Welcome to Ecological Relationships and Energy Flow!
Welcome to one of the most exciting topics in Biology! In this chapter, we explore how living organisms live together, how they depend on one another to survive, and how energy from the Sun powers almost all life on Earth.
Don't worry if ecology terms seem a bit overwhelming at first. We will break down every single idea step-by-step with real-life examples, memory tricks, and easy calculations so you feel completely confident for your exam!
1. Key Ecological Terms: The Building Blocks
Before exploring ecosystems, we need to speak the language of an ecologist. Here are the core definitions you need to master:
Environment: The total surroundings of an organism, including both living and non-living factors.
Habitat: The specific place where an organism lives (for example, a freshwater pond, a woodland floor, or a rocky shore).
Population: All the individuals of one particular species living in a habitat at the same time (for example, all the red squirrels in a forest).
Community: All the populations of different species living and interacting in the same habitat at the same time (for example, the red squirrels, oak trees, birds, and earthworms all together).
Ecosystem: A community of living organisms interacting with each other and with the non-living parts of their environment (for example, a whole woodland ecosystem, including the soil, water, air, plants, and animals).
Biodiversity: A measure of the variety of different species of organisms living in an area. High biodiversity generally means an ecosystem is healthier and more stable.
Memory Trick: The Address Analogy
Think of ecological terms like sending a letter:
- Habitat: Your street address (where you live).
- Population: Your family members living in your house.
- Community: All the different families and pets living on your street.
- Ecosystem: The whole neighbourhood, including the houses, roads, weather, and people.
Abiotic vs Biotic Factors
An ecosystem is shaped by two types of factors:
1. Abiotic factors (Non-living factors):
- Light intensity: Affects the rate of photosynthesis in plants.
- Temperature: Affects enzyme activity and metabolic rates.
- Soil moisture / water availability: Essential for plant growth and animal survival.
- Soil pH: Affects which plants can grow and nutrient availability in the soil.
- Wind speed and direction: Influences transpiration rates in plants.
2. Biotic factors (Living factors):
- Competition: Organisms competing for limited resources (such as food, light, water, territory, or mates).
- Predation: Predators hunting and eating prey.
- Disease: Pathogens infecting plants or animals.
- Food availability: More food allows populations to increase.
Quick Review: Key Takeaway
An ecosystem equals all the living organisms (biotic community) interacting with their physical, non-living environment (abiotic factors).
2. Investigating Ecosystems: Sampling Techniques
Scientists cannot count every single blade of grass or every beetle in an entire forest—it would take far too long! Instead, ecologists take representative samples to estimate population sizes and observe distribution patterns.
Measuring Abiotic Factors
To measure the non-living parts of an ecosystem, ecologists use specialised equipment:
- Light Meter: Measures light intensity. Top exam tip: Take care not to cast your own shadow over the sensor when taking a reading!
- Soil Moisture Meter / Soil Thermometer: Pushed into the ground at a constant depth to measure water content or temperature. Always wipe the probe clean between readings.
- pH Meter / Chemical Test Kit: Measures the acidity or alkalinity of the soil or water.
Measuring Living Organisms: Using Quadrats
A quadrat is a square frame (usually \(0.5\text{ m} \times 0.5\text{ m} = 0.25\text{ m}^2\) or \(1\text{ m} \times 1\text{ m} = 1\text{ m}^2\)) placed on the ground to sample stationary or slow-moving organisms, such as dandelions, daisies, or barnacles.
Random Sampling (Estimating Population Size)
To avoid bias (e.g. accidentally placing the quadrat only where pretty flowers grow), sampling must be completely random.
Step-by-Step Method for Random Sampling:
1. Lay out two long tape measures at right angles across the study area to create a coordinate grid (e.g. \(10\text{ m} \times 10\text{ m}\)).
2. Use a random number generator (on a calculator or app) to select pairs of coordinates.
3. Place the quadrat at the selected coordinates.
4. Count and record the number of individual organisms (or estimate the percentage cover) inside the quadrat.
5. Repeat this process for a large number of quadrats (e.g. at least 10 times) to ensure the sample is reliable and representative.
