Threats to Biodiversity

Habitat Loss and Sustainability

Through increased adoption of sustainable practices, we can reduce habitat loss and its consequences.

Learning Objectives

Describe the effects of habitat loss to biodiversity and concept of sustainability

Key Takeaways

Key Points

  • Habitat destruction renders entire habitats functionally unable to support the species present; biodiversity is reduced in this process when existing organisms in the habitat are displaced or destroyed.
  • Clearing areas for agricultural purposes is the main cause of habitat destruction; other principal causes include mining, logging, and urban sprawl.
  • The primary cause of species extinction worldwide is habitat destruction.
  • Sustainability is a term that describes how biological systems remain diverse and productive over time, creating the potential for long-term maintenance of human well-being.
  • Reducing negative human impact requires three concepts: environmental management, management of human consumption of resources, and awareness of cultural and political concerns to increase sustainability.

Key Terms

  • sustainability: Configuring society so that each person can meet their own needs and greatest potential, while preserving biodiversity and natural ecosystems, and planning for future generations to maintain this potential.
  • endemism: The ecological state of a species being unique to a defined geographic location, such as an island, nation, country or other defined zone, or habitat type; organisms that are indigenous to a place are not endemic to it if they are also found elsewhere.
  • biodiversity: The diversity (number and variety of species) of plant and animal life within a region.

Habitat Loss

Humans rely on technology to modify their environment and replace certain functions that were once performed by the natural ecosystem. Other species cannot do this. Elimination of their ecosystem – whether it is a forest, a desert, a grassland, a freshwater estuary, or a marine environment – will kill the individuals within most species. Remove the entire habitat within the range of a species and, unless they are one of the few species that do well in human-built environments, the species will become extinct.

Effects of Habitat Loss on Biodiversity

Habitat loss is a process of environmental change in which a natural habitat is rendered functionally unable to support the species present. This process may be natural or unnatural, and may be caused by habitat fragmentation, geological processes, climate change, or human activities such as the introduction of invasive species or ecosystem nutrient depletion. In the process of habitat destruction, the organisms that previously used the site are displaced or destroyed, reducing biodiversity.


Biodiversity loss in Sumatra: (a) One sub-species of orangutan is found only in the rain forests of Borneo, while the other sub-species of orangutan is found only in the rain forests of Sumatra. These animals are examples of the exceptional biodiversity of (c) the islands of Sumatra and Borneo. Other species include the (b) Sumatran tiger and the (d) Sumatran elephant, both of which are critically endangered. Rainforest habitat is being removed to make way for (e) oil palm plantations such as this one in Borneo’s Sabah Province.

Human destruction of habitats has accelerated greatly in the latter half of the twentieth century. Natural habitats are often destroyed through human activity for the purpose of harvesting natural resources for industry production and urbanization. Clearing habitats for agriculture, for example, is the principal cause of habitat destruction. Other important causes of habitat destruction include mining, logging, and urban sprawl. Habitat destruction is currently ranked as the primary cause of species extinction worldwide.

Consider the exceptional biodiversity of Sumatra. It is home to one sub-species of orangutan, a species of critically endangered elephant, and the Sumatran tiger; however half of Sumatra’s forest is now gone. The neighboring island of Borneo, home to the other sub-species of orangutan, has lost a similar area of forest, and forest loss continues in protected areas. The orangutan in Borneo is listed as endangered by the International Union for Conservation of Nature (IUCN), but it is simply the most visible of thousands of species that will not survive the disappearance of the forests of Borneo. The forests are being removed for their timber, and to clear space for plantations of palm oil, an oil used in Europe for many items including food products, cosmetics, and biodiesel.

A five-year estimate of global forest cover loss for the years 2000–2005 was 3.1 percent. In the humid tropics where forest loss is primarily from timber extraction, 272,000 km2 was lost out of a global total of 11,564,000 km2 (or 2.4 percent). In the tropics, these losses also represent the extinction of species because of high levels of endemism.

Since the Neolithic Revolution, about 47% of the world’s forests have been lost to human use. Present-day forests occupy about a quarter of the world’s ice-free land, with about half of these occurring in the tropics. In temperate and boreal regions, forest area is gradually increasing (with the exception of Siberia), but deforestation in the tropics is of major concern.

