Sciences - Co-ordinated (Double) (0654) | Organisms and their environment

Hello IGCSE Scientists! Welcome to Organisms and their Environment

This chapter, B18 and B19, takes us out of the cell and into the world around us. We are diving into Ecology—the study of how living things (organisms) interact with each other and their surroundings (environment).

Understanding these interactions is crucial. It helps us see how energy flows to keep life going and, importantly, how human actions impact the planet. Don't worry if you find big ecological systems confusing; we'll break them down step-by-step using clear definitions and relatable examples!

B18: Energy Flow in Biological Systems

B18.1 Sources of Energy

Where does all the energy that powers life come from?

The vast majority of energy input into biological systems starts with the Sun.

• The Sun provides light energy.
• Plants (and other producers) capture this light energy during photosynthesis and convert it into chemical energy (stored in glucose/carbohydrates).
• This chemical energy is then passed along when organisms eat each other.
• Eventually, all this energy is released back into the environment, usually as heat energy, which is lost and cannot be recycled.

B18.2 Food Chains and Food Webs

Defining Key Roles (The Cast of Characters)

To understand how energy moves, we need to know the roles organisms play in an ecosystem:

1. Producer: An organism that makes its own organic nutrients (food), typically using energy from sunlight through photosynthesis.
   Example: Grass, trees, algae.
2. Consumer: An organism that obtains its energy by feeding on other organisms.
3. Decomposer: An organism that gets its energy from dead or waste organic material (like fallen leaves, dead animals, or faeces). They play a vital role in recycling nutrients.
   Example: Bacteria and fungi.

Food Chains (Core)

A food chain shows the transfer of energy from one organism to the next, starting with a producer.

It uses arrows ($\rightarrow$) to show the direction of energy flow (i.e., "is eaten by").

Example:
Grass $\rightarrow$ Rabbit $\rightarrow$ Fox

• The Rabbit eats the Grass, so energy flows from Grass to Rabbit.

Food Webs (Core)

A food web is much more realistic! It is a network of interconnected food chains. Most organisms eat, and are eaten by, multiple different species.

Think of a food web like a complex family tree showing all possible meal options in an ecosystem.

Consumers and Trophic Levels (Core & Supplement)

The position an organism holds in a food chain or food web is called its trophic level.

• Trophic Level 1: Producers (always plants/algae).
• Trophic Level 2: Primary Consumers (eat producers). These are herbivores (animals that eat plants).
• Trophic Level 3: Secondary Consumers (eat primary consumers). These are usually carnivores (animals that eat other animals).
• Trophic Level 4: Tertiary Consumers (eat secondary consumers).
• Trophic Level 5: Quaternary Consumers (eat tertiary consumers).

Did you know? An animal that eats both plants and animals is called an omnivore, and it can occupy multiple trophic levels!

B18.2 Energy Transfer Efficiency (Supplement Focus)

If a mouse eats grass, does it gain 100% of the grass's energy? Unfortunately, no!

The transfer of energy from one trophic level to the next is highly inefficient.

Typically, only about 10% of the energy stored in one trophic level is actually transferred and stored in the next trophic level.

Why is the Energy Transfer Inefficient (B18.2.13)?

Energy is lost at every step through natural biological processes:

• Heat loss: Energy is lost through respiration (aerobic and anaerobic) used for movement, growth, and maintaining body temperature. This heat energy is transferred to the environment.
• Waste products: Energy is lost in faeces (undigested food) and urine (excretion).
• Uneaten parts: Energy is stored in parts of the organism that the consumer cannot or does not eat (e.g., roots, bones, fur).

Limiting Trophic Levels (B18.2.14)

Because so much energy is lost (around 90%) at each transfer, food chains usually have fewer than five trophic levels. If there were six or seven levels, the top consumer would receive almost no energy from the original producer, making survival impossible.

Efficiency and Human Diet (B18.2.15)

Understanding this energy loss helps explain why certain diets are more efficient:

It is more energy efficient for humans to eat crop plants (Primary Consumers) than to eat livestock (Secondary Consumers) that have been fed on those crops.

Analogy: Imagine photocopying a document. If you photocopy the original, it's clear (Producer $\rightarrow$ Human). If you photocopy the photocopy (Producer $\rightarrow$ Cow $\rightarrow$ Human), a lot of detail (energy) is lost.

If we eat wheat directly, we are Primary Consumers (Level 2). If we eat a cow that ate the wheat, we are Secondary Consumers (Level 3), and 90% of the energy from the wheat has already been lost by the cow!

