2011 Volume 7 Pages A13-A31
The estimation of population sizes and habitat ranges, and the stability of those phenomena, plays an important role in conserving endangered species, especially on volcanic island chains. In 2002 and 2003, we studied the population sizes and the area of suitable habitat (forest) for three subspecies (two endemic and one common) of Varied Tits Parus varius that breed on the Izu Islands, a chain of seven volcanic islands located between 110 to 360 km south of Tokyo. Based on the study results, we estimated the distributional stability of the three subspecies between 1955 and 2009. P. v. varius, a common subspecies, inhabited only Oh-shima Island, the northernmost island of the chain. The population size was 12 to 1140 birds, and the area of suitable habitat was approximately 60 km2. P. v. namivei, an endangered subspecies, occurred on Nii-jima and Kozu-shima islands, which are located in the middle of the Izu Island Chain. The population size was 1760 to 1850 birds, and the area of suitable habitat was 20 km2. P. v. owstoni, an endangered subspecies, was distributed on Miyake-jima, Mikura-jima and Hachijo-jima islands, which are located in the southern part of the Izu Island Chain. The population size was 4420 to 5290 birds, and the area of suitable habitat was 65 km2. The model selection of generalized mixed models showed that the area of an island functioned as a factor affecting the distributional stability of Varied Tits on the Izu Islands. Calculations based on the estimated parameters of the selected model showed that the population stability of each island (number of presence years/ total of survey years) was more than 0.9 when an island area was larger than 14.0 km2, and less than 0.1 when the area was smaller than 1.7 km2.
