(1) Introduction to ecosystems, definitions and terms used in ecology

Reminder: Sunlight energy is the principal source of energy input into biological systems and this flow of energy through living organisms is transferred as chemical energy which is eventually transferred to the environment.

Brief reminders of some important definitions (in alphabetical order, explained in more detail where appropriate, you should have come across many of these already)

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.

Ecology is the scientific study of the relationships between living organisms including all plants and animals and their physical environment

Ecology looks at how biotic (living) and abiotic (non-living) factors interact to influence the distribution, abundance, and adaptation of plant and animal life, ranging from individual organisms to entire communities and their interaction.

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.

Indigenous species - one that occurs naturally in a given environment

i.e. it is not there by accident or deliberate introduction into an area.

Interdependence - organisms in food chains and food webs are dependent on each other, a change in one organism population results in changes in other organism populations.

Non-indigenous species - one that does NOT naturally live in a particular habitat.

Meaning it is not native to an area and they maybe beneficial or harmful to an environment.

Their occurrence may be accidental or deliberate introduction.

The main problem they cause is out competing native species e.g. grey squirrels outcompete red squirrels, an example of reducing biodiversity of native species.

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.

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.

 

What is an ecosystem?

Ecology is the study of how all organisms (plant and animals) survive in their physical environment, how they relate to other organisms and what makes an organism successful or unsuccessful as the case may be.

An ecosystem is the interaction of a community of living things (biotic) and non-living (abiotic) factors affecting their environment.

Therefore an ecosystem includes all the populations of all the living organisms (plant or animal) in a specified area and such a description must also include all the non-living conditions/factors such as temperature, soil quality-nutrients, water sources (or lack of them).

Typical ecosystems include:

Natural ecosystems like ancient rainforest-jungle, tundra, swamps, oceans, lakes and even the humble pond.

Artificial ecosystems like planted highly managed forests, fish farms, horticulture in a greenhouse.

 

Within an ecosystem there is continuous competition between organisms for food and other resources and coping with conditions such as availability of water, food or the climate of the environment.

You can consider that ecosystems have different levels of organisation - important terms listed below.

 

A habitat means a specified area of the ecosystem involved e.g. a wood, a pond, a sea shore etc. where the organism lives to give fertile offspring.

 

Biotic factor conditions are the 'living' factors of the environment e.g. availability of food, availability of water, a new pathogen/predator and competition for food between organisms - all of these affect the distribution of organisms.

 

Abiotic factor conditions means the 'non-living' factors like temperature, water/soil pH, presence and concentration of soil nutrients, moisture level, light intensity and other climate conditions - again, all of these affect the distribution of organisms.

Species is defined as a group of similar organisms that can reproduce with each other to produce fertile offspring.

The abundance or population size of an organism is how many individuals are present in a given area.

The distribution of an organism is where you find it in its habitat e.g. part or the whole of a river, stream, field, heathland etc.

 

Levels of organisation - definitions in increasing size

1. Individual - a single organism of any species in its population in the ecosystem habitat.

e.g. a squirrel living in a wood.

2. Population - the total number of one particular organism species in a specified ecosystem habitat.

The number of squirrels that live in the wood.

3. Community - all the populations of organisms of different species living in a specified ecosystem habitat.

The squirrels live along side other animals like rabbits, foxes, insects, plants.

4. Ecosystem - the community of organisms and the conditions under which they live - biotic and abiotic.

This community live in the specified environment, the individuals and species interact with each other (biotic factors) and their lives are affected by e.g. soil type in the woodland (pH, nutrient content), ambient seasonal temperatures, water resources (rain, pond) and light intensity.

 

A stable community is where the biotic and abiotic factors in balance and the population sizes remain fairly constant.

Examples include ancient oak woodland in England or tropical rainforests in South America.

 

Interdependence

Interdependence is the term to describe the fact that ALL organisms depend on other organisms for resources such as food or shelter in order to survive and reproduce.

As a consequence, any change in the population of one species, can have knock on effect on another species in the same community.

This can be best understood by studying food webs. See section (d) Food webs on Biomass page

 

Biodiversity and ecosystems

Since very ecosystem has a range of different plants and animals, the term biodiversity expresses 'width' of diversity of species.

Biodiversity is extremely important for the 'health' of any ecosystem and phrases like 'high biodiversity' or 'low biodiversity' sum up the good or bad state of an ecosystem.

The living organisms in an ecosystem can be described as producers, consumers and decomposers.

Producers initiate a food chain e.g. plants producing food from photosynthesis.

Consumers are eating from a previous level in the food chain, up which the biomass is transferred.

Decomposers are live organisms that break down other dead organisms, recycling materials in the ecosystem.

An ecosystem with a high biodiversity has many advantages over a low biodiversity e.g.

a wider variety of food resources, reducing the dependency of a species on limited choices,

for our own needs, we are provided with food supplies, medicines derived from natural products, oxygen in the atmosphere and a water supply.