B18.3 The Carbon Cycle

The carbon cycle describes how carbon, a fundamental element for life, moves between the air, water, land, and living organisms. It ensures carbon is constantly recycled.

Processes in the Carbon Cycle (Core)

There are six main processes you must know:

1. Photosynthesis: Producers (plants) take carbon dioxide (\(\text{CO}_2\)) from the atmosphere to make organic compounds (glucose). This removes \(\text{CO}_2\) from the atmosphere.
2. Feeding: Consumers eat producers or other consumers, passing carbon compounds along the food chain.
3. Respiration: All living organisms (producers, consumers, and decomposers) break down organic compounds to release energy. This process releases carbon dioxide (\(\text{CO}_2\)) back into the atmosphere.
4. Decomposition: Decomposers break down dead organic matter and waste. This process releases \(\text{CO}_2\) through their respiration.
5. Formation of Fossil Fuels: Organic matter from ancient plants and animals sometimes gets buried under pressure over millions of years, forming fossil fuels (coal, oil, gas). This locks carbon away from the atmosphere.
6. Combustion (Burning): Burning fossil fuels (or wood) releases the stored carbon back into the atmosphere as \(\text{CO}_2\).

B19: Human Influences on Ecosystems

B19.1 Habitat Destruction

Ecosystems and Biodiversity (Core)

Before looking at destruction, let's define the terms:

• Ecosystem: A unit containing the community of organisms (all living things) and their environment (all non-living things), interacting together.
  Example: A forest, a pond, or even a small rotting log.
• Biodiversity: The number of different species that live in an area. High biodiversity means a healthy, stable ecosystem.

Reasons for Habitat Destruction (Core)

Human activities often lead to the destruction of natural habitats, reducing biodiversity. Key reasons include:

1. Increased Area for Production: Clearing land for housing, crop plant production (farming), and livestock production (ranching).
2. Extraction of Natural Resources: Mining, quarrying, and logging destroy ecosystems.
3. Pollution:
   • Freshwater and Marine Pollution: This contaminates water bodies. (Note: Detailed description of eutrophication is not required for the exam).

Deforestation: Undesirable Effects (Core & Supplement)

Deforestation is the large-scale cutting down of trees. This is a crucial example of habitat destruction.

The undesirable effects (and their explanations for the Supplement) are:

1. Reducing Biodiversity / Extinction: Trees provide specialized habitats and food sources. Removing them kills or displaces many species, leading to reduced biodiversity and potentially extinction if the species cannot survive anywhere else.
2. Loss of Soil (Erosion): Tree roots hold the soil together. When trees are removed, heavy rain washes the topsoil away into rivers (soil erosion), making the land infertile for future use.
3. Flooding: Forests absorb large amounts of water. Without trees, rainfall runs quickly over the surface, increasing the speed and volume of water entering rivers, which causes flooding downstream.
4. Increase of Carbon Dioxide in the Atmosphere:
   • Trees absorb large amounts of \(\text{CO}_2\) during photosynthesis (acting as carbon sinks). Cutting down trees stops this uptake.
   • If the trees are burned or left to decompose, the carbon stored inside them is released back into the atmosphere as \(\text{CO}_2\), contributing to global warming.

B19.2 Conservation

Why Organisms Become Endangered or Extinct (Core)

A species is endangered if it is at risk of extinction. It is extinct if there are no individuals of that species left anywhere in the world.

The key human-caused factors leading to endangerment are:

• Habitat Destruction (as discussed above).
• Climate Change: Changes in temperature and rainfall patterns make existing habitats unsuitable for some species.
• Hunting/Poaching: Illegal or excessive hunting that reduces populations to unsustainable levels.
• Overharvesting: Taking too many individuals (especially food species like fish) from the wild faster than they can reproduce. (This links back to B18.2.10).
• Pollution: Introduction of harmful substances into the environment.
• Introduced Species: Non-native (foreign) species brought into an ecosystem by humans. These species often compete with, prey upon, or introduce disease to native species, pushing them towards extinction.

How Endangered Species are Conserved (Core)

Conservation aims to protect biodiversity and stop extinction. Methods include:

1. Monitoring and Protecting Species and Habitats:

   This involves setting up national parks or reserves, monitoring population numbers, and enforcing anti-poaching laws.

2. Education:

   Teaching the public and local communities about the importance of biodiversity and sustainable practices to gain support for conservation efforts.

3. Captive Breeding Programmes:

   Breeding endangered animals in zoos or specialized facilities, often with the goal of releasing them back into the wild later.

4. Seed Banks:

   Storing seeds from rare and important plant species to protect the genetic diversity of plants in case of natural disasters or extinction in the wild.