Preserving biodiversity is an extraordinary challenge that must be met by a greater understanding of biodiversity, changes in human behavior and beliefs, and various preservation strategies.

Change in Biodiversity through Time

The number of species on the planet, or in any geographical area, results from an equilibrium of two ongoing evolutionary processes: speciation and extinction. When speciation rates begin to outstrip extinction rates, species will increase. Likewise, the reverse is true when extinction rates overtake speciation rates. Throughout the history of life on Earth, as reflected in the fossil record, these two processes have fluctuated to a greater or lesser extent, sometimes leading to dramatic changes in the number of species on the planet as reflected in the fossil record (Figure 1).

Graph plots percent extinction occurrences versus time in millions of years before present time, starting 550 million years ago. Extinction occurrences increase and decrease in a cyclical manner. At the lowest points on the cycle, extinction occurrences were between two to five percent. Spikes in the number of extinctions occurred at the end of geological periods: end-Ordovician (450 million years ago), end-Devonian (374 million years ago), end-Permian (252 million years ago), end-Triassic (200 million years ago), and end-Cretaceous (65 million years ago). During these spikes, extinction occurrences ranged from approximately twenty-five to fifty percent.
Figure 1. 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.

Paleontologists have identified five layers in the fossil record that appear to show sudden and dramatic losses in biodiversity. Mass extinctions are characterized by more than half of all species disappearing from the fossil record. There are much lesser, yet still dramatic, extinction events, but the five mass extinctions have attracted the most research into their causes. An argument can be made that the five mass extinctions are only the five most extreme events in a continuous series of large extinction events throughout the fossil record (since 542 million years ago). The most recent extinction 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 this 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.

Recent and Current Extinction Rates

Many scientists say that we are currently experiencing a sixth mass extinction, and it mostly has to do with the activities of humans. Numerous recent extinctions of individual species are recorded in human writings. Most of these coincided with the European colonies’ expansion since the 1500s.

One of the earlier and popularly 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. It was hunted for its meat by sailors and was easy prey because the dodo, which did not evolve with humans, would approach people without fear. Introduced pigs, rats, and dogs brought to the island by European ships also killed dodo young and eggs (Figure 2).

Photo shows a dodo taxidermy exhibit at the Museum of Natural History in London, England. Distinguishing features include a large heavy beak colored dark brown at the end; a large, plump body; tiny wings with very few, short-flight feathers; a few curled tail feathers; a large feathered head and featherless face.
Figure 2. The dodo bird was hunted to extinction around 1662. (credit: Ed Uthman, taken in Natural History Museum, London, England)Steller’s sea cow became extinct in 1768; it was related to the manatee and probably once lived along the northwest coast of North America. Steller’s sea cow was discovered by Europeans in 1741, and it was hunted for meat and oil. A total of 27 years elapsed between the sea cow’s first contact with Europeans and the extinction of the species. The last Steller’s sea cow was killed in 1768. In another example, the last living passenger pigeon died in a zoo in Cincinnati, Ohio, in 1914. This species had once migrated in the millions but declined in numbers because of overhunting and loss of habitat through the clearing of forests for farmland. These are only a few recorded extinctions in the past 500 years. The International Union for Conservation of Nature (IUCN) keeps a list of extinct and endangered species called the Red List. The list is incomplete, but it describes 380 vertebrates that became extinct after 1500 AD, 86 of which were driven extinct by overhunting or overfishing.

Estimates of Present-day Extinction Rates

Estimates of extinction rates are hampered by the fact that most extinctions are probably happening without being observed. Humans often notice the extinction of a bird or mammal, especially if it has been hunted or used in some other way. But many organisms are less noticeable to humans (not necessarily of less value), and many are undescribed.

The background extinction rate is estimated to be about 1 per million species years (E/MSY). One “species year” is one species in existence for one year. One million species years could be one species persisting for one million years or a million species persisting for one year. If it is the latter, then one extinction per million species years would be one of those million species becoming extinct in that year. For example, if there are 10 million species in existence, we would expect ten species to become extinct in a year. This is the background rate.

One contemporary extinction-rate estimate uses the extinctions in the written record since 1500. This method yields an estimated 26 E/MSY for birds alone, almost three times the background rate. However, this value may be underestimated for three reasons. First, many existing species would not have been described until much later in the time period, and so their loss would have gone unnoticed. Second, we know the number is higher than the written record suggests because now extinct species are being described from skeletal remains that were never mentioned in written history. And third, some species are probably already extinct, even though conservationists are reluctant to name them as such. Considering these factors raises the estimated extinction rate to nearer 100 E/MSY. The predicted rate by the end of the century is 1500 E/MSY.

