Although a large number of factors can make a species vulnerable to extinction, conservation biologists have observed that the species most vulnerable to extinction belong to one of six main groups:
• Species with small populations: some species have very small populations, consisting of a few individuals. These small populations are highly vulnerable to random changes in demographics or environmental conditions, loss of genetic diversity, all of which increase the risk of extinction.
Species whose population sizes naturally fluctuate between large and small populations also fall into this category, they are more at risk of extinction during small population phases than these fluctuations.
• Species with declining populations: trends in population size tend to persist, hence declining populations abundance faces a high risk of extinction unless conservation managers identify and address the causes of decline.
• Restricted-range species: certain species, such as those that are restricted to the ocean; mountain peaks; or isolated lakes, which are found only in a limited geographical area. A significant condition, such as acyclone/hurricane or drought could easily affect the range of the entire species, potentially driving the species to extinction.
• Species with only one or a few populations: a large enough disturbance, such as a fire, a storm or an epidemic can wipe out a single population of a species.
For a species with a single population, this means its extinction.
Species in this category (few populations) overlap with those in the previous category (restricted ranges) because species with few populations tend to have narrow ranges.
• Human-exploited species: Overfishing can easily reduce the population to the point of extinction.
Even if overexploitation stopped shortly before the point of extinction, it may still have reduced the population to a dimension in which it becomes sensitive to one or more of the three additional pressures to which small populations are subject to.
• Species with critical symbiotic relationships: species that are members of obligatory symbiotic relationships (where one species cannot survive without another) will go extinct if its host dies.
For example, rhinoceros fly (Gyrostigma rhinocerontis) larvae mature in the stomach mucosa of African rhinoceros and no other species.
Thus, if the host species (the rhinoceros) were to disappear, this largest species of fly in Africa, would also disappear.
This phenomenon in which the extinction of one species leads to the extinction of another is called co-extinction, while a series of linked co-extinctions is called an extinction cascade.
The following characteristics are also linked to extinction, although the links are not as strong as in the case of the previous six categories:
• Animal species with large body sizes: large animals generally require large ranges and more food, have lower reproductive rates, and have smaller population sizes than small animals.
They are often harvested by humans for material benefits. Therefore, within groups of related species, the largest are usually also the most vulnerable to extinction, i.e. a greater carnivore, ungulate or whale species are more likely to become extinct than a smaller carnivore, ungulate or whale.
• Species requiring a large habitat: individuals or social groups of certain species must forage for food over large areas to meet their needs.
When portions of their range are being degraded or fragmented, the remaining area will eventually be too small to support a viable population.
• Species that are poor dispersers: Moving to more suitable habitat is a common survival response following altered environmental conditions. But species with poor dispersal abilities could be doomed if they are unable to move to more suitable areas elsewhere.
• Seasonal migrants: A migratory species depends on intact ecosystems in two or more places to complete its life cycle.
If these ecosystems, whether at migratory stopover sites along migratory routes and/or at migratory arrival points, are damaged, the species could be threatened with extinction.
• Species with low genetic diversity: while genetic diversity allows species to adapt to changing environmental conditions, species with low genetic diversity are more vulnerable to extinction because they lack the ability to adapt to new diseases, new predators, or recent changes in their ecosystems.
• Species having evolved in stable ecosystems: species having evolved in relatively stable environments (ex.tropical ecosystems) are often threatened with extinction because, under stable conditions, a species is unlikely to retain the ability to adapt to environmental changes such as altered microclimates.
• Species with specialized needs: Specialized species are often threatened with extinction because they are unable to adapt to altered ecosystems.
• Species living in groups: A number of factors put species living in groups at risk of extinction.
For example, a herd of ungulates, a flock of birds at their nighttime roost, or a school of fish can be fished out entirely by people using very efficient techniques.
Even if a few individuals remain, removal may leave the population below a critical threshold necessary for effective foraging, mating, or territorial defense.
This link between population size/density and individual fitness is called the Allee effect.
• Species that have never been in contact with humans: species for which they encounter humans the first time lack avoidance strategies that promote survival during these encounters.
Species that never met people therefore have a higher probability of extinction than species that have already survived human contact.
• Species closely related to species that recently went extinct: Groups of closely related taxa, where some members are threatened or already extinct, often sharing characteristics that increase their threat of extinction.
Groups of relative taxa that include many threatened species include monkeys, cranes, sea turtles, and cycads.
• Island species: Island species generally exhibit many of the characteristics mentioned above.
Moreover, the mere fact that an island is surrounded by ocean means that species is unable to swim or fly and have nowhere to go when they need to escape danger.