ENERGY FLOW IN ECOSYSTEMS

Photosynthesis explains how energy from the sun is captured by green plants and used to make food. Most of this energy is used to carry on the plant's life activities. The rest of the energy is passed on as food to the next level of the food chain.






The figure at right shows energy flow in a simple food chain. At each level of the food chain, about 90% of the energy is lost in the form of heat. The total energy passed from one level to the next is only about one-tenth of the energy received from the previous organism. Therefore, as you move up the food chain, there is less energy available. Animals located at the top of the food chain need a lot more food to meet their energy needs.

The most fundamental law governing our planet's finite resources states that matter can neither be created nor destroyed. However, it can be reshaped or transformed. In converting matter from one form to anotherm energy is needed. Much human activity is devoted to the taming and transformation of matter from its "raw" or natural form into products which human beings find more convenient for their use, survival and pleasure. Such conversion has impacts on man himself, including negative impacts.

The material of the planet stays on the planet and undergoes continuous transformations through the energy provided by the earth and the sun. No form of life and no human technology can create something out of n othing and neither can anything discarded become nothing.

The laws of thermodynamics govern energy processes in biological systems. The first law of energy or thermodynamics, also known as the law of conservation of energy, states that the total amount of energy in all its forms remains constant. Thus energy is neither created nor destroyed; it can only be transformed.
Example:

When a goat eats a garden plant, the plant vanishes from sight. But the matter and chemical energy of the plant is converted to chemical energy and different forms of matter in the goat's body. In this case, matter and energy have been changed to different forms, but no matter or energy has been created or destroyed.

The second law of thermodynamics also known as the law of degradation of energy quality states that each time energy is transformed, it tends to go from a more organized and concentrated form to a less organized or more dispersed form. In this case the energy transfers are never 100% efficient. In any conversion of energy from one form to another, some of the initial energy input is always degraded to lower quality, less useful energy, usually low temperatures heat that flows into the environment. Energy can be changed in only one direction - from usable to unusable.

Example: When a car is driven, only about 10% of the high-quality chemical energy available in its gasoline fuel is converted to mechanical energy to propel the vehicle and electrical energy to run its electrical systems. The remaining 90% is degraded to low-quality heat that is released into the environment and eventually dissipated or "lost" to space.

FEEDING RELATIONSHIPS

Energy in the form of energy-rich molecules stored by the producers is taken in or eaten by a series of consumers. Ultimately any energy fixed by producers or accumulated by consumers and not used by them is released by decomposers. This feeding relationship is either by a FOOD CHAIN or a FOOD WEB.

Food chain in an ecosystem refers to the transfer of food energy from the source (e.g. plants) through a series of organisms, in a process of sequential or repeated eating and being eaten. Each time one organism eats another in the food chain, energy is transferred. For every transfer about 90% of the potential energy is lost as heat. The transfer or movement of energy is in one direction only- usually from plants to a series of animals eating each other. Man is ultimately at the highest trophic level.

Trophic level - each step in the flow of energy through an ecosystem. Producers (plants) constitute the first trophic level, and hervibores constitute the second trophic level. Carnivores that eat hervibores are the third trophic level, and carnivores that eat other carnivores are the fourth tropic level. Omnivores, parasites and scavengers occupy different trophic levels, depending on what they happen to be eating at the time. If we eat a piece of beef steak, we are at the third trophic level; if we eat rice, we are the second trophic level.

IMPORTANT POINTS:

In our last lecture we examined the creation of organic matter by primary producers. Without autotrophs, there would be no energy available to all other organisms that lack the capability of fixing light energy. However, the continual loss of energy due to metabolic activity puts limits on how much energy is available to higher trophic levels (this is explained by the Second Law of Thermodynamics). Today we will look at how and where this energy moves through an ecosystem once it is incorporated into organic matter.
Most of you are now familiar with the concept of the trophic level (see Figure 1). It is simply a feeding level, as often represented in a food chain or food web. Primary producers comprise the bottom trophic level, followed by primary consumers (herbivores), then secondary consumers (carnivores feeding on herbivores), and so on. When we talk of moving "up" the food chain, we are speaking figuratively and mean that we move from plants to herbivores to carnivores. This does not take into account decomposers and detritivores (organisms that feed on dead organic matter), which make up their own, highly important trophic pathways.

Figure 1: Trophic levels.

Food chains can be classified into two types:

• Grazing Food Chain - this starts from a green plant base from which the chain goes to grazing hervibores (the organisms eating living plants), and then to carnivores (the organisms eating animals).
• Detritus Food Chain - this goes from dead organic matter into microorganisms and then to detritus-feeding organisms (detrivores) and their predators.

Food webs - refer to the interconnected or interlocking relationships among various food chains in an ecosystem. In a food web, an organism may feed on several members of the web. In complex natural communities, organisms whose food is obtained from plants by the same number of steps are said to belong to the same trophic level.

The food pyramid constitutes the over all structure of dependency among the biotic or living elements in the ecosystem. At the lowest level of the food pyramid are the food sources (generally plants). One may even go further down and say that the base of the food pyramid constitutes organic matter that serves fertilizer or food sources.

SUMMARY:

The concepts of food chains, food web and food pyramid are important to consider in monitoring effects of activities in ecosystems. Any activity not naturally or originally part of the ecosystem would certainly cause disruptions in interactions related to taking in food.

Example: Construction of a bridge may result in river siltation that damages marine plants which are used as food by fishes. Sometimes these activities even poison fishes due to the toxic construction materials, which eventually leads to human food poisoning as man takes in fish from the food chain.