The ecological factors controlling zonation are complex and involve both 

 physical and biological mechanisms. Interactions between dominant rocky shore 

 species have recently been studied in Maine by Menge (1976) and Grant (1977). 



Physical factors are especially important in the upper intertidal zone. 

 Barnacles, which dominate the high intertidal zone of rocky shores, are 

 controlled only by competition with each other and by environmental 

 conditions, such as desiccation and wave action (Menge 1976). Recruitment is 

 greatest in open areas that stay moist at low tide. Dense growths of the 

 macroalgae Ascophyllum and Fucus inhibit barnacle settlement and survival 

 (Menge 1976 and Grant 1977). 



In the rockweed zone (lower in the intertidal zone) , interspecific competition 

 becomes important. On exposed shores the mussel always competes for space 

 more successfully than the barnacle, although less quickly so on vertical 

 surfaces than on horizontal surfaces (Menge 1976). The upper limit of the 

 barnacle zone is set by a physical factor - desiccation. At slightly less- 

 exposed sites, predation by the whelk, Thais lapillus , moderates the 

 population growth of both mussels and barnacles and allows them to coexist. 

 At very exposed sites the lower limit of the barnacle zone is controlled by 

 competition with the mussel, which can compete more successfully than the 

 barnacle but is less tolerant to exposure to the atmosphere, and therefore 

 cannot penetrate to so high a level in the intertidal zone as the barnacle. 

 This pattern is modified by predation on the mussel by the whelk, which 

 prevents the absolute dominance of the mussel. The intertidal distribution of 

 other species is undoubtedly controlled by similar mechanisms. 



In early spring, filamentous algae ( Ulothrix , Urospora , and Bangia ) cover 

 available space on rocky shores. Barnacles settle in May and June in space 

 remaining after a die-off of the algae. High settlement density of barnacles 

 is correlated with early mortality. If barnacles are as dense as 60/cm , 90% 

 mortality can be expected in 5 months. In summer, foliose algae Fucus 

 establishes itself in any remaining free space. Mussels settle from June to 

 September in areas occupied by barnacles, Fucus, or other mussels (Grant 

 1977). 



By August through October, the mussels dominate the barnacles on the more 

 horizontal surfaces of the midintertidal zone (Menge 1976). Clearing of space 

 occurs in the fall and winter and is density dependent (i.e., the hummocks of 

 elongated and weakly-attached barnacles produced by crowding and mats of 

 mussels are prone to removal by winter surf) . Space that is cleared in fall 

 and winter remains clear until settlement by filamentous algae in the spring. 



Normandeau Associates (1975) report a high degree of variation of biomass 

 seasonally at intertidal sites on Sears Island in Penobscot Bay. This 

 variation is a function of the presence and density of mussels. 



NWI data indicate that boulder beaches (boulder rock shores) comprise about 6% 

 of the intertidal area of the characterization area. Boulder beaches are 

 limited to relatively short expanses and are usually adjacent to or associated 

 with bedrock shores. Boulder shorelines provide relatively stable surface for 

 attachment. Shelter is present between and under boulders and the sediment 

 between and under the boulders provides habitat space for infaunal species. 



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