A line graph with number of species on the Y axis, and forest area in kilometers squared on the X axis. The line starts at 0,0, and curves up quickly at first, then more gradually as the values on the X and Y axis increase until the line reaches 100 on the X axis and just below 100 on the Y axis. A vertical dotted line extending up from the value of 10 on the X axis meets the line at just below 50 on the Y axis.
Figure 3. A typical species-area curve shows the cumulative number of species found as more and more area is sampled. The curve has also been interpreted to show the effect on species numbers of destroying habitat; a reduction in habitat of 90 percent from 100 km2 to 10 km2 reduces the number of species supported by about 50 percent.

A second approach to estimating present-time extinction rates is to correlate species loss with habitat loss, and it is based on measuring forest-area loss and understanding species–area relationships. The species-area relationship is the rate at which new species are seen when the area surveyed is increased (Figure 3). Likewise, if the habitat area is reduced, the species seen will also decline. This kind of relationship is also seen in the relationship between an island’s area and the number of species present on the island: as one increases, so does the other, though not in a straight line. Estimates of extinction rates based on habitat loss and species–area relationships have suggested that with about 90 percent of habitat loss, an expected 50 percent of species would become extinct. Figure 3 shows that reducing forest area from 100 km2 to 10 km2, a decline of 90 percent, reduces the number of species by about 50 percent. Species–area estimates have led to present-day species extinction rates of about 1000 E/MSY and higher.

Go to this website ( for an interactive exploration of endangered and extinct species, their ecosystems, and the causes of their endangerment or extinction.

Conservation of Biodiversity

The threats to biodiversity have been recognized for some time. Today, the main efforts to preserve biodiversity involve legislative approaches to regulating human and corporate behavior, setting aside protected areas, and restoring habitats.

Changing Human Behavior

Legislation has been enacted to protect species throughout the world. The legislation includes international treaties as well as national and state laws. The Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) treaty came into force in 1975. The treaty, and the national legislation that supports it, 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 listed species that are protected by the treaty number some 33,000. The treaty is limited in its reach because it only deals with the international movement of organisms or their parts. It is also limited by countries’ ability or willingness to enforce the treaty and supporting legislation. The illegal trade in organisms and their parts is probably a hundreds of millions of dollars market.

Within many countries, some laws protect endangered species and regulate hunting and fishing. The Endangered Species Act (ESA) was enacted in the United States in 1973. When the Act lists an at-risk species, the U.S. Fish & Wildlife Service must 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, but it suffers because it is often difficult to get a species listed or an effective management plan once a species is listed.

The Migratory Bird Treaty Act (MBTA) is an agreement between the United States and Canada signed into law in 1918 in response to declines in North American bird species caused by hunting. The Act now lists over 800 protected species. It makes it illegal to disturb or kill the protected 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 warming is expected to be a major driver of biodiversity loss. Many governments are concerned about the effects of anthropogenic global warming, primarily on their economies and food resources. Because greenhouse gas emissions do not respect national boundaries, the effort to curb them is international. The international response to global warming has been mixed. The Kyoto Protocol, an international agreement from the United Nations Framework Convention on Climate Change that committed countries to reduce greenhouse gas emissions by 2012, was ratified by some countries but spurned by others. Two countries that were especially important in terms of their potential impact that did not ratify the Kyoto Protocol were the United States and China. Some goals for reduction in greenhouse gasses were met and exceeded by individual countries, but worldwide, the effort to limit greenhouse gas production is not succeeding. A renegotiated 2016 treaty, called the Paris Agreement, once again brought nations together to take meaningful action on climate change. But like before, some nations are reluctant to participate.


Photo of Grand Teton National Park shows an oxbow bend in a river with a grassy bank and a variety of deciduous and coniferous trees. Snowcapped mountains are in the background.
Figure 4. National parks, such as Grand Teton National Park in Wyoming, help conserve biodiversity. (credit: Don DeBold)

Conservation in Preserves

Establishing wildlife and ecosystem preserves is one of the key tools in conservation efforts (Figure 4). A preserve is an area of land set aside with varying degrees of protection for the organisms that exist within the preserve’s boundaries. In 2003, the IUCN World Parks Congress estimated that preserves of various kinds covered 11.5 percent of Earth’s land surface. This area is large but only represents 9 out of 14 recognized major biomes, and research has shown that 12 percent of all species live outside preserves.

A biodiversity hotspot is a conservation concept developed by Norman Myers in 1988. Hotspots are geographical areas that contain high numbers of endemic species. The concept aimed to identify important locations on the planet for conservation efforts, a kind of conservation triage. By protecting hotspots, governments can protect a larger number of species. The original criteria for a hotspot included 1500 or more species of endemic plants and 70 percent of the area disturbed by human activity. There are now 34 biodiversity hotspots (Figure 5) that contain many endemic species, including half of Earth’s endemic plants.

