A second gradient of association involves the symmetry of benefits to the interacting species (Figure 1). Associations range from parasitism, in which one party benefits at the expense of the other through mutualism, in which both parties benefit. Between these extremes lies com-mensalism, in which one party benefits while the other is unaffected.
Parasites include not only microbes and intestinal worms but also plant-feeding insects, which are estimated to make up more than half of all animal species. The most familiar parasites associate closely and permanently with their hosts, and are strongly modified to this end. But others have a less intimate association. Among the more bizarre such parasites are certain cichlid fish of the African Great Lakes that feed solely on the scales or eyes of other fish, which they obtain by surreptitiously attacking and sucking from the living victim.
Many associations in nature are commensal, benefiting one party with little or no impact on the other. Commensalism commonly involves a larger host species and a much smaller guest species that exploits the host's organic products or structure. Examples include many animals that live on plants or corals, as well as certain microbes that associate with the human gut or skin with no appreciable effect on the host's fitness. Sessile organisms, such as trees, kelps, or corals, are referred to as foundation species when their dominance of a habitat provides physical structure and environmental conditions that support many other plant and animal species.
Mutualistic associations, like parasitic ones, take a variety of forms, from casual to intimate and obligate. The foraging mutualisms among terrestrial vertebrates mentioned above represent the casual end. Mutualisms based on protection from enemies are common in tropical ecosystems. For example, certain crabs and shrimp live only on corals, protected by the host's stony branches and feeding on its secretions; in return, the crustaceans attack predatory starfish that attempt to eat their coral hosts. The most familiar and ecologically important non-symbiotic mutualisms are those between flowering plants and the insects and other animals that pollinate them. Although not symbiotic, many plant-pollinator associations are highly specialized and coevolved, and have profound effects on terrestrial ecosystem structure and functioning.
Finally, the most highly developed mutualisms are intimate symbiotic associations between plant or animal hosts and microbes that live within the host's body and provide it with novel physiological capacity. These include the associations between plants and their mycor-rhyzal fungi, certain insect groups and their mycetomes, corals and their algae (zooxanthellae), giant tube-worms of hydrothermal vents and their chemosynthetic bacteria, and the complex microbial ecosystems housed within the gut of ruminant ungulates.
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