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The movement of nutrient elements through the various components of an ecosystem is called nutrient cycling. Another name of nutrient cycling is biogeochemical cycles (bio: living organism, geo: rocks, air, water). Nutrient cycles are of two types:

(a) gaseous and

(b) sedimentary. The function of the reservoir is to meet with the deficit which occurs due to imbalance in the rate of influx and efflux. For the sedimentary cycle (e.g., sulphur and phosphorus cycle), the reservoir is located in Earth’s crust.

The interaction between the living organism and the non-living environment is called ecosystem.

Circulation of nutrients within the ecosystem or biosphere is known as biogeochemical cycles and also called as cycling of materials. 

There are two basic types: 

1. Gaseous cycle – It includes atmospheric Oxygen, Carbon and Nitrogen cycles. 

2. Sedimentary cycle – It includes the cycles of Phosphorus, Sulphur and Calcium – Which are present as sediments of Earth.

The movement of decomposed, water soluble organic and inorganic compounds from the surface to the lower layer of soil or the carrying away of the same by water is called leaching or eluviation.

Primary Producer’s (First trophic level): Algae, Hydrilla, maize plant, Phytoplankton, Nymphaea, Spirogyra. 

Primary Consumer (Second trophic level): Crustaceans, grasshopper, deer, mouse, squirrel, rabbit, elephant, goat.

Secondly Consumer (Third trophic level): Spider, cockroach, lizard, wolf, snake, toad, fish, crane. Top carnivore (Fourth trophic level): Lion, Tiger.

The base of the pyramid is broad and it narrows down at the apex. The similar shape is obtained when food or energy relationship between organisms at different trophic level.

The relationship can be expressed in terms of number, energy or biomass.

The base of the pyramid represented by producer and apex is the top consumer; other trophic levels are in between.

In most ecosystems, all the pyramids, of number, of energy and biomass are upright.

The pyramid of number in a tree ecosystem is inverted. 

The pyramid of biomass in sea also inverted because the biomass of fishes is far exceeds that of phytoplankton.

Pyramid of energy is always upright, can never be inverted, because when energy flows from a particular trophic level to the next, some energy is always lost as heat at each step.

The gradual and fairly predictable change in the species composition of a given area is called ecological succession.

Limitations of ecological pyramids:

• It does not take into account the same species belonging to two or more trophic levels. 
• It assumes a simple food chain, it never exits in nature. 
• It dose not accommodate food web.
• Saprophytes are not given place in ecological pyramids.

Arrangement of trophic levels from producers to top carnivores forms pyramid like structure 3 types – 

(i) Pyramid of number 

(ii) Biomass 

(iii) Energy 

Limitations – 

(i) Assumes simple food chain 

(ii) Single species may operate at two or more trophic levels. 

The four functional aspects of an ecosystem are

• Productivity
• Decomposition
• Energy flow
• Nutrient cycling.

1. In some cases same species may belong to two or more trophic levels

2. It does not represent food web

3. Saprophytes are not given any place

(d) fresh water ecosystem Pond ecosystem

The ecological pyramid assumes a simple food chain and does not accommodate food webs. Thereby, it does not take into account the fact that species may belong to two or more trophic levels at a time. Also saprophytes despite their vital role in ecosystem are given no place in the ecological pyramids.