Yuri Maesako

Effects of burrowing by streaked shearwaters (Calonectris leucomelas) on the lucidophyllous forest (association : Arachniodo-Castanopsietum sieboldii) were studied on Ohshima Island in Mie, Japan in 1991. The association was divided into a Damnacanthus indicus variant (differential species : Damnacanthus indicus and Eurya japonica) and a Cocculus orbiculatus variant (differential species : Cocculus orbiculatus, Pueraria lobata and Menispermum dauricum, etc.) which were non-burrowing sites and burrowing sites, respectively. The D. indicus variant was characterized by forest floor species including character species of Camellietea japonicae, while the C. orbiculatus variant was characterized by forest edge species including character species of Rosetea multiflorae. The vegetative differences between the D. indicus variant and the C. orbiculatus variant clearly reflected the ecological characteristics of non-burrowing and burrowing sites. The burrow density of the seabird was low (6.0 + 2.0/100 m^2) on the island. The burrows were found mostly under the roots of trees (62.2%) , and their distribution pattern was aggregated (1_δ=1.67). This may suggest some selection of habitat by streaked shearwaters. The percentage of therophyte (annual plant) was significantly high in the burrowing sites, while the percentages of character species of Camellietea japonicae and canopy cover were significantly low. It is suggested that biotic interactions by streaked shearwaters tended to modify vegetation and destroy forest structure, even though the rookery density was low. The relationship between vegetative structure and habitat of streaked shearwaters, and rookery site selection in the vegetation are discussed.

Machilus thunbergii forests were studied in Kanmurijima Island, Kyoto Pref., to clarify the influence of disturbance by birds' nesting and streaking on the community structure of the forest. Density of nests was negatively correlated with the number of plant species and individuals, the skewness of frequency distribution of height and DBH, Shannon H' and the relative basal area of Aucuba japonica, respectively. Species composition, species diversity index, dominance-diversity curves, skewness of height distribution, crown projection diagrams and other ecological characteristics showed that the disturbance by birds resulted in poor vegetation and the development of simple stratification. The species diversity became slightly higher as the aboveground biomass increased. In these forests Mallotus japonicus very often filled up gaps of canopy trees, and seedlings and saplings of Machilus thunbergii were growing around patches of Carex matsumurae and some herbs as stable spots for germination on the forest floor.