(3) Important ecology definitions, food chains, trophic levels - examples explained

Some important definitions (in alphabetical order, explained in more detail where appropriate)

Biomass is measured as the dry,  mainly organic matter derived from recently living organisms in a population.

Carnivore - an animal that only eats other animals, not plants, to obtain food and energy.

Community - is all of the populations of different species in an ecosystem.

Consumer - not plants or algae, all animals that eat plants or other animals i.e. an organism that gets its energy (via biomass) by feeding of another consumer.

In a food chain, plants or algae are trophic level 1 producer, then 2 is a primary consumer, 3 is secondary consumer, 4 is tertiary consumer, 5 is quaternary consumer.

Decomposer - an organism that breaks down dead or waste plant and animal organic material to obtain food and energy and in doing so nutrients are recycled back to the soil or water.

Ecosystem - the interaction of all the living things in a given area and their non-living environmental conditions.

You can think of it as a unit containing the community of organisms and their environment, interacting together, e.g. a wood, a lake or a decomposing body, so you can consider smaller ecosystems within a much larger one.

Egestion - the act or process of discharging undigested or waste material from a cell or organism e.g. the discharging undigested food as faeces, contributes to wasted mass and energy in a food chain.

Environment - The surrounding air, water, rocks and soil where an organism lives.

Food chain - a linear part of a food web, starting with a producer, ending with a top predator.

Food web - shows how food chains in an ecosystem are linked - shows interdependence - interconnections.

Herbivore - an animal that only eats plants - e.g. aphids, caterpillars, rabbits, tadpoles as sources of food and energy.

Interdependence - organisms in food chains and food webs are dependent on each other,

change in one organism population results in changes in other organism populations.

Omnivore - an animal that eats both plants or animals.

Population -  The number of a group of the same species living in the same area at the same time.

Primary consumer - an animal that eats producers e.g. eats plants or algae.

Producer - usually plants or algae - initial producers of organic nutrients (providing food and energy), from sunlight energy (+H₂O +CO₂) using photosynthesis.

Pyramid of biomass - Biomass is the dry mass of living organisms in an area (the habitat) at a particular time. The biomass pyramid is constructed from the mass of organisms at each trophic level with a proportionate bar to represent each mass. This biomass diagram is always shaped like a pyramid because the biomass always goes down from one trophic level to the next along a food chain.

Pyramid of numbers - Along a food chain, the number of individual animals changes at each trophic level (usually decreases). Arranging these numbers on top of each other using proportionate bars, gives you a diagram called the pyramid of numbers (BUT, it isn't always a pyramid shape!)

Secondary consumer - an animal that eats primary consumers i.e. another animal

Tertiary consumer -  an animal that eats secondary consumers i.e. another animal

Top carnivore - an animal not eaten by any other animal.

Trophic level is the position of an organism in a food chain, food web, pyramid of numbers or pyramid of biomass.

 

Food chains are a simple linear way of showing 'what eats what' in an ecosystem.

Food chains must start with a producer, usually a green plant on land, or algae and phytoplankton in water.

You need a source of food to get any food chain going!

Trophic level refers to the stage or level in a food chain

e.g. pond weed ==> tadpole ==> water beetle ==> perch ==> otter

This food chain involves four stages and five trophic levels.

Apart from the highest trophic level (5), the top predator:

each trophic level provides food and energy for the next higher trophic level,

some food is used immediately in respiration, so energy is lost as heat to the surroundings, and some becomes biomass of the organism including protein, carbohydrates for respiration and an energy store of fat.

The energy resource is used to power all the life processes of the organism, but much energy is lost as heat to the surroundings (before being used to keep a warm bloodied animal warm).

The energy that isn't stored as biomass isn't transferred to the next trophic level and is lost in the food chain - this can be as much as 90% loss i.e. only around 10% may be passed on from one trophic level to another.

For more on this last point see Pyramid of Biomass

There are usually no more than 4 or 5 trophic levels because so much biomass and energy is lost up the food chain at each trophic level that higher levels can't be supported.

 

Producers make their own food usually using sunlight energy in photosynthesis (trophic level 1).

All plants and plankton like algae are examples of food producers.

All the subsequent consumers cannot make their own food, but may become the food for another organism next in the food chain.

 

Producers are initially eaten by the primary consumer (trophic level 2)

Consumers are organisms that eat other organisms (plants or animals) in a food chain.

The primary consumer is the first to eat the producer e.g. a herbivore - plant eater.

