differences along coastlines occupied by 

 kelp. A better knowledge of these 

 distributional scales would provide a 

 context for assessing the general 

 importance of grazing by echinoids. 



Within sites where sea urchins are 

 abundant, their effects have been 

 generally documented in three categories: 

 (1) wholesale removal of algae; (2) the 

 alteration of species diversity via 

 feeding preferences and selective removal 

 of algal species; and (3) the provision of 

 cleared primary substratum suitable for 

 kelp recruitment. We will discuss these 

 below. 



It is commonly observed worldwide 

 that dense aggregations of sea urchins may 

 remove large tracts of algae, creating 

 so-called "barren grounds" (see Lawrence 

 1975 for review). After the dense 

 vanguard of sea urchins has passed, their 

 densities may decline, but may remain high 

 enough to prevent successful kelp 

 recruitment for many years in particular 

 depth strata (Chapman 1981, Andrew and 

 Choat 1982, Breitburg 1984). Thus, large 

 persistent patches without kelp may occur 

 in areas where sea urchins are abundant. 

 These areas devoid of large brown algae 

 often support a high cover of encrusting 

 organisms (Ayling 1981, Choat and Schiel 

 1982, Breitburg 1984). There is no 

 conclusive evidence for generalizations 

 about the more subtle effects of grazers 

 in kelp forests, as most investigations 

 have focused on "barren" areas. Cowen et 

 al. (1982) suggested that, at low 

 densities, sea urchins may indirectly 

 increase bottom cover of red algae by 

 removing overstory brown algae that shade 

 the bottom. Results of intertidal studies 

 suggest that the effects of grazing on 

 algal cover and diversity are dependent 

 upon grazer density (Lubchenco 1978). We 

 can find no published account, however, of 

 an experiment where sea urchin densities 

 were artificially increased to various 

 levels in a kelp forest, and their 

 subsequent behavior, movement, and feeding 

 activities recorded. 



Dean et al . (1984) used a series of 

 observations and experiments to assess the 

 effects of two species of sea urchins on 

 Macrocystis in the San Onofre kelp forest. 

 Two different modes of feeding were seen 



for S. f ranciscanus . Over 3 years, 

 aggregations were either relatively small 

 and stationary, or large and mobile, 

 advancing at the rate of 2 m/month. 

 Stationary aggregations fed mainly on 

 drift kelp and had no significant effect 

 on kelp recruitment and abundance. Mobile 

 aggregations of red sea urchins, however, 

 removed most macroalgae in their path. 

 Small transplanted Macrocystis were 

 consumed over a 2-day period in the mobile 

 aggregation, but remained intact amongst 

 stationary echinoids and in a contral area 

 with no sea urchins. The results of a 

 similar experiment for Lytechinus anamesus 

 were equivocal, with small Macrocystis 

 being consumed in some trials and ignored 

 in others. Of particular interest in this 

 study, however, was the careful 3-year 

 observations of kelp and echinoid 

 abundances along several transects through 

 the kelp forest. Stationary and mobile 

 aggregations of echinoids occurred within 

 100 m of each other, and feeding fronts of 

 sea urchins were seen only twice during 

 the course of the study. These quite 

 different modes of feeding activity were 

 very local-scale events, and apparently 

 were dictated by the unavailability of 

 drift algae leading to a change in the 

 foraging behavior of the sea urchins. 

 Dean et al. (1984) also concluded that 

 both types of aggregations appeared to be 

 unrelated to predation pressure from 

 lobsters and fishes, although density 

 estimates for these predators were 

 anecdotal . 



Many studies have shown that a 

 preference hierarchy can be established 

 for sea urchins consuming algal species in 

 laboratory experiments (Leighton 1961, 

 Lawrence 1975, Vadas 1977). A major 

 question is whether these preferences 

 reflect the manner in which algae are 

 removed j_n situ by the same sea urchin 

 species. Vadas (1977), for example, found 

 that Strongylocentrotus droebachiensis 

 clearly preferred Nereocystis 1 uetkeana to 

 Agarum cribosum in laboratory experiments. 

 Sea urchins grew faster, and had a greater 

 reproductive output when fed Nereocystis 

 for long periods. He postulated optimal 

 feeding strategies for sea urchins in 

 nature, and argued for the coevolution of 

 algae and urchins based on selective 

 removal, plant defenses and benefits to 

 urchins. This study, however, indicated 



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