Noriko TAMURA’s scientific contributions

A hundred of woods, ranging in size from 0.17 to 39.17 ha, were surveyed in Ohuna, Kanagawa Prefecture, and presence/absence data of the invasive Formosan squirrel were collected from December 2001 to December 2002. The Formosan squirrel was ascertained in 42 woods of them. We measured 11 environmental variables of each wood, those are, wood area, distance to the nearest neighbor wood, distance to δha wood with squirrels, percentage of broad-leaved evergreen trees, conifer trees, deciduous trees, or others, and percentage surrounded by cultivated field, residential quarter, road, or others. The most important variable for the occupancy of squirrel was the woods area. The percentage of broad-leaved evergreen trees in woods and the percentage of cultivated field in surrounding area had also positive effects on the presence of squirrel. By adding latter two variables to the woods area, the logistic regression model was improved to account for the probability of the Formosan squirrel in isolated woods (Cox & Snell R²=0.45, P<0.001). Current expansion of this species in Kanagawa Prefecture was explained by these environmental variables; large woods with broad-leaved evergreen trees have been reserved in Kamakura and Zushi City, a center of distribution of the Formosan squirrel. Further studies using the present model will be necessary to know how the Formosan squirrel expands in future through urban environments. © 2004, Ecology and Civil Engineering Society. All rights reserved.

There is growing concern about the impact on the habitat of the native Japanese squirrel (Sciurus lis) in relation to the spread of the Formosan squirrel (Callosciurus erythraeus thaiwanensis), an alien species that was introduced and became naturalized in Kamakura City, Kanagawa Prefecture, in the 1950s. We used a logistic regression model to predict the probability of occurrence of the Formosan squirrel throughout Kanagawa Prefecture. Wooded areas, the proportion of cultivated fields surrounding a wood and the proportion of evergreen trees in a wood, factors that influence the presence of Formosan squirrels, were used as environmental variables in the model. As a result, we found that woods suitable for the Formosan squirrel were distributed not only around Kamakura, which Formosan squirrels currently inhabit, but throughout Kanagawa Prefecture. Furthermore, a comparison of its current habitat and its predicted suitable habitat suggested that the assemblage of large woods along the Sagami River and Sagami Bay would be conductive to increasing its population and expanding its distribution to western Kanagawa. Because there are forests in western Kanagawa which are extensive and are expected to be a suitable habitat for the Formosan squirrel, we should prevent Formosan squirrels from invading the west side of the Sagami River.