14.2 Applications and skills
14.2.2 Pyramids, energy and nutrient diagrams
The shape of a pyramid of net productivity is shown in Figure 14.2.2a. The amount of energy stored as biomass is very high at the lowest trophic level, because a lot of producers are needed to support consumers.
Figure 14.2.2a – General shape for all pyramids of productivity
Since consumers depend on producers for energy, the distribution and abundance of life in any ecosystem is determined by its net primary productivity (NPP). The average NPP of each of the world’s major biomes is shown in Figure 14.2.2b.
Figure 14.2.2b – Net primary productivity of the Earth’s major ecosystems
Activity 1: Interpreting energy diagrams
- Explain how the shape of all energy pyramids illustrates the second law of thermodynamics.
- Predict which of the ecosystems shown in Figure 14.2.2b can support the longest food chains. Explain your prediction.
- Discuss the relationship between NPP and climate conditions in the major biomes. Refer to [link] Figure 4.1.2d [end link] and Figure 14.2.2b in your answer.
Application: Sustainable food
At each trophic level, energy is used in respiration, so the rates at which food is converted to biomass differ. Food conversion rates are expressed as ratios of feed input to protein output, and are important considerations in agriculture.
Figure 14.2.2c – Food conversion ratios
Amount of plant feed necessary to produce different types of food for human consumption. Cattle have the least efficient food conversion rates: approximately 7kg of feed is needed to produce 1kg of cow protein.
Representing nutrient flows in Gersmehl diagrams
Nutrients such as carbon, nitrogen and phosphorus flow between three major stores in an ecosystem. The nutrient stores are:
- biomass (B)
- litter (L)
- soil (S)
Each ecosystem has a unique profile for the amount of nutrients in each of the stores, as well as the rate of flow, or flux, between the stores. Some of the processes that contribute to rate of flux are shown in the following table.
Leaching, erosion, runoff, sedimentation, weathering, precipitation, decomposition, death, precipitation, feeding/assimilation
Fertiliser use, sewage (eutrophication), harvesting, clear cutting (removal of nutrients)
Table 1: Processes that contribute to nutrient flow
Gersmehl diagrams are useful in visualising the inter-relationships between nutrient stores and flows. In a Gersmehl diagram:
- Stores (B, L, S) are represented by circles. The size of the circle represents the amount of biomass held there.
- Fluxes are represented by arrows. The thickness of the arrow represents the rate of flux, but the arrows themselves may represent many processes.
The Gersmehl diagrams for three different ecosystems are shown in Figures 14.2.2d–f.
Figure 14.2.2d – Nutrient flow in a rainforest: the largest store is biomass (B)
There is high precipitation (P) all year round, so rapid transfer between stores and environment because moisture and heat are ideal for decomposers.
Figure 14.2.2e – Nutrient flow in the taiga (boreal forest): the largest store is litter (L)
There is very little transfer between stores.
Figure 14.2.2f – Nutrient flow in a desert: the largest store is soil (S)
There is a high supply of litter but little transfer between stores.
- Compare pyramids of energy from different ecosystems
- Construct of Gersmehl diagrams to show relationships between nutrient stores and flows
Depending on the discipline, an energy pyramid may be called a pyramid of ‘biomass’ or ‘productivity’ or simply ‘energy’. The general shape of all of these pyramids is the same when the following units are used:
Science and social responsibility (Aim 8)
Forests are often cleared to make room for grazing livestock, which then emit carbon dioxide, a greenhouse gas, as a product of respiration. Eating less meat significantly reduces a person’s carbon footprint. Is a vegetarian diet more socially responsible?
Figure 14.2.2g – Rainforest
In a rainforest, high precipitation and moisture lead to high productivity and biomass.
Figure 14.2.2h – Taiga
In a boreal forest, or taiga, the main store of nutrients is litter – mostly leaf needles.
In a desert, biomass does not decompose due to dry conditions. The main nutrient store is soil.
On the Gersmehl diagrams, identify which arrows represent the processes in the table.