Biodiversity hotspots are indicated on a world map. Most hotspots occur in coastal regions and on islands.
Figure 5. 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.

There has been extensive research into optimal preservation designs for maintaining biodiversity. The fundamental principles behind much of the research have come from the seminal theoretical work of Robert H. MacArthur and Edward O. Wilson, published in 1967 on island biogeography.1 This work sought to understand the factors affecting biodiversity on islands. Conservation preserves are “islands” of habitat within “an ocean” of non-habitat. In general, large preserves are better because they support more species, including species with large home ranges; they have more core areas 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 because there is more core habitat unaffected by less hospitable ecosystems outside the preserve boundary. For this reason, preserves in a square or circle shape will be better than those with many thin “arms.” If preserves must be smaller, providing wildlife corridors (narrow strips of protected land) between two preserves is important so species and their genes can move between them. All of these factors are considered when planning the nature of a preserve before the land is set aside.

In addition to the physical specifications of a preserve, there are a variety of regulations related to the use of a preserve. These include timber extraction, mineral extraction, regulated hunting, human habitation, and non-destructive human recreation. Many decisions to include these other uses are based on political pressures rather than conservation considerations. On the other hand, in some cases, wildlife protection policies have been so strict that subsistence-living indigenous populations have been forced from ancestral lands that fell within a preserve. In other cases, even if a preserve is designed to protect wildlife, if the protections are not or cannot be enforced, the preserve status will have little meaning in the face of illegal poaching and timber extraction. This is a widespread problem with preserves in the tropics.

Climate change will create inevitable problems with the location of preserves as the species within them migrate to higher latitudes as the preserve’s habitat 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.

Finally, an argument can be made that conservation preserves reinforce the cultural perception that humans are separate from nature, can exist outside of it, and can only operate in ways that damage biodiversity. Creating preserves reduces the pressure on human activities outside the preserves to be sustainable and non-damaging to biodiversity. Ultimately, the political, economic, and human demographic pressures will degrade and reduce the size of conservation preserves if the activities outside them are not altered to be less damaging to biodiversity.

Check out this interactive global data system ( of protected areas. Review data about specific protected areas by location or study statistics on protected areas by country or region.

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 6) function to suppress elk and coyote populations and provide more abundant resources to the detritivores. Reducing elk populations has allowed the revegetation of riparian (the areas along the banks of a stream or river) areas, which has increased the diversity of species in that habitat. The reduction of coyote populations by wolves has increased the prey species previously suppressed by coyotes. In this habitat, the wolf is a keystone species that maintain diversity within an ecosystem. Removing a keystone species from an ecological community causes a collapse in diversity. The results from the Yellowstone experiment suggest that restoring a keystone species effectively can restore biodiversity in the community. Ecologists have argued for identifying keystone species where possible and focusing protection efforts on these species. It makes sense to return the keystone species to the ecosystems where they have been removed.

Photo shows a pack of wolves walking on snow.
Figure 6. This photograph shows the Gibbon wolf pack in Yellowstone National Park on March 1, 2007. Wolves have been identified as a keystone species. (credit: Doug Smith, NPS)

Other large-scale restoration experiments underway involve dam removal. In the United States, since the mid-1980s, many aging dams have been 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. In other regions, such as the Atlantic coast, dam removal has allowed the return of other spawning anadromous fish species (species born in fresh water, live most of their lives in salt water, and return to freshwater to spawn). Some of the largest dam removal projects have yet to occur or have happened too recently for the consequences to be measured, such as the Elwha Dam on the Olympic Peninsula of Washington State. The large-scale ecological experiments these removal projects constitute will provide valuable data for other dam projects slated for removal or construction.

The Role of Zoos and Captive Breeding

Photo shows the head and neck of a golden lion tamarin, a small monkey with a bare, flesh-colored face and plentiful long golden hair like a lion’s mane.
Figure 7. Zoos and captive breeding programs help preserve many endangered species, such as this golden lion tamarin. (credit: Garrett Ziegler)

Zoos have sought to participate in conservation through captive breeding programs and education (Figure 7). The transformation of the missions of zoos from collection and exhibition facilities to organizations dedicated to conservation is ongoing. In general, it has been recognized that, except in some specifically targeted cases, captive breeding programs for endangered species are inefficient and often prone to failure when the species are reintroduced to the wild. Zoo facilities are far too limited to contemplate captive breeding programs for the number of at-risk species. On the other hand, education is a potential positive impact of zoos on conservation efforts, particularly given the global trend to urbanization and the consequent reduction in contact between people and wildlife. A number of studies have been performed to look at the effectiveness of zoos on people’s attitudes and actions regarding conservation, and at present, the results tend to be mixed.


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Introduction to Environmental Sciences and Sustainability Copyright © 2023 by Emily P. Harris is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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