Climatic changes

Changes in climate have occurred on shorter timescales than the movements of land.

Much of what we see in the present distribution of species represents phases in a recovery from past climatic shifts. Changes in climate during the Pleistocene ice ages, in particular, bear a lot of the responsibility for the present patterns of distribution of plants and animals. The extent of these climatic and biotic changes is only beginning to be unraveled as the technology for discovering, analyzing and dating biological remains becomes more sophisticated (particularly by the analysis of buried pollen samples). These methods increasingly allow us to determine just how much of the present distribution of organisms represents a precise local match to present environments, and how much is a fingerprint left by the hand of history.

Techniques for the measurement of oxygen isotopes in ocean cores indicate that there may have been as many as 16 glacial cycles in the Pleistocene, each lasting for about 125,000 years It seems that each glacial phase may have lasted for as long as 50,000–100,000 years, with brief intervals of 10,000–20,000 years when the temperatures rose close to those we experience today. This suggests that it is present floras and faunas that are unusual, because they have developed towards the end of one of a series of unusual catastrophic warm events!

During the 20,000 years since the peak of the last glaciation, global temperatures have risen by about 8°C, and the rate at which vegetation has changed over much of this period has been detected by examining pollen records. The woody species that dominate pollen profiles at Rogers Lake in Connecticut have arrived in turn: spruce first and chestnut most recently. Each new arrival has added to the number of the species present, which has increased continually over the past 14,000-year period. The same picture is repeated in European profiles.

As the number of pollen records has increased, it has become possible not only to plot the changes in vegetation at a point in space, but to begin to map the movements of the various species as they have spread across the continents. In the invasions that followed the retreat of the ice in eastern North America, spruce was followed by jack pine or red pine, which spread northwards at a rate of 350–500 m year-1 for several thousands of years. White pine started its migration about 1000 years later, at the same time as oak. Hemlock was also one of the rapid invaders (200–300 m year-1), and arrived at most sites about 1000 years after white pine. Chestnut moved slowly (100 m year-1), but became a dominant species once it had arrived. Forest trees are still migrating into deglaciated areas, even now. This clearly implies that the timespan of an average interglacial period is too short for the attainment of floristic equilibrium. Such historical factors will have to be borne in mind when we consider the various patterns in species richness and biodiversity.

‘History’ may also have an impact on much smaller space and time scales. Disturbances to the benthic (bottom dwelling) community of a stream occurs when high discharge events (associated with storms or snow melt) result in a very small-scale mosaic of patches of scour (substrate loss), fill (addition of substrate) and no change. The invertebrate communities associated with the different patch histories are distinctive for a period of months, within which time another high discharge event is likely to occur. As with the distribution of trees in relation to repeating ice ages, the stream fauna may rarely achieve an equilibrium between flow disturbances.

The records of climatic change in the tropics are far less complete than those for temperate regions. There is therefore the temptation to imagine that whilst dramatic climatic shifts and ice invasions were dominating temperate regions, the tropics persisted in the state we know today. This is almost certainly wrong. Data from a variety of sources indicate that there were abrupt fluctuations in postglacial climates in Asia and Africa. In continental monsoon areas (e.g. Tibet, Ethiopia, western Sahara and subequatorial Africa) the postglacial period started with an extensive phase of high humidity followed by a series of phases of intense aridity.

In South America, a picture is emerging of vegetational changes that parallel those occurring in temperate regions, as the extent of tropical forest increased in warmer, wetter periods, and contracted, during cooler, drier glacial periods, to smaller patches surrounded by a sea of savanna. Support for this comes from the present-day distribution of species in the tropical forests of South America. There, particular ‘hot spots’ of species diversity are apparent, and these are thought to be likely sites of forest refuges during the glacial periods, and sites too, therefore, of increased rates of speciation.

On this interpretation, the present distributions of species may again be seen as largely accidents of history (where the refuges were) rather than precise matches between species and their differing environments.

Evidence of changes in vegetation that followed the last retreat of the ice hint at the consequence of the global warming (maybe 3°C in the next 100 years) that is predicted to result from continuing increases in atmospheric carbon dioxide. But the scales are quite different. Postglacial warming of about 8°C occurred over 20,000 years, and changes in the vegetation failed to keep pace even with this. But current projections for the 21st century require range shifts for trees at rates of 300–500 km per century compared to typical rates in the past of 20–40 km per century (and exceptional rates of 100–150 km). It is striking that the only precisely dated extinction of a tree species in the Quaternary, that of Picea critchfeldii, occurred around 15,000 years ago at a time of especially rapid postglacial warming.

Clearly, even more rapid change in the future could result in extinctions of many additional species.