The causes of background extinction and past mass extinctions are not well understood, but it appears that biological and environmental factors ' were involved. A major climate change could have triggered the mass extinction of species. Marine  ' organisms arc particularly vulnerable to tempera­ture changes; if the Earth's temperature changed by just a few degrees, fur example, it is likely that many marine species would have become extinct.

It is also possible that mass extinctions of the past were triggered by catastrophes, such as the col­lision of the Earth and a large meteorite. The im­pact of a meteorite could have forced massive quan­tities of dust into the atmosphere, blocking the sun's rays and cooling the planet.

Extinctions Today

Although extinction is a natural biological process, it can be greatly accelerated by human activities. The burgeoning human population has forced us to spread into almost all areas of the Earth, and when-

plants and animals are disrupted or destroyed, which can lead to their extinction. For example, the dusky seaside sparrow, a small bird that was found only in the marshes of St. Johns River in Florida, became extinct in 1987, largely due to human destruction of its hahirat.

Currently, the Earth's biological diversity is dis­appearing at an alarming rate (Table 16-1). Con­servation biologists estimate that at least one spe­cies becomes extinct each day and that it is likely that a substantial portion of the Earth's biological diversity will be eliminated within the next few decades. As many as one-fourth of the higher plant

Families may be extinct by the end of the 21^st century and countless animals’ species that depend upon those plants for food and habitat will probably become extinct. Some biological fear is that we are entering the greatest period of mass extinction in the Earth's history.

The current mass extinction differs from previ­ous periods of mass extinction in several respects. First, it is directly attributable to human activities. Second, it is occurring in a tremendously com­pressed period of time {just a few decades as op­posed to millions of years). Perhaps even more sobering, larger numbers of plant species are be­coming extinct than in previous mass extinctions. Because plants are the base of the food chain, the extinction of animals that depend on planes cannot be far behind.

Endangered and Threatened Species

A species is endangered when its numbers are so severely reduced that it is in danger of becoming extinct. When extinction is less imminent but the population of a particular species is quite low, the species is said to be threatened. Endangered and threatened species represent a decline in biological diversity, because as their numbers decrease, their genetic variability is severely diminished. Long-term survival and evolution depend upon genetic diversity, so its loss adds to the risk of extinction for endangered and threatened species as compared to species that have greater genetic variability.

Characteristics of Endangered Species Endan­gered species share certain characteristics that seem to have made them more vulnerable to extinction. These include (1) an extremely small (localized) range or (2) a large (extended) range that has been extensively modified by humans; (3) living on is­lands; and (4) low reproductive success, usually the result of a small population size.

The area of the Earth in which a particular spe­cies is found is its range. Many endangered species have a very limited natural range, which makes them particularly prone to extinction if their habi­tat is altered. The Tiburon mariposa lily, for exam­ple, is found nowhere in nature except on a single hilltop near San Francisco. Development of that area would almost certainly cause the extinction of this species.

Species that require extremely large territories in order to survive may be threatened with extinc­tion when all or part of their territory is modified. For example, the California condor, a scavenger bird that lives off of carrion and requires a large, undisturbed territory (hundreds of square kilome­ters) in order to find adequate food, is on the brink of extinction. During the five-year period from 1987 to 1992, it was no longer found in the wild.

In 1992 two zoo-bred California condors were released in the Los Padres National Forest north of Los Angeles. These young condors have been raised by a hand-operated condor puppet so that they would not come into direct contact with humans. Initially they had lived on a netted platform to protect them from falling while they learned to fly; the net was then dropped.

Many island species that are endemic to certain islands (that is, they are not found anywhere else in the world) are endangered. These organisms often have small populations that cannot be replaced by immigration should their numbers be destroyed. Because they evolved in isolation from competi­tors, predators, and disease organisms, they have few defenses when such organisms are introduced (usually by humans) to their habitat.

In order for a species to survive, its members must be present within their range in large enough numbers for males and females to mate. The mini­mum population density and size that ensure repro­ductive success vary from one type of organism to another. However, for all organisms, if the popula­tion density and size fall below a critical minimum level, the population declines, becoming suscepti­ble to extinction.

Endangered species often share other charac­teristics. Some have low reproductive rates (the female blue whale, for example, produces a single calf every other year). Some endangered species breed only in very specialized areas (the green sea turtle, for example, lays its eggs on just a few beaches). Highly specialized feeding habits can also endanger a species. The giant panda eats only bamboo, a plant all of whose members periodically flower and die together; when this oc­curs, panda populations face starvation. (Like many other endangered species, giant pandas are also endangered because of habitat destruction.)