A native species is an organism that is living in an area for entirely natural reasons, with no human intervention involved. This may be because the organism evolved in that environment or because it migrated by a variety of means. By contrast, a non-native species is introduced by humans and may become invasive, taking over the natural environment and choking out native species. Invasive species often develop quickly and spread aggressively, making it difficult for natives to compete.

One reason for a species to be native to a region is because it evolved there. This is seen most commonly on islands, where comparative isolation allows unique species to develop over extended periods of time. Islands are subsequently very vulnerable because of their distinctive ecologies. When island environments are disrupted, the native species found nowhere else may rapidly go extinct.

For plants, transportation of plants and seedlings can occur on wind currents, on the bodies and in the bellies of animals in their natural range, and as a result of putting out shoots and runners. As plants evolve to fit an evolutionary niche, they will spread in other areas where that niche remains unfilled. Plants can end up covering very large territories as they are helped along by birds, mammals, and other animals.

Animals spread natively as their populations increase and they range further in search of territory. Native animals may follow seasonal migrational patterns or may relocate their populations periodically in response to various pressures, natural curiosity, or changing landscapes. Some animals may travel vast distances in search of new territory. Birds and some insect species are particularly famous for their lengthy migrations.

Over time, a native species usually evolves to perfectly fit in the environment where it has settled. Subspecies that have refined themselves to take advantage of subtle variations in the environment can also develop. Some of these organisms can become invasive when relocated and conversely, can be vulnerable to invasive species in their natural environments.

Humans have been manipulating plant and animal species for thousands of years, whether they are domesticating grains or using iceplant for ballast and discarding it on foreign shores. Recognition of the importance of native species and unique environments has led to the development of a number of organizations that promote native plants and animals. These groups may encourage people to use natives in their gardens, participate in eradication campaigns for invasive species, and educate people about the risks to native animal species posed by imported livestock and pets.