 9. Catharanthus roseus, the Madagascar periwinkle, has various medicinal properties. Among other uses, it is a source of vincristine, a drug used in the treatment of lymphomas. (Credit: Forest and Kim Starr)
Finally, it has been noted by several studies that the mental health of humans is improved from living in a biodiverse world, with nature itself acting as a medicine for the human condition. One proponent of this idea is famed entomologist E. O. Wilson, who argued that human evolutionary history has adapted us to live in a natural environment that promotes human health and well-being.
Research has shown that there are measurable psychological benefits of spending time in natural landscapes. Happiness and good health have been shown to depend on having access to nature.
Agriculture
Since the beginning of human agriculture more than 10,000 years ago, human groups have been breeding and selecting crop varieties first derived from nature. Every plant, animal, and fungus that has been cultivated by humans was bred originally from wild ancestors.
The ability to create new, disease resistant crop varieties relies on the diversity of varieties available (biodiversity!), especially wild forms of the crop plant. These ancestral wild forms are often the only source of new gene variants that can be bred with existing varieties to create new varieties with needed attributes, such as pest resistance. Loss of wild species means the loss of potential for crop improvement. Maintaining the genetic diversity of wild species related to domesticated species ensures our continued supply of food.
The potato demonstrates a well-known example of the risks of low crop diversity: during the tragic Irish potato famine (1845–1852 AD), the single potato variety grown in Ireland became susceptible to a potato blight – wiping out the entire crop. This loss led to famine, death, and mass emigration out of the area. Resistance to disease is a chief benefit to maintaining crop biodiversity – the lack of diversity in contemporary crop species carries similar risks.
The following video explores various ways that maintaining biodiversity also supports human health:
Convention on Biological Diversity (2020, April 9) Biodiversity and Health. [Video – YouTube] https://youtu.be/WZvUUTSxanc
8.4 Threats to Biodiversity and Extinction
Extinction, the loss of a species forever, decreases biodiversity. When speciation rates are higher than extinction rates, biodiversity increases. When speciation rates are lower than extinction rates, biodiversity decreases.
Both speciation and extinction are natural ecological events that have occurred throughout the history of life. However, recent human activities have greatly affected extinction rates. Biologists estimate that species extinctions are currently 500–1000 times higher than the normal (background) rate seen previously in Earth’s history. The current high rates may cause a precipitous decline in the biodiversity of the planet in the next century. The loss of biodiversity will include many species we know today. Although it is sometimes difficult to predict which species will become extinct, many are already listed as endangered (at great risk of extinction if conditions do not change). Many extinctions will affect species that biologists have not yet even discovered and described. Threatened species are those that are likely to become endangered within the foreseeable future, if conditions do not change.
Biodiversity loss refers to the reduction of biodiversity due to displacement or extinction of species. The loss of a particular individual species may seem unimportant to some, especially if it is not a charismatic species like the Bengal tiger or the bottlenose dolphin. However, the current accelerated extinction rate means the loss of tens of thousands of species within our lifetimes. Much of this loss is occurring in tropical rainforests like the one pictured in Figure 10. which are very high in biodiversity but are being cleared for timber and agriculture. This may have dramatic negative effects on human welfare as these ecosystems are lost.
Figure 10. Habitat destruction through deforestation, especially of tropical rainforests as seen in this satellite view of the Amazon rainforests in Brazil, is a major cause of the current decline in biodiversity. (Credit: modification of work by Jesse Allen and Robert Simmon, NASA Earth Observatory)
Biologists recognize that human populations are embedded in ecosystems and are dependent on them, just as is every other species on the planet. Agriculture began after early hunter-gatherer societies first settled in one place and heavily modified their immediate environment. This cultural transition has made it difficult for humans to recognize their dependence on living things other than crops and domesticated animals on the planet. Today our technology smooths out the harshness of existence and allows many of us to live longer, more comfortable lives, but ultimately the human species cannot exist without its surrounding ecosystems. Our ecosystems provide us with food, medicine, clean air and water, recreation, and spiritual and aesthetical inspiration.
