Classification
Any organism including plants or animals can become endangered. Organisms are classified by the International Union for Conservation of Nature (IUCN) into the following categories.
Extinct (EX) – These species no longer exist and have become extinct.
Extinct in the wild (EW) – Survives only in captivity, cultivation and/or outside the native range, as presumed after exhaustive surveys.
Critically endangered (CR) – These species are at an extremely high risk of extinction in the wild and their population is declining at an alarming rate.
Endangered (EN) – These species are at a very high risk of extinction in the wild and their population is declining rapidly.
Vulnerable (VU) – Meets one of the 5 red list criteria and thus considered to be at high risk of unnatural (human-caused) extinction without further human intervention.
Near threatened (NT) – These species are at risk of becoming endangered in the near future.
Least concern (LC) – These species are considered to have a low risk of extinction and are not considered endangered.
Conservation Status Classification
IUCN uses five criteria to classify organisms and assess their conservation status. The conservation status of a species refers to its risk of extinction in the wild. These criteria are:
A) Reduction in population: The population size of the species has decreased significantly over a certain period of time.
B) Geographic range: The species has a restricted geographic range or habitat.
C) Population size: The species has a small population size, estimated to number fewer than 2,500 mature individuals, and is in decline.
D) Extremely small population size: The species has an extremely small population size, estimated to number fewer than 250 mature individuals.
E) Probability of extinction: A quantitative analysis has shown that the probability of the species becoming extinct in the wild is at least 20% within 20 years or five generations.
These criteria are used to classify species into the different categories of the IUCN Red List, such as Least Concern, Vulnerable, Endangered, Critically Endangered, Extinct in the Wild, and Extinct. The criteria are intended to provide a consistent and objective way to assess the conservation status of a species, and to help prioritize conservation efforts.
Check out https://www.iucnredlist.org to look at the population status of thousands of plants and animals.
Risk Factors
Certain organisms are inherently more at risk for extinction than others due to their biology and ecology. The difference in reproductive rate, as well as the ability of an organism to adapt to environmental change, are two key factors that can influence the survival and extinction risk of a species.
R-selected species, such as mice, have a high reproductive rate and can quickly rebound from population losses. These species are typically short-lived and have many offspring that have a low chance of survival.
K-selected species, such as elephants, have a lower reproductive rate and take longer to recover from population losses. These species are typically long-lived and have fewer offspring that have a higher chance of survival.
Specialist species, such as orangutans, have a very specific niche and are adapted to a particular set of conditions. They can be particularly vulnerable to environmental changes and pressures, such as habitat loss, as they may not be able to adapt to new conditions.
Generalist species, such as pigeons, are able to adapt to a variety of different climates, food sources, and environmental challenges. They tend to be more resilient to environmental changes and pressures, but they can also compete with native species and alter the ecosystem.
Overharvesting
Hunting, fishing, and other forms of harvesting are the most direct human influences on wild populations of plants and animals. Overharvesting occurs when individuals are removed from a population at a rate faster than the population can replace them. This can lead to population declines and even extinction, as seen in the case of the dodo bird on Mauritius.
Historically, market hunting has led to the overharvesting of many species, such as the American bison and passenger pigeon, causing dramatic declines in population numbers. However, with the implementation of regulations and legal protections, many species have been able to recover, such as the American bison.
Regulations such as hunting and fishing restrictions and limits on the number of animals that can be harvested have been implemented in many countries to prevent overharvesting. However, in some regions of the world, harvest regulations are not enforced, and illegal poaching continues to threaten species with extinction.
Illegal harvesting of rare animals, such as tigers, rhinoceroses, and apes, and rare plants, birds, and coral reef dwellers for private collections has become an increasing problem, and it's threatened the persistence of these species.
Plant/Animal Trade
The trade in plants and animals can represent a serious threat to the persistence of some species. Laws such as the Lacey Act in the United States and the United Nations Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) were developed to control and regulate the trade of threatened and endangered plants and animals.
