Community level systems Study Guide

In every ecosystem, organisms are organised into feeding levels called trophic levels. The flow of energy and nutrients through an ecosystem starts with producers. Producers, such as plants and algae, form the first trophic level. They convert light energy into chemical energy through photosynthesis, forming the base of the food web. Organisms that obtain their energy by consuming other organisms are called consumers. Consumers are categorised based on what they eat.

The second trophic level consists of primary consumers, also known as herbivores. These organisms feed directly on producers. Examples include rabbits eating grass or deer browsing on leaves.

Secondary consumers occupy the third trophic level. They are carnivores or omnivores that feed on primary consumers. For instance, a fox hunting a rabbit or a bird eating an insect would be secondary consumers. Tertiary consumers form the fourth trophic level and feed on secondary consumers. These are often top predators within a food web, such as an eagle eating a snake.

Sometimes, a fourth level of consumer, quaternary consumers, can exist. These organisms consume tertiary consumers. Throughout all trophic levels, decomposers play a vital role. Organisms like bacteria and fungi break down dead organic matter from all trophic levels, recycling nutrients back into the ecosystem. This cycling of nutrients is essential for the continuous functioning of an ecosystem.

Energy transfer between trophic levels is inefficient; only about 10% of the energy from one trophic level is transferred to the next. The rest is lost as heat during metabolic processes or in undigested waste. This inefficiency explains why there are fewer organisms and less biomass at higher trophic levels in an ecosystem.

In a food chain, energy is transferred from one trophic level to the next. This transfer is not 100% efficient due to several factors. Producers (like plants) convert light energy into chemical energy stored in biomass. When a primary consumer eats a producer, only a fraction of the producer's biomass is converted into the consumer's biomass. Energy is lost at each trophic level through metabolic processes such as respiration, movement, and heat. Furthermore, not all parts of an organism are consumed, and not all consumed biomass is digested and assimilated; some is egested as waste.

The efficiency of biomass transfer between trophic levels is calculated using the formula: (biomass in higher trophic level / biomass in lower trophic level) × 100%. Typically, only about 10% of the biomass, and thus energy, from one trophic level is transferred to the next. This low transfer efficiency has a significant impact on the length of food chains. Because so much energy is lost at each step, there is less and less energy available to support organisms at higher trophic levels. This limits the number of trophic levels a food chain can have, usually to three or four. A large base of producers is needed to support a much smaller biomass of primary consumers, who in turn support an even smaller biomass of secondary consumers, and so on. This reduction in biomass at successive trophic levels is often represented by a biomass pyramid.