Maarg Manthan · Topic 2.6

Ecological Productivity and Ecological Pyramid

Environment & Ecology › Ecosystem Structure and Function · Topic 2.6

Ecological productivity is the rate at which organic matter is produced in an ecosystem. Primary productivity is the rate of production by producers: gross primary productivity, GPP, is the total rate of photosynthesis, and net primary productivity, NPP, is GPP minus the producers' respiration. Secondary productivity is the rate at which consumers form new organic matter. It is measured in grams per square metre per year or in kilocalories. Tropical rainforests, coral reefs, estuaries and wetlands are the most productive, and deserts and open oceans the least. An ecological pyramid is a graphic model of the number, biomass or energy at each trophic level, with producers at the base. Pyramids of number and biomass can be upright or inverted, but the pyramid of energy is always upright, as shown by Charles Elton, who first described them.

5 min read · UPSC Prelims · Environment & Ecology · Ecosystem Structure and Function

At a Glance

GPP Total photosynthesis
NPP GPP minus respiration
Most productive Rainforest, reef, estuary
Always upright Energy pyramid
First described by Charles Elton
Exam link UPSC Prelims, Environment & Ecology: Ecosystem Structure and Function

Where Does It Fit?

This topic follows the account of food webs and energy flow models. It measures how much energy ecosystems produce and shows the loss by trophic level as pyramids. Previous topic: Food Web and Models for Energy Flow. Next topic: Biomagnification.

What Is Ecological Productivity?

Term	Meaning
Primary productivity	Rate at which solar energy is captured by producers and stored as organic matter
Gross primary productivity (GPP)	Total rate of photosynthesis, including the part the plants use in respiration
Net primary productivity (NPP)	GPP minus plant respiration; the food available to consumers and decomposers
Secondary productivity	Rate of formation of new organic matter by consumers
Net community productivity	NPP minus respiration of consumers and decomposers
Units	Grams of dry matter per square metre per year, or kilocalories per square metre per year
Factors	Light, temperature, water, nutrients, plant type and age. Nutrient shortage often limits productivity in oceans

Which Ecosystems Are Productive and What Are Ecological Pyramids?

High productivity: tropical rainforests, coral reefs, estuaries, swamps and marshes, and intensive crops
Low productivity: deserts, tundra and the open ocean, though the ocean as a whole contributes much because of its size
Pyramid of numbers: shows number of individuals at each level; usually upright, but inverted in a tree ecosystem where one tree supports many insects
Pyramid of biomass: shows total dry weight at each level; upright in forests and grasslands, but inverted in the sea where phytoplankton have small standing biomass but fast turnover
Pyramid of energy: shows energy flow at each level in a given time; always upright since energy decreases at each step
Trophic levels: producers at the base, then herbivores, then carnivores
Limitations: pyramids do not show detritus food chains or species at several levels
Uses: show the loss of energy and the structure of an ecosystem

What Else Should You Remember?

Elton: Charles Elton described the pyramid of numbers in 1927
Tree ecosystem: one tree supports many insects and birds, giving an inverted pyramid of numbers
Parasitic chain: a pyramid of numbers is inverted since many parasites live on one host
Pond pyramid of biomass: can be inverted since phytoplankton are eaten almost as fast as they grow
Pyramid of energy: always upright, reflecting the second law of thermodynamics
Standing crop: the biomass at one time
Productivity of crops: high in sugarcane and rice with inputs
Oceans: cover 70 percent of Earth and contribute about half of net primary production
Upwelling zones: bring nutrients and raise productivity, as off Peru and Somalia
Mangrove productivity: very high because of nutrient trapping
Eutrophication: excess nutrients cause algal blooms that change productivity
Global NPP: about 170 billion tonnes of dry matter per year
Productivity and climate: rise in temperature and CO2 affects NPP differently in places
Human appropriation of NPP: people use a large share of the land NPP
Ecological efficiency: the percentage of energy passing between levels
Link: energy transfer is in Flow of Energy in an Ecosystem
Link to webs: see Food Web and Models for Energy Flow
Link to toxins: see Biomagnification
Pyramids in exams: a favourite question is which pyramid can be inverted
Forest NPP numbers: tropical rainforests have NPP of about 2,000 to 2,200 grams per square metre per year, against less than 100 in deserts
Measuring methods: productivity is measured by harvest, oxygen change in water, carbon dioxide uptake and satellite greenness indices
Light and dark bottle method: used to estimate aquatic productivity by oxygen change
Secondary productivity limits: usually much lower than primary because of energy loss
Net community production: what remains for storage in the ecosystem and export
Carbon sink: ecosystems with positive net production store carbon

