Associations between species vary from fleeting and casual to mutually obligatory and coevolved (Figure 1). At the casual end of this spectrum, associations may arise with little or no interaction, simply because species have similar environmental requirements and tolerances, a view championed by Gleason in his individualistic concept of plant associations. A variety of essentially fortuitous associations among species can enhance fitness of one or both parties. For example, among both terrestrial plants and seaweeds, toxic or otherwise herbivore-resistant species provide small-scale refuges for the germination and growth of species that would otherwise be eaten. Although both species have equal or greater fitness when living alone in appropriate environments, such associational defenses broaden the range of conditions under which the edible species can thrive, and often increase diversity in the immediate vicinity of the defended plant. Among animals, casual associations form when they enhance food availability or reduce risk of predation or parasitism. For example, foraging in mixed-species flocks can facilitate higher food intake by individual birds as activities of one species flush out prey, or open up tough foods (nuts, vertebrate carcasses) that are then available to others. A more specialized association occurs between oxpecker birds and African ungulates. By removing ticks from their hosts, oxpeckers get a steady source of food, while the host gets relief from parasitism. A similar relationship holds among large coral-reef fish and the cleaner shrimps and fishes that feed on their parasites and necrotic tissues.
More persistent and regular interactions among species can lead to the evolution of specialized associations, often involving adaptations in one or both of the interacting species. A conspicuous example, noted by Darwin, is the coevolution of tubular flowers with hummingbirds and long-tongued insects. At the far end of the gradient of intimacy is symbiosis ('living together') between species that live in close association for most or all of their lives. Symbiosis may be facultative, as in the case of lichens whose algal and fungal components can each live alone, or obligate, as in the case of reef-building corals that cannot survive long without their symbiotic algae. The most extreme case of symbiosis is the eukaryotic cell itself, which is the product of an ancient association among formerly free-living prokaryotes that have become fully integrated physiologically and genetically into a single organism.
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