To maximize opportunities for colonization and reduce the probability of extinction, Diamond recommended large, compact, closely spaced, connected reserves. However, species with limited ability to disperse and requirements for specific habitats found in small, dispersed patches may need several small reserves, and corridors may facilitate the spread of introduced species, diseases, or fire. Preserves in ecotones are likely to be valuable for organisms with low mobility as the climate changes. A coarse-filter approach to protection prioritizes ecosystems that contain high biodiversity, such as biological hotspots. In gap analysis, the distributions of selected taxa are compared to the locations of protected areas to identify priority areas for additional protection. Reserves may have a variety of levels of protection. Some conservation biologists advocate using the absence of human influence as the benchmark for strictly protected reserves. Restoration should conserve key processes, structures, and interactions such as fire in native prairie. Restoration should also address both abiotic and biotic elements of an ecosystem. Advocates of rewilding support the restoration of animals, especially megafauna, to their former ranges through reintroductions .

New information and evidence challenge equilibrium models. Fluctuations among livestock in African rangelands and northern fur seals in the Pacific Ocean do not conform to models of competition for resources maintaining populations in equilibrium. Rather than the self-interested competition for resources predicted by the tragedy of the commons, many societies regulate resource use through cooperative arrangements. Evidence from several disciplines undermines the concept of a stable climax association that perpetuates itself indefinitely. Some so-called climax communities depend on disturbances, including natural or anthropogenic burning. Some ecosystems have multiple stable states, and some reach irreversible tipping points, which are becoming more likely because of climate change. Viewing nature as in flux rather than in balance addresses these issues, while the current scale of environmental change infuses a sense of urgency into these discussions.

People are part of ecosystems. Even places that are sometimes considered pristine have been influenced by the activities of people for millennia, and the local and traditional knowledge of those peoples contains important insights. Conservation that strives to maintain or restore biological diversity and cultural diversity, which often have similar geographical distributions, takes many forms. People live and pursue their traditional livelihoods in Indigenous reserves, extractive reserves, and biosphere reserves. Collaborations between Western science and parataxonomists or other local experts combine the insights and skills of people trained in different cultures. Some programs strive to create economic incentives for local participation in conservation through ecotourism, bioprospecting, or harvests of nontimber forest products. Co-management of resources has resulted from some cases in which Indigenous or local people took initiative in managing their resources, challenged Western wildlife management, or formed cooperatives to promote sustainable use and equitable working conditions. The success of these endeavors depends on economic, social, political, ecological, and historical conditions.

Economic, technological, and demographic changes after World War II created new pressures on species, habitats, and human environments. Concerns about human impacts on the environment mounted. Rachel Carson, Charles Elton, Barry Commoner, and others articulated concerns about pesticides and other harmful substances in air and water, introduced species, escalating extinction rates, and the modification and fragmentation of habitats. Ecologists, economists, and philosophers like Paul Ehrlich, Garrett Hardin, John Kenneth Galbraith, Lynne White, Norman Myers, and Arne Naess attributed these problems to varied causes, including population growth, tragedies of the commons, excessive resource consumption and disparities in consumption, militarism, the misuse of science and technology, externalities, and anthropocentrism. In response to these developments and to increasing awareness of the limitations of utilitarian conservation, a preservationist approach to conservation that seeks to protect species and regardless of their economic value became prominent.

Darwin and Wallace proposed that natural selection is the process responsible for the evolution of adaptive features. Mutations provide the raw material of evolution. Interacting species influence each other’s evolution through coevolution. Evolution offers insight into many past and current controversies including Proximate (extrinsic) and ultimate (intrinsic) factors influence species’ vulnerability to extinction. Going through a bottleneck results in low genetic diversity and the high risk of becoming extinct due to inbreeding, catastrophes, and fluctuations in birth and death rates and the sex ratio. The theory of island biogeography states that extinction risk is high in small, isolated populations. Understanding evolution has practical implications for managing the evolution of resistance to pesticides, problems from hybridization, and populations at risk of extinction.

