Factors Restricting the Distribution of Frogs in a Rural Landscape Characterized by Dispersed Settlement in Isawa, Iwate Prefecture, Northeast Japan

Abstract

The belt-transect survey at a scale of 1:25000 was investigated in order to clarify the relation between the inhabitation of frogs and the landscape structure in the Isawa alluvial fan. The index value of frog abundance was investigated and obtained by counting egg masses and by the hearing density of calling. The invasion situation from the hill of each frog to a large paddy field region has been understood. The breeding site of brown frogs Rana japonica and R. ornativentris, was restricted to the edge of the alluvial fan. Also under the cliff of the terrace side where water oozes out in the spring, the breeding site for both brown frogs existed in a detached district of the fan. As for R. rugosa and Rlacophorus schlegelii, the inhabitation was found only in the southern fan region. It was hypothesized that the existence and quantity of a ground waterway network and small pieces of wood were the distribution restriction factors for each species. In contrast, Rana porosa porosa inhabited in the whole region of the fan by a high density; especially it was remarkable in the northern fan region where it currently improved by rectangular field plotting. For this reason, the investigation year of 2001 has many land consolidation paddy fields, which are comparatively old now, and it is possible that the majority of the ground waterway still remained.