The Lacey Act, first passed in 1900, originally prohibited the transport of illegally harvested game animals across state lines. Today, the Act prohibits the interstate shipping of all illegally harvested plants and animals. CITES is an international agreement among 182 countries worldwide, which aims to control the international trade of threatened and endangered species.
Despite these laws and agreements, illegal trade in plants and animals continues to occur worldwide. Illegal trade in wildlife is estimated to be worth between 5 billion to 220 billion dollars annually. Illegal trade in wildlife and wildlife parts have been sent to United States ports over a 10-year period. The illegal trade in wildlife is driven by demand for fur, body parts thought to have medicinal value, and rare animals as pets.
Even legal trade in certain species can pose a potential long-term threat to species persistence. In the southwestern United States, for example, the growing movement to reduce water used by replacing grass lawns with desert landscapes has led to an increased demand for cacti and other desert plants collected from the wild, causing concern for these plant populations in the wild.
Conservation
Countries have implemented a variety of strategies to address the decline in species and protect endangered and threatened species. One of the most important strategies is the implementation of legislation to protect habitats. The Endangered Species Act of 1973 in the United States, for example, aims to protect endangered and threatened species by preserving habitats from development.
Endangered Species Act
The Endangered Species Act (ESA) of 1973 is a federal law in the United States that aims to protect and recover species that are in danger of extinction throughout all or a significant portion of their range. The Act is administered by the U.S. Fish and Wildlife Service (FWS) and the National Oceanic and Atmospheric Administration (NOAA) and it is one of the strongest laws in the world for protecting endangered and threatened species.
The ESA provides for the listing of species as either "endangered" or "threatened" based on their risk of extinction. Once a species is listed, the Act requires the federal government to take actions to protect and recover it. This includes the designation of "critical habitat," which is defined as specific areas that are essential for the conservation of the species, as well as the prohibition of harming the listed species and their trade, including their fur or body parts.
The Act also implements the international CITES agreement, which regulates the trade of threatened and endangered species. To assist in the conservation of threatened and endangered species, the Act authorizes the government to purchase habitat that is critical to the conservation of the species and to develop recovery plans to increase the population of threatened and endangered species.
The ESA has been the center of much debate and controversy in recent years, as it can restrict certain human activities in areas where listed species live, including how landowners use their land. Despite this, the Act has had significant success in protecting and recovering endangered and threatened species, with several species being removed from the list as their populations have increased. However, efforts to weaken the ESA have been ongoing and it's important to continue to support and advocate for its protection.
Education
Another key strategy is education, raising public awareness about the potential extinction of species and the impact of human actions on biodiversity. Through education, people can learn about the importance of conservation and take steps to support it.
Breeding Programs
Captive breeding programs in zoos and aquariums can also play a role in species conservation. These institutions can serve as education centers, while at the same time using the animals in captive breeding programs to rebuild populations of critically endangered species. Programs like the Smithsonian National Zoo and the San Diego Zoo have been instrumental in establishing populations of species like the black-footed ferret, red panda, and clouded leopard.
Organizations
In addition, conservation organizations, non-governmental organizations, and research institutions have been working in collaboration with governments to address the decline in species, through research, monitoring, and conservation actions in the field, and also by creating protected areas, corridors, and reintroduction programs.
Frequently Asked Questions
What exactly makes a species endangered vs just threatened?
Being "threatened" vs "endangered" is about degree of risk. A threatened species is likely to become endangered in the near future if current trends continue; an endangered species is at high risk of extinction now or very soon. Agencies and lists (like the IUCN Red List or the U.S. Endangered Species Act) use criteria such as population size, rate of decline, geographic range (habitat fragmentation), and likelihood of extinction to place species in categories. Small populations, fast declines, very limited diets or habitats, heavy poaching, or invasive competitors push a species from threatened to endangered. For the AP exam, mention specific causes (poaching, habitat loss, invasive species, genetic bottlenecks, bycatch) and protection strategies (habitat protection, captive breeding, reintroduction, legal protections like ESA or CITES). For extra review on topic 9.9, see Fiveable’s study guide (https://library.fiveable.me/ap-environmental-science/unit-9/endangered-species/study-guide/nPHB7UlREP5mNGlxRUfD) and practice questions (https://library.fiveable.me/practice/ap-environmental-science).