6. Calculate the mean (average) number of organisms per quadrat.
7. Scale up to estimate the total population across the entire area using the formula:
\(\text{Total Population Estimate} = \text{Mean count per quadrat} \times \left(\frac{\text{Total Study Area}}{\text{Area of one quadrat}}\right)\)
Worked Example:
A student uses a \(0.25\text{ m}^2\) quadrat to count buttercups in a field with a total area of \(200\text{ m}^2\).
The mean number of buttercups per quadrat is \(6\).
\(\text{Total Population} = 6 \times \left(\frac{200\text{ m}^2}{0.25\text{ m}^2}\right) = 6 \times 800 = 4800\text{ buttercups}\).
Systematic Sampling: Belt Transects
Sometimes abiotic conditions change steadily across an area (this is called an environmental gradient—for example, moving from the open shore into sand dunes, or from a sunny field into a dark forest).
To study how the distribution of organisms changes along a gradient, ecologists use a transect:
1. Lay a long tape measure in a straight line across the environmental gradient.
2. Place quadrats at regular intervals along the line (e.g. every \(2\text{ metres}\)). This is a belt transect.
3. Record the abundance of the organism and measure the abiotic factor (e.g. light intensity) at each interval.
4. This reveals how changes in the abiotic factor directly affect where species live.
Common Mistakes to Avoid
- Mistake: Saying you "throw" the quadrat randomly. (Throwing is dangerous and not truly random!).
- Correction: Always explain that you use a coordinate grid and random number generator.
3. Feeding Relationships and Energy Flow
Every living organism needs energy to survive. The ultimate source of energy for almost all ecosystems on Earth is sunlight.
Feeding Roles in an Ecosystem
1. Producers (Autotrophs): Green plants and algae that produce their own glucose using light energy during photosynthesis.
2. Primary Consumers: Herbivores (animals that eat only plants) that feed on producers.
3. Secondary Consumers: Carnivores (meat-eaters) or omnivores (eat plants and meat) that feed on primary consumers.
4. Tertiary Consumers: Top predators that feed on secondary consumers.
5. Decomposers: Microorganisms (fungi and bacteria) that break down dead plant and animal matter, returning vital mineral nutrients to the soil.
Food Chains and Trophic Levels
A food chain shows a single path of feeding relationships. Each feeding stage in a food chain is called a trophic level.
Example of a simple food chain:
\(\text{Grass (Producer)} \longrightarrow \text{Rabbit (Primary Consumer)} \longrightarrow \text{Fox (Secondary Consumer)}\)
Crucial Rule for Food Chains:
The arrows in a food chain represent the direction of energy flow, NOT who is eating whom! The arrow points from the organism being eaten into the mouth of the organism eating it.
Food Webs and Interdependence
In nature, animals rarely eat just one thing. A food web consists of many interconnected food chains showing the complex feeding relationships in an ecosystem.
Organisms in a food web are interdependent—a change in the population of one species has direct and indirect knock-on effects on others.
Worked Example: Predicting Changes in a Food Web
Imagine a food web where: \(\text{Grass} \longrightarrow \text{Slugs} \longrightarrow \text{Thrushes (birds)} \longrightarrow \text{Hawks}\)
What happens if a disease reduces the population of thrushes?
1. Slugs will increase because fewer thrushes are hunting and eating them.
2. Grass will decrease because a larger slug population will eat more grass.
3. Hawks will decrease because their food source (thrushes) has dropped.
Quick Review: Key Takeaway
Arrows in a food chain show the flow of energy. If one species changes in number, it creates a ripple effect across the entire food web due to interdependence.
4. Energy Loss Between Trophic Levels
Have you ever wondered why food chains are rarely longer than four or five trophic levels? The reason is that energy is lost at every single step!
How is Energy Lost?
Producers absorb only about \(1\%\) to \(3\%\) of the total sunlight that reaches them (the rest reflects off leaves, passes straight through, or is the wrong wavelength).
At each subsequent trophic level, roughly \(90\%\) of the energy is lost, meaning only about \(10\%\) is passed on to build new biomass in the next organism.
