The density of predators and grazers can be subject to wide natural variations. The fluctuations of the northern sea urchin, Strongylocentrotus droebachiensis, have been well documented (Hagen, 1995). The endoparasitic nematode Echinomermella matsi appears important in mediating these fluctuations (Skadsheim et al., 1995; Hagen, 1996). The major role which grazers can play in controlling infralittoral algal populations has been discussed. A grazer reduction can allow the lower depth limit of canopy algae to extend downwards, though only to the levels which they are naturally capable of reaching, which will not impact on the main body of the circalittoral zone. Predators such as Asterias are also suspected to have importance in structuring communities, but less is known of their fluctuations in number. More needs to be known concerning the dynamics of predator and grazer numbers in CFT communities - very little is known of the factors which regulate their numbers. Until this understanding is available there will be substantial difficulties in separating natural variability from anthropogenic impacts.

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