Island ecology is the study of island organisms and their interactions with each other and the environment. Islands account for nearly 1/6 of earth's total land area, yet the ecology of island ecosystems is vastly different from that of mainland communities. Their isolation and high availability of empty niches lead to increased speciation. As a result, island ecosystems comprise 30% of the world's biodiversity hotspots, 50% of marine tropical diversity, and some of the most unusual and rare species. Many species still remain unknown.

The diversity of species on islands is highly impacted by human activities such as deforestation and introduction of the exotic species. In response, ecologists and managers are directing attention towards conservation and restoration of island species. Because they are simple systems, islands provide an opportunity to study processes of extinction that can be extrapolated to larger ecosystems.

Islands are attractive sites for ecological research because they provide clear examples of evolution in action. They show interesting patterns of colonization, adaptation, and speciation.

Islands are surrounded by water, and may or may not exist as part of a continental land mass. Oceanic islands arise due to volcanic activity or reef growth, and usually subside over time due to erosion and changing sea levels. When islands emerge, they undergo the process of ecological succession as species colonize the island (see theory of island biogeography). New species cannot immigrate via land, and instead must arrive via air, water, or wind. As a result, organisms with high dispersal capabilities, such as plants and birds, are much more common on islands than are poorly dispersing taxa like mammals. However, some mammals are present on islands, presumably from swimming or riding on natural "rafts" that are washed away from the mainland.

Of the species that arrive, only some will be able to survive and establish populations. As a result, islands have fewer species than mainland habitats. Island populations are small and exhibit low genetic variability (see founder effect), but are isolated from the predators and competitors that they initially evolved with. This can lead to a process called ecological release, where a species is released from its ancestral community interactions and then colonizes new niches.

In response to these changing ecological pressures, island species can become much more docile than their mainland counterparts, and may grow larger (see island gigantism) or smaller (see island dwarfism). Some of these unique adaptations are reflected in charismatic island species such as the Malagasy hippopotamus, Komodo dragon, or pygmy mammoths. Although, the giant tortoises of the Galápagos Islands and the Seychelles (the Galápagos tortoise and Aldabrachelys respectively) are sometimes given as examples of insular gigantism, they are now thought to represent the last remaining populations of historically widespread giant tortoises i.e. gigantism is an ancestral trait that occurred in the absence of insular selection pressures. The collection of differences in morphology, ecology, physiology and behaviour of insular species compared to their continental counterparts is termed Island syndrome.

After immigration, birds, and some reptiles or mammals, tend to become larger and predatory, showing relaxed intraspecific competition. For mammals, small species will increase in size and large species will decrease in size. This is referred to as the "island rule," and is suggested to minimize energy expenditure.

Other adaptations to life on islands include increased poikilothermy, relaxed anti-predator behavior, and reduced sexual selection in animals, and loss of herbivore defenses and reduced dispersal in plants.