Human activities can impose far-reaching effects on the environment

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Christopher Magee                Monday 6th October 2003                

Over the last two hundred years, the human population has grown exponentially. In the process of trying to satisfy the needs of these growing numbers, we have changed our planet; for example, millions of hectares of forests have been cleared to supply timber and land for homes and agriculture, deserts have grown, some rivers have dried up, and the air and oceans have been polluted.

        The Second World War reminded many countries of the importance maintaining self-sufficiency in basic foods, In the UK, between 1945 and the mid-1980s; the overriding aim of agricultural policy was to increase production. Food production is the oldest industry of all. Without a source of food and energy, there can be no life, and it is perhaps not surprising that humans have drastically altered the natural environment in order to grow more food. The nature of agriculture in the UK changed more during these 50 years than it had done in the previous two centuries. Such changes had profound environmental effects (Figure 1.1 and 1.2)

The result of all these changes has been that agriculture has become more intensive, producing higher yields per acre by relying on greater chemicals use and technological inputs. It also has become more expensive, relying on purchase of machinery and chemicals to replace the heavy labour requirements of the past. To remain competitive, farmers have been forced to become more efficient, farming ever-larger acreages with bigger equipment and more fertilizers and pesticides. Much larger farms consisting of extensive fields of a single crop have in large part replaced small farms growing a wide variety of crops.

Farmers manipulate the natural environment so that they get as much food as possible from their land. In doing so, they create new ecosystems – agroecosystems, which are very different from natural unmanaged ecosystems. Agroecosystems are simpler than wilder ecosystems – they have less plant and animal genetic diversity and a less complex spatial structure. There are fewer and shorter food chains and this means that more of the available solar energy is channelled into crop and, either directly or via livestock into humans. Humans effectively remove this energy, along with many nutrients, from the ecosystem when they harvest the crop. As a result, a much smaller fraction of the energy and nutrients in the ecosystem passes along detritus/decomposer food chains.

        Nutrient cycles are therefore distorted. In order to maintain fertility, the nutrients that are removed in the harvested crop must be replaced either by adding artificial fertilisers or by adding organic matter in the form of manure. Artificial fertilisers such as ammonium sulphate contain no organic matter and are applied as powders that, being soluble, can easily leach through the soil before plant roots can absorb them. Plants that might compete with the crop are removed or killed by herbicides. Soil organisms, flying insects – which may feed on the plants, and disease-causing pathogens are killed by pesticides.

        Thus, humans radically alter natural ecosystems to produce agro-systems. This alters the nature and number of plant and animal species, the flow of energy and the cycling of nutrients.

As earth's population grows so does the demand for food, and the use of pesticides has become essential in meeting this demand. Improved agricultural yields, reduced energy costs for producers and lowered costs of food for consumers are attributed to the controlled use of pesticides. But pesticides are not without cost to wildlife, livestock and even people unless proper care is taken in their use. Pesticide use skyrocketed after World War II. At the same time, significant numbers of wildlife deaths began appearing.

Levels of pesticides in soils are constantly changing and as a result causing many problems (see Figure 1.3). So many variables and processes are involved, that rates of accumulation of even the most persistent insecticides are quite variable and difficult to determine. The problem of accumulation in soils arises because the tiny organisms in soil are not capable of degrading many pesticides at rates sufficiently high to prevent soil and also water pollution. Thus, the persistent types, such as DDT and other chlorinated hydrocarbons, remain available for absorption by higher animals (including human beings) and for causing harm to non-target organisms.

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For example the pesticide DDT accumulates in the fatty tissues of carnivorous animals, inhibiting cytochrome oxidase and limiting reproductive success (especially thin eggshells in birds of prey). Thus, the overuse of persistent pesticides may lead to accumulation of these compounds in food chains. The top carnivores consume many organisms from lower down the food chain, and so ingest high levels of pesticide. Other organic molecules from the prey are respired, so the relative concentration of the pesticide increases. Hence DDT's ability to spread through the food chain was especially insidious because the chemical was able to bioaccumulate and bio-magnify.

Bio-magnification is ...

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