Biology | Ecosystems

Welcome to the World of Ecosystems!

Hey there! Ready to dive into one of the most exciting topics in Biology? In this chapter, we'll explore Ecosystems. Think of it as being a detective, uncovering the secrets of how living things interact with each other and their environment. Understanding ecosystems is super important because it helps us see the big picture of life on Earth and why we need to protect our beautiful planet. Don't worry if some ideas seem big at first – we'll break everything down into easy-to-understand pieces. Let's get started!

1. From a Single Organism to the Whole World

To understand ecosystems, we first need to know how ecologists organise the living world into different levels. It's like sorting your folders on a computer, from smallest to largest!

The Levels of Organisation

Imagine you're looking at a single fish in a pond. That's our starting point.

• Species: A group of similar organisms that can reproduce to have fertile offspring. For example, all the clownfish are one species.


• Population: All the members of a single species living in a particular area at the same time. For example, all the clownfish living in one coral reef.


• Community: All the different populations of different species living and interacting in the same area. For example, the clownfish, corals, sea anemones, and sea turtles all living together in the reef. It's the entire living part!


• Ecosystem: The community of organisms (all the living things) interacting with their non-living environment (like the water, sunlight, and rocks). For example, the entire coral reef community PLUS the water, sunlight, temperature, and sand.


• Biosphere: The part of the Earth where life exists. It's the sum of all the ecosystems on our planet!

A Look Around Hong Kong

We are lucky to have so many different types of ecosystems right here! The syllabus wants you to appreciate this variety, which includes:

• Freshwater streams (e.g., in Tai Po Kau Nature Reserve)
• Rocky shores (e.g., at Cape D'Aguilar)
• Mangroves (e.g., in Mai Po Nature Reserve)
• Grassland and woodland (e.g., in our country parks)

Key Takeaway

Think of it like an address: A species is like a person. A population is their family in one house. A community is all the families in the neighbourhood. An ecosystem is the neighbourhood plus the houses, roads, and weather.

2. What's in an Ecosystem? The Living and Non-Living Parts

Every ecosystem has two main types of components that work together. It's like a stage play – you have the actors (living) and the stage set (non-living).

The Non-Living Stuff: Abiotic Factors

Abiotic factors are the non-living chemical and physical parts of the environment that affect living organisms. They set the stage for life.

• Light intensity: Affects the rate of photosynthesis. More light generally means more plant growth.
• Temperature: Affects the rate of metabolic reactions (like enzymes!). Most organisms have an optimal temperature range to live in.
• Water: Essential for all life. Factors like humidity (moisture in the air) and salinity (saltiness of water) determine who can live where.
• pH: The acidity or alkalinity of the soil and water. It can affect nutrient availability and enzyme function.
• Oxygen: Crucial for aerobic respiration. The amount of dissolved oxygen in water is very important for aquatic life.

The Living Community: Biotic Factors

Biotic factors are all the living organisms within an ecosystem. This includes everything from the tiniest bacterium to the largest tree.

Habitat vs. Niche: An Organism's Address and Job

These two terms are often confused, but they're quite different. Don't worry, here's a simple way to remember them!

• Habitat: This is simply where an organism lives. It's its 'address'. For example, a panda's habitat is a bamboo forest.


• Niche: This is the role or 'job' an organism has in its ecosystem. It includes what it eats, what eats it, how it behaves, and how it affects its environment. For example, a panda's niche includes eating bamboo, dispersing seeds, and being a food source for predators (if any).

Memory Aid: Habitat is the Home. Niche is the Nine-to-five job.

Species Diversity and Dominant Species

• Species diversity: A measure of the number of different species in a community. High diversity usually means a healthier, more stable ecosystem.
• Dominant species: The species that is the most abundant or has the highest biomass in a community. For example, pine trees might be the dominant species in a pine forest.

Key Takeaway

An ecosystem is a delicate balance between the abiotic (non-living) factors like sunlight and water, and the biotic (living) community. Each organism has a habitat (address) and a niche (job).

3. Life's Big Drama: How Organisms Interact

In any community, organisms are constantly interacting. These relationships are crucial for the balance of the ecosystem.

