change (MacArthur, 1955). However, diverse communities have shown 

 high susceptibility to perturbations (Paine, 1969; May, 1975). In 

 addition, experiments (e.g., Hairston, et al., 1968) and models 

 (Strobeck, 1973; Maynard- Smith, 1974) have shown that stability is not 

 a necessary consequence of increased complexity. Steele (1974) concluded 

 that neither a system's diversity or efficiency is a definite reflection 

 of its dynamic stability; thus, there is a lack of general predictive 

 capacity for evaluating the consequences of large-scale man-induced 

 disturbances. The likelihood that multiple stable points may exist 

 within natural communities (Lewontin, 1969; MacArthur, 1972; Sutherland, 

 1974) further complicates this issue. 



5. Southern California and Imperial Beach Sediments . 



The Continental Shelf off southern California is more complex 

 geographically and has more mixed substrate patterns than typical 

 shelf sediments from other regions (Emery, 1960). The shelf off 

 Imperial Beach is wider than off other southern California areas and is 

 typified by 8.8-percent rock, 76.4-percent sand, and 14.6-percent silt 

 (Stevenson, Uchipi, and Gorsline, 1959). As the subtidal delta of the 

 Tijuana River is approached at the southern end of Imperial Beach, 

 sediments become coarser and better sorted, ranging from very fine to 

 medium sand with median grain sizes between 150 to 300 micrometers 

 (Intersea Research Corporation, 1978). Muslin (1978) reported inter- 

 tidal beach sediment median grain size to be about 200 micrometers 

 (fine sand) . Mineralogical composition of beach sediments is dominated 

 by hornblende (Intersea Research Corporation, 1978) and characterizes 

 a metamorphic source rock (Pettijohn, 1957). Imperial Beach lies 

 within a zone of wave convergence and also experiences longshore 

 currents flowing northward. These two sources of energy create a high 

 rate of sediment transport to the north. The potential transport rate 

 is 76,500 cubic meters per year. Erosion loss has been estimated at 

 22,900 cubic meters per year (Intersea Research Corporation, 1978; 

 Muslin, 1978). These sediments must be constantly replenished if 

 erosion of the beach is to be stabilized. Previously, Tijuana River 

 basin runoff was the major source of sediment input, but this has been 

 reduced by an estimated 70 percent during this century due to a 

 reduction in precipitation, creation of upstream dams, and use of 

 water for agricultural purposes (Intersea Research Corporation, 1978). 

 Further details of the erosion problem at Imperial Beach are presented 

 by Inman (1973) . 



6. Southern California and Imperial Beach Infauna . 



Knowledge of distribution patterns of the southern California 

 mainland shelf macrofauna is almost entirely derived from the exten- 

 sive surveys between Point Conception and the Mexican border conducted 



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