The amelioration hypothesis maintains that aggregations of foundation species (large-bodied, abundant species that create a habitat for smaller species, e.g., massive corals and their associated reef species) increase their abundance and expand their range into areas of greater predation or physical stresses through positive effects of aggregation. The experimental evidence for this hypothesis was provided by studies of salt marsh mussels, Geukensia demissa. Similar to the mussel bed model described above, risk of predation on the mussels by fishes and crabs increases toward lower shore levels, but large, older adult mussels are resistant to predation and shield the vulnerable juveniles and small adults under their aggregations. As the small mussels grow the aggregation expands, collecting new recruits, and continuing the expansion until a bound ary is achieved at lower shore levels than would have occurred without the cooperative interaction. The physical matrix of mussels provides habitat for many smaller invertebrates that would not occur without the mussels. The concept of positive feedbacks associated with aggregations now extends to many terrestrial and marine foundation species.
See also: Rocky Intertidal Zone; Suspension Feeders;
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