CHAPTER 5 



HYPOTHESES CONCERNING COMMUNITY 

 STRUCTURE AND DYNAMICS 



The ecosystem must first be accurately described using proper 

 methods of sampling and measuring the relevant environmental 

 variables. Once this is done, the ecologist looks for patterns of 

 correlation between the distribution and abundance of the dif- 

 ferent organisms and their abiotic environment. By asking ques- 

 tions about the causes of the patterns, answers suggest 

 themselves which can be formulated as testable hypotheses. 

 Then experiments can be designed to test these hypotheses. 

 Connell (1974). 



5.1 INTRODUCTION 



Previous chapters have been primarily 

 descriptive, discussing various biotic and 

 abiotic factors associated with the 

 distribution and abundance of Macrocystis 

 and other organisms in giant kelp forests. 

 The availability of light, nutrients, and 

 suitable substrata, temperatures and water 

 motion put broad limits on where kelp 

 forests might occur, but they reveal 

 little about the mechanisms and 

 interactions which structure these types 

 of communities. In many cases, subtidal 

 workers have substituted a rubber diving 

 suit for the tweed coat and gumboots of 

 the early intertidal natural historian, 

 attempting to describe habitats and the 

 organisms present in terms of associations 

 and observed patterns. Only in relatively 

 recent times has the technology of SCUBA 

 diving allowed researchers to explore 

 subtidal communities in a routine fashion, 

 permitting them to identify the factors 

 and patterns that form the basis of 

 experimentally testable hypotheses. 

 Consequently, there are only a few 

 published accounts of studies that have 

 tested for the processes that affect the 



distribution 

 species, and 

 interactions 



and abundances 

 fewer still that 

 of factors. 



of kelp 

 assess the 



Putatively important processes such 

 as competition and grazing (= predation of 

 plants by animals) may have strong effects 

 in some places, at some times. The 

 interactions between kelps and other 

 organisms may have different effects at 

 various stages of the life cycles of 

 plants (Figure 25). The abundances of 

 individual species change along a depth 

 gradient, which may affect the intensity 

 of interactions. Analyses of many of 

 these problems are now tractable 

 experimental ly. 



A critical examination of the methods 

 used to assess the important processes 

 that structure communities is not unique. 

 Currently, there is much debate about the 

 importance of competition in terrestrial 

 and marine intertidal communities: when 

 and where is competition important, or 

 does it even occur at all in many 

 situations (Connell 1983; Simberloff 1983; 

 see American Naturalist 1983, 122(5))? 

 When and where is predation important? 

 Methodological problems have been 



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