Ecological Pyramids
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You should understand the concept of trophic levels and recall that energy transfer between them is inefficient, typically around 10%.
Why this matters
Ecological pyramids provide a visual framework for understanding energy flow and biomass distribution in ecosystems.
While the 10% rule gives us a quantitative measure of energy loss, ecological pyramids offer a structural representation of how organisms and energy are organised within a community. By examining the shape of these pyramids, ecologists can infer the stability of an ecosystem, the relative abundance of species at different trophic levels, and the potential for population fluctuations. This lecture moves beyond simple energy calculations to explore how biomass, numbers, and energy relate to one another, and why these relationships sometimes appear counter-intuitive.
The Intuitive Picture
Imagine a pyramid built out of blocks. At the base, you have a massive number of producers, such as grass or phytoplankton. Above them, a smaller stack of herbivores, like rabbits or zooplankton. At the very top, a single block representing a top predator, like a hawk or a shark. This is the classic image of an ecological pyramid. In most terrestrial ecosystems, this shape holds true. The base is wide because producers capture a large amount of solar energy, and each subsequent level supports fewer individuals due to the energy lost as heat during metabolism. This structure reflects the fundamental constraint that energy is not created but transformed, and a significant portion is dissipated at every step. However, this simple visual metaphor can be misleading. Similarly, a single large tree (producer) might support hundreds of insects (primary consumers) and dozens of birds (secondary consumers), creating an inverted pyramid of numbers. Therefore, while the pyramid shape is a useful heuristic, it is not a universal law. The key is to understand what is being measured: energy, biomass, or individual counts. Each metric tells a different story about the ecosystem's structure and function.
Pyramids of Energy
The pyramid of energy is the most robust and universally applicable form of ecological pyramid. It plots the rate of energy flow through each trophic level, typically measured in joules per square metre per year (\(\mathrm{J\,m^{-2}\,yr^{-1}}\)). Because energy is constantly being lost as heat due to the second law of thermodynamics, the pyramid of energy is always upright. It never inverts. This is because the energy available to the next trophic level is strictly a fraction of the energy assimilated by the current level. From the previous lectures, \( E_n \approx E_0\,\varepsilon^{\,n-1} \), where \(\varepsilon\) is the ecological efficiency. If \(\varepsilon\) is 0.1, then the second level has 10% of the first, the third has 1% of the first, and so on. This exponential decay ensures that the base is always the largest. For example, if a grassland ecosystem has a net primary production of \(10,000\ \mathrm{J\,m^{-2}\,yr^{-1}}\), the herbivore level might support \(1,000\ \mathrm{J\,m^{-2}\,yr^{-1}}\), and the carnivore level \(100\ \mathrm{J\,m^{-2}\,yr^{-1}}\). This consistent shape allows ecologists to compare vastly different ecosystems, from tropical rainforests to arctic tundra, on a common quantitative basis. The energy pyramid directly reflects the metabolic constraints of life, making it the gold standard for assessing ecosystem productivity and stability.
Pyramids of Biomass
The pyramid of biomass plots the total mass of living organisms at each trophic level, usually expressed in grams per square metre (\(\mathrm{g\,m^{-2}}\)) or tonnes per hectare. Unlike the energy pyramid, the biomass pyramid can be inverted. This occurs in ecosystems where producers have a high turnover rate but low standing crop. A classic example is the open ocean. Phytoplankton are small, short-lived, and reproduce rapidly. Their total biomass at any given moment is often lower than that of the zooplankton that feed on them. Despite this inverted shape, the energy flow is still upright because the phytoplankton are constantly being replaced. The biomass pyramid is a snapshot of standing stock, whereas the energy pyramid is a measure of flow. In terrestrial forests, the biomass pyramid is typically upright, with trees forming a massive base. However, in some grasslands, the biomass of herbivores can sometimes approach or even exceed that of the grasses, depending on the season and grazing pressure. Understanding the distinction between standing biomass and energy flow is crucial for interpreting these shapes correctly. An inverted biomass pyramid does not violate thermodynamic laws; it simply indicates a high rate of production relative to the standing stock.
Pyramids of Numbers
The pyramid of numbers plots the actual count of individual organisms at each trophic level. This is the most variable and often the most misleading of the three pyramids. It depends heavily on the size of the organisms. In a forest ecosystem, a single oak tree (producer) may support thousands of insects (primary consumers) and hundreds of birds (secondary consumers). This creates an inverted pyramid of numbers, even though the energy and biomass pyramids are upright. Conversely, in a grassland, a single grass plant may support only a few grasshoppers, resulting in an upright pyramid of numbers. The pyramid of numbers is useful for understanding population dynamics and species interactions but is poor for assessing energy flow or ecosystem productivity. It is sensitive to the definition of a "trophic level" and the size of the organisms involved. For instance, a single baleen whale (a secondary consumer) may consume a vast number of krill (primary consumers), creating a highly skewed number ratio. Because it does not account for the energy content or mass of individuals, the pyramid of numbers is rarely used in quantitative ecological modelling. It serves primarily as a descriptive tool for illustrating the relative abundance of species in a community.
The concept
Trophic levels and energy flow
Energy dissipates as it passes up a food chain.
A common misconception is that an inverted pyramid of biomass implies an inverted flow of energy. This is incorrect. The energy pyramid is always upright because energy must flow from producers to consumers, and a significant portion is lost as heat at each step. The inversion in biomass is a result of high turnover rates in producers, not a violation of thermodynamic principles. Confusing standing stock with flow is a frequent error in ecological analysis. C
Worked example
Problem. Consider a simplified aquatic ecosystem with the following annual energy flows: Producers (phytoplankton) assimilate \(5,000\ \mathrm{J\,m^{-2}\,yr^{-1}}\). Primary consumers (zooplankton) assimilate \(500\ \mathrm{J\,m^{-2}\,yr^{-1}}\). Secondary consumers (small fish) assimilate \(50\ \mathrm{J\,m^{-2}\,yr^{-1}}\). Calculate the ecological efficiency between each transfer and determine the shape of the energy pyramid.
- Calculate the efficiency from producers to primary consumers: \(\varepsilon_1 = \frac{500}{5000} = 0.10\) or 10%.
- Calculate the efficiency from primary to secondary consumers: \(\varepsilon_2 = \frac{50}{500} = 0.10\) or 10%.
- Observe that the energy values decrease by a factor of 10 at each step: \(5,000 \to 500 \to 50\).
- Since each level is smaller than the one below it, the pyramid of energy is upright.
- Confirm that the total energy lost as heat accounts for the remaining 90% at each transfer, consistent with the second law of thermodynamics.
Units check. The ratio of energy flows is dimensionless, and the pyramid shape is a qualitative descriptor based on the magnitude of the energy values.
Three quick tests
- Why is the pyramid of energy always upright?
Answer
Because energy is lost as heat at each trophic level, so the energy available to the next level is always less than the previous one. - Can a pyramid of biomass be inverted?
Answer
Yes, in ecosystems with high producer turnover rates, such as the open ocean, the standing biomass of producers can be lower than that of consumers. - Which pyramid is most useful for comparing ecosystem productivity?
Answer
The pyramid of energy, as it directly measures the rate of energy flow and is independent of organism size or turnover rates.
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Lecture notes checked line by line by the editor; every number in the worked example was recomputed.