Herbivores only eat things like plants or plankton e.g. rabbits eat grass or small fish eat algae

 

Primary consumers are then eaten by a secondary consumer (trophic level 3).

Secondary consumers are the second consumers in the food chain and are carnivores.

Carnivores are 'meat' eaters - here carnivores eat herbivores e.g. fox eats rabbit.

At trophic level 3, carnivores eat the primary herbivore consumers from trophic level 2.

Omnivores eat both plants and animals depending on the availability of food.

 

Secondary consumers are eaten by tertiary consumers (trophic level 4). etc.

Tertiary consumers are the third consumers in the food chain etc.

We now have carnivores eating carnivores!

Consumers that hunt and eat other animals are called predators and their prey are the animals they eat.

Note that if a predator eats all their chosen prey, it will die out for lack of food or move to another habitat.

In stable communities the numbers of predators and preys rises and falls but in a balanced way.

Carnivores that not hunted themselves by another predator are at the top of the food chain, and therefore they are always the highest trophic level - they are known as the apex predator.

 

When you have several food chains interacting/overlapping you get a food web, which is part of a much wider description of an ecosystem.

 

NOTE organisms that don't get eaten, eventually die, and the dead plants and animals are broken down and eaten by decomposers.

 

Decomposer organisms like bacteria and fungi play are a very important part of ecosystems - they break down dead plants and animal material.

To decompose dead plant or animal material, these organisms secrete enzymes that break the dead organic material down into smaller soluble food molecules which can diffuse into the microorganisms i.e. absorption of nutrients - sometimes referred to as extracellular digestion.

 

Summary of trophic levels

As already mentioned, each stage of a food chain is called a trophic level.

(The modern scientific word trophic comes from 'trophikos' in the late 19th century from the ancient Greek word 'trophe' which meant 'nourishment'.)

A specific trophic level consists of one or more organisms that perform a specific role in one or more food chains - see food web examples later.

The trophic level number increases up the food chain.

The first four trophic levels of a food chain etc. can be shown as ...

producer  ==>  primary consumer  ==>  secondary consumer  ==>  tertiary consumer etc.

trophic level 1  ==>  trophic level 2  ==>  trophic level 3  ==> trophic level 4 etc.

The arrows indicate the direction of biomass transfer, that is also the direction of chemical potential energy transfer up the food chain.

AND the decomposers take of everything in the end!!!!

 

Three examples of food chains and trophic level

(a) One of many food chains in a river or lake ecosystem

Pondweed ==> eaten by tadpoles ==> tadpoles eaten by water beetle ==> perch fish eats water beetle ==> otter eats the perch

This food chain involves five trophic levels.

The pondweed is the photosynthesising producer (trophic level 1).

Tadpoles are the primary consumer (trophic level 2, herbivore).

The water beetle is the secondary consumer (trophic level 3, carnivore).

The perch is the tertiary consumer (trophic level 4, carnivore).

The otter is trophic level 5 at the top of the food chain, which must always be the highest trophic level - known as the apex predator ('top predator' - top carnivore!).

 

(b) One of many food chains involving a garden and field ecosystem

Cabbage ==> butterflies lays eggs - resulting caterpillars feed off cabbage ==> butterflies eaten by blue/great tit birds ==> bird of prey eg kestrel, catches smaller birds eg blue/great tits

This food chain involves four trophic levels.

The cabbage is the producer.

The caterpillar is the primary herbivore consumer.

The blue tit (carnivore) is the secondary consumer .

The bird of prey is the tertiary consumer - the top of this food chain (the apex predator).

The word apex refers to the top of the 'triangle' of the pyramid of biomass e.g. like the one for four trophic levels shown below.

More on the pyramid of biomass in part 5.

In this pyramid of biomass: 1 = cabbage, 2 = caterpillar, 3 = blue tit bird, 4 = bird of prey (raptor)

Each bar on a biomass pyramid represents the relative amount of mass for the organisms of that trophic level in a food chain and because of natural losses (including energy molecules), it always decreases up the pyramid (take care - this is not always so for a pyramid of numbers).

 

(c) food chain: green plants ===> rabbits ==> foxes

This food chain involves three trophic levels.

The photosynthesising grass is the producer.

The rabbits are the primary herbivore consumers.

The foxes are the secondary carnivore consumers - the top of this food chain.