• Predation (+/-): One organism (the predator) hunts and kills another (the prey). Example: A snake (+) eating a mouse (-).


• Competition (-/-): When two or more organisms need the same limited resource (like food, water, or space). Both are negatively affected because they have to share. Example: Two lions fighting over a kill.


• Parasitism (+/-): One organism (the parasite) lives on or inside another organism (the host) and harms it. The parasite gets food and shelter. Example: A tick (+) feeding on a dog (-).


• Mutualism (+/+): A relationship where both organisms benefit. It's a "win-win"! Example: Bees (+) get nectar from flowers, and the flowers (+) get pollinated.


• Commensalism (+/0): One organism benefits, and the other is neither harmed nor helped (it's neutral). Example: A bird (+) builds a nest in a tree (0). The bird gets a home, and the tree is unaffected.

Key Takeaway

Relationships between organisms define the structure of a community. They range from harmful (predation, competition) to helpful (mutualism).

4. Who Eats Whom? The Flow of Energy

Energy is the fuel for life, and it flows in one direction through an ecosystem. Where does it all start?

The Source of Energy

For almost all ecosystems on Earth, the ultimate source of energy is the sun.

The Roles in Energy Flow

• Producers (Autotrophs): These are the "chefs". They produce their own food, usually through photosynthesis, converting light energy into chemical energy. Examples: Plants, algae.


• Consumers (Heterotrophs): These are the "customers". They get energy by eating other organisms.
  • Primary consumers: Herbivores that eat producers. (e.g., rabbits eating grass)
  • Secondary consumers: Carnivores or omnivores that eat primary consumers. (e.g., snakes eating rabbits)
  • Tertiary consumers: Carnivores or omnivores that eat secondary consumers. (e.g., eagles eating snakes)


• Decomposers: These are the vital "cleanup crew". They break down dead organic matter (dead plants, animals, waste) and return nutrients to the soil. This is crucial for recycling! Examples: Bacteria and fungi.

Mapping the Flow: Food Chains and Food Webs

• A Food Chain shows a simple, single pathway of energy flow. The arrows show the direction of energy transfer (from the organism being eaten to the organism that eats it).
  Example: Grass → Rabbit → Fox → Wolf


• A Food Web is more realistic. It consists of many interconnected food chains and shows the complex feeding relationships in an ecosystem.

Common Mistake Alert! Always draw the arrows in a food chain pointing from the organism that is eaten to the one that eats it. The arrow means "is eaten by" and shows where the energy is going!

The Energy Loss Problem: The 10% Rule

As energy flows from one trophic level (feeding level) to the next, a huge amount is lost! Only about 10% of the energy from one level is stored in the next. The other 90% is lost, mainly as heat during respiration, or is uneaten or undigested.

This is why food chains are usually short (4-5 levels). There just isn't enough energy left to support more levels!

Visualising Ecosystems: Ecological Pyramids

Ecologists use pyramids to represent the relationships between trophic levels.

• Pyramid of Numbers: Shows the total number of individual organisms at each trophic level. The base is always the producers. It's usually pyramid-shaped, but can be inverted. For example, one large oak tree (producer) can support thousands of caterpillars (primary consumers).


• Pyramid of Biomass: Shows the total dry mass (biomass) of all organisms at each trophic level. This is a better indicator of the energy available at each level and is almost always a true pyramid shape.

Key Takeaway

Energy flows through an ecosystem, starting from the sun. It is transferred through food chains, but about 90% is lost at each step. This flow can be visualized using pyramids of numbers and biomass.

5. The Great Recycle: How Nature Reuses Materials

Unlike energy, which flows through and is lost, essential materials like carbon and nitrogen are cycled within an ecosystem. Decomposers are the superstars of this process!

The Carbon Cycle

Carbon is the building block of life. Here's how it cycles:

1. Photosynthesis: Producers (plants) take carbon dioxide (CO₂) from the atmosphere and use it to make organic compounds (food).
2. Consumption: Animals get carbon by eating plants or other animals.
3. Respiration: All living organisms (plants, animals, decomposers) release CO₂ back into the atmosphere through respiration.
4. Decomposition: Decomposers break down dead organisms, releasing the carbon they contain. Some of this carbon is released as CO₂ through their respiration.
5. Combustion: Humans burn fossil fuels (which are ancient, compressed organic matter), releasing huge amounts of CO₂ into the atmosphere.

