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A-level Geography - Ecosystems: Change and Challenge: Nature of Ecosystems

Structure of Ecosystems

An ecosystem is a community of living organisms (plants, animals and microbes) in conjunction with the nonliving components of their environment (things like air, water and mineral soil), interacting as a system. These biotic (living) and abiotic (nonliving) components are regarded as linked together through nutrient cycles and energy flows. As ecosystems are defined by the network of interactions among organisms, and between organisms and their environment, they can be of any size but usually encompass specific, limited spaces.


The energy that flows through ecosystems is obtained primarily from the sun. It generally enters the system through photosynthesis, a process that also captures carbon from the atmosphere. By feeding on plants and on one another, animals influence the quantity of plant and microbial biomass present. By breaking down dead organic matter, decomposers release carbon back to the atmosphere and facilitate nutrient cycling by converting nutrients back to the ecosystem.

Energy Flows

All the energy used by living things comes ultimately from the sun. Energy enters living systems as a result of photosynthesis by plants and some bacteria and protists. Less than 4% of the incident sunlight is captured. More than half of the energy captured by plants is used in respiration for maintenance. Energy used in respiration is lost as heat and therefore unavailable to other organisms. The other half is converted to plant tissues.


There are 2 types of organisms that have direct access to the energy in plant tissues: herbivores, that feed on the plant while it is alive, and decomposers, that feed on the plant after it is dead. In most ecosystems, the majority of the energy goes to the decomposers. In a grassland, for example, only 10% of the energy in plants is taken by grazing animals such as buffalo. Herbivores use almost all of their energy intake on respiration for body maintenance; the rest goes to herbivore biomass. Much of the energy in herbivore biomass is taken by carnivores, such as wolves, while some goes to decomposers. Almost all of the energy taken in by carnivores goes to maintenance. The decomposers, which receive most of the plant energy, use up over half of it in maintenance. The rest may be locked up in soil organic material or taken by organisms that feed on decomposers.


Ultimately, all of the energy originally captured by plants is transformed and lost as heat; energy is not recycled. The following animated diagram gives some details of how energy flows.

Energy Pyramid

An energy pyramid is a graphical model of energy flow in a community. The different levels represent different groups of organisms that might compose a food chain. From the bottom-up, they are as follows:

  • Producers (or autotrophs)  — bring energy from nonliving sources into the community.
  • Primary consumers — eat the producers, which makes them herbivores in most communities.
  • Secondary consumers — eat the primary consumers, which makes them carnivores.
  • Tertiary consumers — eat the secondary consumers, they may be.
  • Decomposers  — operate at each trophic level, organism that breaks down plants, animals and waste matter. Fungi and becteria are decomposers.


Energy pyramid depicts the energy is minimum as the highest trophic level and is maximum at the lowest trophic level. At each trophic level, there is successive loss of energy in the form of heat and respiration, etc. Other than this, there are also:

  • Pyramid of numbers — depicts the relationship in terms of the number of producers, herbivores and the carnivores at their successive trophic levels.
  • Pyramid of biomass — represents the relative amounts of biomass at each trophic level.


Nutrient Cycles

The following presentation shows the pathways of Gershmels nutrient cycles:

Within all ecosystems, nutrients are required for plant growth and are recycled from one store to another. For example, leaves fall from trees and as they decompose nutrients are returned to the soil. The model of the nutrient cycle was first developed in 1976, by P.F. Gersmehl, who attempted to show differences between ecosystem regards nutrients, transferred and stored between three areas. Circles of proportionate size are drawn to represent the stores of nutrients within the biomass, litter and soil. Nutrient transfers, inputs and outputs are represented by arrows of varying thickness, depending on the relative rates of transfer between the stores.


In all nutrient cycles there are interactions between the atmosphere and soil and many food chains are involved. Nutrient cycles vary greatly between ecosystems, as the rate of nutrient transfer is dependent on the amount of moisture, heat, vegetation and the length of the growing season. The diagrams above show the model of nutrient cycling, and the variation between different nutrient cycles within the Taiga, Steppe and Equatorial Rain Forest.

More ecological pyramids

The following slide shows a set of energy pyramids, number pyramids and biomass pyramids with more details, you can right click to view the original full scale image in a new tab/window.

Trophic Levels

Trophic level

Trophic Level is a  nutritive series, or food chain ecosystem. The organisms of a chain are classified into these levels on the basis of their feeding behaviour. The first and lowest level contains the producers, green plants. The plants or their products are consumed by the second-level organisms—the herbivores, or plant eaters. At the third level, primary carnivores, or meat eaters, eat the herbivores; and at the fourth level, secondary carnivores eat the primary carnivores. These categories are not strictly defined, as many organisms feed on several trophic levels; for example, some carnivores also consume plant materials or carrion and are called omnivores, and some herbivores occasionally consume animal matter.