Biodiversity and food chains on the littoral bottoms of Lakes Baikal, Biwa, Malawi and Tanganyika: working hypotheses
摘要
Boreal Lake Baikal, temperate Lake Biwa, and tropical Lakes Malawi and Tanganyika each possess high biodiversities of benthic organisms in their littoral areas. Two general mechanisms influencing the evolutionary development of these high biodiversities can be hypothesized; i.e., a bottom-up constraint depending on the availability of food resources, and a top-down effect exerted by predators especially within the fish assemblage. The bottom-up constraint may be prominent where the abundance of benthic organisms in the grazing-food-chain is restricted by a seasonally lowest production, referring chiefly to attached algae, and large fluctuations in the abundance of primary producers provide a resource that is eventually utilized by benthic organisms of the detritus-food-chain. The top-down effect may be realized where the most of benthic organisms are supported by a constantly large primary production. These hypotheses help to explain, for the lakes we cite, the facts that diverse grazing-food-chain organisms live on the littoral bottom of tropical lakes where there is relatively constant large algal production due to suitable conditions of water temperature and light penetration through the year; while on the other hand, detritus-food-chain organisms are predominant on the littoral bottom of a boreal lake with seasonally fluctuating algal production and its pronounced lowest production in a year. In fish communities, segregations in habitat and food resource are important mechanisms allowing development of high species packing, while cohabitation within predatory fishes increases their feeding success due to the decrease of defensive efficiency by the threat of a variety of attacking methods. A further top-down effect may be exerted by scale-eating fishes, especially in Lake Tanganyika. The presence of such predatory and harmful members may promote more dense packing of prey members to increase their mutual defense efficiency.