Feeding more than seven billion human bodies takes a heavy toll on the earth’s resources. This begins with the appropriation of about 38 percent of the earth’s land surface and about 20 percent of its net primary productivity. Added to this are the resource-hungry activities of industrial agribusiness: everything from crops’ need for irrigation water, synthetic fertilizers, and pesticides, to the resource costs of food packaging, transport (now a major part of global trade), and retail.


Sustainability and deforestation: Since the Neolithic Revolution, nearly half of the world’s forests have been destroyed for human use. Sustainable practices, which preserve environments for long-term maintenance and well-being, can help preserve habitats and ecosystems for greater biodiversity.


Sustainability is a concept that describes how biological systems remain diverse and productive over time. Long-lived and healthy wetlands and forests are examples of sustainable biological systems. For humans, sustainability is the potential for long-term maintenance of well-being, which has ecological, economic, political, and cultural dimensions. Sustainability requires the reconciliation of environmental, social, and economic demands, which are also referred to as the “three pillars” of sustainability.

Healthy ecosystems and environments are necessary for the survival and flourishing of humans and other organisms, and there are a number of ways to reduce humans’ negative impact on the environment. One approach is environmental management, which is based largely on information gained from earth science, environmental science, and conservation biology. A second approach is management of human consumption of resources, which is based largely on information gained from economics. A third, more recent, approach adds cultural and political concerns into the sustainability matrix.

Loss of biodiversity stems largely from the habitat loss and fragmentation produced by human appropriation of land for development, forestry and agriculture as natural capital is progressively converted to human-made capital. At the local human scale, sustainability benefits accrue from the creation of green cities and sustainable parks and gardens. Similarly, environmental problems associated with industrial agriculture and agribusiness are now being addressed through such movements as sustainable agriculture, organic farming, and more-sustainable business practices.


Overharvesting threatens biodiversity by degrading ecosystems and eliminating species of plants, animals, and other organisms.

Learning Objectives

Explain why overharvesting is a threat to biodiversity

Key Takeaways

Key Points

  • Until recently, human populations harvested resources in limited quantities. Today, new methods of harvest and capture contribute to overharvesting and overexploitation.
  • Overharvesting stems from several factors, including an exponential increase in the human population, expanding markets, increasing demand, and improved access and techniques for capture.
  • Overharvesting natural resources for extended periods of time depletes these resources until they cannot recover within a short period of time; some may never recover.
  • Overharvesting is one of five primary activities threatening global biodiversity; others include pollution, introduced species, habitat fragmentation, and habitat destruction.
  • Aquatic species are especially threatened by overharvesting, due to a situation known as the tragedy of the commons.

Key Terms

  • overexploitation: Excessive and damaging use of natural resources, including plants and animals.
  • trawler: A fishing boat that uses a dragnet, or “trawl net,” to catch fish.
  • apex predator: An animal at the top of the food chain, preying on other species but not prey itself.


Overharvesting, also called overexploitation, refers to harvesting a renewable resource to the point of diminishing returns. Ecologists use the term to describe populations that are harvested at a rate that is unsustainable, given their natural rates of mortality and capacities for reproduction. The term applies to natural resources such as wild medicinal plants, grazing pastures, game animals, fish stocks, forests, and water aquifers. Sustained overharvesting can lead to the destruction of the resource, and is one of the five main activities – along with pollution, introduced species, habitat fragmentation, and habitat destruction – that threaten global biodiversity today.

All living organisms require resources to survive. Overharvesting these resources for extended periods of time can deplete natural resources to the point where they are unable to recover within a short time frame. Humans have always harvested food and other resources they have needed to survive; however, human populations, historically, were small and methods of collection limited to small quantities. Exponential increase in human population, expanding markets, and increasing demand, combined with improved access and techniques for capture, are causing the exploitation of many species beyond sustainable levels.

Effects of overharvesting

As mentioned above, sustained overharvesting is one of the primary threats to biodiversity. Overharvesting can lead to resource destruction, including extinction at the population level and even extinction of whole species. Depleting the numbers or amount of certain resources can also change their quality; for example, the overharvesting of footstool palm (a wild palm tree found in Southeast Asia, the leaves of which are used for thatching and food wrapping) has resulted in its leaf size becoming smaller.