Energy is lost through:
1. Respiration: A large amount of energy is released as heat during cellular respiration to fuel body processes and movement.
2. Excretion: Energy is lost in metabolic waste products (such as urea in urine).
3. Egestion: Undigested food that cannot be absorbed is passed out as faeces (e.g. cellulose, bones, hair).
4. Uneaten parts: Not all of an organism is eaten (e.g. plant roots, woody stems, animal bones and fur).
Calculating the Efficiency of Energy Transfer
You can calculate how efficiently energy is passed from one trophic level to the next using this formula:
\(\text{Efficiency of Energy Transfer (\%)} = \left(\frac{\text{Energy transferred to the next level}}{\text{Total energy received from previous level}}\right) \times 100\)
Worked Example:
A caterpillar consumes \(500\text{ kJ}\) of energy from eating leaves. It uses \(300\text{ kJ}\) in respiration, loses \(150\text{ kJ}\) in faeces, and stores \(50\text{ kJ}\) in new body tissue.
Calculate the efficiency of energy transfer to the caterpillar's body tissue:
\(\text{Efficiency} = \left(\frac{50\text{ kJ}}{500\text{ kJ}}\right) \times 100 = 0.10 \times 100 = 10\%\)
Because so much energy is lost at each level, by the time you reach a 4th or 5th trophic level, there is simply not enough energy left to support another population of predators.
5. Ecological Pyramids: Numbers vs Biomass
Ecologists use diagrams called ecological pyramids to visually represent trophic levels in a food chain. The producer is always placed at the bottom (base) of the pyramid.
1. Pyramids of Numbers
A pyramid of numbers represents the actual count of individual organisms at each trophic level.
- In many food chains, the pyramid has a normal pyramid shape (wide base, narrow top), such as: \(1000\text{ grass plants} \longrightarrow 50\text{ grasshoppers} \longrightarrow 5\text{ frogs} \longrightarrow 1\text{ hawk}\).
- However, pyramids of numbers can sometimes look inverted or irregular! This happens when organisms are very different in size.
Example of an irregular pyramid of numbers:
\(\text{One single large oak tree} \longrightarrow \text{Thousands of caterpillars} \longrightarrow \text{Five blue tits} \longrightarrow \text{One sparrowhawk}\)
Here, the base of the pyramid is a tiny bar (just 1 tree), while the second bar is very wide (thousands of caterpillars)!
2. Pyramids of Biomass
Biomass is the total mass of living biological material in an organism. To be completely accurate, scientists measure dry biomass (the mass of the organism with all water removed), measured in units such as \(\text{g/m}^2\) or \(\text{kg/m}^2\).
Key rules about Pyramids of Biomass:
- A pyramid of biomass represents the total mass of living tissue at each trophic level.
- Pyramids of biomass are ALWAYS pyramid-shaped (broadest at the base and tapering evenly to the top) in terrestrial ecosystems.
- They are always pyramid-shaped because energy and material are lost at each trophic level, so there must always be more biomass at the bottom than at the top.
Rules for Drawing Ecological Pyramids in Exams
When drawing pyramids of numbers or biomass:
1. Producer at the bottom: Always place trophic level 1 at the base.
2. Symmetry: Draw each bar centrally stacked on top of the one below it.
3. Labels: Clearly label each bar with the organism's name and its trophic level.
4. Scale: Make sure the height of each bar is uniform and the width is drawn accurately to scale.
Summary Checklist: Are You Exam Ready?
Make sure you can comfortably answer the following before your exam:
- Can you define ecosystem, community, population, habitat, and biodiversity?
- Can you explain how to set up a random sampling investigation using a grid and quadrats?
- Can you describe when and why to use a belt transect along an environmental gradient?
- Do your food chain arrows correctly show the flow of energy?
- Can you list at least four ways energy is lost between trophic levels (respiration, heat, egestion, excretion)?
- Can you calculate the percentage efficiency of energy transfer?
- Can you explain why a pyramid of numbers can be irregular, while a pyramid of biomass is always pyramid-shaped?