Why do some species go extinct while others survive when their environment changes?
When environments change, species survive or go extinct based on how well they can handle new selective pressures. Risk factors that push species toward endangerment include overhunting/poaching, very specific habitat needs, limited diets, small population size (genetic bottlenecks), being outcompeted by invasive species, or high bycatch rates (CED EK EIN-4.B.1, EIN-4.B.4). Species that fare better are generalists (broad diet, flexible habitat), have high reproductive rates, lots of genetic diversity, or can move to new habitats (EK EIN-4.B.2, EIN-4.B.3). Conservation tools—habitat protection, captive breeding and reintroduction, habitat corridors, anti-poaching laws (Endangered Species Act, CITES), and reducing fragmentation—reduce extinction risk (EK EIN-4.B.5). For AP prep, you should be able to explain selective pressures and cite strategies on a free-response; review the Topic 9.9 study guide (https://library.fiveable.me/ap-environmental-science/unit-9/endangered-species/study-guide/nPHB7UlREP5mNGlxRUfD) and practice questions (https://library.fiveable.me/practice/ap-environmental-science).
I'm confused about selective pressures - can someone explain what these actually are?
Selective pressures are any environmental factors that change an organism’s fitness—i.e., how likely it is to survive and reproduce. In APES terms (EK EIN-4.B.3), these can be natural (predation, drought, disease) or human caused (poaching, habitat loss, pollution, invasive species). For example: heavy hunting selects for animals that are more nocturnal or wary; habitat fragmentation selects for species that can use small patches or move across corridors; invasive competitors can reduce food for natives with specialized diets. Over time selection shifts trait frequencies (behavioral or genetic)—if a population can’t adapt or move, those pressures can push it toward endangerment or extinction (EK EIN-4.B.1–2). On the exam, expect questions asking you to link a pressure to changes in behavior, fitness, or population outcomes and to propose conservation strategies (e.g., protected habitat, anti-poaching laws, captive breeding). For more review, see the Topic 9.9 study guide (https://library.fiveable.me/ap-environmental-science/unit-9/endangered-species/study-guide/nPHB7UlREP5mNGlxRUfD) and Unit 9 overview (https://library.fiveable.me/ap-environmental-science/unit-9).
What's the difference between a species being endangered because of hunting vs because of habitat loss?
Hunting-driven endangerment and habitat-loss-driven endangerment both reduce population size, but they work differently and need different solutions. Hunting (poaching, bycatch) is a direct selective pressure that removes individuals—often the largest or rarest—fast, causing rapid declines and skewed age/sex structures. Conservation responses focus on law enforcement (Endangered Species Act, CITES), anti-poaching patrols, trade bans, and captive breeding/reintroduction. Habitat loss (fragmentation, edge effects) reduces available resources, isolates populations, and causes gradual declines, increased competition, and genetic bottlenecks that lower long-term fitness. Solutions target protecting and reconnecting habitat (corridors, conservation easements, protected areas) and land-use planning. On the AP exam you should be able to explain both causes and propose matching strategies (criminalize poaching vs protect habitat)—this maps to EIN-4.B in the CED. For a quick review, check the Topic 9.9 study guide (https://library.fiveable.me/ap-environmental-science/unit-9/endangered-species/study-guide/nPHB7UlREP5mNGlxRUfD), Unit 9 overview (https://library.fiveable.me/ap-environmental-science/unit-9), and practice questions (https://library.fiveable.me/practice/ap-environmental-science).
How do invasive species actually outcompete native species for resources?