Core Threats to Biodiversity
The core threat to biodiversity on the planet is the combination of rapid human population growth and the loss of habitats and degradation of natural resources because of the large human population. Humans require resources to survive and grow, and many of those resources are being removed unsustainably from the environment. The four greatest proximate threats to biodiversity are:
Habitat loss
Overharvesting
Introduction of exotic species
Anthropogenic (human-caused) climate change.
Other environmental issues, such as toxic air, water, and soil pollution, have specific targeted effects on species, but are not generally seen as threats at the magnitude of the others.
Habitat Loss
Habitat loss is the primary reason for loss of biodiversity on our planet, as species become extinct. Humans use technology to modify their environment for agriculture or other reasons, destroying the natural habitat in the process. Elimination of habitat (the part of an ecosystem required by a particular species) – whether a forest, coral reef, grassland, or flowing river – causes the loss of all species within that former habitat, so has a much greater impact on biodiversity than hunting, for example. Human destruction of habitats accelerated in the latter half of the twentieth century, as the world’s human population added billions of people across the globe.
Consider the exceptional biodiversity of Borneo, which has lost a significant amount of its forest to timber and palm oil plantations (Figure 11). Palm oil is used in many products including food products, cosmetics, and biodiesel fuel. Cutting down the forests for timber or to expand palm plantations results in the loss of numerous species unique to those affected areas.
Figure 11. An oil palm plantation in Sabah province Borneo, Malaysia, replaces native forest habitat that a variety of species depended on to live. (Credit: Lian Pin Koh)
Rivers and streams are important ecosystems and are also frequently the target of habitat modification, such as the damming of rivers. Altering the flow of a river can reduce or eliminate populations that are adapted to seasonal changes in flow. An estimated 91% of riverways in the United States have been modified with damming or significant stream bank modification. Many fish and other riparian (river-dependent) species in the United States, especially rare species with restricted distributions, have seen declines caused by river alteration or loss. Many species of amphibians that must carry out parts of their life cycles in both aquatic and terrestrial habitats are at greater risk of population declines, because of their dependence on both aquatic and terrestrial habitats.
A short but informative video that could not be embedded in this book, but is available for public use (from PBS) is “Science Trek – Habitat Loss, found at: https://www.pbs.org/video/habitat-habitat-loss-encakw/
Overharvesting
Overharvesting involves removing species from a habitat in an unsustainable manner – more are removed than can be replaced by reproduction. Overharvesting is a serious threat to many species, but particularly to aquatic species. For example, the western Altlantic ocean was a hugely productive fishery for almost 400 years, but the introduction of modern factory trawlers in the 1980s removed too many fish and damaged habitat to the point where the cod populations crashed. Fishery collapse is both an economic and political phenomenon, since only strongly enforced government regulation can maintain the fisheries for those who depend on this for their livelihood or for their source of protein.
Most fisheries are managed as a common resource, available to anyone willing to fish, even when the fishing territory lies within a country’s territorial waters. Common resources are subject to an economic pressure known as the tragedy of the commons, in which fishers have little motivation to exercise restraint in harvesting a fishery when they do not own the fishery, resulting in overexploitation.
Bushmeat is the generic term used for wild animals killed for food (Figure 12). Hunting is practiced throughout the world, but hunting practices, particularly in equatorial Africa and parts of Asia, are believed to threaten several species with extinction. Traditionally, bushmeat in Africa was hunted to feed families directly. However, recent commercialization of the practice now has bush meat available in grocery stores in Africa and abroad, which has increased harvest rates to the level of unsustainability. Additionally, human population growth has increased the need for protein foods that are not being met from agriculture. Species threatened by the bushmeat trade are mostly mammals including many monkeys and the great apes living in the Congo basin.
Figure 12. Harvesting of pangolins for their scales and meat, and as curiosities, has led to a drastic decline in population size for this fascinating creature. (Credit: This work by David Brossard is licensed under CC BY 4.0)
Invasive Species
Invasive species are those that have been intentionally or unintentionally introduced by humans into a new ecosystem. Invasive species can threaten other species through competition for resources, predation, or disease. Human transportation of people and goods, including the intentional transport of organisms for trade, has dramatically increased the introduction of invasive species to new areas. These new introductions are often at distances that are well beyond the capacity of the species to ever travel itself and are often outside the range of the species’ natural predators.
