In the northeastern Pacific, intertidal zones of the most wave-beaten shores receive more energy from breaking waves than from the sun. Despite severe mortality from winter storms, communities at some wave-beaten sites produce an extraordinary quantity of dry matter per unit area of shore per year. At wave-beaten sites of Tatoosh Island, WA, sea palms, Postelsia palmaeformis, can produce >10 kg of dry matter, or 1.5 x 108 J. per m2 in a good year. Extraordinarily productive organisms such as Postelsia are restricted to wavebeaten sites. Intertidal organisms cannot transform wave energy into chemical energy, as photosynthetic plants transform solar energy, nor can intertidal organisms "harness" wave energy. Nonetheless, wave energy enhances the productivity of intertidal organisms. On exposed shores, waves increase the capacity of resident algae to acquire nutrients and use sunlight, augment the competitive ability of productive organisms, and protect intertidal residents by knocking away their enemies or preventing them from feeding.

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The intertidal mussels Mytilus edulis and M. californianus compete for space on the west coast of North America. An analysis of differences in size, growth rate, age at first reproduction, life span, mode of reproduction, niche and habitats between these two species demonstrates that their life history strategies are different in several important ways. M. californianus is a larger, sturdier, slower growing mussel which has effective predator-deterring mechanisms and is an overall superior competitor for space in the intertidal. Its reproductive strategy is based on continually spawning at a very low level throughout a yearly cycle. M. edulis is a classic fugitive species which rarely attains a large size but matures early and is characterized by a single massive reproductive output each year. The differences noted in their life history strategies are very likely the result of either a) a "pre-adaptation" for coexistence, or b) a competitively-induced regional evolutionary divergence.In Washington a band of small M. edulis (removed from most predation and competition) exists in a high intertidal spatial refuge, and contributes relatively little to the gene pool. Larger individuals exist in disturbed areas lower in the intertidal zone. These lower zones represent a relatively high risk, and usually only temporary, habitat for M. edulis because of mortality due to competition and predation. Individuals, however, colonizing these two intertidal patches contribute 43 times as much as their high intertidal counterparts in terms of reproductive fitness (calculated on a populationwide basis).

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