Natural Ecosystems: Understanding Ecosystems (Syllabus 9.1)

Hello future Environmental Manager! This chapter is the foundation of understanding how the natural world works. Don't worry if some of the terms sound complicated—we'll break them down with simple analogies. By the end, you'll see the world as a huge, interconnected network, which is exactly what an ecosystem is!

1. The Basic Building Blocks: Key Definitions

Before we look at energy flow, we need to know the official language of ecology. Getting these definitions right is essential for exam success!

1.1 Core Terms Defined

Think of your local town or school when learning these:

  • Organism: A single living thing (e.g., one student, one tree, one fish).

  • Population: All organisms of one species living in a specific area (e.g., all the students in one specific class, or all the deer in a forest).

  • Community: All the different populations (different species) interacting in an area (e.g., all the students, teachers, insects, and plants living together in the school yard).

  • Habitat: The specific physical place where an organism lives (e.g., the pond is the habitat of the frog, or the classroom is the habitat of the student).

  • Ecosystem: The complex system formed by a community of living organisms (biotic) interacting with their non-living surroundings (abiotic) (e.g., a desert, a rainforest, or a pond). This is the big picture!

  • Niche: The exact role or job an organism has within its ecosystem, including what it eats, where it lives, and how it interacts with other species. (e.g., The niche of a bee is to collect nectar and pollinate flowers.)

Quick Takeaway: An ecosystem is the community (living things) plus the environment (non-living things) working together.

2. Components of an Ecosystem

An ecosystem is made up of two main groups of things: the living and the non-living.

2.1 Biotic Components (The Living Parts)

These are the organisms that can be grouped by how they get their energy:

  1. Producers: These make their own food, usually through photosynthesis. (e.g., plants, algae). They start the food chain.

  2. Consumers: These eat other organisms. We classify them by what they eat:
    • Primary Consumers: Eat producers (herbivores). (e.g., rabbits, cows).
    • Secondary Consumers: Eat primary consumers (carnivores or omnivores). (e.g., foxes eating rabbits).
    • Tertiary Consumers: Eat secondary consumers. (e.g., eagles eating snakes).

  3. Decomposers: These break down dead organic matter and waste, recycling nutrients back into the soil. (e.g., bacteria and fungi). They are incredibly important!

Did You Know? If decomposers stopped working, the world would be knee-deep in dead leaves and animals!

2.2 Abiotic Components (The Non-Living Parts)

These are the physical and chemical factors that influence the organisms in the ecosystem.

  • Temperature: Affects metabolic rates and determines where species can survive.
  • Humidity: The amount of water vapour in the air, crucial for plants and animals to avoid drying out.
  • Water: Essential for all life processes. (Availability, flow, and depth).
  • Oxygen: Needed for respiration by most organisms.
  • Salinity: The salt content (especially important in aquatic systems like oceans and estuaries).
  • Light: Necessary for photosynthesis in producers.
  • pH: Measures the acidity or alkalinity of the soil or water, affecting nutrient availability.

Key Takeaway: Biotic things rely completely on Abiotic things (like water and temperature) to survive.

3. Interactions within the Community

Different populations within a community interact in many ways. These interactions shape the ecosystem.

  • Competition: When organisms fight for the same limited resources (like food, light, water, or mates). This can happen between different species or within the same species.

  • Predation: One organism (the predator) hunts, kills, and eats another organism (the prey). This keeps populations in balance.

  • Pollination: The transfer of pollen, essential for the reproduction of many plants. This often involves an interaction between a plant and an animal (e.g., a bee, bird, or bat).

4. The Flow of Energy: Food Chains, Webs, and Trophic Levels

Energy is not recycled; it flows through the ecosystem in one direction, starting with the sun.

4.1 Photosynthesis and Respiration

These two processes are the engine of almost all ecosystems.

Photosynthesis (Making Food)

This is how producers (like plants) use sunlight to convert carbon dioxide and water into food (glucose) and oxygen.

Word Equation:

\(\text{Carbon dioxide} + \text{Water} \xrightarrow{\text{light / chlorophyll}} \text{Glucose} + \text{Oxygen}\)

Importance of Chlorophyll: This green pigment, found in producers, is absolutely necessary as it captures the light energy required to power the process.