In terms of numbers we are talking ....

thousands of blades of grass ===> dozens of rabbits ===> a few foxes

 

(d) food chain:  grass/leaves ===> grass hopper ===> rat ===> snake

producer ==> primary consumer (herbivore) ===> secondary consumer (carnivore) ===> tertiary consumer (carnivore)

trophic level 1 ===> trophic level 2 ===> trophic level 3  ==> trophic level 4

In this pyramid of biomass: 1 = grass, 2 = grass hopper, 3 = rat, 4 = snake

---

Key points - Summary of ideas

Based on the syllabus-specifications for students taking the AQA, Edexcel and OCR GCSE level biology examinations (~US grades 9-10).

A food chain is a sequence that shows how energy and nutrients flow from one organism to another in an ecosystem.

It starts with a producer (usually a plant or algae) that makes its own food through photosynthesis.

Then, consumers (animals) eat the producers and other consumers, transferring energy up the chain.

Trophic Levels in a Food Chain

Trophic levels describe an organism's position in a food chain:

1. Trophic Level 1 – Producers: Plants and algae that make their own food using sunlight.

2. Trophic Level 2 – Primary Consumers: Herbivores that eat producers.

3. Trophic Level 3 – Secondary Consumers: Carnivores or omnivores that eat primary consumers.

4. Trophic Level 4 – Tertiary Consumers: Carnivores that eat secondary consumers.

5. Apex Predators: The top of the food chain, with no natural predators.

6. Decomposers: Bacteria and fungi that break down dead organisms, recycling nutrients back into the ecosystem.

Each level loses energy as it moves up the chain, meaning fewer organisms exist at higher trophic levels.

---

Summary of learning objectives and key words or phrases

Explain what is a food chain?

Explain what we mean by a producer, primary consumer, secondary consumer, tertiary consumer.

Explain what is a trophic level?

Be able to understand and construct a food chain from given information.

---

WHAT NEXT?

TOP OF PAGE

Index of Biomass, energy. pyramids, food web notes

INDEX of all my BIOLOGY NOTES

HOME PAGE of Doc Brown's Science website

UK KS3 Science Quizzes for KS3 science students aged ~11-14, ~US grades 6, 7 and 8

Biology * Chemistry * Physics UK GCSE level students aged ~14-16, ~US grades 9-10

Advanced Level Chemistry for pre-university age ~16-18 ~US grades 11-12, K12 Honors

Find your GCSE/IGCSE science course for more help links to all science revision notes

email doc brown - comments - query?

---

Explaining the importance of food chains & biomass in GCSE level biology, What you need to know about food chains & biomass for GCSE level biology, Explaining the use of food chains & biomass knowledge in GCSE level biology, Examples of food chains & biomass explained when studying GCSE level biology, What is the significance of food chains & biomass in GCSE level biology, describing and explaining the theory of food chains & biomass when studying GCSE level biology, revision notes for food chains & biomass in exams, online help for understanding food chains & biomass, exam revision notes for food chains & biomass, what do I need to learn about food chains & biomass? revision summary for food chains & biomass, learning notes for food chains & biomass, help to pass the food chains & biomass topic in an exam question, how to prepare for questions on food chains & biomass in a GCSE biology examination? Based on the syllabus-specifications for students taking the IGCSE/GCSE level biology examinations summary revision notes and key points on food chains & biomass for students taking the AQA igcse/gcse biology notes on food chains & biomass, Edexcel gcse biology notes on food chains & biomass,  OCR 21st century GCSE biology notes on food chains & biomass, OCR gateway GCSE biology notes on food chains & biomass, WJEC gcse biology notes on food chains & biomass, CCEA gcse biology notes on food chains & biomass for students taking CIE Cambridge igcse biology, or any other GCSE or IGCSE level biology exams notes on food chains & biomass, useful for US grade 9-10 biology courses

---

Doc Brown's school biology revision notes: GCSE biology, IGCSE  biology, O level biology,  ~US biology science grade 8, grade 9 and grade 10 school science courses or equivalent for ~14-16 year old students of biology IGCSE AQA GCSE Biology Edexcel GCSE Biology IGCSE OCR Gateway Science Biology OCR 21st Century Science Biology  Some of these biology revision notes might be suitable for UK KS3 Science-Biology courses for ages 12-14 (~US biology science grades 6, grade 7 and grade 8)

---

SITEMAP Website content © Dr Phil Brown 2000+. All copyrights reserved on biology revision notes, images, quizzes, worksheets etc. Copying of website material is NOT permitted. Exam revision summaries and references to science course specifications are unofficial.