Do all mass extinctions represent an ecological crisis? Evidence from the Late Ordovician

The high rate of species extinction in recent decades is seen by many ecolo gists as heralding an extinction of catastrophic magnitude in the near futu re. The ecological consequences of such a biodiversity crisis are hard to p redict, but some indication of likely effects can be gained from the knowle dge of mass extinctions in the past. The Late Ordovician extinction was one of the five great extinctions in the geological record. It occurred in two phases about 0.5-1 million years (Ma) apart and resulted from climatic and related environmental changes associated with the rapid growth and decay o f the large Gondwanan ice cap. Overall, an estimated 86% of species became extinct, 61% of genera and 12-24% of families, but few or no orders or high er taxa. The extinction severely affected both marine benthos and plankton. Using brachiopod data as a measure of ecological change, it can be seen th at the number of within-habitat species (alpha diversity) was severely redu ced and the number and distinctness of benthic communities (beta diversity) on marine shelves also declined sharply. Concurrently the number of palaeo geographic provinces fell from ten to five, possibly as a result of a loss of endemic species and preferential survival of cosmopolitan species. At th e peak of extinction, following the second extinction phase, the ecological structure of both benthic and planktonic ecosystems had been severely disr upted and downgraded in complexity as a wide variety of niches were 'vacate d'. In spite of the profound biodiversity and ecological crisis within this 'survival' interval, communities returned to their pre-extinction levels o f alpha and beta diversity during the subsequent 'recovery' interval. In sp ite of the large amount of vacant ecospace to be filled there was very litt le innovation in terms of adaptive strategy, so that the structure of the e merging Silurian communities was similar to that of the Ordovician. In thes e terms the ecological recovery was remarkable, but it was also prolonged o ver about 4-5 Ma. On a geological time scale the biosphere returned to 'nor mal', but on a human time scale the mass extinction produced a very severel y degraded biosphere. Copyright (C) 2001 John Wiley & Sons, Ltd.