Respiration (Releasing Energy)

This is the process used by all living organisms (plants and animals) to break down glucose (food) to release the energy needed for life functions.

Word Equation:

\(\text{Glucose} + \text{Oxygen} \longrightarrow \text{Carbon dioxide} + \text{Water} + \text{Energy}\)

4.2 Food Chains and Food Webs
  • Food Chain: Shows the direct flow of energy from one organism to the next. The arrows show the direction of energy flow (always pointing to the organism that eats the other).

    Example: Grass \(\longrightarrow\) Rabbit \(\longrightarrow\) Fox


  • Food Web: A complex network showing how multiple food chains are interconnected. Food webs are much more realistic than single food chains and show the ecosystem's stability (if one prey source disappears, a consumer can switch to another).
4.3 Trophic Levels and Food Chain Length

A trophic level is the position an organism occupies in a food chain.

  1. Trophic Level 1 (T1): Producers (plants).
  2. Trophic Level 2 (T2): Primary Consumers (herbivores).
  3. Trophic Level 3 (T3): Secondary Consumers (carnivores/omnivores).
  4. Trophic Level 4 (T4): Tertiary Consumers (top predators).

Why are food chains limited in length? Food chains rarely have more than 4 or 5 trophic levels because around 90% of energy is lost at each level. After several trophic steps, there is simply not enough energy left to support another viable population of consumers.

4.4 Ecological Pyramids

Ecological pyramids are graphical representations of the structure of an ecosystem.

Pyramids of Numbers

Show the number of individual organisms at each trophic level.

Note: These pyramids can sometimes be inverted. Example: One large oak tree (producer) supports thousands of caterpillars (primary consumers).

Pyramids of Biomass

Show the total dry mass of biological material (biomass) at each trophic level at a given time.

  • In terrestrial ecosystems, these are typically upright as biomass decreases up the food chain.
  • They can occasionally be inverted in some aquatic ecosystems if producers (like phytoplankton) reproduce and are consumed very rapidly.
Pyramids of Energy

Show the total amount of energy stored or transferred at each trophic level over a period of time.

  • Crucial Rule: Pyramids of energy are always upright. They can never be inverted.
  • The 10% Rule: Only about 10% of the energy from one trophic level is passed on to the next.

Where does the other 90% go? It is lost at each step as:
1. Heat released during respiration.
2. Waste and undigested material (faeces and excretion).
3. Uneaten parts of organisms (such as roots, bones, or fur).

Quick Review: Energy pyramids are always upright because energy is lost at every transfer!

5. Recycling Life: The Carbon Cycle

Unlike energy, matter (like carbon) must be recycled for life to continue. The carbon cycle describes the movement of carbon atoms through the biotic and abiotic parts of the ecosystem.

5.1 Key Processes in the Carbon Cycle (Step-by-Step)
  1. Atmospheric Carbon: Carbon exists in the atmosphere primarily as carbon dioxide (CO₂).

  2. Uptake (Photosynthesis): Plants (producers) take CO₂ from the atmosphere to make glucose (sugar). Carbon is now locked into the plant biomass.

  3. Transfer (Feeding): When animals (consumers) eat plants, the carbon moves up the food chain.

  4. Release (Respiration): All living things (plants, animals, decomposers) break down food for energy (respiration), releasing CO₂ back into the atmosphere.

  5. Release (Decomposition): Decomposers break down dead organisms and waste, releasing carbon back into the soil and atmosphere.

  6. Storage (Fossil Fuels): Over millions of years, dead plant and animal matter can be buried and compressed, forming fossil fuels (coal, oil, gas), which act as a massive carbon store.

  7. Human Impact (Combustion): When humans burn fossil fuels or wood (combustion), they release huge amounts of stored carbon rapidly back into the atmosphere as CO₂. This upsets the natural balance and drives climate change (enhanced greenhouse effect).

Memory Trick: Think of the 4 main Carbon Cycle movements: Photo-Respire-Decompose-Combust (PRDC).