A pyramid of numbers is a graphical representation that shows the number of organisms at each trophic level. It is an upright pyramid in light of the fact that in an ecosystem, the producers are always more in number than other trophic levels.
The pyramid of numbers was advanced by Charles Elton in 1927. Charles pointed out the huge difference in the number of organisms involved in each level of the food chain. Succeeding links of the trophic structure reduce rapidly in number until there’s a very small number of carnivores at the top.
The reason for the pyramid shape is that there must always be enough plants to produce food at the bottom. Otherwise, the entire food chain would collapse. At the higher level, no predator can be as common as its prey. Otherwise, the population of both animals would soon be wiped out. Sparrowhawks, for example, can never be common than blue tits if they live in the same ecosystem.
This pyramid doesn’t consider the biomass of organisms. Neither does it indicate the energy transmitted or the utilization of energy by the organisms involved. Numbers pyramid can be convenient since counting is usually a simple task and can be undertaken over the years to track the changes in a given ecosystem.
The pyramid can also be used to figure out how the population of a given species can affect another. Additionally, it can serve as a basis for an ecosystem’s quantitative analysis.
But it is worth noting that some types of organisms can be difficult to count, especially in the case of some juvenile forms. The unit of measurement in the pyramid of numbers is the number of organisms. This pyramid varies between ecosystems and is of three types.
Types of Pyramid of Numbers
- Upright pyramid of number
- Partly upright pyramid of number
- The inverted pyramid of number
1. Upright Pyramid of Number
This type of number pyramid is found in the grassland ecosystem. This ecosystem is characterized by numerous autotrophs that support lesser herbivores. The herbivores, in turn, support a smaller number of carnivores.
Therefore, this pyramid is upright. Namely, with every higher trophic level, the number of organisms decreases.
For example, the grasses sit at the lowest trophic level or the base of the number pyramid because of their abundance. The primary consumer, such as a grasshopper, occupies the next higher trophic level. Grasshoppers are fewer in number than grass. The next trophic level is a primary carnivore, such as a rat.
There are fewer rats than grasshoppers because they consume grasshoppers. Secondary carnivores, such as snakes, occupy the next higher trophic level. Snakes feed on rats and snakes are eaten by hawks, which occupy the highest trophic level and are the least in number.
A pond ecosystem also depicts an upright pyramid of numbers. Phytoplankton like algae and bacteria are the producers here and hence the highest in number. The smaller herbivorous fishes are fewer in number compared to producers.
Likewise, the small carnivorous fishes are less in number than the herbivorous ones. Lastly, the apex consumers or largest carnivorous fishes are the least in number.
2. Partly Upright Pyramid of Number
This type of number pyramid is typical of the forest ecosystem. In this ecosystem, the producers are large-sized trees, which sit at the base of the number pyramid. The herbivores, such as elephants and fruit-eating birds, make the primary consumers. They are more in number than the producers. Afterward, the number of individual organisms reduces at each successive trophic level.
3. Inverted Pyramid of Number
An inverted number pyramid is found in parasitic food chains. In these food chains, there’s normally one producer supporting numerous parasites. The parasites, in turn, support more hyper-parasites. In short, in this pyramid, number of individuals at each level is increased from lower level to higher level.
Examples of Pyramid of Number
1. Clover → Snail → Thrush → Hawk
Being a plant, clover is the producer of this food chain and hence sits at the bottom of the pyramid. Energy is lost to the environment as you go up from one level to the next. That means there are fewer organisms at each step in this food chain. There is a need for plenty of clovers to support the snail population.
A thrush feeds on plenty of snails. A hawk, in turn, eats plenty of thrushes. Hence, the population of hawks is very small.
2. Phytoplankton → Zooplankton → Small Crustaceans → Predator Insects → Small Fish → Large Fish → Kingfisher
In this aquatic food chain, the phytoplankton is consumed by the zooplankton, which is in turn eaten by the small crustaceans. Then the predator insects feed on the small crustaceans. The predator insects are in turn consumed by the small fish, which are eaten by the large fish. Lastly, the large fish are eaten by the kingfisher. The Kingfisher is the least in number in the food chain and sits at the apex of the pyramid.
In some cases, the pyramid may not look like a pyramid at all. That’s why we have a partly upright pyramid of numbers. This normally happens if the producer is a large plant or if one of the animals is very small. Whatever the situation, however, the producer still occupies the bottom of the pyramid.
Some examples of this case include:
3. Oaktree → Caterpillars → Blue Tit → Sparrowhawk
A large number of caterpillars can feed on a single oak tree. The caterpillars provide food for several blue tits that in turn are eaten by a sparrowhawk.
4. Oaktree → Insects → Woodpecker
Many insects can feed on an oak tree since it is very large.
5. Grass → Rabbit → Flea
Fleas are very small and plenty of them can feed on one rabbit.