Overharvesting not only threatens the resource being harvested, but can directly impact humans as well – for example by decreasing the biodiversity necessary for medicinal resources. A significant proportion of drugs and medicines are natural products which are derived, directly or indirectly, from biological sources. However, unregulated and inappropriate harvesting could potentially lead to overexploitation, ecosystem degradation, and loss of biodiversity; further, it can negatively impact the rights of the communities and states from which the resources are taken.

Tragedy of the commons

Overharvesting is a serious threat to many species, especially aquatic ones. Common resources – or resources that are shared, such as fisheries – are subject to an economic pressure known as “the tragedy of the commons,” in which essentially no harvester has a motivation to exercise restraint in harvesting from a certain area, because that area is not owned by that harvester. The natural outcome of harvesting common resources is their overexploitation.

For example, most fisheries are managed as a common resource even when the fishing territory lies within a country’s territorial waters; because of this, fishers have very little motivation to limit their harvesting, and in fact technology gives fishers the ability to overfish. In a few fisheries, the biological growth of the resource is less than the potential growth of the profits made from fishing if that time and money were invested elsewhere. In these cases (for example, whales) economic forces will always drive toward fishing the population to extinction.


Cod trawler and net: Overharvesting fisheries is an especially salient problem because of a situation termed the tragedy of the commons. In this situation, fishers have no real incentive to practice restraint when harvesting fish because they do not own the fisheries.

Cascade Effects

Overexploitation of species can also result in cascade effects, particularly if a habitat loses its apex predator. Because of the loss of the top predator, a dramatic increase in their prey species can occur. In turn, the unchecked prey can then overexploit their own food resources until population numbers dwindle, possibly to the point of extinction.

Exotic Species

Exotic species introduced into foreign ecosystems can threaten native species through competition for resources, predation, and disease.

Learning Objectives

Describe the impact of exotic and invasive species on native species

Key Takeaways

Key Points

  • Exotic species introduced to new environments often reset the ecological conditions in that new habitat, threatening the species that exist there; this is the reason that they are also termed invasive species.
  • Invasive species that are closely related to rare native species have the potential to hybridize with the native species; harmful effects of hybridization have led to a decline and even extinction of native species.
  • Biologists studying frogs and toads may be inadvertently responsible for the worldwide spread of a fungus deadly to amphibians.

Key Terms

  • invasive species: any species that has been introduced to an environment where it is not native and has since become a nuisance through rapid spread and increase in numbers, often to the detriment of native species

Exotic Species

Exotic species are those that have been intentionally or unintentionally introduced by humans into an ecosystem in which they did not evolve. Such introductions probably occur frequently as natural phenomena. For example, Kudzu (Pueraria lobata), which is native to Japan, was introduced in the United States in 1876. It was later planted for soil conservation. Problematically, it grows too well in the southeastern United States: up to one foot each day. It is now a pest species, covering over seven million acres in the southeastern United States. If an introduced species is able to survive in its new habitat, that introduction is now reflected in the observed range of the species. Human transportation of people and goods, including the intentional transport of organisms for trade, has dramatically increased the introduction of species into new ecosystems, sometimes at distances that are well beyond the capacity of the species to ever travel itself and outside the range of the species’ natural predators.


Exotic threats: The brown tree snake, Boiga irregularis, is an exotic species that has caused numerous extinctions on the island of Guam since its accidental introduction in 1950.

Most exotic species introductions probably fail because of the low number of individuals introduced or poor adaptation to the ecosystem they enter. Some species, however, possess preadaptations that can make them especially successful in a new ecosystem. These exotic species often undergo dramatic population increases in their new habitat, resetting the ecological conditions in the new environment, while threatening the species that exist there. For this reason, exotic species, also called invasive species, can threaten other species through competition for resources, predation, or disease.

Exotic Species Threaten Native Species

Invasive species can change the functions of ecosystems. For example, invasive plants can alter the fire regimen, nutrient cycling, and hydrology in native ecosystems. Invasive species that are closely related to rare native species have the potential to hybridize with the native species. Harmful effects of hybridization have led to a decline and even extinction of native species. For example, hybridization with introduced cordgrass, Spartina alterniflora, threatens the existence of California cordgrass in San Francisco Bay. Invasive species cause competition for native species. Four hundred of the 958 endangered species under the Endangered Species Act are at risk due to this competition.


Global decline in amphibian species: 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.

