Threatened Species

Threatened Species Definition

A threatened species is any species which is vulnerable, endangered, or critically endangered. The International Union for Conservation of Nature, or IUCN, is commonly referenced as a leading organization in determining if a species can be considered a threatened species or not. The IUCN also defines the category nearly-threatened species, for any animals which are likely to become threatened species within the foreseeable future. The IUCN designation of threatened species can be seen below.

IUCN risk categories

Other organizations may define a threatened species slightly differently. For instance, the United States Endangered Species Act defines a threatened species as any species which is likely to become endangered within the foreseeable future. However, most international organizations and conservation societies tend to use the definition of the IUCN. The difference is only subtle, and either way, a threatened species is one which is likely to go extinct if nothing is done to protect it.

Criteria for a Threatened Species

Population Size Decline

While the criteria for each organization differs, the following are general criteria developed by the IUCN and other organizations to determine if a species can be labeled a threatened species. One of the most important markers of a threatened species is the population size, and its overall direction. A small, decreasing population is much more likely to be threatened than a small, increasing population. Scientists typically measure a population using mark-recapture studies. In these studies, a portion of the population is captured and marked somehow. They are then released. After time has passed, another portion is captured. Based on the percentage of the recapture which have marks, the scientists can estimate the total population. Thus, they can see if a population is very small by capturing the same individuals repeatedly.

Further, the population must be observed over several generations, ideally, to have enough data to truly see decline in a population. Sometimes, there are not enough animals left for 3 generations, and these species would be a critically endangered threatened species. In larger populations it is helpful to observe multiple generations to be sure that the population is not simply adjusting to a predator and prey dynamic, or other natural cycle which. Some natural cycles can severely affect population numbers, but the general trend in the population can still be increasing. Data which effectively shows that this is not true, and that the population is declining generation-over-generation will be more effective in showing that species truly is threatened.

Number of Mature Individuals

Along those lines, scientist must also measure and separate the number of sexually active adults in the population. A sea-turtle, for instance, can lay hundreds of eggs each season. It would be inaccurate, however, to consider all of those offspring as contributing to the next generation. The fact for sea-turtles, and many other animals, is that many offspring will not make it to sexual maturity. In calculating a threatened species, it is more important to focus on the individuals which have already reached maturity and are actively reproducing.

Due to the limitations of a small genetic pool, the number of reproducing individuals must be relatively high for any population to have a good chance at long term survival. Anything less than 10,000 mature, reproducing adults is usually considered a threatened species, when the other criteria are also met. This is usually based on genetic models. Small populations are typically subject to conditions like genetic drift and population bottlenects, which can easily drive them to extinction in only a few generations. When a threatened species has less than 250 known mature individuals, it is typically considered critically endangered. Further, if a declining trend is seen in the reproducing adults, this can also be grounds for labeling a threatened species.

Geographic Range

Almost as important as the number of individuals is the geographic range of a threatened species. A species with a cosmopolitan distribution (found globally) is unlikely to be in danger of going extinct. However, if the reproducing adults in that range are actually separated and cannot reproduce, the possibility of extinction increases. This phenomenon is known as fragmentation. It can be natural, such as an impassable mountain range, or man-made, like a highway. Either way, if a species cannot cross the barrier, the populations on each side are effectively on their own.

In terms of designating a threatened species, fragmentation decreases the chances of two adults being able to cross the barrier and divide. Effectively, it creates two smaller populations, each with a lower number of breeding adults. The smaller population size increases the chance of extinction in each group, which raises the total chance of extinction for the species. Thus, fragmentation is an important consideration for threatened species. Generally, animals found in only 1 small area are considered critically endangered. The total size of the area depends on the animal, and the typical range it needs to survive.

As with population size, scientist quantify future and current risks to geographic range. A threatened species may also be designated in future actions to their habitat put part or all of their population in danger. Deforestation and ocean acidity are two events which are greatly affecting many species right now. The drastic effects will alter their ranges and ultimately their population sizes. Many animals are being added to the threaten species list because of large-scale events like this which affect their range.

Statistical Analysis

To put together a view of the “big-picture”, scientist often use computer modeling and simulations to estimate the risk presented to a species. In general, if the chance of the species going extinct over the next 100 years is greater than 10%, the label threatened species can be used. When that threshold begins to reach 50% or greater in the next 10 years, the threatened species is considered critically endangered. The vaquita, a small porpoise which lives in the Gulf of California, is considered a critically endangered threatened species. There are fewer than 30 individuals left, they are restricted to one location, and they have been declining steadily for decades. This is the kind of devastation that gets and animal to the critically endangered list.

