CHAPTER 4. ECOLOGICAL PATTERNS AND PROCESSES 



This chapter discusses the ecological 

 processes that determine the distribution 

 and abundance of flora and fauna 

 associated with rubble structures in the 

 South Atlantic Bight. In general, we 

 restrict our attention here to waters of 

 relatively high salinity, characterized by 

 predominately marine organisms. In 

 several cases, the processes organizing 

 these communities have not been 

 intensively studied on rubble structures, 

 and we must, therefore, infer their 

 importance from studies conducted in other 

 similar habitats. In this chapter, we 

 outline our general conceptual framework 

 and then concentrate on the organization 

 of intertidal communities, sunlit, 

 subtidal communities (which tend to be 

 dominated by seaweeds), and shaded, 

 subtidal communities (which tend to be 

 dominated by benthic animals). We also 

 discuss the effects of indirect, complex 

 interactions among organisms since these 

 interactions are probably more important 

 than has been generally appreciated. 



4.1 CONCEPTUAL FRAMEWORK 



In addition to the seasonal 

 temperature effects discussed previously, 

 the two major physical gradients affecting 

 the plant and animal communities of rubble 

 structures are tidal level and 

 availability of sunlight. 



The animal communities in particular 

 are well segregated by tidal level for 

 reasons discussed by Jackson (1977). In 

 the intertidal zone solitary sessile 

 animals dominate. These animals usually 

 possess hard external coverings (e.g., 

 shells) which confer superior resistance 

 to the harsher physical conditions 

 experienced when exposed during low tide. 

 Subtidal ly, colonial animals are more 



abundant because they have indeterminate, 

 vegetative growth and are less susceptible 

 to overgrowth and grazing. Solitary 

 animals survive subtidally through 

 morphological or behavioral attributes 

 (large size, aggregative behavior) which 

 protect them in competition with colonial 

 animals and from subtidal predators. 



Jackson (1977) has also argued that 

 when light is sufficient and all else is 

 equal, colonial animals lose in 

 competition with plants. This is because 

 many plants, like colonial animals, have 

 characteristics that are important in 

 competition (e.g., asexual reproduction 

 and indeterminate growth). Additionally, 

 with adequate light, plants have a further 

 advantage in that they can 

 photosynthetical ly derive energy for 

 growth and reproduction. In contrast, 

 colonial animals must depend on external 

 food sources. Therefore, seaweeds 

 generally dominate shallow sunlit 

 habitats, and colonial animals generally 

 dominate deeper, darker water and shaded 

 habitats beneath docks and bridges. 



Wave action appears less important 

 than tidal level and sunlight, but some 

 organisms, for example the oyster 

 Crassostrea virqinica , are intolerant of 

 the higher wave action of the open coast 

 (Ortega 1981). 



4.2 ORGANIZATION OF INTERTIDAL COMMUNITIES 



In spite of 

 experimental studies, 

 the organization of 

 community on rubble 

 processes similar to 

 other rocky intertidal 

 (Connell 1972; Menge 1976) 



the paucity of 



it is likely that 



the invertebrate 



structures is by 



those operating in 



communities 



On exposed 



shores, the upper limits of blue-green 



36 