Lakes and islands are particularly vulnerable to extinction threats from introduced species. In Lake Victoria, as mentioned earlier, 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 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. Several other species are still threatened. The brown tree snake is adept at exploiting human transportation as a means to migrate; one was even found on an aircraft arriving in Corpus Christi, Texas. Constant vigilance on the part of airport, military, and commercial aircraft personnel is required to prevent the snake from moving from Guam to other islands in the Pacific, especially Hawaii. Islands do not make up a large area of land on the globe, but they do contain a disproportionate number of endemic species because of their isolation from mainland ancestors.

It now appears that the global decline in amphibian species recognized in the 1990s is, in some part, caused by the fungus Batrachochytrium dendrobatidis, which causes the disease chytridiomycosis. There is evidence that the fungus, native to Africa, may have been spread throughout the world by transport of a commonly-used laboratory and pet species: the African clawed toad (Xenopus laevis). It may well be that biologists themselves are responsible for spreading this disease worldwide. The North American bullfrog, Rana catesbeiana, which has also been widely introduced as a food animal, but which easily escapes captivity, survives most infections of Batrachochytriumdendrobatidis and can act as a reservoir for the disease.

Climate Change and Biodiversity

The global warming trend is recognized as a major biodiversity threat, especially when combined with other threats such as habitat loss.

Learning Objectives

Evaluate climate change and its impact on biodiversity

Key Takeaways

Key Points

  • This warming trend is persistently shifting colder climates further toward the north and south poles, forcing species to move with their own adapted climate norms, while also facing habitat gaps along the way.
  • Climate shifts will move up mountains, resulting in the crowding of species higher in altitude and eliminating the habitat for those species adapted to those high elevations; indeed, some climates will completely disappear.
  • Global warming will also raise ocean water levels due to melted water from glaciers and the greater volume of warmer water; shorelines will be flooded, affecting many species; many islands will disappear altogether.

Key Terms

  • anthropogenic: having its origin in the influence of human activity on nature
  • biodiversity: the diversity (number and variety of species) of plant and animal life within a region

Climate Change

Climate change, specifically, the anthropogenic (caused by humans) warming trend presently underway, is recognized as a major extinction threat, particularly when combined with other threats such as habitat loss. Scientists disagree about the probable magnitude of the effects, with extinction rate estimates ranging from 15 percent to 40 percent of species by 2050. Scientists do agree, however, that climate change will alter regional climates, including rainfall and snowfall patterns, making habitats less hospitable to the species living in them.

Impact of Climate Change on Biodiversity


Grizzly-polar bear hybrid: Since 2008, grizzly bears (Ursus arctos horribilis) have been spotted farther north than their historic range, a possible consequence of climate change. As a result, grizzly bear habitat now overlaps polar bear (Ursus maritimus) habitat. The two kinds of bears, which are capable of mating and producing viable offspring, are considered separate species as historically they lived in different habitats and never met. However, in 2006 a hunter shot a wild grizzly-polar bear hybrid known as a grolar bear, the first wild hybrid ever found.

The warming trend will shift colder climates toward the north and south poles, forcing species to move with their adapted climate norms while facing habitat gaps along the way. The shifting ranges will impose new competitive regimes on species as they find themselves in contact with other species not present in their historic range. One such unexpected species contact is between polar bears and grizzly bears. Previously, these two species had separate ranges. Now, with their ranges are overlapping, there are documented cases of these two species mating and producing viable offspring. Changing climates also throw off species’ delicate timing adaptations to seasonal food resources and breeding times. Many contemporary mismatches to shifts in resource availability and timing have recently been documented.

Range shifts are already being observed. For example, some European bird species’ ranges have moved 91 km northward. The same study suggests that the optimal shift based on warming trends was double that distance, suggesting that the populations are not moving quickly enough. Range shifts have also been observed in plants, butterflies, other insects, freshwater fishes, reptiles, 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; seals are the only source of protein available to polar bears. A trend to decreasing sea ice coverage has occurred since observations began in the mid-twentieth century. The rate of decline observed in recent years is far greater than previously predicted by climate models.

Finally, global warming will raise ocean levels due to glacial melt and the greater volume of warmer water. Shorelines will be inundated, reducing island size, which will have an effect on many species; 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 also be jeopardized. This could result in an overabundance of salt water and a shortage of fresh water.