Revised 24 May 2019

All ecosystems are solar based and have some common characteristics that abide by the Laws of Thermodynamics. Within an ecosystem there are food producers in the form of plants which use sunlight to produce food and food consumers in the form of animals which use this food. Both producers and consumers maintain an ecosystem by recycling the by-products given off by each other. Figure 1 shows a balanced ecosystem.

Figure 1: Balanced Ecosystem (Odum & Odum, 1976, p95)

Sunlight supports the photosynthetic process used by producers whereby heat disperses into the atmosphere and oxygen is produced. Consumers use this oxygen and feed on the food provided by producers and give off heat and carbon-dioxide as they do so. Carbon dioxide plus sunlight forms the food of the producers. The producer-consumer interaction of the two processes of photosynthesis and respiration are complementary - one provides what the other needs. This complementary interaction is called symbiosis. 

Figure 2 shows this producer-consumer (P-R) symbiosis. Chemical components are isolated to show how they cycle in the P-R process. Each component contributes part of the energy requirement. The nutrients are shown coming out with the heat, but going on separately to be cycled and reused.

Figure 2: Producer-consumer symbiosis (Odum & Odum, 1976, p100)

Plants develop living biomass from the process of photosynthesis. Upon dying, this biomass falls to the ground as wood and forest litter. Some of this biomass is eaten by consumers before dying, or is consumed by insects and microbes after dying. These processes form a food chain. A plant eaten by one animal might in turn be eaten by another animal, and so on. Most ecological systems develop food chains with at least five stages. A food chain is linear and follows the generalized form of Plant - Herbivore – Carnivore. The quality of energy is upgraded at each step of the chain, but with an inevitable accompanying loss of energy to heat due to the Second Law of Thermodynamics.

In nature, the food and feeding relationships of plants and animals are rarely in the form of a simple linear food chain, but instead interconnects with a large number of other food chains to form a food web. This web can become extremely complex, and the discovery and description of a food web in any given habitat is an enormous task. Tampering with a food web can have some unexpected and undesirable effects.

Animals at the base of a food chain are relatively abundant, while those at the other end are relatively few in numbers. There is a progressive decrease between the two extremes. When comparing the population of other carnivores and herbivores with that of humankind, humankind does not have the same symbiotic relationship within the ecosystem. The reason why is because humankind has been able to tap resources of energy which other parts of the ecosystem cannot use. By utilising this energy, humankind has been able to sustain a larger population than that of other similar sized animals. 

All members of an ecosystem have inflows and outflows of mineral nutrients. Figure 3 is representative of a typical ecosystem with inflows of both organic matter and mineral nutrients. Both photosynthesis and respiration are stimulated to higher levels than they would reach without the inflows.

Figure 3: Steady state inflow and outflow of nutrients (Odum & Odum, 1976, p104)