Migration: Species' Response to Climatic Variations Caused by Changes in the Earth's Orbit

Tree populations change in distribution, abundance and association in response to long-term changes in climate. Isopoll maps for selected taxa in western Eurasia and eastern North America illustrate features of these 'migrations' which can be seen on both continents despite their major geographical contrasts and the different taxa involved. These maps show differential rates and directions of migration among taxa and illustrate different behaviour among taxa that today share similar distributions and/or grow together in communities. Communities are thus shown to be merely temporary assemblages of species brought togheter by the environmental conditions prevailing at any particular time. Migration of plant taxa is considered as a response to orbitally-forced cimate changes at timescales of 105years, and is compared to bird migration as a response to seasonal climate changes. The two phenomena are seen to be essentially similar and to have many of the same consequences, although they involve organisms that respond differently to climatic changes at different frequencies. Plant taxa have shown only limited macroevolutionary response to the climatic fluctuations of the last$1.6 \time 10^6$years. Their migratory responses to these climate changes have, however, led to alternating allopatry and sympatry, and alson often to isolated small populations, both situations which are believed to favour rapid macroevolution.