偶见种

分析了该流域淡水贝类与栖息环境的关系及该流域的常见种和偶见种，并探讨了该流域淡水贝类的分布区划。

The relationship of freshwater mollusk s with their environment , common species and occasional species in this river system were analyzed and the division of mollusk distribution in the investigated waters was also discussed .

通过测定论述了成都平原区荒芜草坪2个群落及20个（直径30cm）随机分布不同样圈中优势种之间及优势种与伴生种、偶见种之间的数量特征。

Through the analysis of the quantity characteristic between the advantages species and between the companion living species and the seldom-seen species in the 2 communities and 20 different circles at random in the Chengdu plain deserted lawn .

优势种5种，常见种6种，偶见种28种。

Species of birds are dominant species , 6 species are common species and 28 species are rare species .

其中，偶见种或稀有种的频度出现的百分比均高于常见种和优势种，体现了该区域植物种质资源种类的均匀性。

Among them , occasional and rare species that appear more frequently than the common and dominant ones , which reflected the blance of plant germplasm resources in this area .

候鸟中常见种19种、一般种72种，少见及偶见的58种。

Among migrant are 19 kinds of common species ; 72 kinds of general species ;