Biodiversity is reduced by removing too much of a species from an ecosystem e.g. overfishing, overhunting, deforestation, draining swamps or bogland.

Unless conservation-replacement measures are not taken, the population of affected species are not sustainable - possibly to the point of extinction.

 

A high level of biodiversity in a small (e.g. field) or huge (a forest, country) is very good, especially if environmental conditions change.

Some natural ecosystems like rainforests and tropical reefs have a high biodiversity, providing a variety of food and shelter for many species all year round.

Other natural ecosystems have a low biodiversity e.g. arctic tundra, deserts or deep-sea volcanic thermal vents, where a relatively few highly adapted species of plants or animals can survive.

All natural ecosystems are self-supporting, meaning all the resources an organism needs survive and reproduce are all present in their habitats.

However, such systems do need the input of energy, usually from sunlight and,

all animals need plants for oxygen and food,

and plants need carbon dioxide, pollination and seed dispersal from animals.

These connections are called interdependence.

 

Distribution

Distribution is a measure of how high or low the population of species is from one area compared to another in an ecosystem i.e. how they are dispersed around an ecosystem.

Herbivores such as rabbits and sheep will tend to congregate where there is good grazing land.

BUT, there are other consequences for the distribution of plants e.g.

If the intensity of grazing is low, just a few plants will dominate their habitat and out-compete the others.

As the grazing increases, more plant species can thrive because the dominant plant population is controlled by the grazers and the weaker species can grow, but they are often specially adapted plants that can resist the intensive grazing of the herbivores.

The same argument applies to other wild animals such as wild cattle living on the e.g. the African savannah.

For more on these see:

Food chains, food webs, trophic levels, pyramids of biomass, transfer efficiency, pyramids of numbers

Biodiversity and ecological surveying - using quadrats and transects, methods of trapping animals

Biodiversity, land management, waste management, maintaining ecosystems - conservation

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Key ideas and technical terms for the 7 sections on ecology

Ecosystem and Associated Terms

Definition of an Ecosystem

An ecosystem is a biological community of interacting organisms and their physical environment. It encompasses both biotic factors (living organisms such as plants, animals, and microbes) and abiotic factors (non-living components such as temperature, water, soil, and sunlight).

Ecosystems function as complex systems where energy flows and nutrients cycle between organisms and their surroundings.

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

Key Concepts and Technical Terms

1. Habitat

A habitat is the place where an organism lives.

It provides food, water, shelter, and other conditions necessary for survival.

Different organisms thrive in various habitats, such as forests, oceans, and deserts.

2. Population

A population refers to a group of individuals of the same species living in a specific area at the same time.

For example, all the oak trees in a woodland form a population.

3. Community

A community consists of all the populations of different species living and interacting in the same habitat.

It includes various producers, consumers, and decomposers.

4. Biotic Factors

These are the living components of an ecosystem that affect organisms. Examples include:

• Predation (one organism hunting another)

• Competition (organisms competing for resources like food and space)

• Symbiosis (mutually beneficial interactions between species)

5. Abiotic Factors

These are the non-living physical and chemical components of an ecosystem. Examples include:

• Temperature (affects metabolic rates)

• Light intensity (influences photosynthesis)

• Water availability (essential for life)

• pH levels (affects soil and aquatic environments)

6. Producers, Consumers, and Decomposers

• Producers (autotrophs): Organisms that make their own food using sunlight (e.g., plants via photosynthesis).

• Consumers (heterotrophs): Organisms that obtain energy by consuming other organisms. They can be:

  • Primary consumers (herbivores like rabbits)

  • Secondary consumers (carnivores like foxes)

  • Tertiary consumers (top predators like eagles)

• Decomposers: Organisms that break down dead organic material and recycle nutrients back into the ecosystem (e.g., bacteria and fungi).

7. Food Chains and Food Webs

• A food chain shows the flow of energy through organisms in an ecosystem (e.g., grass → rabbit → fox).

• A food web is a more complex representation showing multiple interlinked food chains.

8. Nutrient Cycles

Nutrient cycles ensure the continuous movement of essential elements:

• Carbon cycle: Involves photosynthesis, respiration, decomposition, and fossil fuel combustion.

• Nitrogen cycle: Includes nitrogen fixation, nitrification, and denitrification.

• Water cycle: Involves evaporation, condensation, precipitation, and transpiration.

9. Biodiversity

Biodiversity refers to the variety of living organisms in an ecosystem. Greater biodiversity increases ecosystem stability and resilience against environmental changes.

10. Trophic Levels

Trophic levels represent an organism’s position in a food chain:

1. Level 1: Producers (plants)

2. Level 2: Primary consumers (herbivores)

3. Level 3: Secondary consumers (carnivores)

4. Level 4: Tertiary consumers (top predators)

Each level transfers energy, but only around 10% of energy is passed on, while the rest is lost as heat.

