It is often stated that the upper limits of organisms are set by physical factors, whereas the lower limits are set by biological interactions but there are many exceptions to this rule. The specific causes of the zonation seen on most rocky shorelines vary with geographic location, but zona-tion results primarily from behavior of larvae and adults, tolerance to physiological stress, the effects of consumers, and the interplay between production and the presence of neighbors.
Adult movements and larval behavior during settlement from the plankton onto rocky shores have major effects on the distribution of animals. For example, studies of barnacles have shown that vertical zonation of larvae in the water column contributes to corresponding vertical zonations of both larval settlement and adults on the shore, a pattern previously ascribed solely to interspecific competition. For seaweeds, behavior is a relatively unimportant cause of their zonation since adult seaweeds are sessile and settling spores are mostly passively transported.
Marine organisms living higher on the shore are faced with more frequent and extreme physiological challenges than their lower shore counterparts, and the upper limits of intertidal distributions for most species are set by cellular dehydration. Dehydration can occur either from freezing during winter or simply desiccation associated with long emersion times. High temperatures and wind, which accelerate the rate of water loss from tissues, exacerbate the effects of desiccation.
Primary and secondary production by sessile organisms can be limited at higher tidal elevations because nutrients and other resources can be acquired only when immersed. Respiration rates of seaweeds and invertebrates are temperature dependent and thus can be greater when an organism is exposed at low tide. For seaweeds, prolonged exposure to dehydration also reduces photosynthesis.
The reduced productivity associated with increased exposure at higher tidal elevations modifies intra- and interspecific interactions. For instance, competition between seaweeds, which may be intense lower on the shore, is reduced at higher tidal elevations and enables coexistence. Competition among intertidal seaweeds is hierarchical with lower shore species dominating those of the higher shore. Thus, fucoid species of the mid intertidal zone are outcompeted for space in the low zone by foliose red seaweeds that pre-empt space with an encrusting perennial holdfast. There is also a competitive hierarchy among mid intertidal zone fucoids with those typically occurring lower on the shore competitively dominant to those higher up. This is most apparent on European rocky shores where the diversity of intertidal fucoids is greatest.
Grazing rates tend to be greater lower on the shore, although there are cases of herbivory by insects setting the upper limits of ephemeral green algae. Grazing by sea urchins at the interface with the sub-littoral zone can limit the lower distributions of macroalgae, but there is little evidence for grazing on perennial seaweeds setting the lower limits of those taxa within the intertidal zone. Grazing of perennial seaweeds is most intense at the sporeling stage soon after settlement. Grazing by gastropods and small crustaceans certainly contributes to losses of biomass of established individuals, but does not affect distributions within the intertidal zone. In contrast, the grazing of established ephemeral species both on emergent rock and tidepools is intense during spring and summer in many regions eventually eliminating those algae from their respective habitats. There are also many examples of consumers using seaweeds as habitat as well as food.
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