As mentioned in the article “The IUCN Red List of Threatened Species: conservation status of species“, published recently in this blog, the International Union for the Conservation of Nature (IUCN) has updated the number of species included in the Red List. It now consists of a total of almost 100,000 different plants and animals, 25% of which are endangered.
The number of species included in the Red List continues to increase year after year – and 100,000 are already far too many. But are all these species equally endangered? The answer is no. Although most extinctions are caused by human activity (namely, of edge effects and habitat fragmentation), the impact of mankind on fauna varies from one environment or ecosystem to another. So what species are more likely to go extinct?
Species with a limited geographical distribution
Species with a limited geographical distribution tend to be at a higher risk of extinction, as they lack alternatives when their territory is fragmented, destroyed or degraded (or when exotic species are introduced). Moreover, they usually have a low population density, a phenomenon that can be observed in species endemic to islands, such as the Tenerife blue chaffinch (Fringilla teydea).
Species that live on islands are at a higher risk of extinction, for several reasons: the competition for resources and space with other species (which may involve displacement), the fact that they live in small populations (which reduces reproductive success), a higher occurrence of hybridization, the possible arrival and naturalization of invasive species, the introduction of diseases and parasites, etc. The dodo, for example, a bird that inhabited the islands of Mauritius and Réunion, suffered some of these effects, which led to its extinction (although hunting was the main reason for its disappearance).
Migratory and wide-roaming species
Migratory species are also at a higher risk of extinction than other animals, as they are dependent on the proper conservation of all the environments they inhabit. Therefore, they need their new territories to present specific characteristics: an adequate size, a series of natural resources, etc. A classic example are migratory birds, which travel thousands of kilometres in search of new habitats. However, there are many other species that can face a higher extinction risk, such as salmonids; they are dependent on the maintenance and protection of the seas, lakes and rivers where they spawn.
Wide-roaming species are also at a high risk of extinction, as these animals claim large territories that are difficult to control. Wide-roaming species do not just wander around; they travel long distances every day to maintain territorial control. They can do this on foot (terrestrial species) or flying (birds). Birds of prey and canids are examples of this type of species, in which each couple can claim several hectares of territory.
Many of these migratory species – which travel kilometres and kilometres – must eat a lot of food in order to obtain the necessary energy for their daily activities, so they often have large bodies. Thus, large carnivores and herbivores must seek food during their journeys, increasing the risk of encountering threats and with no guarantees that they will be successful in their search. This group – which includes polar bears (Ursus maritimus) – is also exposed to greater risk.
Specialist species or k-strategists
Specialist species or k-strategists are one of the most endangered groups; they usually have very few descendants and invest a lot of resources in taking care of their young. These species rarely explore other territories or environments, and they settle close to their birthplace and parents.
Thus, in the event of a threat, they do not have the ability to move to other places, which would allow them to avoid danger. This means they often do not have the necessary flexibility to adapt to other environments. Gorilas are an example of specialized species.
Species typical of stable environments
Species characteristic of stable and homogeneous environments are more sensitive to environmental changes of any kind (physical, chemical, biological…), so a small variation in their habitat can pose an irreparable threat to such species, as they are unable to survive, for example, a possible displacement. In this sense, amphibians are a significantly sensitive group.
Species that form small groups or flocks
Species that form small groups or flocks are more visible to predators, so they are more sensitive to hunting. Furthermore, when these groups especially small, they may even suffer from “Allee effects“: when population size or density falls below a certain threshold, there may be a rapid decline in population viability or vigor, and an increase in the extinction risk.
The island fox (Urocyon littoralis), for example, is a species that has suffered the negative consequences of these effects: difficulty in finding a couple, lower odds of hunting success, no social thermoregulation… consequences that have made it virtually extinct.
Translated by Carlos Heras