Fortunately, most exotic species introductions fail because of the low number of individuals introduced or poor adaptation to the ecosystem they enter. Some species, however, have characteristics that can make them especially successful in a new ecosystem. These exotic species often undergo dramatic population increases in their new habitat. When this happens, the exotic species also is thought of as an invasive species.
Lakes and islands are particularly vulnerable to extinction threats from introduced species. In Lake Victoria, the intentional introduction of the Nile perch was largely responsible for the extinction of about 200 species of cichlids. The accidental introduction of the brown tree snake via aircraft (Figure 13) from the Solomon Islands to Guam in 1950 has led to the extinction of three species of birds and three to five species of reptiles endemic to the island.
Figure 13. The brown tree snake, Boiga irregularis, is an exotic species that has caused numerous extinctions of birds and reptiles on the island of Guam since its accidental introduction in 1950. (Credit: US Department of the Interior)
Invading exotic species can also be disease organisms. It now appears that the global decline in amphibian species recognized in the 1990s was caused largely by the fungus Batrachochytrium dendrobatidis, which causes the disease chytridiomycosis (Figure 14). There is evidence that the fungus is native to Africa and may have been spread throughout the world by transport of a commonly used laboratory and pet frog species.
Figure 14. This Limosa harlequin frog (Atelopus limosus), an endangered species from Panama, died from a fungal disease called chytridiomycosis. The red lesions are symptomatic of the disease. (Credit: Brian Gratwicke)
Climate Change
Human-caused climate change is a major extinction threat, particularly when combined with other threats such as habitat loss. Scientists have noted that climate change has already altered regional climates around the globe, including rainfall and snowfall patterns, making habitats less hospitable to the species living there, eventually leading to extinctions. Caused by the emissions of greenhouse gases, primarily carbon dioxide and methane, into the atmosphere due to burning fossil fuels and deforestation, scientists overwhelmingly agree the present climatic trends are caused by humans. Warming trends will shift colder climates toward the north and south poles, forcing species to move (if possible) with their adapted climate norms or go extinct.
The shifting ranges will impose new competitive and predatory pressures on species as they come into contact with species not present in their historic range. Changing climates also throw off the delicate timing adaptations that species have to seasonal food resources such as insect blooms and breeding times. Range shifts have been observed in birds, plants, butterflies, other insects, freshwater fishes, reptiles, amphibians, and mammals.
Climate gradients will also move up mountains, eventually crowding species higher in altitude and eliminating the habitat for those species adapted to the highest elevations. Some climates will completely disappear. The rate of warming appears to be accelerated in the arctic, which is recognized as a serious threat to polar bear populations that require sea ice to hunt seals during the winter months.
Finally, global warming will raise ocean levels due to meltwater from glaciers and the greater volume occupied by warmer water. Shorelines will be inundated, reducing island size and a number of islands will disappear entirely. Additionally, the gradual melting and subsequent refreezing of the poles, glaciers, and higher elevation mountains—a cycle that has provided freshwater to environments for centuries—will be altered. This could result in an overabundance of salt water and a shortage of fresh water (Figure 15).
Figure 15. The effect of global warming can be seen in the continuing retreat of Grinnell Glacier. The mean annual temperature in Glacier National Park has increased 1.33°C since 1900. The loss of a glacier results in the loss of summer meltwaters, sharply reducing seasonal water supplies and severely affecting local ecosystems. (Credit: USGS, GNP Archives)
8.5 Changes in Biodiversity through Time and Mass Extinction Events
The number of species on the planet, or in any geographical area, is the result of an equilibrium of two evolutionary processes that are ongoing: (1) speciation (forming new species over evolutionary time, so increasing biodiversity) and (2) extinction (loss of species so decreasing biodiversity). When speciation rates are greater than extinction rates, the number of species will increase and vice versa. Throughout the history of life on Earth, as reflected in the fossil record, these two processes have always fluctuated to some extent. Usually these changes are relatively minor, but at other times the changes in biodiversity are more dramatic, such as the “Cambrian explosion” 541 million years ago, during which speciation gave rise to almost all major phyla of animals. On the other side are the periods of mass extinctions (Figure 16).
