Human impacts on the environment–such as habitat loss, over-exploitation, species introductions, and pollution–do not threaten all groups of species equally. At greatest risk are species with small population sizes, species whose populations vary greatly and species with slow rates of population growth. More specifically, the following groups of organisms are particularly susceptible to extinction:
Species at higher trophic levels
Species high in the food chain tend to be large, rare animals with slow rates of population growth. Particularly susceptible to over-exploitation or habitat loss, these species include the Caribbean monk seal, great auk (Alca impennis), Steller’s sea cow (Hydrodamalis stelleri), thylacine (Thylacinus cynocephalus, also known as the Tasmanian wolf), and many endangered birds of prey, cats, wolves, foxes, and whales.
Local endemics
Species with restricted ranges are often threatened by habitat loss. For example, the entire natural habitat of the Devil’s Hole pupfish (Cyprinodon diabolis) is a spring measuring roughly 3 by 15 meters. Water development, pollution, or habitat alteration could easily drive the species to extinction in the wild. The extremely high rate of island species extinction further testifies to the threat that local endemics face.
Species with chronically small populations
This category overlaps the first. Since many species at higher trophic levels have sparsely distributed populations, habitat restriction or fragmentation may reduce their populations to very small levels. However, the population sizes of species at lower trophic levels may also be extremely small in a given habitat or region. Many tropical forest tree species, for instance, have very low population densities and are thus threatened by habitat loss and fragmentation.
Largest members of a guild
Whether or not a species is on the top trophic level, large species have high metabolic demands, require large habitats, and tend to occur in low densities. Thus, the largest species within a group of species sharing similar food sources (a guild) tend to be a high risk of extinction. For example, all of the lemurs that have died out since Madagascar was colonized by human beings were as large or larger than the surviving species.
Species with poor dispersal and colonization ability
As with local endemics, species with narrow habitat requirements and species that can’t disperse easily to new habitats are at high risk of extinction, even if their population is widespread. For example, in the face of a warming climate, some of the most threatened species will be those that can’t disperse as fast as suitable habitat moves to higher elevations and latitudes.
Species with colonial nesting habits
Colonial nesting species are particularly susceptible to over-exploitation or the loss of breeding habitat. Several species of marine turtles, for example, have been threatened by the development of nesting beaches for tourism and other purposes. Similarly, the vulnerability of the extinct passenger pigeon (Ectopistes migratorius) to hunters was increased by the birds’ tendency to nest in enormous colonies.
Migratory species
Migratory species depend upon suitable habitat in their summer and winter range and along the course of their migratory route. Thus, the potential for adverse effects of habitat changes on migrant populations is high. In North America, Kirtland’s warbler (Dendroica kirtlandi), Backman’s warbler (Vermivora backmanii), and the Whooping Crane (Grus americana) are endangered because habitat in both their breeding and wintering ranges has shrunk.
Species dependent on unreliable resources
These species’ populations fluctuate greatly, so they face increased threats of extinction when their population is low. Populations of frugivores and nectarivores, for example, may be reduced significantly during years when fruit or nectar crops are low.
Species with little evolutionary experience with disturbances
In regions where human beings have a longstanding presence, the species most sensitive to human disturbance have already been lost and many of the remaining species have adapted to the additional disturbance. In contrast, species are extremely vulnerable where human disturbance has no historical precedent. Thus, the loss of the great auk, passenger pigeon, elephant birds, and moas, and the endangerment of the musk oxen (Ovibos moschatus) were all related to the introduction of a predator where none had existed before or to the introduction of a predator with a new means of hunting – say, the rifle – that the prey had not previously experienced.
