As with previously discussed aspects 

 of experimental work in algal forests, the 

 interactions of physical and biological 

 factors are important determinants of 

 spatial and temporal heterogeneity within 

 a depth stratum, yet studies have 

 previously been constrained to looking at 

 only main effects. Large physical 

 disturbances such as severe winter storms 

 can differentially affect kelp species 

 with long-term consequences for community 

 composition (Dayton and Tegner 1984b, 

 Ebeling et al. MS., Schiel and Foster in 

 prep.). There are many important factors 

 which have not been tested, and which 

 could be important in many of the studies 

 mentioned in this chapter (see Figure 24). 

 For example, the abundance of echinoids 

 and grazing gastropods can be positively 

 correlated (Ayling 1981, Simenstad et al. 

 1978) and may be synergistic in some of 

 their grazing activities. Alteration of 

 kelp canopies and the abundances of 

 fleshy, encrusting or articulated red 

 algae not only alters the available space 

 and the irradiance levels, but also the 

 local grazing regimes, particularly small 

 echinoids and herbivorous micro- 

 invertebrates. The size of kelp stands 

 could also be important in determining 

 community structure (c.f., Dayton and 

 Tegner 1984a), although there is little 

 experimental evidence for this. Suitably 

 designed field experiments testing 

 specific hypotheses offer a promising 

 approach to addressing these problems and 

 removing the answers from the equivocal 

 realm of anecdote, conjecture, and 

 correlation. 



5.7 INDIRECT EFFECTS: SEA OTTER FORAGING 



importance of sea otter-sea urchin-kelp 

 relationships is based on three types of 

 evidence: (1) When large numbers of sea 

 urchins are present in an area, kelp 

 abundance may be low (see references in 

 previous section). (2) Sea otters 

 preferentially feed on large sea urchins 

 when urchins are available. In areas 

 where otters are abundant, sea urchins 

 tend to be scarce and small in size (Estes 

 et al. 1978, Breen et al. 1982, Van 

 Blaricom in press). (3) Historical 

 evidence indicates that sea otters were 

 once abundant along the west coast of 

 North America and were important predators 

 in kelp forests (Estes and Van Blaricom in 

 press) . 



Estes et al. (1978) examined the 

 distribution and abundances of kelp and 

 sea urchins at different sites in the 

 Bering Sea, some of which had populations 

 of sea otters. Strongylocentrotus 

 polyacanthus were particularly abundant 

 only in the site without otters, where 

 there was little macroalgae. Sea urchins 

 were larger at this site when compared to 

 the site where otters actively foraged. 

 Estes et al. (1981) found that in recently 

 repopulated areas of the Aleutian Islands, 

 sea otter diets consisted mainly of sea 

 urchins, whereas epibenthic fish were the 

 most important prey to an established 

 otter population. Duggins (1980) provided 

 experimental information from Alaska 

 showing that when dense aggregations of 

 echinoids are removed, a lush algal flora 

 may develop. At the sites examined in 

 Alaska, the evidence is that otters may be 

 a "keystone species" (c.f. Paine 1966, 

 Estes et al. 1978) in shallow communities. 



General Hypothesis: 

 enhance kelp abundance by 

 urchins, the major grazers. 



Sea otters 

 removing sea 



As already discussed in Section 

 4.6.2.2, sea otters ( Enhydra lutris ) 

 consume up to a fourth of their body 

 weight in food per day, feeding on a wide 

 range of invertebrate species. Attempts 

 at experimentally assessing their effects 

 on nearshore communities have been 

 hampered by the obvious logistic 

 constraints of dealing with a mobile 

 predator and the use of "natural 

 experiments," that is, comparing areas 

 with and without sea otters. The 



The evidence is not so clear for the 

 population of sea otters in California. 

 Van Blaricom (in press) reviewed the 

 literature concerning the recent expansion 

 of the range of sea otters along the 

 central coast of California. Dense sea 

 urchin populations were reduced along the 

 Monterey Peninsula, leaving generally 

 small, concealed individuals (McLean 1962, 

 Lowry and Pearse 1973). Indeed, there are 

 no examples of dense aggregations of sea 

 urchins persisting where otters are 

 present. Van Blaricom noted that 

 Nereocystis , an annual plant, tends to 

 persist in the presence of sea urchins 

 while Macrocystis does not. From recent 



106 



