Species interactions 



Resources 



Light 



Nutrients 



Space 



may be allected b\ 



Competition 



I.For resources 



2. Interference 



3. Pre-emption of space 



Grazing 



Life history 

 features 



expressed through 



L 4' 



Recruitment 

 Growth 

 Reproduction 

 Survival 



Other factors 



also affected by 



Temperature 

 Water motion 



Figure 26. Summary of important features of kelp forest dynamics and the temporal and 

 spatial scales at which they occur. 



scale of a few years at most. The large 

 prominent surface canopy of Macrocystis 

 pyrifera has tended to obscure the 

 variability of the communities it 

 dominates at local sites. Kelp forests, 

 however, may be as variable within areas 

 as they are among different areas along a 

 coast (Rosenthal et al . 1974, Foster et 

 al. 1983; see Chapter 3). Experimental 

 studies of the factors producing 

 variability between and within sites are 

 possible on relatively restricted spatial 

 and temporal scales. 



Finally, an examination of 

 experimental studies involving kelp 

 abundances and distribution shows that in 

 most cases only one factor has been 

 assessed, in one place, at one time. 

 Factorial experiments are necessary to 

 test the relative importance of factors, 

 their interactions, and unaccounted for 

 variability. Many more areas will have to 

 be examined before any generalizations 

 assume validity. In addition, many 

 studies suffer from pseudorepl ication 

 (inferential statistical testing using 

 subsamples from within one "experimental" 

 plot and one "control" plot; Hurlbert 



1984). Replication of treatments and 

 sites within each area is a powerful means 

 of assessing local variability (Hurlbert 

 1984). We do not go into the details of 

 experimental design and sampling, but 

 suggest Underwood (1981), Hurlbert (1984) 

 and Green (1979) for good discussions of 

 these. 



5.2 EFFECTS OF TEMPERATURE 



General hypothesis: The distri- 

 butional range of Macrocystis is limited 

 by its inability to survive higher temper- 

 atures (ca. 20 °C) and by the inability of 

 gametophytes to become fertile at lower 

 temperatures (ca. 5 °C). 



The most common explanation for 

 limits to the geographical distributions 

 of large algal species is their suscepti- 

 bility to temperature. Susceptibilities 

 can occur at several stages in the life 

 histories of plants. The topic has been 

 extensively reviewed by Van den Hoek 

 (1982) and is discussed in Section 2.3. 

 This reference states that at least six 

 different boundaries can be postulated: 



91 



