Rocky intertidal shores often display a vertical zonation of fauna and flora associated with the strong environmental gradient produced by the rise and fall of the tides. For example, most moderately exposed rocky shores of the northern hemisphere have kelps at the littoral sublittoral interface, followed by rhodophyte algae dominating the low intertidal zone, by fucoid algae, mussels, and barnacles dominating the mid-intertidal zone, and by cyanobacteria, lichens, and a variety of small tufted, encrusting, or filamentous ephemeral seaweeds occurring in the high intertidal zone. While species from many phyla may be found together, often a single species or group is so common; vertical zones are named according to the dominant group (e.g. the intertidal balanoid zone named after barnacles in the family Balanidae).
Combinations of various physical factors acting upon different inhabitants in intertidal zones that vary in their exposure to waves can lead to complex patterns of distribution and abundance along shorelines in a particular region. Nevertheless, some general patterns are evident at a regional scale. Geographically, vertical zonation patterns are most pronounced on temperate rocky shores where species diversity is high and tidal amplitudes tend to be greatest. On rocky shores in the tropics, biotic zones are compressed into narrow vertical bands because of small tidal amplitudes. In polar regions, annual ice scour and low species diversity tend to obscure any conspicuous vertical zonation.
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