Theory predicts that mutualistic associations should be most favored under conditions where risk ofmortality from abiotic stress or predation is high, or in nutrient-poor environments where the symbiosis provides the host with nutritional benefits. Under such conditions association can provide escape from mortality or starvation that outweighs the costs imposed by competition for resources with the associate. These predictions are generally consistent with evidence from the distribution of mutualistic and commensal associations. For example, predation pressure is generally higher, and nutrient availability is generally lower, in tropical than in temperate marine waters. Accordingly, symbiosis with algae is much more common in tropical corals than in temperate species, and protective commensalism with sedentary invertebrates is much more common among tropical than temperate shrimp. Protective mutualisms between plants and ants also occur primarily in the tropics. On a more local scale, positive associations among plant species are best developed in abio-tically stressful habitats. In salt marshes of northeastern North America, for example, the dominant plant Spartina alterniflora stabilizes the shoreline, allowing numerous other plant species to colonize and thrive where they could otherwise not establish in the shifting substratum.
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