Figure 16. Occurrence of mass extinction events. Extinction intensity as reflected in the fossil record has fluctuated throughout Earth’s history. Sudden and dramatic losses of biodiversity, called mass extinctions, have occurred five times in the past. (Credit: Sandhya Ramesh – A Brief History of Earth)
The background extinction rate refers to the normal level of extinction that is expected to happen over time due to natural selection. By studying the fossil record, paleontologists have also identified five layers in the fossil record that appear to show sudden and dramatic losses in biodiversity. These five events are called mass extinctions and are characterized by more than half of all species disappearing from the fossil record. There are many lesser, yet still dramatic, extinction events, but the five mass extinctions have attracted the most research into their causes. Each of these mass extinction events was likely caused by a major event that changed Earth’s landscape.
The most recent mass extinction event in geological time, about 65 million years ago, saw the disappearance of most dinosaur species (except birds) and many other species. Most scientists now agree the main cause of that extinction was the impact of a large asteroid in the present-day Yucatán Peninsula and the subsequent energy release and global climate changes caused by dust ejected into the atmosphere. Biologists believe that the earth is currently experiencing a sixth mass extinction due to the activities of humans, especially widespread habitat loss.
There are numerous recent extinctions of individual species that are recorded in human writings. Most of these are coincident with the expansion of the European colonies since the 1500s. One of the earlier known examples is the dodo bird. The dodo bird lived in the forests of Mauritius, an island in the Indian Ocean. The dodo bird became extinct around 1662, after being hunted for its meat (it had no fear of humans because its island had been predator free). Introduced pigs, rats, and dogs brought to the island by European ships also killed dodo young and eggs (Figure 17).
Figure 17. The dodo bird was hunted to extinction around 1662. (Credit: Ed Uthman, taken in the Natural History Museum, London, England)
A large manatee called the Steller’s sea cow became extinct in 1768, just 27 years after this large mammal was first discovered in the North Atlantic. The sea cow was slow and easy to capture, and was killed for its meat and oil. The passenger pigeon used to exist in the millions in the eastern United States, but a combination of habitat destruction (its roosting trees were cut down for timber and farming) and hunting brought it to complete extinction by 1914. The International Union for Conservation of Nature (IUCN) keeps a list of extinct and endangered species called the Red List. Although likely incomplete, the list describes 380 vertebrates that have become extinct in recent years.
The 6th Mass Extinction (Anthropocene or Holocene Extinction)
Unlike previous extinction events caused by natural phenomena, the sixth mass extinction is driven by human activity.. This extinction event is primarily (though not limited to) the unsustainable use of land, water and energy use, and climate change. Habitat loss due to human activities (thus the term “anthropocene” or human caused) is the primary driving force of this 6th mass extinction. Currently, 40% of all land has been converted for food production. Agriculture is also responsible for 90% of global deforestation and accounts for 70% of the planet’s freshwater use, devastating the species that inhabit those places by significantly altering their habitats.
It is evident that where and how food is produced is one of the biggest human-caused threats to species extinction and our ecosystems. Unfortunately, unsustainable food production and consumption are significant contributors to greenhouse gas emissions that are causing atmospheric temperatures to rise, wreaking havoc across the globe. The climate crisis is causing everything from severe droughts to more frequent and intense storms. It also exacerbates the challenges associated with food production that stress species, while creating conditions that make their habitats inhospitable. Increased droughts and floods have made it more difficult to maintain crops and produce sufficient food in some regions. The intertwined relationships among the food system, climate change, and biodiversity loss are placing immense pressure on our planet, and causing this latest mass extinction.
8.6 Preserving Biodiversity
Threats to biodiversity have been recognized for some time. Today, the main efforts to preserve biodiversity involve (1) legislative approaches to regulate human and corporate behavior, (2) setting aside protected areas, and (3) habitat restoration. Fortunately, most countries have enacted various laws to protect species. Some of this legislation is international, such as the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES), a treaty that came into force in 1975. The treaty and the national legislation that supports it in each country provide a legal framework for preventing “listed” species from being transported across nations’ borders, thus protecting them from being caught or killed when the purpose involves international trade. The illegal trade in organisms and their parts is still probably a market in the hundreds of millions of dollars, but was greatly reduced by this treaty.