The Nitrogen Cycle

Nitrogen is essential for making proteins and DNA. But the nitrogen gas (N₂) in the air is unusable for most organisms. It needs to be 'fixed' by special bacteria. Don't worry, this seems complex, but let's focus on the key roles.

• Nitrogen Fixation: Nitrogen-fixing bacteria (in soil or plant roots) convert unusable nitrogen gas from the air into usable compounds like ammonia.
• Nitrification: Nitrifying bacteria in the soil convert ammonia into nitrates, which are the best form of nitrogen for plants to absorb through their roots.
• Assimilation: Plants absorb nitrates from the soil to make proteins. Animals then get nitrogen by eating plants.
• Decomposition: Decomposers break down dead organisms and waste products, returning nitrogen compounds to the soil.
• Denitrification: Denitrifying bacteria convert nitrates in the soil back into nitrogen gas, which returns to the atmosphere.

Did you know? Decomposers are the most important link in material cycles. Without them, nutrients would be locked up in dead matter, and life would grind to a halt!

Key Takeaway

Essential materials like carbon and nitrogen are constantly cycled through ecosystems. Bacteria and fungi (decomposers) play a critical role in breaking down dead material and making these nutrients available again.

6. How Ecosystems Change Over Time: Succession

Ecosystems are not static; they change over time in a predictable process called ecological succession.

Starting from Scratch: Primary Succession

This happens in a place where no life or soil existed before. Think of bare rock after a volcano erupts.

1. Pioneer Species: Hardy species like lichens and mosses are the first to arrive. They can grow on bare rock.
2. Soil Formation: The pioneers slowly break down the rock, and as they die and decompose, they create the very first thin layer of soil.
3. Colonisation: As the soil gets richer, larger plants like grasses and shrubs can grow. They attract insects and small animals.
4. Maturation: Over hundreds or even thousands of years, larger trees move in, and the ecosystem eventually becomes a stable, mature forest.

Bouncing Back: Secondary Succession

This happens when an existing community has been cleared by a disturbance (like a forest fire or logging), but the soil remains intact.

Because soil is already present, secondary succession is much faster than primary succession. Grasses and weeds grow back quickly, followed by shrubs and trees.

The Final Stage: Climax Community

This is the final, stable stage of succession. A climax community is mature, has high species diversity, and doesn't change much over time unless there's another major disturbance.

Key Takeaway

Ecosystems develop and change over time through succession. Primary succession starts from bare rock, while secondary succession is a faster recovery process where soil already exists.

7. Our Role in the Ecosystem: Field Studies and Conservation

As biologists, we need tools to study ecosystems. This helps us understand them and figure out how to protect them.

Being an Eco-Detective: Studying a Habitat

It's impossible to count every single organism in a habitat, so we use sampling methods to estimate.

• Quadrat Sampling: A quadrat is a square frame of a known area (e.g., 1m²). To estimate the population of a plant, you place the quadrat randomly in an area several times, count the number of plants inside each time, and then calculate the average. This is great for plants and slow-moving animals.


• Transect Sampling: A transect is a line (like a measuring tape) laid across a habitat. It's used to study how the distribution of organisms changes across an environmental gradient.
  • In a line transect, you record every organism that touches the line.
  • In a belt transect, you place quadrats at regular intervals along the line to get more data.


• Measuring Abiotic Factors: We use scientific instruments to measure the non-living conditions, such as a light meter (light intensity), a pH meter (pH), a thermometer (temperature), and so on.

Our Impact and the Need for Conservation

Human activities are having a massive impact on ecosystems worldwide. This has led to habitat destruction, pollution, and loss of biodiversity. Therefore, there is an urgent need for conservation – the protection and management of Earth's biodiversity and ecosystems.

By understanding how ecosystems function, we can make better decisions to protect them for future generations.

Key Takeaway

We use sampling techniques like quadrats and transects to study populations and communities. This data helps us understand the impact of human activities and highlights the critical need for conservation.