Invasive species outcompete natives in a few clear ways that tie directly to the CED idea of competition and selective pressures (EIN-4.B, EIN-4.B.4). Common mechanisms: - Broader niche or generalist diet: invasives eat lots of things so they use more food resources and survive in varied habitats. - Faster growth and reproduction: they reach high population sizes quickly, monopolizing space, light, or nutrients. - Lack of predators/pathogens: without natural controls, their populations stay high (enemy release), increasing competitive pressure on natives. - Resource monopolization: they alter soil nutrients, water use, or shading so native species can’t get what they need. - Chemical interference or disease: some release allelopathic chemicals or bring new pathogens; others hybridize with natives, reducing genetic integrity. On the AP exam, you might be asked to explain how these pressures cause endangerment (EIN-4.B.1–4). For a Topic 9.9 review, see Fiveable’s study guide (https://library.fiveable.me/ap-environmental-science/unit-9/endangered-species/study-guide/nPHB7UlREP5mNGlxRUfD) and try related practice questions (https://library.fiveable.me/practice/ap-environmental-science).
Can you give me specific examples of animals that went extinct because they had limited diets?
Short answer: yes—species with very specialized diets are much more likely to go extinct when their food source disappears. A clear AP-aligned example is the Xerces blue butterfly (Glaucopsyche xerces): its larvae fed on native coastal lupines, and habitat loss + removal of those host plants contributed to its extinction in the early 20th century. Other real-world patterns: many island endemics and specialist nectar-feeders (for example, several extinct Hawaiian honeycreepers) declined or went extinct after their specific food plants/habitats were lost or altered by humans, invasive species, or disease. The CED lists “limited diet” as a risk factor (EK EIN-4.B.1), so on the exam you can cite specialist insects or birds that depended on single plant species or narrow food chains as classic examples of vulnerability. Want to review more examples and CED-aligned notes? Check the Topic 9.9 study guide (https://library.fiveable.me/ap-environmental-science/unit-9/endangered-species/study-guide/nPHB7UlREP5mNGlxRUfD) and practice questions (https://library.fiveable.me/practice/ap-environmental-science).
Why are some animals more adaptable to environmental changes than others?
Some species handle environmental change better because of traits that increase their fitness and flexibility. Generalists (broad diet, wide habitat tolerance), high reproductive rates, large population size, high genetic diversity, phenotypic plasticity (ability to change behavior/physiology), and good dispersal ability let species adapt or move when conditions change. Specialists (narrow diet, specific habitat), small populations, low genetic diversity (genetic bottlenecks), limited dispersal, or being heavily hunted make species more likely to become endangered (CED EK EIN-4.B.1–B.2). Invasive species and competition can also outcompete less adaptable species (EIN-4.B.4). Conservation strategies that reduce extinction risk include habitat protection and corridors, captive breeding/reintroduction, anti-poaching laws, and legislation like the Endangered Species Act to boost population size and genetic diversity. For a focused review on endangered species for APES, check the Topic 9.9 study guide (https://library.fiveable.me/ap-environmental-science/unit-9/endangered-species/study-guide/nPHB7UlREP5mNGlxRUfD) and practice questions (https://library.fiveable.me/practice/ap-environmental-science).
What are the main strategies scientists use to protect endangered species from extinction?
Scientists use a mix of legal, ecological, and population-level strategies to prevent extinction. Key approaches: enforce laws and treaties (Endangered Species Act, CITES) and use IUCN Red List status to prioritize species; protect and restore habitat (protected areas, conservation easements, habitat corridors to reduce fragmentation and edge effects); reduce direct human threats (criminalize poaching, limit bycatch, control harvesting); manage populations genetically and demographically (captive breeding, genetic rescue to avoid bottlenecks, careful reintroduction programs); control invasive species and diseases that outcompete natives; and conserve biodiversity hotspots to maximize impact per dollar. Many exam questions ask you to link a threat to the right strategy (e.g., habitat loss → corridors/protected areas). For a quick review see the Topic 9.9 study guide (https://library.fiveable.me/ap-environmental-science/unit-9/endangered-species/study-guide/nPHB7UlREP5mNGlxRUfD) and Unit 9 overview (https://library.fiveable.me/ap-environmental-science/unit-9). Practice applying these on Fiveable’s practice problems (https://library.fiveable.me/practice/ap-environmental-science).
I don't understand how competition for mates can lead to a species becoming endangered - can someone explain?
