Trophic levels are the feeding position in a food chain such as primary producers, herbivore, primary carnivore, etc. Green plants form the first trophic level, the producers. Herbivores form the second trophic level, while carnivores form the third and even the fourth trophic levels. In this section we will discuss what is meant by food chains, food webs and ecological pyramids.
The feeding of one organism upon another in a sequence of food transfers is known as a food chain. Another definition is the chain of transfer of energy (which typically comes from the sun) from one organism to another. A simple food chain is like the following:
rose plant -- aphids -- beetle -- chameleon -- hawk.
In this food chain, the rose plant is the primary producer. The aphids are the primary
consumers because they suck the juice from the rose plant. The beetle is the primary
carnivore because it eats the aphids. The chameleon, a secondary carnivore, eats the
beetle. The hawk is the tertiary carnivore because it eats the secondary carnivore, the
chameleon. The hawk eventually dies and its remains are broken down by decay-causing
bacteria and fungi.
Except in deep-sea hydrothermal ecosystems, all food chains start with photosynthesis and will end with decay.
In an ecosystem there are many different food chains and many of these are cross-linked to form a food web. Ultimately all plants and animals in an ecosystem are part of this complex food web.
Trophic levels and the energy flow from one level to the next, can be graphically depicted using an ecological pyramid. Three types of ecological pyramids can usually be distinguished namely:
It is easily understood that many grass plants are needed to feed fewer snails on which, in turn, even fewer chickens would be able to feed. This in turn requires only a few people to eat the chickens that ate the snails. The Number pyramid shows the number of organisms in each trophic level and does not take into consideration the size of the organisms and over-emphasizes the importance of small organisms. In a pyramid of numbers the higher up one moves, so each consecutive layer or level contains fewer organisms than the level below it.
The Number Pyramid
This pyramid indicates the total mass of the organisms in each trophic level. The size of the organism is over-emphasized and it can happen that the mass of level 2 is greater than that of level 1, because the productivity of level 1 is not taken into consideration. Thus an enormous mass of grass is required to support a smaller mass of buck, which in turn would support a smaller mass of lions.
The Biomass Pyramid
The Energy pyramid indicates the total amount of energy present in each trophic level. It also shows the loss of energy from one trophic level to the next. An energy pyramid shows clearly that the energy transfer from one trophic level to the next is accompanied by a decrease due to waste and the conversion of potential energy into kinetic energy and heat energy. The energy pyramid is more widely used than the others because comparisons can be made between trophic levels of different ecosystem. It is, however, more difficult to compile an energy pyramid than it is compile the other types of pyramids.
The Energy Pyramid