The IUCN and many other organizations and governments maintain databases tracking the status of species. Legislation and international agreements regulate immediate causes of rarity such as exploitation and habitat loss. Preservationist species conservation aims to minimize proximate causes of mortality with measures such as reducing insect herbivory on young Pitcher’s thistles or nest parasitism by cowbirds on Kirtland’s warblers. Intrinsic limits on population growth are managed through interventions such as headstarting marine turtles, replacement clutching and cross-fostering whooping cranes and brush-tailed rock wallabies, removing raptor chicks at risk of fratricide, as well as genetic management combined with captive breeding, reintroduction, and translocation of red kites, brown bears, and California condors. Genetic management of rare populations strives to minimize problems from both inbreeding and genetic swamping. Designations of species as indicators, umbrellas, keystones, or flagships are used to prioritize species for protection and intervention.

Utilitarian conservation attempts to minimize or eliminate populations of species that are viewed as pests because they cause economic damage or threaten the health and safety of people or domesticated organisms. People disagree about what constitutes a pest. Pigeons, English sparrows, wolves, tigers, and many other organisms inspire strong positive and negative feelings. Pest control is most acceptable if it is non-lethal and has little effect on non-target species. Integrated pest management strives to use biological pest control, habitat manipulation, and education to maximize the efficiency of pest control. Historically, predators were targeted by government control programs including bounties, but Aldo Leopold changed his attitude about wolves in the influential essay Thinking Like a Mountain. Human behavior toward dangerous animals such as bears may exacerbate pest problems.

Information about the natural world comes from many sources. In controlled experiments, the responses of similar groups to a treatment are compared, and differences in the responses suggest that the treatment may have had an effect. Where controlled experiments would be impossible or unethical studies that compare conditions in two or more similar situations that differ in place or time may be appropriate. Statistical analysis allows investigators to evaluate the probability that observed results are due to chance. Historical records, natural records such as fossils, oral traditions, traditional ecological knowledge, and observations from citizen scientists and parataxonomists are also important. Researchers often develop models to predict how a system behaves under specified conditions. This is useful when a system, such as the Earth’s climate, cannot be observed directly. Science provides a framework within which results can be compared to predictions and conclusions can be modified as new evidence becomes available. Arguments and information about the natural world should be evaluated critically for misleading statements and potential bias.

By the late twentieth century, changing social, economic, and political conditions along with new scientific insights and trends in ethics and philosophy presented challenges not fully addressed by utilitarian and preservationist conservation. Indigenous rights activists, advocates for animal rights and the rights of nature, ecofeminists, scholars in the social sciences and humanities, legal experts, and representatives of nongovernmental organizations, national governments, and international development agencies offered diverse perspectives and agendas. Many disputed the idea that people are not part of nature, while others suggested that Indigenous peoples should be considered guardians of nature. Some promoted sustainable development along with attention to the social, political, and cultural consequences of conservation, particularly for the survival of threatened cultures and marginalized groups that have often been displaced by reserves. These developments led to the emergence of a stewardship approach to conservation that sustains complex ecosystems characterized by ecological and cultural diversity.

Utilitarian managers regulate harvests and manipulate habitats for wild species of interest. Wild products can be legally harvested for recreation, commerce, or subsistence under certain circumstances. Illegal traffic in wild plants, animals, or their products for commerce threatens many species. In populations that are regulated by density-dependent processes near carrying capacity, sustained yield can be maximized if harvests take place when populations are near half the environment’s carrying capacity. Harvests may be sustainable if mortality from the harvest is compensatory. Habitats can be modified by changing the arrangement and structure of habitat components to provide cover, by conserving soil, and by modifying succession, for instance, with the use of fire. The amount of edge or interspersion between different habitats is an important component of habitat quality.