Edaphic factors are those related to the soil. In terms of plant geography we can look at soils in two perspectives. One views broad patterns of the great soil groups, reflecting the pedogenic regimes of podzolization, laterization, calcification, gleization, and salinization. These pedogenic regimes are largely controlled by the climatic regimes and will be found closely correlated with the global patterns of the vegetation formation classes. A second viewpoint is in terms of plant habitats-the small-scale mosaic of place-to-place variations of the earth's surface. Here the edaphic factors also act as important controls.
Although this book treats the systematic principles of soil science ahead of those of natural vegetation, a good argument might be made for reversing this order of treatment on the grounds that vegetation plays a leading role in the development of soil characteristics. Given a barren habitat, recently formed by some geologic event such as the outpouring of lava or the emergence of a coastal zone from beneath the sea, the gradual evolution of a soil profile goes hand in hand with the occupancies of the habitat by a succession of plant communities. The plants profoundly alter the soil by such processes as contributing organic matter, or by producing acids, which act upon the mineral matter. Animal life, feeding upon the plant life, also makes its contribution to physical and chemical processes of soil evolution.
Biotic factors. The sustained activity within a particular plant community functioning as a stable ecosystem and the gradual modification of the vegetation at a given place with time throughout a succession of stages require that the plants and animals within the community contribute to its operation through their own physical and chemical process and through their life cycles of growth and subsequent decay.
Among the biotic factors may be mentioned the activity of bacteria in consuming the dead tissue of larger plants; of earthworms in altering and aerating the soil; of insects and herbivorous animals which attack and consume plants; and of insects and birds which perform such functions as pollination of plants and the dispersal of seeds. A whole realm of biotic influence lies in the diseases of plants, the spread of which may drastically alter the plant associations over wide areas. The geographer is particularly interested in the way in which large herds of grazing and browsing animals, whether wild or domesticated, have contribution to the development of certain formation classes of vegetation.
Man is himself perhaps the most potent biotic factor influencing vegetation over the globe today. His role has been dominantly destructive with respect to the plant associations and formation classes that might otherwise be expected to cover the lands in response to the various factors of climate, soil and geomorphology that we have thus far reviewed.
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