Competition for mates can push a species toward endangerment because it directly reduces reproductive success—a key part of population viability. If few individuals (or one sex) secure most mates, effective breeding population shrinks. That creates an Allee effect: at low densities, individuals struggle to find mates, so birth rates drop and the population can fall faster. Strong mate competition can also skew sex ratios or lead to selective pressures favoring showy traits that increase mortality (predation, energy costs), lowering survival. Smaller breeding pools cause genetic bottlenecks and inbreeding, reducing genetic diversity and adaptive potential—making the species less able to respond to habitat change or disease (CED EKs EIN-4.B.3, EIN-4.B.4). This combination feeds an extinction vortex unless conservation (habitat protection, captive breeding, reintroduction, laws like ESA/CITES) increases population size and genetic diversity. For more AP-aligned review, see the Topic 9.9 study guide (https://library.fiveable.me/ap-environmental-science/unit-9/endangered-species/study-guide/nPHB7UlREP5mNGlxRUfD) and extra practice problems (https://library.fiveable.me/practice/ap-environmental-science).
How does criminalizing poaching actually help save endangered species in the real world?
Criminalizing poaching helps in three practical ways: deterrence, enforcement, and population recovery. Laws and penalties (national laws, the Endangered Species Act, and international agreements like CITES) raise the risk and cost for poachers and traffickers, which reduces illegal take and market supply. Strong enforcement (rangers, patrols, prosecutions) physically prevents kills and intercepts smuggled wildlife or parts. That lowers selective pressure from overhunting so small populations can stabilize, recover, or be paired with other strategies (habitat protection, captive breeding, reintroduction). Laws also enable funding and coordination for conservation, and make prosecution of wildlife crimes possible across borders. On the APES exam, criminalizing poaching is listed as a strategy to protect species (EK EIN-4.B.5)—link your answer to enforcement, reduced harvest, and giving populations time to rebound. For a quick review, check the Topic 9.9 study guide (https://library.fiveable.me/ap-environmental-science/unit-9/endangered-species/study-guide/nPHB7UlREP5mNGlxRUfD) and practice questions (https://library.fiveable.me/practice/ap-environmental-science).
What happens when an endangered species has very specific habitat requirements that we can't protect?
If a species needs a very specific habitat we can’t protect, we use a mix of in-situ and ex-situ strategies and triage. Ex-situ options: captive breeding, seed/genetic banks, and managed breeding to avoid genetic bottlenecks, then reintroduction if habitat can be restored. Assisted migration (moving populations to suitable areas) or creating habitat corridors can help if climate shift or fragmentation is the issue. If protection/restore is impossible, conservation managers may prioritize (triage) species that provide biggest biodiversity benefits (keystone/umbrella species) and document the lost species on lists like IUCN. Legal tools (Endangered Species Act, CITES) and reducing direct threats (poaching, invasive species, pollution) still matter. These topics map to EKs and solution-focused FRQs on the AP exam—review the Topic 9.9 study guide (https://library.fiveable.me/ap-environmental-science/unit-9/endangered-species/study-guide/nPHB7UlREP5mNGlxRUfD) and Unit 9 resources (https://library.fiveable.me/ap-environmental-science/unit-9) for examples and practice.
Why do species with limited habitat requirements become endangered faster than generalist species?
Species with limited (specialized) habitat needs become endangered faster because they have fewer options when conditions change. If a species needs a specific plant, microclimate, or nesting site, habitat loss, fragmentation, invasive species, or climate shifts remove those limited resources and the species can’t easily move or switch diets (EK EIN-4.B.1–B.2). Specialists usually have smaller population sizes and ranges, so a single disturbance (disease, extreme weather, or human activity) can cause big declines and genetic bottlenecks that reduce adaptive potential. Generalists tolerate a wider range of conditions and food, so they’re more resilient to change. On the AP exam you should connect these ideas to concepts like habitat fragmentation, edge effects, invasive species, and genetic bottleneck, and propose protections (habitat corridors, legal protection, captive breeding). For more AP-aligned review, see the Topic 9.9 study guide (https://library.fiveable.me/ap-environmental-science/unit-9/endangered-species/study-guide/nPHB7UlREP5mNGlxRUfD) and Unit 9 overview (https://library.fiveable.me/ap-environmental-science/unit-9). Practice questions: (https://library.fiveable.me/practice/ap-environmental-science).
