第四紀の気候変動と種多様性 : 氷期-間氷期サイクルに対する海生生物の応答(<特集>生物多様性と古生物学)

抄録

Quaternary climatic changes are characterized by orbitally induced glacial-interglacial cycles with periods of about 23,000, 41,000 and 100,000 years and sea-level oscillations of 70-130m. These changes influence the biogeography of species through changes in physical and chemical parameters and the formation and destruction of environmental barriers, and create opportunities for increase of species diversity through the insolation of populations. However, the fossil record indicates that rates of speciation and extinction during the Quaternary were not significantly higher than during the pre-Quaternary. It is therefore thought that most evolutionary changes that take place in some populations over thousands of years are likely to be wiped off by recombination of populations with the following climatic change. Species have responded to rapid environmental change by migration without adaptive evolution. In other words, the Quaternary glacial-interglacial cycles strongly affected species diversity at a locality or in a region. This conclusion is consistent with the history of Pleistocene molluscs in the Sea of Japan. The warm Tsushima Current has come into the Sea of Japan recurrently during each interglacial stage since oxygen isotope stage 59 (ca. 1.7Ma). After the opening of the Tsushima Strait, the regional species diversity in the sea has varied drastically in response to glacial-interglacial cycles. It is worthy that benthic communities with a very low species diversity of molluscs temporarily appeared at offshore environment during warming associated with inflow of the warm Tsushima Current. This inner-to outer-shelf environment existed with a lateral scale of a few kilometers and a vertical scale of a few tens of meters during at least three transitions from oxygen isotope stages 48 to 47, 44 to 43 and 32 to 31. These deglaciation periods coincided with the three highest peak of summer solar insolation at Northern Hemisphere between oxygen isotope stages 50 and 26. Thus, anomalously high seasonality induced by orbital-insolation cycles may have an important influence on regional species diversity in the early Pleistocene Sea of Japan. On the other hand, the fossil records indicate that there is not significant variance in rates of speciation and extinction through the Quaternary, aside from the extinction of endemic species during the middle to late Pleistocene.