At the various stages of their life 

 cycles, seaweeds in giant kelp forests 

 fall prey to different species of 

 herbivorous invertebrates. Plants, 

 however, may grow too large to be consumed 

 by particular grazers, so that the number 

 of grazing species that may actually 

 remove entire plants decreases as plants 

 get larger. There are only a few species 

 of grazers that directly remove adult 

 plants, but many species live on the 

 plants, feed upon their tissues, and 

 indirectly cause the removal of all or 

 parts of the plants. These indirect 

 effects include grazing of plant tissue, 

 which may provide centers for fungal and 

 bacterial infections that can sever 

 blades, fronds, or holdfasts; this 

 severing can provide sites for epiphyte 

 growth or can weaken parts of the plants, 

 rendering them vulnerable to removal by 

 increased water motion or entanglement 

 with other plants. 



This section discusses the more 

 common species of invertebrates known to 

 have direct or indirect effects on the 

 removal of seaweeds, particularly kelps 

 (Table 6). There are many more herbivores 

 present in Macrocystis forests than will 

 be mentioned or listed here. More 

 comprehensive species lists can be found 

 in Leighton (1971), Smith and Carlton 

 (1975), and Morris et al . (1980). 



4.4.3.1 Sea urchins and sea stars 



(Echinodermata) . Sea urchins are 

 generally the most obvious grazers, and 

 may significantly affect the distribution 

 and abundance of macroscopic algae. Their 

 extensive grazing effects have been 

 recorded in tropical, temperate and boreal 

 regions (Lawrence 1975). There has been 

 extensive local removal of plants by sea 

 urchins in some Macrocystis pyrif era 

 forests in southern California (Leighton 



1971; see also Chapters 3 and 5), and kelp 

 distribution increased after sea urchins 

 declined at one site in central California 

 (Pearse and Hines 1979). 



In most kelp forests, there are often 

 great numbers of sea urchins, but these 

 commonly have little effect on attached 

 kelp (Lowry and Pearse 1973, Foster 1975a, 

 Cowen et al . 1982), feeding mostly on 

 drift material (Mattison et al . 1977, 

 Vadas 1977, Duggins 1980, Harrold and Reed 

 in press). Extensive feeding by sea 

 urchins on attached plants appears to be 

 related to the dispersion of individuals 

 and their density on patches of substratum 

 (e.g., Schiel 1982), and to behavioral 

 changes associated with the availability 

 of drift algae (Dean et al. 1984, Harrold 

 and Reed in press), or even large zoo- 

 plankton (Duggins 1981a). 



There is a general pattern for cases 

 where patches of kelp are completely 

 removed by sea urchins. Dense 

 aggregations of animals converge, and a 

 "feeding front" is formed. The urchins in 

 the vanguard of movement are often large 

 individuals, tightly packed together 

 (Leighton 1971, Dean et al. 1984). Most 

 or all of the plants in the path of these 

 dense aggregations are consumed or else 

 detached from the substratum by grazing 

 through the holdfasts or lower fronds. 

 Sea urchins further back in the 

 aggregations feed on this newly-freed 

 plant material . 



Large-scale removal of Macrocysti s on 

 the west coast has been primarily by two 

 species, Strongylocentrotus franciscanus 

 (Plate 2D), and the smaller j>. purpuratus . 

 Another species, the white urchin 

 Lytechinus anamesus , may occasionally 

 graze large kelp (Clarke and Neushul 1967) 

 but is probably more important as a grazer 

 of juveniles (Dean et al . 1984). 



The large red sea urchin 

 Strongyl ocentrotus franciscanus occurs on 

 rocky substrata throughout the range of 

 Macrocystis on the west coast (Morris et 

 <TL 1980). Leighton (1971) described 

 large individuals of this species forming 

 the advancing edge of a feeding front that 

 removed a large tract of Macrocystis 

 (^ 100 x 200 m) in the Point Loma kelp 

 forest during 1960 (see Table 7). The 



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