Climate extremes and ecological responses in the paleo-record

Background/Question/Methods

Climate varies at all timescales, so for an individual, population, or species to survive, it must be able to accommodate the climate variations likely to occur during its lifetime at its location.  The climate variability to be accommodated includes both high-frequency climate variability (daily to interannual) and the superposition of this high-frequency variability upon longer-term modes of variability (interdecadal to multi-millennial).  Climate extremes then are episodes that a) exceed the acclimatory capacity of individuals, populations, or species (Gutschick and BassiRad, 2003) and b) initiate rapidly and last briefly relative to the timescale of interest.  Ecological responses to high-frequency climate extremes can be readily observed in modern systems, or tested experimentally.  However, paleoecological data are necessary to study ecological responses to a) climatic variability and extremes at longer timescales and b) climatic extremes overlaid on mean climate states different from present.  Here we review well-documented instances of climatic extremes during the late Quaternary and the ecological responses to these extremes. 

Results/Conclusions

During the end-Pleistocene and early Holocene, the most ecologically significant climate variability was temperature-related, including the Bølling-Allerød (manifested in most places as abrupt warming), the beginning and end of the Younger Dryas (manifested in most places as abrupt cooling, then warming), and the 8.2 event (manifested in many places as a 200-year transient cooling; in others as a step change in climate).  These climatic extremes triggered large changes in species abundances and community structure, and either accelerated or temporarily reversed the long-term responses of species and communities to orbitally-paced deglaciation and warming.  During the Holocene, the most ecologically severe climatic extremes were hydrological, with multiple instances of droughts triggering large changes in species abundances and vegetation structure.  These include the sub-continental-scale megadroughts of the last 2000 years.  Many, perhaps most, abrupt vegetation changes during the late Quaternary can be linked to climatic extreme events, and the ecological responses are highly mediated by disturbance regime and by local site variables.  Some abrupt vegetational changes appear not to be directly forced by climatic extremes but instead manifest as abrupt local responses to progressive climatic changes, suggesting that non-linearities in ecological processes also play a role in ecologically abrupt change.