Question

What are ecological pyramids? Mention its limitations.

Answer 1

Hint: Number, biomass and energy relations of an ecosystem at various trophic levels are graphically represented in a form of pyramids called ecological pyramids.

Complete answer:
1.Ecological pyramids are graph style representations of various ecological parameters at the successive trophic levels of food chains with producers at the base, top carnivores at the apex and intermediate levels in between.
2.The three ecological pyramids that are usually studied are:

A.Pyramid of number: It can be upright, inverted or spindle shaped.
In a grassland, the pyramid of number is upright whereas in a forest it is inverted and called a parasitic food chain.
Parasites in a forest ecosystem act as secondary consumers and may include lice, ticks on birds; the hyperparasites are bacteria, fungi and as secondary consumers. Depending on the organism of various trophic levels, the shape may differ in forest ecosystems. The trees are lesser in number as compared to herbivores which may be fruit eating birds and the carnivorous birds make up the secondary consumer level. Tertiary consumers or top carnivores may be absent.
Secondary consumers are lesser than primary consumers giving a spindle shape to the pyramid.
In the pond ecosystem, the pyramid of numbers is evidently upright.

B.Pyramid of biomass: The amount of living matter present in an organism at particular trophic level is called biomass. It is measured precisely in ms of dry weight as compared to fresh weight. In grassland and forest ecosystems, In pond ecosystems, the pyramid of biomass is inverted as the biomass increases from producer level to top carnivore level.

C.Pyramid of energy: It is always upright in all the ecosystems as the amount of energy stored at various successive trophic levels becomes less and less. It is well known that only a part of energy is transferred to the next trophic level from the preceding trophic level and a major amount is lost as heat.

4.Limitations are:
a.It does not take into account the same species belonging to two or more trophic levels, e.g., insectivorous plants.
b.It assumes a simple food chain and does not accommodate a food web.
c.Moreover organisms like Saprophytes, decomposers, microbes and detritivores are not given any place in ecological pyramids.

Note: At the top of pyramids less number of organisms are present, this is because very little food is left for secondary consumers compared to primary consumers. indicating there are fewer consumers than producers.