Biotic response to rapid climatic changes during the Late Glacial : high resolution biostratigraphies and biological processes

Abstract. Organisms can respond to rapid climatic changes in three ways: 1) adaptation by evolution, affecting physiology and morphology), 2) migration and population dynamics including biogeographical changes) and 3) extinction local or global). Here, the focus is on examples of the second type. Organisms, whether algae, trees, or animals, find their ecological niches in a multi- dimensional space of gradients such as temperature winter, summer, means or extremes), humidity soil or air), pH, various nutrients, light. Presence or absence of taxa species, genera, families) can be related to such gradients. With training sets based on current gradients, they can also be related to environmental changes of the past e. g. summer mean temperatures or pH). The relationships between the occurrence of taxa and environmental variables can also be used to examine the biotic response to changes based on other proxies, for example, changes in temperature inferred from oxygen- isotope ratios in carbonates or from the content in organic matter of lake sediments. The groups of organisms referred to here are plants pollen), insects chironomids) and other aquatic invertebrates. The three Late Glacial periods with very high rates of change in temperature estimates are the transition from the Oldest Dryas to the Bölling from GS- 2 to GI- 1 in the Late Glacial, ca. 14 670 cal yr BP), and the beginning and the end of the Younger Dryas ca. 12 600 cal yr BP, 11 500 cal yr BP respectively).

The « classical » hypothesis was that trees represented in pollen diagrams) respond more slowly to climatic change than invertebrates aquatic or terrestrial) because of differences in life cycles. But it is shown here that terrestrial vegetation) and aquatic invertebrate) ecosystems may respond synchronously. Three major biological processes are involved in the responses to climatic change:
1) Migration – can be slow if, for example, a longliving tree migrated back from a southern refugium.
2) Build- up of populations – intermediate velocity, for the process needs time depending on the life cycles of the organisms.
3) Productivity – can change rapidly, within a year or a few years e. g. pollen productivity, tree rings). The first two of these processes occur on the organisational level of populations, the last one on the level of the individual. These processes develop also in various combinations.