Welcome to Populations in Ecosystems!

Hello! You’ve reached one of the most fascinating parts of your AQA A Level Biology course. This chapter is all about how living things interact with each other and their "neighborhood." Whether you're aiming for an A* or just trying to get your head around the basics, these notes are designed to make the complex world of ecology simple and easy to remember.

Don't worry if this seems like a lot of terms at first! We will break them down one by one using real-world examples.


1. The Basics: What is an Ecosystem?

Before we look at how populations grow, we need to know where they live. An ecosystem is a dynamic system made up of a community (all the living things) and the abiotic (non-living) factors of their environment.

Key Definitions to Learn:

  • Population: A group of individuals of the same species occupying a particular space at a particular time that can potentially interbreed.
  • Community: All the populations of different species living and interacting in a particular place at the same time.
  • Habitat: The place where an organism normally lives.
  • Niche: This is a common exam favorite! A niche is a species' "role" within its habitat. It includes where it lives, what it eats, and how it behaves.
The "Job and Address" Analogy:

Think of a habitat as an organism’s address (where it stays) and its niche as its job (what it does there). No two species can have exactly the same niche in the same habitat because they would compete until one was forced out!

Key Takeaway: Ecosystems are made of living (biotic) and non-living (abiotic) parts. Every species has a unique role called a niche.


2. Factors Affecting Population Size

No population can grow forever. Eventually, it hits a limit called the carrying capacity. This is the maximum population size that an ecosystem can support.

Abiotic Factors (The Non-Living Stuff)

Population size varies because of things like:

  • Temperature: Each species has an optimum temperature. If it's too cold, enzymes work slowly; too hot, and they denature.
  • Light intensity: More light means faster photosynthesis in plants, which means more food for the rest of the web.
  • pH and Water: Specific levels are needed for chemical reactions in cells to work properly.

Biotic Factors (The Living Stuff)

Living things interact in three main ways that limit population size:

  1. Interspecific Competition: Competition between different species (e.g., lions and hyenas fighting for the same zebra).
  2. Intraspecific Competition: Competition between members of the same species (e.g., two stags fighting for a mate).
  3. Predation: This creates a "cycling" effect. As prey numbers go up, predators have more food and their numbers go up. Then, the predators eat too many prey, prey numbers fall, and the predators start to starve.
Memory Aid: "Inter" vs "Intra"

Inter-specific = Inter-national (between different countries/species).
Intra-specific = Intra-mural (within the same school/species).

Quick Review: Population size is limited by non-living factors (like heat) and living factors (like competition and predators).


3. How Do We Measure Populations?

Ecologists can't count every single blade of grass or every single rabbit. Instead, we use sampling. For the exam, you need to know three main methods.

A. Quadrats (For non-motile or slow-moving organisms)

A quadrat is a square frame. To make it fair, you must place them randomly to avoid bias. You can measure:

  • Percentage cover: How much of the square is covered by a specific plant.
  • Frequency: How many of the little squares within the quadrat contain the species.

B. Belt Transects (To see how things change)

If you want to see how species change as you move from a forest into a field, you use a transect. You lay a tape measure down and place quadrats at regular intervals along it.

C. Mark-Release-Recapture (For motile/moving organisms)

To estimate the size of a population of animals (like mice), we use this formula:

\( Estimated Population Size = \frac{n_1 \times n_2}{m} \)

Where:
\( n_1 \) = Number caught and marked in the first sample.
\( n_2 \) = Total number caught in the second sample.
\( m \) = Number of marked animals in the second sample.

Crucial Exam Tip: The Assumptions

AQA loves to ask about the "assumptions" of this method. For the math to work, you must assume:

  • The mark didn't hurt the animal or make it easier for predators to see.
  • The marked animals had enough time to mix back into the population.
  • There were no births, deaths, or migrations during the study.

Key Takeaway: Use quadrats for plants and Mark-Release-Recapture for animals. Always ensure sampling is random to avoid bias!


4. Succession: How Ecosystems Change Over Time

Ecosystems are dynamic—they are constantly changing. Succession is the process by which an ecosystem changes over time.

Step-by-Step: Primary Succession

  1. Colonisation: The first species to arrive on bare rock (like lichens) are called pioneer species.
  2. Environment Change: Pioneer species die and decompose, adding organic matter (humus) to the soil.
  3. Less Hostile Environment: The soil can now hold more water and nutrients. This makes the environment less hostile.
  4. New Species: New, larger plants can now grow. These may out-compete the pioneer species.
  5. Climax Community: Eventually, a stable state is reached where the species don't change much anymore. This is the climax community (usually a forest in the UK).
Did you know?

As succession progresses, biodiversity usually increases because more niches become available for different animals and plants!

Conservation and Succession

Sometimes, humans want to stop succession to protect a specific habitat. For example, grazing sheep on a moorland prevents trees from growing. This is called managing succession to maintain a certain stage of biodiversity.

Quick Review: Succession starts with a pioneer species, makes the land less hostile, and ends in a stable climax community.


Common Mistakes to Avoid:

1. Confusion between Interspecific and Intraspecific: Use the mnemonic provided above! "Inter" is between different types.
2. Forgetting "Abiotic": When asked why a population changed, don't just look at predators; check if the temperature or rainfall changed too.
3. Bias in Sampling: Always mention random number generators for placing quadrats to ensure your data is representative.


Final Summary:

1. Populations are limited by the carrying capacity of the environment.
2. Competition and predation are key biotic factors that control numbers.
3. Sampling (Quadrats and Mark-Release-Recapture) allows us to estimate population size.
4. Succession is the natural change from bare land to a stable climax community.

Keep practicing those past paper questions on Mark-Release-Recapture calculations—you've got this!