Landscapes and Geomorphology: A Very Short Introduction by Andrew Goudie & Heather Viles

Author:Andrew Goudie & Heather Viles [Goudie, Andrew]
Language: zho
Format: mobi
Tags: #genre
Publisher: OUP Oxford
Published: 2010-08-25T16:00:00+00:00

10. Longevity and size of some biologically constructed landforms

As can be seen in Figure 10, biologically constructed landforms range widely in scale and longevity, but in comparison with the whole spectrum of landforms illustrated in Figure 2, they are relatively small, and both form and decay quite quickly. Exceptions include large and long-lived coral reef constructions which can persist within a landscape for millions of years and, in the case of the larger ones, such as the Great Barrier Reef, are several thousand kilometres in dimension. One key question here, given the range of biologically produced landforms, is: would the world’s landscapes look different if there were no plants, animals, or micro-organisms? Clearly, they would look superficially different, in that there would be no green mantle, but would the basic lineaments of the landscape be different? This question has been the source of much geomorphological debate in recent years, but the general agreement is that at the large scale, it wouldn’t look much different. This is because of the overwhelming importance of tectonics and climate as the basic controls on landscapes at this scale. However, at the more local scale, there are many instances of landscapes whose very fabric and appearance have been heavily influenced by plant and animal action. This question is not purely of relevance to today’s landscape, as over the long history of the Earth, life has developed through evolution alongside changing geomorphological landscapes, and early plant and animal communities may have imprinted themselves rather differently on the landscape. Such palaeo-landscapes are occasionally visible in geological sections.

Perhaps the more crucial role that plants, animals, and micro-organisms play within the landscape is not directly through creating landforms, but indirectly through their influence on Earth surface processes. There is a bewildering array of biological influences on geomorphic processes, but they largely fall into two types, either accelerating or retarding the rates of process operation. There are many instances where animals accelerate Earth surface processes, as seen, for example, in the way in which grazing of cattle and other large animals can lead to accelerated soil erosion, through removing the protective layer of vegetation. Cattle have also been observed to accelerate the erosion of river banks as they come down in herds to drink. Smaller animals, such as ants, termites, earthworms, and ground-dwelling birds, have huge effects on soil cycling and mineral transformations, through activities which move and alter sediment (for example, through their guts). Micro-organisms are similarly highly effective agents of mineral transformation, and there is much evidence to suggest that they can radically enhance weathering rates. Some species of lichen, for example, are able to etch their way into a rock surface, producing small-scale pits as a result (see Figure 11). At a larger scale, vegetation communities as a whole have been shown to dramatically enhance the rates of rock weathering. If you compare the rate of weathering under soil and vegetation communities (especially forested ecosystems) with that on bare rock surface, you will find that they are several orders of magnitude higher.

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