Conservationists are not necessarily against the extinction of certain species; some of them understand that natural evolution may involve such a consequence. What they strenuously oppose is when animal species are hunted or fished to extinction by humans, and when human development of land deprives animals of their natural habitats, thereby facilitating the endangerment and eventual extinction of those animals.
Whether conservationists should allow some species to die out is entirely up to them. Many Americans feel the environmental movement goes too far when major development projects intended to benefit thousands or hundreds of thousands of people are jeopardized because the construction would eliminate or endanger the habitat of animal species.
A well-known case involving the trade-off between development and conservation involved a tiny fish called the snail darter, the natural habitat of which was threatened by the construction of a dam in Tennessee. Conservationists sued to stop the project from going forward. The case became a major cause celebre for environmentalists and their supporters in Congress. Because the dam's construction was being funded through the federal Tennessee Valley Authority, and because its construction was seen by many as in violation of the congressionally-passed Endangered Species Act, high-profile politicians and activists on both sides of the issue were visible. Eventually, in 1978, the case reached the Supreme Court, which decided in favor of the conservationists. Tennessee Senator Howard Baker and Representative John Duncan responded by introducing legislation to exempt the dam, which was by now nearing completion, from the Endangered Species Act. That legislation became law, and the dam's construction was completed.
Should conservationists have allowed the snail darter to become extinct? That depends upon your perspective. Drawing the line on when or which species should be permitted to disappear from the Earth is a slippery slope. Once certain species are permitted to become extinct by virtue of human interaction with the environment, then it becomes easier to allow additional species to disappear. That is an absolutist approach, but it does illustrate the difficulties involved in arriving at a decision.
Most environmentalists are followers of Charles Darwin, he of the "survival of the fittest" theory of evolution. Darwin, in his seminal 1859 study, On the Origin of Species, concluded that an evolutionary process of "natural selection" determined which species would survive. Whether mankind's determination to expand and develop through the use of technology, with the resultant devastation that has caused through the last 100 years, qualifies under Darwin's theory of evolution is debatable. If human behavior, including the drive to develop land for commercial or residential use, or so that certain vital resources like water can be better exploited by humans, constitutes basic human nature, then one could conclude that conservationists should allow certain species to become extinct. Such an outcome, however, is exceedingly unlikely.