Ecosystems and Communities

An ecosystem is a complex network of living organisms interacting with each other and their environment. In this lesson you will learn the key ecology terms, how energy flows through food chains and webs, and why energy is lost at each trophic level.

---

Key Ecology Definitions

You must know these terms precisely — examiners look for exact definitions:

Term	Definition
Ecosystem	A community of living organisms (biotic) interacting with the non-living (abiotic) components of their environment
Population	All the organisms of one species living in a particular area at a particular time
Community	All the populations of different species living and interacting in a particular area at a particular time
Habitat	The place where an organism lives
Niche	The role an organism plays within its ecosystem — including what it eats, what eats it, and how it interacts with its environment
Biodiversity	The variety of different species of organisms on Earth, or within an ecosystem

How They Relate

• An ecosystem = community + abiotic factors (e.g. a pond ecosystem includes all the fish, plants, algae, bacteria AND the water, rocks, temperature, light)
• A community = all the different populations in an area (e.g. all the fish, all the plants, all the algae, all the bacteria in the pond)
• A population = just one species (e.g. all the perch in the pond)

Exam tip: The difference between a community and a population is a very common question. A population is ONE species; a community is ALL species in an area. Never mix these up.

---

Interdependence

Species in a community are interdependent — they depend on each other for survival. If one species is removed or its population changes, it can affect the entire community.

Examples of Interdependence

Type	Example
Food	Caterpillars depend on plants for food; birds depend on caterpillars
Shelter	Many insects live in trees; removing trees removes their habitat
Pollination	Bees pollinate flowers while collecting nectar — plants depend on bees for reproduction
Seed dispersal	Animals eat fruits and disperse seeds in their droppings
Decomposition	Decomposers break down dead matter, recycling nutrients into the soil for plants

If a species is removed from a food web, it can cause a knock-on effect throughout the community. For example, if a predator is removed, its prey population may increase, leading to overgrazing of plants.

---

Food Chains

A food chain shows the flow of energy from one organism to the next in a feeding relationship.

Example:

grass → rabbit → fox → eagle

Trophic Level	Name	Example	Role
1st	Producer	Grass	Makes its own food by photosynthesis; source of energy for the food chain
2nd	Primary consumer	Rabbit	Herbivore — eats the producer
3rd	Secondary consumer	Fox	Carnivore — eats the primary consumer
4th	Tertiary consumer	Eagle	Top predator — eats the secondary consumer

Key Points

• The arrows in a food chain show the direction of energy transfer (not "what eats what" — the arrow points FROM the organism being eaten TO the organism eating it)
• Producers are always at the start — they convert light energy into chemical energy (glucose) through photosynthesis
• Energy is transferred through the food chain when organisms are eaten

---

Food Webs

A food web is a network of interconnected food chains within an ecosystem. Food webs are more realistic than food chains because most organisms eat more than one type of food and are eaten by more than one predator.

Interpreting Food Webs

When answering questions about food webs, consider:

• What would happen if a species were removed?
• Which organisms compete for the same food source?
• Which species are predators and which are prey?

Exam tip: If asked "what would happen if species X were removed?", trace ALL the connections. Consider: (1) what eats X — those populations may decline (less food), (2) what X eats — those populations may increase (less predation), and (3) knock-on effects further along the chains.

---

Trophic Levels

Trophic levels are the stages in a food chain or food web.

• Trophic level 1 = Producers
• Trophic level 2 = Primary consumers
• Trophic level 3 = Secondary consumers
• Trophic level 4 = Tertiary consumers

Each step up a trophic level, energy is lost, so there is less energy available for the next level.

---

Pyramids of Biomass

A pyramid of biomass shows the mass of all the organisms at each trophic level in a food chain at a given time.

Key Features

• The producer level is always at the bottom and is the widest bar
• Each bar gets narrower as you go up — less biomass at each level
• Pyramids of biomass are always pyramid-shaped (unlike pyramids of numbers, which can sometimes be inverted)

Why Pyramid-Shaped?

At each trophic level, biomass is lost because:

• Organisms use energy from food for respiration (released as heat) — this energy cannot be passed to the next level
• Not all parts of an organism are eaten (bones, roots, shells)
• Some material is lost as waste (faeces and urine)

---

Energy Transfer Between Trophic Levels

Only approximately 10% of the energy at one trophic level is transferred to the next level. The rest is lost.

graph LR
    A["Sun"] -->|"Light energy"| B["Producers - 100%"]
    B -->|"~10% transferred"| C["Primary Consumers"]
    C -->|"~10% transferred"| D["Secondary Consumers"]
    D -->|"~10% transferred"| E["Tertiary Consumers"]

    B -->|"~90% lost"| F["Heat from respiration, waste, uneaten parts"]
    C -->|"~90% lost"| F
    D -->|"~90% lost"| F

    style A fill:#f1c40f,color:#333
    style B fill:#2ecc71,color:#fff
    style C fill:#3498db,color:#fff
    style D fill:#e67e22,color:#fff
    style E fill:#e74c3c,color:#fff
    style F fill:#95a5a6,color:#fff

Where Energy is Lost

How energy is lost	Explanation
Respiration	All organisms respire, releasing energy as heat — this is the biggest loss (especially in warm-blooded animals)
Excretion	Energy is contained in waste products like urea and CO₂
Egestion	Faeces contain undigested material that still has stored energy
Not consumed	Some parts of organisms are not eaten (e.g. bones, bark, roots)
Decomposition	Energy in dead organisms and waste passes to decomposers

Why Food Chains Are Usually Short

Because so much energy is lost at each level, there is not enough energy to support more than about 4–5 trophic levels. This is why food chains rarely have more than 5 organisms.

Calculating Energy Transfer Efficiency

Efficiency = (energy transferred to next level ÷ energy available at previous level) × 100

Example: A plant captures 50,000 kJ of energy. A rabbit eating the plant gains 5,000 kJ.

Efficiency = (5,000 ÷ 50,000) × 100 = 10%

Exam tip: In calculation questions, always show your working and include the percentage sign. A common mistake is dividing the wrong way around — the energy at the NEXT level goes on top (numerator).

---

Producers and Photosynthesis

Producers (mainly green plants and algae) form the base of almost all food chains. They use photosynthesis to convert light energy from the Sun into chemical energy stored in glucose.

Photosynthesis equation:

carbon dioxide + water → glucose + oxygen (using light energy)

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Not all sunlight that reaches a plant is used for photosynthesis:

• Some light is reflected by leaves
• Some light passes through leaves
• Some light hits parts of the plant that do not photosynthesise
• Only certain wavelengths of light are absorbed by chlorophyll

---

Decomposers

Decomposers (mainly bacteria and fungi) break down dead organisms and waste materials. They are essential for:

• Recycling nutrients — returning minerals and elements to the soil for plants to absorb
• Breaking down waste — preventing the accumulation of dead matter

Decomposers feed by secreting enzymes onto dead material, digesting it externally, and then absorbing the soluble products. This is called saprotrophic nutrition.

---

Required Practical: Investigating the Distribution of Organisms Using Quadrats and Transects

This investigation is Edexcel Core Practical 9 (1BI0 / 1SC0). It tests the key ecology skills of random sampling, systematic sampling along an environmental gradient, and estimating population size — skills that often appear in 6-mark exam questions on ecosystems.

Aim: To estimate the population size and distribution of a named organism (for example, daisies or dandelions) in a habitat such as a school field, and to investigate how their distribution changes along an environmental gradient (for example, with distance from a tree or a path).

Hypothesis / prediction: The number of daisies will change with distance from a shaded area because light intensity is a limiting factor for plant growth.