Within many countries, including the United States, there are national laws that protect endangered species and state laws that regulate hunting and fishing. The Endangered Species Act (ESA) was enacted in 1973 to protect threatened or endangered species. When an at-risk species is listed by the Act, the U.S. Fish & Wildlife Service is required by law to develop a management plan to protect the species and bring it back to sustainable numbers. The ESA, and others like it in other countries, is a useful tool, and has helped many species such as the wolf and the bald eagle to recover in numbers.
The Migratory Bird Treaty Act (MBTA) is an agreement between the United States and Canada that was signed into law in 1918 in response to declines in North American bird species caused by hunting. It makes it illegal to disturb or kill the protected bird species or distribute their parts (much of the hunting of birds in the past was for their feathers). Examples of protected species include northern cardinals, the red-tailed hawk, and the American black vulture.
Global climate change is expected to be a major driver of biodiversity loss, with many governments concerned about its effects on their economies and food resources. Because greenhouse gas emissions do not respect national boundaries, the effort to curb them needs to be international. The international response to global warming has been mixed. The Kyoto Protocol, an international agreement that came out of the United Nations Framework Convention on Climate Change that committed countries to reducing greenhouse gas emissions by 2012, was ratified by some countries, but spurned by others, including the United States and China, major greenhouse gas emitters. A renegotiated 2016 treaty, called the Paris Agreement, once again brought nations together to take meaningful action on climate change. But President Trump withdrew the United States’ support of the agreement, leaving it up to individual state governments to try to make meaningful changes. Fortunately, one of President Biden’s first actions as president was to have the United States rejoin this important international agreement.
Conservation in Preserves
Establishment of wildlife and ecosystem preserves is a key tool in conservation efforts (Figure 18). A preserve, such as a national park or wildlife refuge, is an area of land set aside with varying degrees of protection for the organisms that exist within the boundaries of the preserve. In 2016, the IUCN World Parks Congress estimated that almost 15 percent of Earth’s land surface was covered by preserves of various kinds. This area is large but only represents 9 out of 14 recognized major biomes. Also, many species live outside of any of these preserves.
Figure 18. National parks, such as Grand Teton National Park in Wyoming, help conserve biodiversity. (Credit: Don DeBold)
Biodiversity hotspots are geographical areas that contain high numbers of endemic species, which are species that are only found in one location. The purpose of the concept was to identify important locations on the planet for conservation efforts, a kind of conservation triage. By protecting hotspots as preserves or parks, governments are able to protect a larger number of species. The original criteria for a hotspot included the presence of 1500 or more species of endemic plants and 70 percent of the area disturbed by human activity. There are now 34 biodiversity hotspots (Figure 19) that contain large numbers of endemic species, which include half of Earth’s endemic plants.
Figure 19. Conservation International has identified 34 biodiversity hotspots. Although these cover only 2.3 percent of the Earth’s surface, 42 percent of the terrestrial vertebrate species and 50 percent of the world’s plants are endemic to those hotspots. (Credit: Wikimedia Commons – World Hotspots)
There has been extensive research into optimal preserve designs for maintaining biodiversity. In general, large preserves are better because they support more species, including species with large home ranges; they have more core area of optimal habitat for individual species; they have more niches to support more species; and they attract more species because they can be found and reached more easily. One large preserve is better than the same area of several smaller preserves. If preserves must be smaller, then providing wildlife corridors (narrow strips of protected land) between two preserves is important so that species and their genes can move between them.
Climate change will create inevitable problems with the location of preserves as the species within them migrate to higher latitudes as the habitat of the preserve becomes less favorable. Planning for the effects of global warming on future preserves, or adding new preserves to accommodate the changes expected from global warming is in progress, but will only be as effective as the accuracy of the predictions of the effects of global warming on future habitats.
