Welcome to Ecological Relationships!
Have you ever wondered how all living things in a forest, field, or pond survive together? In this chapter, we will explore Ecological Relationships. You will discover how living organisms depend on one another for food, how energy flows through nature, how living things compete for resources, and how scientists measure populations in the wild.
Don't worry if some of the terms seem new at first! We will break everything down into bite-sized chunks with simple analogies and memory tricks.
---1. Key Ecological Terms
Before diving into who eats whom, let's learn the basic vocabulary ecologists use. Think of these as building blocks, starting from a single organism all the way up to an entire environment.
• Habitat: The specific place where an organism lives (for example, a woodland, a rocky seashore, or a garden pond).
• Population: All the individuals of one single species living in a habitat at the same time (for example, all the red squirrels in a forest).
• Community: All the different populations of living organisms (plants, animals, fungi) living together in a habitat.
• Ecosystem: A community of living organisms interacting with each other and with the non-living (physical) environment.
Feeding Roles in an Ecosystem
Every living organism plays a role in getting and transferring energy:
• Producer: A green plant or algae that makes its own food by photosynthesis using sunlight energy. Producers are always at the very start of a food chain.
• Primary Consumer: An animal that eats producers. Because they only eat plants, they are also called herbivores (e.g., rabbits, caterpillars).
• Secondary Consumer: An animal that eats primary consumers. If they eat other animals, they are carnivores (e.g., foxes, blue tits).
• Tertiary Consumer: An animal that eats secondary consumers (usually an apex predator at the top of the food chain).
• Omnivore: An animal that eats both plants and animals (e.g., humans, badgers).
• Decomposer: Microorganisms (like bacteria and fungi) that break down dead plant and animal material, recycling nutrients back into the soil.
Quick Key Takeaway: Producers make food from sunlight; consumers eat other organisms to get energy. A community is all the living things, while an ecosystem includes living things plus their non-living environment.
---2. Food Chains and Food Webs
Understanding Food Chains
A food chain shows the feeding relationships between organisms and the pathway along which energy moves.
Here is a simple example:
Grass \(\rightarrow\) Caterpillar \(\rightarrow\) Blue Tit \(\rightarrow\) Hawk
Common Mistake to Avoid: The direction of the arrow! In an exam, students often point the arrow from the predator to the prey. Always remember: The arrow points in the direction of energy flow (from what is being eaten to what is doing the eating). Think of the arrow as saying: "is eaten by and gives energy to".
Understanding Food Webs
In nature, animals rarely eat just one single type of food. A food web consists of many interconnected food chains linked together.
Predicting Changes in a Food Web
Exam questions often ask what happens if one organism in a food web increases or decreases. Let's look at a simple scenario:
Grass \(\rightarrow\) Slug \(\rightarrow\) Thrush (bird) \(\rightarrow\) Sparrowhawk
What happens if a disease reduces the Thrush population?
1. The Slug population will increase: There are fewer thrushes eating them.
2. The Grass population will decrease: More slugs are eating the grass.
3. The Sparrowhawk population will decrease: Sparrowhawks have less food to eat.
Tip for answering these questions: Always explain your answer in two steps: state what happens to the population (increases or decreases) and explain why (less food, more predation, etc.).
Quick Key Takeaway: Food chains show one feeding path; food webs show many linked chains. Always make sure your arrows show the transfer of energy.
---3. Energy Flow and Ecological Pyramids
Why is Energy Lost Along a Food Chain?
Did you know that only about \(10\%\) of the energy at each level is passed on to the next level? That means roughly \(90\%\) of the energy is lost!
Energy is lost at each trophic (feeding) level because:
• Energy is used in respiration to keep the organism alive and released as heat.
• Energy is used for movement.
• Not all parts of the organism are eaten (e.g., bones, fur, roots).
• Some material is passed out as waste and excretion (urine and faeces/indigestible material).
Because so much energy is lost at each stage, food chains rarely have more than \(4\) or \(5\) levels. There simply isn't enough energy left to support a higher level!
Pyramids of Numbers
A pyramid of numbers represents the actual count of individual organisms at each trophic level of a food chain.
• Usually, a pyramid of numbers is wide at the bottom and narrow at the top (e.g., \(1000\) blades of grass \(\rightarrow\) \(50\) rabbits \(\rightarrow\) \(1\) fox).
• Irregular Shapes: If the producer is very large (like a single oak tree), the base will be narrow because only \(1\) tree supports thousands of caterpillars. Similarly, if an animal has parasites (like fleas on a fox), the top bar will be wider than the bar below it.
Pyramids of Biomass
Biomass is the total mass of living material at each level (dry mass).
• A pyramid of biomass is always a true pyramid shape (widest at the bottom and narrowing at each level above).
