When biotas meet: understanding biotic interchange.

When the barrier between biotas with long separate histories breaks down, species invade from one biota to the other. Studies of episodes of marine and terrestrial biotic interchange that have occurred during the last 20 million years show that large-scale extinction of species before the onset of interchange renders biotas especially prone to invasion. As environments and species are being exploited and eliminated on an ever increasing scale in the human-dominated biosphere, the geographical expansion of species from biotas in which evolution of high competitive, defensive, and reproductive abilities has proceeded the furthest will become more frequent. Historical events and interactions are essential ingredients for understanding the current and future structure and composition of the world's biota.