抄録
Equations of the Shannon-Wiener information theory and the MacArthur's type I distribution, which are fully shown in the text, are applied to synecologic analysis of chydorid thanatocoenosis in Lake Nojiri. Indices of the specific diversity and the equitability (ε) were very low in the late glacial disproportionate population (Hutchinson's type IV distribution). It took about 5, 000 years from the end of the late-glacial period to establish a maximum equitable chydorid population. Two volcanic ash falls seriously disturbed the chydorid biocoenosis in the lake; the first volcanic ash fall (ca. 5, 600 B. P.) led to an extinction of Eurycercus lamellatus and a sharp decrease of the ε value (from 1.0 to 0.47), and the second volcanic ash (ca. 4, 600 B. P.) to a temporary decrease of Alona affinis and of the ε value (from 0.71 to 0.66). Two peaks of Monospilus dispar are also resulted from these disturbances. Agricultural activities intensified at around 1, 500 B. P. led to an increase of the productivity of the lake, without reducing significantly the equitability of chydorid population.
