-101- 



VII. EFFECTS OF ECOSYSTEM MANIPULATION ON 



LIVING RESOURCES 



Ecosystems are dynamic entities of complex relation- 

 ships in which all subtle but influential factors are 

 rarely understood. Biotic components of ecosystems evolve 

 and adapt to survival in each other's presence. Ecological 

 interactions such as competition, predation, and limitation 

 by the physical environment affect each group's distribution 

 and population dynamics. 



Relatively little is known about all of the Antarctic 

 marine ecosystem's relationships. As in any ecosystem, 

 trophic relationships are important in determining inter- 

 actions between the system's biological components. South 

 of the Convergence, Antarctic krill is the dominant prey 

 species at the base of the food web, affecting species 

 groups such as whales, seals, birds, fish, and squid. 

 Although biomass estimates of krill-consiomer populations are 

 somewhat uncertain, there is no doubt that vast amounts of 

 krill are fundamental to the functioning of the Southern 

 Ocean ecosystem. 



A. Ecosystem Changes Following the Decline of Whale Stocks 



The intensity of competition for krill between consumers 

 is uncertain. Parameters affecting krill and krill-consumer 

 populations may include subtle spatial and temporal interactions, 

 If one assumes that before the exploitation of baleen whales 

 there was significant competition between krill-eating 

 species, it follows that reduced whale populations would 

 allow greater use of krill resources by competitors. This 

 may have led to increased populations of seals, seabirds, 

 fish, squid and baleen whale populations which had not been 

 heavily exploited. 



Authors do not agree on population level shifts between 

 krill consumers due to changing levels of available krill. 

 Mackintosh (1970) felt that available information to assess 

 population changes in seals, birds and unexploited baleen 

 whale groups were insufficient to conclude that there had 

 been significant population shifts resulting from trophic 

 interactions. He did, however, acknowledge the possibility 

 that fish and squid populations could have benefited from 

 the krill no longer eaten by whales. Nemoto (1964) felt 

 that the decrease of whales would almost certainly cause 

 major shifts in marine food chains. The following discussion 

 reviews information supporting the idea that various species 

 groups have manifested population responses following the 

 marked decline in baleen whale populations due to harvest. 