• This is because biomass represents the stored energy, and energy is constantly lost at each trophic level.
Quick Review:
• Pyramid of numbers = count of heads (can be inverted or irregular).
• Pyramid of biomass = total mass of living tissue (always pyramid-shaped).
4. Competition in Ecosystems
Organisms in an ecosystem must fight for limited resources to survive and reproduce. This struggle is called competition.
What do Plants Compete for?
Plants in the same area compete with each other for:
• Light (needed for photosynthesis)
• Space (to grow roots and leaves without being overcrowded)
• Water (absorbed from the soil for photosynthesis and support)
• Minerals (nutrients from the soil, such as nitrates for making proteins)
Memory Trick: Remember the acronym LSWM (Light, Space, Water, Minerals).
What do Animals Compete for?
Animals compete with each other for:
• Food: To obtain energy and nutrients.
• Water: Essential for chemical reactions in cells.
• Territory / Shelter: Space to nest, find food, and avoid predators.
• Mates: To reproduce and pass on their genes.
Biotic and Abiotic Factors
The distribution of organisms in an environment is influenced by two types of factors:
• Abiotic factors (non-living): Temperature, light intensity, moisture levels, soil pH, and wind speed.
• Biotic factors (living): Predators, diseases, food availability, and grazing.
Quick Key Takeaway: Competition happens when resources are scarce. Plants compete for light, space, water, and minerals; animals compete for food, water, territory, and mates.
---5. Practical Ecology: Sampling Techniques
Ecologists cannot count every single plant or animal in a large area. Instead, they take samples to estimate population sizes and investigate how organisms are distributed.
1. Using Quadrats (for Plants and Slow-moving Animals)
A quadrat is a square frame (usually \(0.5\text{ m} \times 0.5\text{ m} = 0.25\text{ m}^2\) or \(1\text{ m} \times 1\text{ m} = 1\text{ m}^2\)).
How to do Random Sampling:
1. Lay out two tape measures at right angles to create a grid over the sample area.
2. Use a random number generator (or roll dice) to pick pairs of coordinates.
3. Place the quadrat at the chosen coordinates.
4. Count the number of the target organism inside the quadrat.
5. Repeat the process many times (e.g., \(10\) to \(20\) times) and calculate an average (mean) count per quadrat.
6. Scale up: Multiply the average count per quadrat by the total number of quadrats that would fit into the whole area.
Why random sampling? Random sampling eliminates human bias. If you choose where to put the quadrat, you might pick areas with lots of pretty flowers, giving an inaccurate estimate.
Sample Calculation:
Suppose a field has an area of \(200\text{ m}^2\). A \(1\text{ m}^2\) quadrat is placed randomly \(10\) times. The total number of dandelions counted across the \(10\) quadrats is \(40\).
• \(\text{Mean number of dandelions per quadrat} = \frac{40}{10} = 4\text{ dandelions/m}^2\)
• \(\text{Estimated total population} = 4 \times 200 = 800\text{ dandelions}\)
2. Belt and Line Transects (Investigating Gradients)
When you want to see how the distribution of plants changes across an area (for example, moving from under the shade of a tree out into an open field):
• Lay a measuring tape (a transect line) along the environmental gradient.
• Place quadrats at regular intervals (e.g., every \(2\text{ metres}\)) along the tape.
• Measure both the number of organisms and the abiotic factor (such as light intensity using a light meter) at each interval.
3. Pitfall Traps (for Ground-Crawling Invertebrates)
A pitfall trap is a small container sunk into the ground to catch crawling insects (like beetles, spiders, and woodlice).
Key Precautions for Setting Up a Pitfall Trap:
• Top level with soil surface: So insects fall in easily without having to climb up a lip.
• Raised cover/lid supported by stones: Protects trapped insects from rain (so they don't drown) and stops predators (like birds) from eating them.
• Drainage holes or checking regularly: Prevents the trap from flooding and ensures captured animals are released safely.
Quick Key Takeaway: Quadrats placed randomly estimate total population size. Transects show changes along an environmental gradient. Pitfall traps collect ground-crawling small animals.
---Chapter Summary & Revision Checklist
Before you move on, check that you can:
• Define habitat, population, community, and ecosystem.
• Draw and interpret food chains and food webs with arrows showing the direction of energy flow.
• Explain why energy is lost between trophic levels (respiration, heat, movement, excretion).
• Distinguish between pyramids of numbers (can be irregular) and pyramids of biomass (always pyramid-shaped).
• List what plants (light, space, water, minerals) and animals (food, water, territory, mates) compete for.
• Describe how to use a quadrat randomly to estimate a population, and how to use transects and pitfall traps properly.