that sea urchins in nature fed mainly on 

 drift Nereocystis , plants which had 

 already been removed by other causes. In 

 addition, the densities of urchin 

 aggregations were not mentioned as a 

 factor important to plant removal. Other 

 studies have shown that sea urchin 

 densities can be important in nature. 

 Breen and Mann (1976) found that there was 

 a non-linear effect of sea urchin numbers 

 on algal removal in Laminaria longi cruris 

 beds in Nova Scotia. Schiel (1982) also 

 postulated a non-linear effect of sea 

 urchin feeding for subtidal areas in 

 northern New Zealand. In parallel 

 laboratory and field experiments, he found 

 that the removal of plant material 

 increased exponentially with sea urchin 

 numbers. He also found that there was no 

 correlation between the feeding preference 

 hierarchies found in the laboratory, and 

 those found in experimental situations in 

 the field. The order of removal of algal 

 species by sea urchins from natural stands 

 appeared to be related to holdfast 

 morphology, and was not correlated with 

 hierarchies established in field 

 experiments. Because sea urchins clumped 

 on some replicates, and their feeding 

 effect was non-linear, Schiel (1982) 

 postulated an "all-or-nothing" effect of 

 sea urchins on kelp removal. 



A contrasting result was found by 

 Harrold and Reed (in press) at San Nicolas 

 Island. Red sea urchins ( Strongylocentro- 

 tus franciscanus ) were abundant both in 

 Macro_£y_sti_s -dominated areas and in patches 

 devoid of large brown algae. The movement 

 of the echinoids and their effects on the 

 epibenthic community were affected by the 

 availability of drift Macrocystis . Red 

 sea urchins moved greater distances and 

 fed on benthic organisms in "barren" 

 patches, while they remained relatively 

 stationary and fed on drift kelp in 

 Macrocystis patches. This result is 

 similar to that found by Mattison et al. 

 (1977) in central California. One of the 

 major differences between the activities 

 of echinoid grazers in the eastern Pacific 

 and those elsewhere may therefore be 

 related to the preponderance of large 

 kelps, and hence ample drift material, 

 compared to the smaller stipitate 

 laminarians found in most other parts of 

 the world. 



Much time and money have been, and 

 are being, spent on "the urchin problem" 

 (North and Pearse 1970, North 1983a) in 

 southern California. The "problem" 

 appears to arise primarily from a 

 management viewpoint that Macrocystis 

 forests are desirable, are the unvarying 

 natural state of coastal waters, and the 

 perception that localized aggregations of 

 kelp-destroying sea urchins are somehow 

 man-induced (Bascom 1983). Although this 

 may be the case near large sewage 

 outfalls, it is equally likely that waste 

 discharge caused a reduction in algal 

 biomass, and the urchins are simply eating 

 what is left (see Chapter 6). Moreover, 

 recent observations suggest that urchin 

 "barren" grounds may come and go in kelp 

 forests as natural variations in a dynamic 

 community (see Chapters 3 and 4). 

 Nevertheless, the consequent assaults on 

 the lowly and meddlesome sea urchins have 

 taken epic proportions, from destruction 

 of tests with quicklime to outright 

 mechanical maceration (Chapter 6). There 

 was also a major effort organized through 

 SCUBA diving clubs to smash sea urchins 

 with hammers (North 1972a). These 

 projects have met with only limited 

 success. In some cases, Macrocystis 

 became locally established, while in 

 others, fish grazing and probably 

 limitations in algal spore dispersal 

 prevented establishment (North 1972a, 

 1973). 



Evidence from studies elsewhere 

 indicates that when adult plants are 

 nearby, the removal of sea urchins can 

 result in a large recruitment of kelp 

 (Jones and Kain 1967, Duggins 1980, Andrew 

 and Choat 1982). An experimental study 

 examining this for Macrocystis forests was 

 done by Pearse and Hines (1979) near Santa 

 Cruz, central California. Large numbers 

 of Strongylocentrotus franciscanus died 

 over a wide area along the edge of a large 

 stand of Macrocystis as a result of 

 disease (see Section 4.7). Dense 

 recruitment of Macrocystis , Pterygophora 

 cal ifornica , and Laminaria dentigera 

 occurred during the following spring in 

 the previously urchin-dominated area. The 

 boundary of the kelp forest also was 

 extended seaward by over 100 m due to the 

 removal of echinoids and their legacy of 

 cleared substratum. 



105 