Threatened Species Examples

Black-footed Ferret

The black-footed ferret (Mustela nigripes) is a threatened species on the verge of extinction. While it is though that the ferret once had an extended range, poising of their prey (prairie dogs) and the plague killed off their populaitons. Further, as the plains became more populated, many prairie dog town were destroyed, killing off their food-source. By the 1960’s, the black-footed ferret was extremely rare and limited to a few populations in Wyoming. By 1987, it was considered extinct in the wild.

A massive campaign was undertaken to breed captive black-footed ferrets and reintroduce them to the wild. Since then, the ferrets have been reintroduced to several location, with varying success. There are now 4 populations of self-sustaining wild ferrets, which greatly increased their chances of getting off the endangered species list. With sustained effort and protections, the ferrets could see a day when they are completely removed from the threatened species list, although that is a far way off.

Tawny Nurse Shark

Found off the coast of Australia and Indonesia, the tawny nurse shark (Nebrius ferrugineus) is a threatened species considered vulnerable by the IUCN. This threatened species has mostly declined, like many sharks, due to fishing practices and harvesting them for their fins. Gill nets and other forms of fishing which are non-selective often catch the sharks and kill them in the process. The sharks are also hunted for their fins, which are part of shark-fin soup, an Asian delicacy which is supposed to have mysterious powers. This is nonsense, of course, and the harvesting of sharks for their fins has decimated many species. The tawny nurse shark, which reproduces at a low rate and fails to distribute far from where they are born, takes a lot of time to recover. For this reason it is considered a threatened species, which will need protections in order to succeed in the future.

Other Species

These two species hardly represent all the species on the threaten species list. The IUCN keeps a global list, but every country and even locality may keep their own list or assesment protocol. Further, while these two were vertebrate species, vertebrates represent only a fraction of all threatened species. Below is a graph which breaks down the approximate proportions of species currently on the IUCN Red List.

Percentage of species listed on the IUCN Red List

As you can see, amphibians and other vertebrates do make up a large portion of this list. Also represented are plants, molluscs, and insects. There are even some fungi on the IUCN list (not pictured). There are several reasons for the disparity in the number of species represented from each group. First of all, vertebrates are by far the most studied group, even though their numbers pale in comparison to invertebrates. Secondly, amphibians are currently experiencing the drastic changing effects of climate change and deforestation, which is destroying their habitats and food sources. For this reason, we see a lot of amphibians as threatened species. Plants, molluscs, insects, and other groups with few threatened species are not necessarily doing well, scientist just don’t have enough information to label them as a threatened species.


1. True or False? The threatened species list includes EVERY threatened species known to man.
A. True
B. False

Answer to Question #1
False. The threatened species list includes every animal that has evidence that it is threatened. There are many species we know about, but are not on the list because they have not been studied in enough detail, or at all. Remember that scientists like to have multiple generations of data on an organisms before labeling it as a threatened species.

2. Which of the following is true?
A. A threatened species and an endangered species are exactly the same
B. A threatened species can be vulnerable, endangered, or critically endangered, according to the IUCN
C. A threatened species is NOT endangered

Answer to Question #2
B is correct. Again, this definition can vary, but a threatened species is any species which has a serious possibility of going extinct.

3. You are a conservation scientist. You find a species which is restricted to a small geographic location. You gather data on the population and find that it is small and in serious decline. You submit your data to the IUCN and they classify the species as vulnerable. Is the species now protected?
A. No
B. Yes
C. Maybe

Answer to Question #3
A is correct. Probably not. In fact, the battle to protect the species has likely just begun. Many countries have their own policies for protecting animals and determining their status. While the IUCN does hold significant influence, it is usually not enough to get protections put in place. Likely, lobbying for laws and funds to protect the habitat of the animal in needed to secure actual protection.


  • Blumstein, D. T., & Fernandez-Juricic, E. (2010).A Primer of Conservation Behavior. Sunderland: Sinauer Associates, Inc. Publishers.
  • Feldhamer, G. A., Drickamer, L. C., Vessey, S. H., Merritt, J. F., & Krajewski, C. (2007). Mammology: Adaptation, Diversity, Ecology (3rd ed.). Baltimore: The Johns Hopkins University Press.
  • Internation Union for Conservation of Nature. (2000, February 9). IUCN Red List Categories and Criteria. Retrieved 5 14, 2018, from
  • Sarkar, S. (2005). Biodiversity and Environmental Philosophy: an introduction. New York: Cambridge University Press.
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