Species diversities analyzed by density and cover in an early volcanic succession

Abstract

To evaluate alpha diversities, various variables such as density, cover, volume, and weight have been used. However, density is often a distinct variable from the remaining three. To clarify differences in diversity measured by those two kinds of variables, the data collected in fourteen 2×5 m permanently-marked plots on Mount Usu, Japan, which erupted during 1977 and 1978 in growing seasons from 1983 to 1989 was analyzed, using Shannon's species diversity (H′) that is represented as a result of combination of species richness and evenness (J′). H′ and J′ were evaluated by density (density H′ and J′) and cover (cover H′ and J′). Cover H′ and J′ were significantly lower than density H′ and J′, indicating that cover H′ has different characteristics from density H′. Those differences are due to differences in evenness, because species richness is the same. The rank orders of species density are different from those of cover. The predominance of a few perennial herbs greatly decreases cover evenness, while seedling establishment success influences density evenness. Therefore, I propose that, during the early stages of succession on harsh environments such as volcanoes, density diversity represents seedling establishment success rate while cover diversity expresses vegetative reproduction success rate.