Habitat Restoration
Habitat restoration is the process of bringing an area back to its natural state, before it was impacted through destructive human activities. It holds considerable promise as a mechanism for maintaining or restoring biodiversity. Reintroducing wolves, a top predator, to Yellowstone National Park in 1995 led to dramatic changes in the ecosystem that increased biodiversity. The wolves (Figure 20) function to suppress elk and coyote populations and provide more abundant resources to the detritivores. Reducing elk populations has allowed revegetation of riparian areas (along the banks of streams and rivers), which has increased the diversity of species in that habitat. In this habitat, the wolf is a keystone species, meaning a species that is instrumental in maintaining diversity within an ecosystem. The results from the Yellowstone experiment suggest that restoring a keystone species effectively can have the effect of restoring biodiversity in the community.
Figure 20. This photograph shows the Gibbon wolf pack in Yellowstone National Park, March 1, 2007. Wolves have been identified as a keystone species. (Credit: Doug Smith, National Park Service)
Other large-scale restoration experiments underway involve dam removal. In the United States, since the mid-1980s, many aging dams are being considered for removal rather than replacement because of shifting beliefs about the ecological value of free-flowing rivers. The measured benefits of dam removal include restoration of naturally fluctuating water levels (often the purpose of dams is to reduce variation in river flows), which leads to increased fish diversity and improved water quality. In the Pacific Northwest of the United States, dam removal projects are expected to increase populations of salmon, which is considered a keystone species because it transports nutrients to inland ecosystems during its annual spawning migrations.
The Role of Zoos and Captive Breeding in Maintaining Biodiversity
Zoos have sought to play a role in conservation efforts both through captive breeding programs and education (Figure 21). The transformation of the missions of zoos from collection and exhibition facilities to organizations that are dedicated to conservation is ongoing. Several species have been brought back from the brink of extinction by zoos, such as the Arabian Oryx, which went extinct in the wild but thrives in several zoo facilities. Some attempts have been made to reintroduce oryxes to the wild, but their natural habitats are often in war torn and unstable areas. Zoo facilities are too limited to have captive breeding programs for the numbers of species that are now at risk, but they do play an important role for some species. Education is a potential positive impact of zoos on conservation efforts, particularly given the global trend toward urbanization and the consequent reduction in contacts between people and wildlife.
Figure 21. Zoos and captive breeding programs help preserve many endangered species, such as this golden lion tamarin. (Credit: Garrett Ziegler)
8.7 Summary
Biodiversity exists at multiple levels of organization, and is measured in different ways depending on the goals of those taking the measurements. These include numbers of species, genetic diversity and ecosystem diversity. Maintaining biodiversity is important to humans because living organisms are often the source of medicines and agricultural crop varieties. Natural ecosystems provide services that support human survival such as pollination of crops, nutrient cycling and soil replenishment, fresh water, sources of new medicines, and pest control. Loss of biodiversity threatens these ecosystem services and risks making food production and pharmaceutical research more expensive or even impossible. The core threats to biodiversity are a rapidly growing human population (which causes habitat loss), and unsustainable resource use by more and more humans. Climate change is predicted to be a significant cause of extinction in the coming century.
Preserving biodiversity requires legislative action and international treaties such as CITES treaty that regulates the transportation of endangered species across international borders. In the United States, the Endangered Species Act protects listed species and the The Migratory Bird Act protects birds. Preserving habitats is essential for maintaining biodiversity, and 11 percent of Earth’s land surface is protected in some way. Habitat restoration has the potential to restore ecosystems to previous biodiversity levels before species become extinct. Examples of restoration include reintroduction of keystone species and removal of dams on rivers.
The following video explores some of the ways that biodiversity can be preserved, especially in response to climate change:
Conservation International (2021 Mar 15) [Video – YouTube] Protecting biodiversity, protecting our future. https://youtu.be/XiwkDKBJB4s
Attributions
Content in this chapter includes original work created by Lauren Roberts and Paul Bosch as well as from the following sources, with some modifications:
“Concepts of Biology” by OpenStax is licensed under CC BY 4.0 / A derivative from the original work
“Biology, 2nd edition” by OpenStax is licensed under CC BY 4.0 / A derivative from the original work
Essentials of Environmental Science by Kamala Doršner is licensed under CC BY 4.0.