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Importance of Ecosystems

• Maintain balance in the environment.

• Support biodiversity.

• Regulate climate through processes like carbon storage.

• Provide resources such as food, medicines, and raw materials.

Understanding ecosystems and their dynamics is crucial for conservation efforts and mitigating human impacts like deforestation, pollution, and climate change.

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Learning objectives for this section on ecology and related sections to links above summary of learning objectives and key words or phrases

• Be able to explain what an ecosystem is, and know and understand all the definitions and terms used in ecology listed above.

• Be able to demonstrate an understanding of how some energy is transferred to less useful forms at each trophic level and this limits the length of a food chain.

• Be able to show an understanding that the shape of a pyramid of biomass is determined by energy transferred at each trophic level.

  • Know and understand that the mass of living material (the biomass) at each stage in a food chain is less than it was at the previous stage.

  • Appreciate that the biomass at each stage can be drawn to scale and shown as a pyramid of biomass.

  • Up the food chain: producer ==> primary consumer ==> secondary consumer ==> tertiary consumer etc.

  • The producer is usually a photosynthesising plant or algae.

  • In a biomass pyramid, each horizontal bar (drawn to scale) is proportional to the mass of the living material at that producing level and feeding levels (trophic levels).

  • How to construct a biomass pyramid: To draw to scale, you can keep the vertical height the same for each level and make the horizontal length of the bar proportional to the biomass of that level in the pyramid.

  • Obviously, the bigger the bar, the greater the biomass at the producer/feeding-trophic level.

  • Up the food chain and 'up the pyramid' the biomass gets less because of loss of organic material, waste energy and even the energy from respiration, required to sustain life, eventually becomes waste energy too eg heat energy to the surroundings. More in section (c).

  • Know and understand that the amounts of material and energy contained in the biomass of organisms is reduced at each successive stage in a food chain because:

  • (i) some materials and energy are always lost in the organisms’ waste materials by eg excretion (urine, droppings), fallen leaves from trees etc.

  • (ii) respiration supplies all the energy needs for living processes, including movement and much of this energy is eventually transferred to the surroundings, particularly with warm blooded mammals where much energy is spent in maintaining their raised body temperature.

  • the overall simplistic equation for respiration is the opposite of photosynthesis

  • glucose + oxygen ==> water + carbon dioxide (+ energy)

  • This energy is needed for all life processes, energy to do things like movement of any organism, heat to keep mammals warm,

  • The fact of the matter is, that up a food chain/biomass pyramid, only a small percentage of the mass is passed on eg

  • plants producers (100%) ==> primary consumers (caterpillars, 40%) ==> secondary consumers (small birds 5%) ==> bird of prey (owl, 0.5%)

  • This means in this particular food chain, that of all the mass /energy you start with, only 0.5% (¹/₂₀₀th) eventually ends up as the owl.

  • In the food chain: plants ==> rabbits ==> foxes, all these fields of plants of large areas of grass support a relatively smaller population of rabbits, which in turn support a very small number of foxes - you only get a relatively small numbers of a top predator!

  • This is the reason why you rarely get food chains of more than five stages (feeding/trophic levels) because there is so little mass/energy left in the end.

  • Once the energy is lost, it can't be used by the animal in the next stage of the food chain i.e. the next trophic level.

• Know and understand that changes in the environment affect the distribution of living organisms.

  • Exam question examples might include, but not limited to, the changing distribution of some bird species and the disappearance of pollinating insects, including bees.

• Know and understand that animals and plants are subjected to environmental changes.

  • Realise that such changes may be caused by living or non-living factors

  • Biotic factors - 'living factors': Change in competitor (a new or rise/fall in native ones), spread of an infectious disease from parasites and pathogens, levels of prey available to hunt,

    • One species population might be affected by a 'living' factor. If it is the prey for some other animal, then in turn the predator is affected, so population changes are frequent in the animal world and can rise or fall significantly with the availability of food.

    • The decline in the bee population in many countries is attributed to them carrying pathogens/parasites and their food supply contaminated with pesticides - but nobody is quite sure, what is sure, is that bees immune system can't cope.

    • The spread of Dutch elm disease, and other diseases, are devastating tree populations.

  • Abiotic factors - 'non-living factors': Change in the average temperature or rainfall,

    • The average temperature in some northern European countries has risen, so populations of some bird species from southern areas eg the Mediterranean countries, are beginning to increase in northern Europe.

    • Acid rain, from the industrial revolution onwards, has affected forests and ecosystems in lake by decreasing the pH of water.

    • The English Channel separating England and France has become slightly warmer (only by 0.5^oC in 100 years), so species of animals from warmer waters are moving north-east into warmer water i.e. the geographical distribution of marine life is changing.

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