Biodiversity

Extinction refers to the loss of some species or other taxonomic unit (e.g., subspecies, genus, family, etc.; each is known as a taxon), occurring over all of its range on Earth. (Extirpation refers to a more local disappearance, with the taxon still surviving elsewhere.) The extinction of any species is an irrevocable loss of part of the biological richness of Earth, the only place in the universe known to support living creatures. Extinction can be a natural occurrence caused by an unpredictable catastrophe, chronic environmental stress, or ecological interactions such as competition, disease, or predation. However, there have been dramatic increases in extinction rates since humans have become Earth's dominant large animal and the cause of global environmental change.

Extinction has always occurred naturally. Almost all species that have ever lived on Earth have become extinct. Perhaps they could not cope with changes occurring in their environment, such as climate changes, the intensity of predation, or disease. Alternatively, many extinctions may have occurred simultaneously as a result of unpredictable catastrophes. From the geological record it is known that species, families, and even phyla have appeared and disappeared over time. For example, numerous phyla of invertebrates proliferated during an evolutionary radiation occurring at the beginning of the Cambrian era about 570 million years ago, but most of these animals are now extinct. The 15-20 extinct phyla from that period are known from the Burgess Shale of British Columbia, and they represent unique experiments in invertebrate form and function. Similarly, entire divisions of plants have appeared, radiated, and disappeared, such as the seed ferns Pteridospermales, the cycad-like Cycadeoidea, and woody plants known as Cordaites. Of the 12 orders within the class Reptilia, only three survive today: crocodilians, turtles, and snakes/lizards. Clearly, the fossil record displays a great deal of evidence of natural extinctions.

Overall, the geological record suggests that there have been long periods of time characterized by uniform rates of extinction, but punctuated by about nine catastrophic episodes of mass extinction. The most intense extinction event occurred at the end of the Permian period some 245 million years ago, when 54% of marine families, 84% of genera, and 96% of species are estimated to have become extinct.

Another famous, apparently synchronous, extinction of vertebrate animals occurred about 65 million years ago at the end of the Cretaceous period. The most renowned extinctions were of the last of the reptilian dinosaurs and pterosaurs, but many plants and invertebrates also became extinct at that time. In total, perhaps 76% of species and 47% of genera became extinct in the end-of-Cretaceous crisis. One hypothesis to explain the cause of this mass extinction involves a meteorite impacting the Earth, causing huge quantities of fine dust to be spewed into the atmosphere, and resulting in a climatic deterioration that most large animals could not tolerate. However, some scientists believe that the extinctions of the last dinosaurs were more gradual.

However, humans have been responsible for almost all of Earth's recent extinctions. These extinctions are occurring so quickly that they represent a modern mass extinction of similar intensity to those documented in the geological record. Examples of recent extinctions caused by humans include such well-known cases as the dodo, passenger pigeon, and great auk. Many other high-profile species have been taken to the brink of extinction, including the plains bison, whooping crane, ivory-billed woodpecker, right whale, and other marine mammals. These losses have been caused by insatiable overhunting and intense disturbance or conversion of natural habitats.

Beyond these well-known and tragic cases involving large animals, Earth's biodiversity is experiencing an even larger loss. This ruin is mostly being caused by extensive conversions of tropical ecosystems, particularly rainforest, into agricultural habitats that sustain few of the original species. As was described previously, tropical ecosystems sustain extremely large numbers of species, most of which have restricted distributions. The conversion of tropical forest into other habitats inevitably causes the loss of most of the locally endemic biota. This is a great tragedy, and the lost species will never occur again.

The most important human influences causing the extinction or endangerment of species are: (1) excessive exploitation, (2) effects of introduced predators, competitors, and diseases, and (3) habitat disturbance and conversion. These stressors can result in small and fragmented populations that experience the deleterious effects of inbreeding and population instability and then decline further, ultimately to extirpation or extinction.

The increased rate of extinction and endangerment of biodiversity during the past several centuries is best documented for vertebrates because, as noted previously, most invertebrate species, particularly insects, have not yet been described by scientists. During the last four centuries there have been more than 700 known extinctions globally, including about 100 species of mammals and 160 species of birds, all because of human influences.

A much larger number of species is facing imminent extinction; they are endangered. For example, more than 1,000 species of birds are considered to be threatened with extinction. Of this total, 46% live only on oceanic islands, a situation in which species are especially vulnerable to extinction caused by stresses associated with human activity. Birds of tropical forests account for 43% of the threatened bird species, wetland species for 21%, grassland and savanna species 19%, and other habitats 17%. Only 1.5% of the threatened species are North American, 4.2% are European or Russian, 33% Central and South American, 18% African, 30% Asian, and 14% from Australia and the Pacific.