In semi-quantitative methods used in ecological field censuses, a specified area is surveyed, often within a fixed period of time, and the abundance of a species is evaluated as one of several density classes. It has been suggested that this method can cover a variety of habitats and takes less time compared to widely used quantitative methods involving quadrats and transects. However, low standardization of semi-quantitative surveys often causes fluctuations in survey effort, and this makes it difficult to compare the results from different localities or occasions. In the present study, the distribution of intertidal molluscs was investigated both by semi-quantitative and quantitative methods in the same plot. The time needed to complete the semi-quantitative survey was a third or a fourth of that required for the quantitative investigation, yet the two approaches yielded near-identical results in terms of species richness, diversity, and relative abundances of molluscan functional groups. A theoretical analysis suggests that relative abundance is robust against fluctuation in survey effort, and the present field investigation supported this viewpoint. That is, the number and cumulative density of species comprising a particular biogeographic sub-set of molluscs increased with increasing survey effort while the proportions of these among the total recorded species remained comparatively constant. The present results suggest that semi-quantitative methods can allocate finite human resources to a wider area or for a longer period than quantitative methods. This can be done without decreasing accuracy, provided relative abundance is used as the basis for calculating assemblage parameters.
