Food Chain, Food Web and
the Flow of Energy and Matter
Learn Food Chains and Trophic Pyramids
1. What is the primary energy source for life on earth?
The primary energy source for life on earth is the sun. The sun plays the important role of keeping the planet warmed and it is the source of the luminous energy used in photosynthesis. This energy is converted into organic material by the photosynthetic autotrophic beings and consumed by the other living beings.
2. What is the main means by which autotrophic beings obtain energy?
The main means by which autotrophs obtain energy is photosynthesis. (There are also chemosynthetic autotrophs.)
3. Which is the autotrophic group responsible for the production of most part of the molecular oxygen of earth?
Algae and cyanobacteria of the phytoplankton are the organisms that contribute most to the production of molecular oxygen.
4. In the ecological study of food interactions, what are the autotrophic beings called?
In Ecology autotrophic beings are called producers because they synthesize the organic material consumed by the other living beings of an ecosystem.
An ecosystem cannot exist without producers.
5. How are the heterotrophic beings divided in the ecological study of food interactions?
Heterotrophs are divided into consumers and decomposers. An ecosystem can exist without consumers but it cannot be sustained without decomposers. Without the decomposers the organic material would accumulate causing environmental degradation and later death of the living beings.
6. What is a food chain?
The food chain is the linear not branched sequence in which a living being serves as food for another, starting with the producers and going up to the decomposers.
7. How is energy transferred along a food chain?
The energy flux along a food chain is always unidirectional, from the producers to the decomposers.
8. What are trophic levels? How many trophic levels can a food chain have?
Trophic levels correspond to positions on a food chain. Therefore producers always belong to the first trophic level and decomposers to the last trophic level, consumers that directly eat the producers belong to the second trophic level and so on.
There is no limit regarding the number of trophic levels on a chain, since many orders of consumers can exist.
9. What are primary consumers? Can a food chain present quaternary consumers without having secondary or tertiary consumers? Can a tertiary consumer of one chain be a primary or secondary consumer of another chain?
Primary consumers are living beings that eat autotrophic beings, i.e., they eat the producers. Primary consumers always belong to the second trophic level of a chain.
A food chain cannot have consumers of superior orders without having the consumer of the inferior orders. A consumer however can participate in several different chains not always belonging to the same consumer order in each of them.
10. What is the difference between the concepts of food chain and food web?
The chain concept is a theoretical model to study the energy flux in ecosystems. Actually in an ecosystem the organisms are part of several interconnected food chains, forming a food web. Therefore the chain is a theoretical linear sequence and the web is a more realistic representation of nature in which the food chains interconnect forming a web.
11. What are the three main types of trophic pyramids studied in Ecology?
The three types of trophic pyramids studied in Ecology are the numeric pyramid, the biomass pyramid and the energy pyramid.
Generally the variable dimension of the pyramid is the width, and the height is always the same for each represented strata of living beings. The width therefore represents the number of individuals, or the total mass of these individuals or the available energy in each trophic level.
12. What do numeric pyramids represent?
Numeric pyramids represent the number of individuals in each trophic level of a food chain.
13. In a numeric pyramid to which trophic level does the base always refer? What about the top level?
In a numeric pyramid the base corresponds to the first trophic level, i.e., to the producers. The top level of the pyramid corresponds generally to the last consumer order of the food chain (since the number of individual decomposers, most of them microorganisms, is too large to be represented).
14. In a numeric pyramid is it possible for the base to be smaller than the other levels?
Since the numeric pyramid represents the quantity of individuals in each trophic level of the food chain, inferior trophic levels with less individuals than the superior trophic levels may exist. For example, a single tree can serve as food to millions of insects.
15. In the short term what will happen to the levels above and below a population of secondary consumers of a numeric pyramid if a large number of individuals from this population dies?
If an intermediate level of a numeric pyramid has its variable dimension decreased, i.e., if the number of individuals of that level is reduced, the number of individuals of the level below will increase and the number of individuals of the level above will be reduced. That happens because the individuals of the level below will face less predators and the individuals of the level above will have less available food.
16. What do biomass pyramids represent?
Biomass pyramids represent the sum of the masses of the individuals that participate in each trophic level of a food chain.
17. What is dry mass?
When biomasses are compared often the concept of dry mass is used. The dry mass is the total mass less the water mass of an individual. The total mass is also called fresh mass. To use dry mass instead of fresh mass is useful because among living beings there are differences related to the proportion of water within their body and such differences can distort the quantitative analysis of incorporated organic material.
18. What do energy pyramids represent?
Energy pyramids represent the amount of available energy in each trophic level of the food chain.
19. Into which type of energy is the light used in photosynthesis transformed?
The luminous energy used in photosynthesis is transformed into chemical energy.
20. Can the amount of available energy in a given trophic level be larger than the available energy in inferior trophic levels? What does that condition means to the conformation of the energy pyramids?
A superior trophic level always has less available energy than inferior trophic levels. This is because in each trophic level only a fraction of the organic material of the level below is incorporated into the consumers (into their bodies). The other part is eliminated as waste or is used in the metabolism as energy source. Therefore it is never possible to have energy pyramids with inverted conformation, i.e., with the tip to the bottom and the base to the top. It is also not possible to have superior trophic levels with a variable dimension larger than inferior ones. In every energy pyramid, from the base to the top, the size of the variable dimension decreases.
21. What is the gross primary production of an ecosystem? How does GPP relate to photosynthesis?
Gross primary production of an ecosystem, or GPP, is the quantity of organic material found in a given area in a given period.
Since only autotrophs produce organic material and photosynthesis is the main production process, GPP is a result of the photosynthesis.
22. What are the factors that for influencing photosynthesis also interfere with the gross primary productivity?
Mainly water and light, but also mineral salts, temperature and carbon dioxide are factors that interfere with the gross primary productivity.
23. What are the destinations of the organic material fabricated by the producers?
Part of the organic material synthesized by the producers is consumed as energy source for the metabolism of the own producer individual. The other part is incorporated (into the body) and becomes available to heterotrophic beings of the ecosystem. In each following trophic level part of the organic material is used in the metabolism of the individuals of the level, the other part is eliminated as waste and only a fraction is incorporated and becomes available as food for the following level.
24. What is the formula of the net primary production (NPP)? How does NPP relate to the energy pyramids?
Net primary production is the gross primary productivity less the organic material consumed as energy source in the metabolism of the producers: NPP = GPP – (organic material spent in aerobic respiration). It represents the organic material available in the first trophic level.
The base of the energy pyramids must represent the NPP and not the GPP since the idea of these pyramids is to show the available energy in each trophic level of the food chain.
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