- The pyramid of energy represents how metabolic energy is transferred when organisms eat other organisms in an ecosystem.
- The pyramid consists of several bars that denote different trophic levels.
- Energy starts from the bottom and proceeds through all trophic levels.
- This flow of energy is crucial to maintain a healthy ecosystem.
Many people see food webs as ‘who is eating whom,’ but this doesn’t provide a clear explanation of biological productivity. Food chains are rather complex models of energy flow.
To better explain such models, we use a graphical representation called an Energy Pyramid (or ecological pyramid). It shows how metabolic energy is transferred when organisms eat other organisms in an ecosystem.
What Is An Energy Pyramid?
A pyramid of energy represents the amount of energy retained in the form of new biomass at individual trophic levels in a given ecosystem.
The pyramid consists of several bars, each representing a different trophic level. These bars are organized on the basis of who feeds on whom. All bars have the same height, but their width varies according to the number of components being measured.
The energy pyramid is used to quantify the transfer of energy from one organism to another along the food chain. Energy (as well as the width of the bar) reduces as you proceed through various trophic levels from the bottom to the top of the pyramid.
Where Does This Energy Come From?
Energy enters our planet from the Sun. The producers (plants) at the base of the pyramid use sunlight to produce metabolic energy via photosynthesis. Most of the solar radiation, however, is reflected back into space and converted into heat. Only 1-2% of the solar radiation is absorbed by photosynthetic processes and transformed into food.
An Energy Pyramid
Almost 90% of the energy produced from photosynthesis is used by plants in their own metabolism. The remaining 10% is stored in their tissues. So when herbivores (primary consumers) eat plants, only 10% of energy is transferred to them.
Of that 10% that herbivores eat, about 90% is used in their metabolism, and only 10% is stored in their tissues to be eaten by carnivores (secondary consumers).
The cycle continues up the pyramid, with each subsequent consumer only inheriting 10% of the previous energy level. The apex predators at the top of the pyramid get as little as 0.01% of the original solar energy.
Decomposers like fungi, worms, and bacteria consume a small amount of energy remaining in the tissues of dead animals and plants.
This is also called the 10% rule in ecology. It states that when energy is transferred from one tropical level to the next in a given ecosystem, only 10% of that energy will be passed on.
The flow of energy is crucial to maintain a healthy ecosystem. More energy at the lower trophic levels allows organisms to transfer energy up the pyramid. This energy is usually measured in calories per square meter per year or grams per square meter per year.
Four Levels Of An Energy Pyramid
There could be a different number of levels in different ecosystems. We have listed four main levels of the energy pyramid that show how energy flows in a healthy ecosystem.
Illustration of Energy Pyramid’s trophic level | Image credit: dkfindout
Level 1: Producers
Autotrophs are the produces in a food chain. They manufacture their own food by harnessing natural energy. The best examples of primary producers are plants (on land) and algae (in water).
Instead of eating other organisms, plants and algae pull nutrients from soil and ocean and produce their own food via photosynthesis.
But what about regions on Earth where there is no sunlight, such as deep-sea hydrothermal ecosystems? Here primary producers make their food through a different process known as chemosynthesis. Chemosynthetic microorganisms use methane as a source of energy instead of sunlight, as in photosynthesis.
Level 2: Primary Consumers
The second trophic level energy pyramid consists of herbivores. They are species that cannot produce their own food and depend on other organisms for survival. Animals like buffaloes and giraffes are primary consumers: they only eat producers such as plants, bacteria, and algae.
Primary consumers vary according to the ecosystem or community. While some species of grasshoppers and dear consume forest plants, certain ducks and tadpoles feed on aquatic plants. They also vary in size, ranging from tiny zooplankton to large elephants.
Level 3: Secondary Consumers
Secondary consumers are either carnivores or omnivores that feed on primary consumers.
Carnivores are animals that eat other animals, whereas omnivores are animals that eat both plants and other animals. Humans, for example, are omnivorous: we eat producers (plants) as well as other consumers (animals).
A field mouse, a shrew, a garter snake, and a woodpecker are all examples of secondary consumers. Since a field mouse eats both other animals and plants, it can be both a primary consumer and a secondary consumer. It also gets eaten by other secondary consumers, such as garter snakes.
Level 4: Tertiary Consumers
Tertiary consumers are carnivores that eat other carnivores. They consume both primary and secondary consumers as the main source of food. The golden eagle is a perfect example of a tertiary consumer: it eats foxes that belong to the third trophic level in the energy pyramid.
Since humans have a widely varied diet and consume foods from every trophic level, they can be put in either level 2, 3, or 4.
If people choose to be vegetarian, they would be classed as primary consumers; by eating food like grain-fed chicken, they would fill the role of secondary consumers; however, if that chicken also consumes insects, humans would be classed as tertiary consumers.
Apex predators, also called alpha predators, exist at the very top of the energy pyramid. They are never preyed upon and play an important role in maintaining the population of other organisms in their domain.
The killer whale, lions, tigers, polar bears, crocodiles, snow leopards are all examples of apex predators.
Real Example Of An Energy Pyramid
Ocala Silver River | Wikimedia
Let’s look at an actual energy pyramid from Silver Springs, Florida. They have measured the energy flow in kilocals per meter square per year.
Producers (P) get 20,810 kilocals; as you can see in the picture, trees consume their energy from sunlight. The research team studied that 3,368 kilocals made it to the next level, primary consumers (C1). This means about 16.1% of energy moved to the next level.
Energy pyramid of Silver Springs Florida | Credit: Wikimedia
Similarly, secondary (C2) and tertiary consumers (C3) got 383 and 21 kilocals. This means C2 got 11.3% of energy from the previous level, whereas C3 got only 5.4% from the C2. Furthermore, a class of decomposers called saprotroph consumes about 5,060 kilocals of energy.
All in all, the findings show that ecological efficiency lies somewhere between 5% and 20%, depending on how efficient that ecosystem is.
Other Ways To Measure This Energy
The other way to analyze the ecosystem is biomass. For this, we use a pyramid of biomass that shows the quantity of biomass present at each trophic level of a given ecosystem at a particular time.
The biomass represents the total amount of organic or living matter in an ecosystem. And the energy is usually measured in calories per square meter or grams per square meter.
Biomass pyramids are not necessarily upright. In a pond ecosystem, for example, the mass of producers (like phytoplankton) is lower than the mass of consumers (like insects and fish). This is because phytoplankton have very short lifespans, but they reproduce very quickly.
Pyramid of numbers; a) Grassland b) parasitic food chain
Another way to analyze the ecosystem is the pyramid of numbers. It shows the number of individual organisms (irrespective of their biomass or size) present at each trophic level.
These pyramids can be ‘inverted’ for ecosystems where parasites feed on large host animals or beetles feed on forest trees.