Ecologists have long noted the importance of geology in the global and regional distribution of organisms. Life, ranging from macro- to microscopic, exists on and within a mosaic ofgeologies that vary across both space and time. The contributions of geologic phenomena to maintaining and generating biotic diversity are twofold. First, large-scale geologic events (e.g., continental drift and rising of mountains) create discontinuous or patchy landscapes. Second, within this patchwork of landscapes, parental geologic materials such as igneous, metamorphic, or sedimentary rocks can become exposed, leading to the development of soils differing in chemical and physical characteristics. This creates opportunities for colonization and differentiation of species. The edaphic factor pertains to physical, chemical, and biological properties of soil resulting from these geologic phenomena. Discontinuities in the edaphic factor have contributed to the intriguing patterns of diversity we see in the biotic world. Edaphology is a branch of soil science that studies the influences of soils on organisms, especially plants. It includes agrology, the study of human uses of soils for agriculture, as well as how the features of soils affect human land use decisions.
According to soil ecologist Hans Jenny, soils owe their distinct characteristics to five interacting factors: climate, organisms, topography, parental rock, and time. If all but one factor (e.g., parental rock) remain unchanged, then variation in a soil body can be attributed to that one factor. Botanists have long recognized that the distribution, habit, and composition of vegetation are greatly influenced by the edaphic factor. The striking effects on vegetation of unusual and often extreme substrates (e.g., serpentine, limestone, dolomite, shale, gypsum) are apparent even to amateur naturalists. Whereas climate broadly defines major biomes (e.g., tropical rainforests, temperate deciduous forests, deserts, tundra), it is geology that enriches diversity within these zones. The role played by the edaphic factor in the distribution of plant species was keenly observed and recorded by many eighteenth- and nineteenth-century plant ecologists, who considered soils second only to climate as the major ecological determinant of plant distribution. It was in the twentieth century, however, that ecologists fully appreciated the role of the edaphic factor in generating habitats within which plants and their associated organisms live, interact, reproduce, and diverge over time.
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