Quantification of biotic responses to rapid climatic changes around the Younger Dryas - a synthesis

To assess the presence or absence of lags in biotic responses to rapid clim atic changes, we: (1) assume that the delta(18)O in biogenically precipitat ed carbonates record global or hemispheric climatic change at the beginning and at the end of the Younger Dryas without any lag at our two study sites of Gerzensee and Leysin, Switzerland; (2) derive a time scale by correlati ng the delta(18)O record from these two sites with the delta(18)O record of the GRIP ice core; (3) measure delta(18)O records in ostracods and mollusc s to check the record in the bulk samples and to detect possible hydrologic al changes; (4) analyse at Gerzensee and Leysin as well as at two additiona l sites (that lack carbonates and hence a delta(18)O record) pollen, plant macrofossils, chironomids, beetles and other insects, and Cladocera; (5) es timate our sampling resolution using the GRIP time scale for the isotope st ratigraphies and the biostratigraphies; and (6) summarise the major pattern s of compositional change in the biostratigraphies by principal component a nalysis or correspondence analysis. We conclude that, at the major climatic shifts at the beginning and end of the Younger Dryas, hardly any biotic la gs occur (within the sampling resolution of 8-30 years) and that upland veg etation responded as fast as aquatic invertebrates. We suggest that the min or climatic changes associated with the Gerzensee and Preboreal oscillation s were weakly recorded in the biostratigraphies at the lowland site, but we re more distinct at higher altitudes. Individualistic responses of plant an d animal species to climatic change may reflect processes in individuals (e .g. productivity and phenology), in populations (e.g. population dynamics), in spatial distributions (e.g. migrations), and in ecosystems (e.g. trophi c state). We suggest that biotic responses may be telescoped together into relatively short periods (50 to 150 years), perhaps disrupting functional i nteractions among species and thus destabilising ecosystems. (C) 2000 Elsev ier Science B.V. All rights reserved.