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The results obtained by using a wire gauze shovel were analyzed statistically to determine the reliability of the figures. The place selected for the test was a broad and apparently uniform riffle on Hatsukagawa Creek near Sanda City, Hyogo prefecture. <BR>Judging from the coefficient of variation among thirteen samples collected at random, the number of taxa in each unit area has a relatively small variability; on the other hand, both the total individual numbers and the total biomass have a great variability. Moreover, both of the two diversity indices - Shannon's and Simpson's - have very stable values. The greater variability of figures is, the more repeatly samples must be collected to obtained any specified level of precision. For example, in order to estimate the total number of individuals within 10% of the true value at the 95% level of significance, no less than 39 repetitive samples with a 50 cm × 50 cm quadrat are required in contrast to only six samples for the mean number of taxa. Practically speaking, error of the order of 30% must be permitted to estimate the number of individuals and biomass. <BR>As to each single species, the variability of samples depends on the degree of aggregation of individuals. For example, <I>Eubrianax sp., Baetiella japonica, Dugesia gonocephala</I> and <I>Hydropsyche ulmeri</I> show a very contagious distribution, so the variability is great. In opposition, <I>Mystrophola inops, Antocha spp., Stenopsyche griseipennis</I> and <I>Ecdyonurus yoshidae</I> show a nearly random distribution and consequently, the variability is relatively small. <BR>As regards quadrat size, sampling with a 25 cm × 25 cm quadrat may be better than with a 50 cm × 50 cm quadrat, considering the balance of variability of samples and effort for sampling. <BR>This paper deals chiefly with sampling error in the statistical sense. Another important point in sampling procedure is selectivity of a sampling method. Further study on this point is needed.
- 日本陸水学会の論文
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