ABSTRACT

As with all elements of the natural landscape – rocks, slopes, rivers, soils – vegetation communities have a history of development. The changes which occur in the characteristics of a particular species over many generations are called evolution. Evolutionary change takes place over millions of years and is achieved by the mechanism of natural selection, first proposed by Charles Darwin in 1859. In this way, the modern characteristics of plant and animal species result from a long history. The present distribution of species is greatly influenced by past events, too. The theory of continental drift explains why some species are widely distributed across continents now separated by thousands of miles. The changing patterns of distribution over time are preserved in the fossil record. The latter may take the form of hard-rock fossils or, for Quaternary peat and lake sediments, the plant pollen grains and spores which they contain. Vegetation communities are also subject to short-term changes over the order of, say, thirty to 100 years. Natural disturbances by floods, volcanic activity, hurricanes, disease and fire can completely alter the vegetation of an area. The habitats of the plants are changed, favouring a new set of communities. These short-term changes reflect the dynamic nature of vegetation; the processes occurring in the successional development of plant communities will be discussed in this chapter.