The organisms living in a garden pond interact with each other in a wide variety of ways (see figure 7). Plants and algae constitute the basic source of nutrition for the ecological system that exists in the pond. They are fed upon by micro-organisms which, in turn, are eaten by the organisms on the next level of the ecological order. Fish feed off small organisms and micro-organisms (plankton), or off the succeeding members of the food chain. There are also types of fish which are herbivorous. Fish excrement consisting of dead plankton and plant remains are further degraded by organisms in the bed of the pond before finally being mineralized by micro-organisms. During the process of mineralization, organic compounds are oxidized and nutrients such as nitrate and phosphate set free. These nutrients are again made available to growing plants and algae.
The cycle has thus been completed. The sequence of processes demonstrates how much the individual organisms depend on one another. This system of interdependences is referred to as the food chain, or, better still, the food network. The stability of this food network is vital for the maintenance of a healthy biological balance. The greater the diversity of the species, the more stable the food network (and thus the whole ecological system) will be. If the network is disrupted in any way by external influences, all the organisms in the system will be affected.
The food chain is often represented in the form of a pyramid, since during each round of devouring and being devoured losses of energy and biomass of up to 90 % take place (see figure 6). This is well illustrated by the following simple fact: It takes 100 kg of soya meal to produce 10 kg of pork for human consumption. If humans fed off the soya meal direct they would consume only one tenth of the amount required to fatten the pigs.
Carbon Cycle
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