BIOLOGY Volume 1 - A Guide to General Biology - 2004

10. ORGANISMS AND THE ENVIRONMENT

10.8. Human Impact on Ecosystems

10.8.3. Destruction of Terrestrial Ecosystems

Since prehistoric times, humans have modified ecosystems to secure food, shelter, fuel, and other resources, as well as to dispose of waste. Early hunter-gatherer communities had a minimal impact on the natural environment, but as human populations grew larger, more sedentary, and technologically advanced, the resulting changes to the biosphere steadily intensified, in some places reaching catastrophic proportions over the past few centuries. For example, deforestation in Britain began as early as the Neolithic period, but progressed at an increasing pace, and by the early 20th century, forests survived on only 3% of the country's area. Today, the global scale of deforestation is a source of mounting concern.

The destruction of forest vegetation is typically driven by the demand for new agricultural fields and pastures, leading to The Emergence of entirely new anthropogenic ecosystems. The mismanagement of such ecosystems creates further problems, including soil erosion, desertification, and the Adverse effects of synthetic pesticide use.

Global Deforestation

Forests represent climax communities across a significant portion of the planet and covered one-third of the land surface until relatively recently. Today, the area of temperate forests is shrinking only slightly, but these are merely remnants of the continuous prehistoric tracts once cleared by humans. Many developed countries strictly protect their remaining forest resources and engage in large-scale reforestation. Conversely, tropical rainforests continue to be cleared at an alarming rate. While they accounted for approximately 15% of the world's land area in 1950, this share dropped to 7% by 2000, even though this still corresponds to 300 million hectares. Current estimates suggest that 12 million hectares of forest disappear annually—an area equivalent to England—while another 10 million hectares degrade due to the harvesting of high-value timber species, poor management practices, and the neglect of conservation issues.

Where population density is low, traditional shifting cultivation (slash-and-burn agriculture), which involves clearing tropical forests, does not cause significant long-term damage. Trees in a small plot are felled, stumps are uprooted, the timber is burned, and the cleared soil is plowed and sown. The ash acts as a fertilizer, and the fire destroys potential pests. Crop yields are quite good in the initial years, but then decline rapidly because tropical soils are inherently nutrient-poor (see below). At the same time, the field begins to be invaded by regenerating forest vegetation. After 3–4 years, the plot is abandoned because clearing and cultivating a new site is easier. Within 30–40 years, a mature tropical forest canopy is restored on the fallow land.

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Fig. 10.30. A farmer planting cassava in a plot cleared by slash-and-burn agriculture amidst the Malagasy rainforest.

This final point is crucial for maintaining soil fertility. In tropical rainforests, the bulk of the ecosystem's organic matter and mineral nutrients is concentrated in the above-ground parts of plants. Litter, like any POM, mineralizes very rapidly, and within 5–6 weeks, if not sooner, its nutrients are reincorporated into the living biomass—almost nothing potentially useful accumulates in the soil. Trees form a dense surface network of roots that absorbs 99.9% of mineral salts leaching through the ground. Many forest species belong to the legume family, hosting nitrogen-fixing Bacteria in ROOT nodules. Other species typically associate with mycorrhizal Fungi, which transfer vital minerals directly from decomposing litter to the plants as part of their own Nutrition. Such rapid nutrient cycling maintains high ecosystem productivity despite low soil fertility. This system is well-suited for shifting cultivation, where small plots are comparable to natural canopy gaps («windows») that periodically form and quickly regrow. However, large-scale deforestation for agricultural land under such conditions is clearly unsustainable.

For the shifting cultivation system to be sustainable, the original vegetation must regenerate on the fallow land. If a previously used plot is cleared again before a mature forest has time to develop, The amount of ash produced will be lower than before, meaning the soil will be poorer in nutrients, crop yields will drop faster, and the farmer will be forced to abandon the field sooner and cut down neighboring forest.

As the global population grows, a further expansion of land devoted to food crops is quite likely. Deforestation will also be driven by increasing demands for fuel—firewood and brushwood. Further compounding the situation is the threat of changing land use, particularly the clearing of large areas for non-food crops or pastures. In some countries, commercial logging of valuable species such as teak and mahogany is a primary driver of forest degradation.

Deforestation seriously impairs the state of the biosphere for numerous reasons.

1. The loss of a traditional source of products utilized by local communities—timber, firewood, vines, honey, fruit, herbs, game, etc.

2. A threat to the long-term satisfaction of the growing global demand for structural timber, furniture wood, and raw Materials for the paper industry.