How do scientists decide which endangered species to prioritize for conservation efforts?
Scientists prioritize species using a mix of biological risk, ecological importance, and practical feasibility—exactly the kinds of ideas in EK EIN-4.B. Key criteria include: - Extinction risk and trends (IUCN Red List status, rate of decline) - Unique traits or endemism (limited range or specialized habitat) - Ecological role (keystone or foundation species that support ecosystems) - Genetic health (small populations/genetic bottlenecks) - Main threats (poaching, habitat fragmentation, invasive species) and whether threats are fixable - Feasibility and cost-effectiveness (can captive breeding, habitat protection, corridors, or legislation like the ESA/CITES realistically recover them?) - Conservation return-on-investment (saving many species by protecting habitat or hotspots vs. single-species focus) On the AP exam, connect this to strategies in EK EIN-4.B.5 (habitat protection, captive breeding, legislation). For a crisp review, check the Topic 9.9 study guide (https://library.fiveable.me/ap-environmental-science/unit-9/endangered-species/study-guide/nPHB7UlREP5mNGlxRUfD) and try practice questions (https://library.fiveable.me/practice/ap-environmental-science).
What's the connection between selective pressures and natural selection when it comes to endangered species?
Selective pressures are the environmental factors (hunting, habitat loss, invasive species, limited food) that change organisms’ fitness; natural selection is the process by which individuals with traits that survive those pressures leave more offspring. For endangered species this matters because strong new pressures (rapid habitat fragmentation, poaching, or an invasive competitor) can quickly favor traits that many individuals don’t have. If a species can’t adapt genetically or move to new habitat, its population declines and may hit a genetic bottleneck, reducing variation and making future adaptation even harder (EKs EIN-4.B.1–3, EIN-4.B.4). Conservation strategies—habitat protection, corridors, captive breeding and reintroduction, anti-poaching laws—either reduce the selective pressures or increase the population’s ability to evolve (EK EIN-4.B.5). On the AP exam you might be asked to explain how a specific pressure changes fitness or to justify a management strategy that reduces extinction risk. For a quick review, see the Topic 9.9 study guide (https://library.fiveable.me/ap-environmental-science/unit-9/endangered-species/study-guide/nPHB7UlREP5mNGlxRUfD) and Unit 9 overview (https://library.fiveable.me/ap-environmental-science/unit-9). For practice, try problems at (https://library.fiveable.me/practice/ap-environmental-science).
Can legislation really prevent species extinction or is it too little too late?
Short answer: yes—legislation can prevent extinctions, but it’s only part of the solution and sometimes comes too late if other factors aren’t addressed. Why: laws like the Endangered Species Act, CITES, and protected-area policies create legal protection against poaching, habitat destruction, and trade; they fund recovery tools (habitat protection, captive breeding, reintroduction, habitat corridors) that increase population size and genetic diversity. But limits exist: delayed listing, weak enforcement, habitat fragmentation, invasive species, and rapid climate change can make recovery much harder—small populations face genetic bottlenecks and extinction debt even after protection. What matters for success: early action, strong enforcement, habitat protection/restoration, corridors to allow movement, and integrated strategies (poaching laws + captive breeding + ecosystem management). For AP exam framing, connect causes (poaching, limited diet, invasive species, habitat needs) to specific strategies listed in the CED (criminalize poaching, protect habitat, legislation). Review Topic 9.9 for specifics (https://library.fiveable.me/ap-environmental-science/unit-9/endangered-species/study-guide/nPHB7UlREP5mNGlxRUfD). For more practice, see unit resources (https://library.fiveable.me/ap-environmental-science/unit-9) and practice questions (https://library.fiveable.me/practice/ap-environmental-science).