1. Graduate School of Environment and Information Sciences; Yokohama National University; Yokohama Kanagawa 240-8501 Japan
2. Department of Ecology, Evolution and Behavior; University of Minnesota; Saint Paul MN 55108 USA
3. Field Science Center for Northern Biosphere; Hokkaido University; Otoineppu 438 Otoineppu Hokkaido 098-2501 Japan
4. Center for Ecological Research; Kyoto University; Otsu Shiga 520-2113 Japan