3. Forests are often located on uplands and watersheds, intercepting a significant portion of rainfall. The forest canopy mitigates The impact of heavy tropical rainfall on the environment in various ways. It returns large amounts of Water to the atmosphere via evaporation and Transpiration while simultaneously minimizing surface runoff, thereby ensuring water infiltration into the soil. This leads to The formation of a stable deep groundwater table that steadily feeds streams and rivers. Destroying the forest canopy dramatically increases surface runoff, drying out watershed areas and causing extreme fluctuations in water levels in lowland rivers. For instance, the unprecedented floods in Bangladesh in the summer of 1988, which affected most of the country, were attributed primarily to deforestation in the mountains of northern India and Nepal.

4. Increased surface runoff leads to soil erosion. The fertile topsoil can be completely washed away, leaving exposed ground that is unsuitable not only for agriculture but also for the regeneration of the original forest stand. Concurrently, reservoirs, estuaries, and harbors silt up, requiring regular dredging. The risk of landslides and mudflows also emerges.

5. Forests are the planet's primary oxygen producers and carbon dioxide consumers; consequently, their destruction leads to rising atmospheric CO₂ concentrations and an enhanced greenhouse effect, which threatens global climate warming (see section 10.8.1).

6. Forests are communities characterized by high species richness and diversity. Their destruction will lead to the extinction of poorly studied life forms, thereby depleting the planet's genetic reserves and reducing bio-resources potentially useful to humanity. Tropical forests have already provided us with anti-malarial and anti-Cancer drugs, and scientists are actively investigating rainforest plants in the search for treatments against AIDS and many other diseases.

Soil Erosion and Desertification

Deforestation is not the only cause of soil erosion. Mismanagement of fields and pastures leads to widespread soil loss. Hilly landscapes with steep, regularly plowed slopes in high-rainfall areas with long-cleared forests are especially vulnerable in this regard. Key measures for soil protection in such situations include terracing—traditionally used, for example, in Southeast Asia and proven to be highly effective; contour plowing (i.e., plowing across the slope); and constructing bunds around fields to retain surface runoff. Although these and other measures have long been known, up to 5 million hectares of arable land are lost to erosion annually. Overgrazing on pastures thwarts the growth of plant cover that anchors the soil. In such cases, rainwater flows freely across the surface, causing sheet erosion that washes away the upper fertile soil horizon. If water flows become concentrated in limited areas, deep gullies form. Every year, 7 million hectares of pasture are lost in this manner, with a significant portion of the abandoned land effectively turning into desert.

Desertification can also occur naturally, such as when arid regions experience below-average rainfall for several consecutive years. Today, however, this process is accelerated by human activity and understood in a broader sense. It refers not to the formation of a typical desert ecosystem, but to the degradation of fertile land leading to the complete loss of its agricultural value. The causes are usually as follows: 1) overgrazing; 2) over-cultivation; 3) deforestation (see above); 4) improper irrigation.

Overgrazing, or exceeding the carrying capacity of pastures, thwarts plant cover and tramples the soil, destroying its Structure. This is typically followed by wind and water erosion. Over-cultivation refers to the intensive use of soil for crop production without adequate measures to restore its fertility. This depletes soil mineral nutrients and humus, destroys soil structure, and ultimately thwarts plant cover, leading to erosion.

On irrigated lands, the main problems are waterlogging and salinization. Waterlogging occurs when groundwater is close to the surface. Improper watering under such conditions leads to prolonged root inundation, which crops such as wheat and cotton cannot withstand. Soil salinization refers to an increase in the concentration of soluble salts within the soil. This can happen for various reasons. In hot regions, wetting the soil during irrigation alternates with upward movement driven by capillary action of evaporating water, which draws dissolved salts up from the depths into the upper horizon. Sometimes deep wells are used for irrigation where the water, although considered fresh, contains an excessive amount of salts. If the soil is insufficiently permeable, these salts accumulate in the topsoil. Only a few crops can tolerate a solution concentration exceeding 0.5–1.0%, making the economic consequences of salinization painfully clear.

Combating desertification is a formidable challenge. A complete temporary ban on land use may be required until the vegetation cover is restored. This entails halting economic activity, rising unemployment, livestock liquidation, and similar hardships. Measures to reduce water erosion have already been discussed. To combat wind erosion, protective fences or shelterbelts can be employed. The problems of waterlogging and salinization are addressed by improving drainage and leaching the soil. Admittedly, all of this comes with economic, social, administrative, and political complexities. However, unless the root causes of unsustainable land use are addressed, there is little hope for the success of desertification control programs.