Why Does It Matter for the Exam?

GPP and NPP: NPP = GPP minus R
Productive systems: rainforest, reef, estuary
Pyramids: energy always upright
Current affairs link: blue carbon and productivity
Common errors: saying energy pyramids can be inverted

As an analytical point, productivity differs widely across ecosystems, and the most productive ones, such as mangroves and reefs, are also under threat. Protecting them safeguards fisheries, carbon stores and coastal protection.

Why Is This Topic Important?

Terms GPP, NPP
Pyramids Three
Upright Energy

What Came Next?

The next topic covers biomagnification. Next topic: Biomagnification.

Key People and Terms

GPP Gross primary productivity
NPP Net primary productivity
Standing crop Biomass at a time
Pyramid of energy Energy at each level
Inverted pyramid Wider at the top
Upwelling Nutrient-rich water rising

Exam Corner

Points to Remember

NPP = GPP minus R.
Energy pyramid: always upright.
Sea biomass pyramid: inverted.
Tree pyramid of numbers: inverted.
Elton: 1927.

Do Not Confuse With

GPP and NPP Total versus net
Pyramid of numbers and energy Can invert versus always upright
Productivity and biomass Rate versus amount

Memory Hook

GPP less respiration is NPP; energy pyramid never flips.

Mains Angle

Questions ask for the pyramid that can be inverted and productive ecosystems. These points help.

Productivity GPP, NPP
Pyramids Types
Ecosystems High and low

A Question You May Face

An original practice question, not a past paper question.

“Explain primary productivity and the types of ecological pyramids, and why the pyramid of energy is always upright.”

How to Answer

Productivity GPP, NPP.
Pyramids Three.
Inversion Cases.
Reason Energy loss.

GS Relevance

Prelims: GPP and NPP, productive ecosystems, pyramid types and inversion. GS Paper 3: ecology.

Frequently Asked Questions

What is the difference between GPP and NPP?

Gross primary productivity is the total rate at which producers capture energy and make organic matter by photosynthesis. Net primary productivity is what is left after the producers use part of it in their own respiration. NPP is the amount of organic matter available to consumers and decomposers.

Which ecosystems have the highest productivity?

Tropical rainforests, coral reefs, estuaries, swamps and marshes and mangroves have the highest productivity per unit area because of abundant light, warmth, water and nutrients. Deserts, tundra and the open ocean have the lowest productivity per unit area.

What is an ecological pyramid?

An ecological pyramid is a graphical representation of the number of organisms, their biomass or the energy at each trophic level of an ecosystem, with producers at the base and top carnivores at the apex. Charles Elton first described pyramids in 1927. There are three types: of numbers, biomass and energy.

Why is the pyramid of energy always upright?

The pyramid of energy is always upright because energy is lost as heat at every trophic level, so each higher level receives less energy than the one below it. This follows the second law of thermodynamics and the ten percent law, so the energy at a higher level can never be greater.

When is a pyramid of biomass inverted?

A pyramid of biomass is inverted in many aquatic ecosystems such as the open sea, where the standing biomass of phytoplankton producers is smaller than that of zooplankton and fish consumers. This happens because phytoplankton reproduce and are eaten very fast, so they have a high turnover rate with low standing crop.

PYQ Practice — Statement Analysis

1 The pyramid of energy is always upright.

True

Because of energy loss.

2 A pyramid of biomass in the sea is generally upright.

False

It is often inverted.

3 NPP is GPP minus plant respiration.