1. Graduate School of Human and Environmental Studies Kyoto University Sakyo Kyoto 606‐8501 Japan
2. Center for Ecological Research Kyoto University 509‐3, 2‐chome, Hirano Otsu Shiga 520‐2113 Japan
3. PRESTO Japan Science and Technology Agency 4‐1‐8 Honcho Kawaguchi Saitama 332‐0012 Japan