In arid and seasonally dry areas such as in Australia and the west-American plains, continuous grazing causes environmental disasters; essentially desertification of the land. However, a return to herding-style animal husbandry with long recovery periods between grazings allows the land to heal. In very dry regions, seeds must be planted deeply so that when they germinate the plant roots can reach groundwater. Only the hooves of animals can do this over millions of hectares. Without these actions by grazing animals, which plant seeds and recycle nutrients, dryland (arid) ecosystems become desert because:
standing dead growth chokes the growth of new plants, instead of mulching the soil;
rather than becoming well-set into the soil, the seeds instead sprout on the soil surface, and then die;
as old plants die, the amount of bare ground increases, and this loses its ability to absorb and store water;
this leads to rivers, springs, and wells becoming dry, and hence droughts become the norm.
Soil may be classified according to the degree of its "Brittleness" for which different kinds of animal management are appropriate:
A scale has been indicated (see link under Related Reading, at end of this article) from 1 to 10:
‘non-brittle tending’ (Rain forest).
‘brittle tending’ (True desert).
Brittleness concerns how evenly the humidity is spread throughout the year, rather than the total rainfall. The more dry months there are compared to humid months, the more brittle tending the location is. Temperate regions are more ‘non-brittle tending’ with a fairly even balance of humidity throughout the year. In an actual rainforest, the humidity is nearly constant. In more brittle tending locations the humidity tends to be seasonal, such that there are are wetter summers and drying summers. The more various and seasonal the humidity is, the total rainfall (precipitation) drops, tending toward a true desert where every day is dry.
Non-brittle tending areas cover only up to one third of the earth's land surface (ca 50 million km^2) overall, and less than this proportion on most continents. In non-brittle regions, there is enough moisture overall to keep healthy populations of microbes, worms, beetles and bugs, which help to aerate the soil and move nutrients around. Carbon cycling is a natural feature of such environments, since grasses and woody plants are efficiently broken down by these organisms, or it is eaten by small animals who move around in small groups and contribute their manure to the system allowing its carbon to be cycled. In conclusion, it is the humid nature of a particular environment that sustains its carbon cycling mechanism.
It is the case, however, that the majority of the earth's surface is seasonal, where there are periods of extensive growth during the growing season (as shown by the Keeling Curve, and its annual oscillation, where during the growing season in the northern hemisphere the atmospheric CO2 concentration falls as the gas is taken up by plant growth through photosynthesis), followed by a period where not much grows and plants either die or become dormant. In order for the large volume of dry, standing vegetation and its carbon to be cycled before the next growing season, since it is now too dry for insects to take part in any decomposition processes, herds of large herbivores make their contribution. In their guts are populations of bacteria and other microbes, and convert the plants they graze into urine and dung, which are returned to the soil.
As they move around in herds, their hooves trample standing plants and so provide a protecting and continually decomposing cover to the soil.
Non-brittle tending environments tend to be fairly resilient to the impact of human farming practices and their carbon cycling ability is unlikely to be damaged in the long term. Any such damage that is incurred is quite quickly healed once the particular practice is stopped. The underlying problem of agriculture is that the types of farming that are fine in non-brittle regions have been adopted in brittle-tending areas which are less forgiving to them, to the level of devastation.
There has been the tendency for individual herds of wild and domestic animals to be reduced in size but more spread-out in their grazing behaviour so that the same region of land is grazed more constantly than in the natural environment. For this reason, ecosystems have broken down along with their intrinsic carbon cycling, in the majority of brittle tending areas of the Earth. But by knowing these facts, humans can change their land management practices to restore the natural brittle environment as a self-sustaining, symbiotic mechanism, by mimicking principal key features.
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