Introduction to Ecosystems, Productivity, and Succession

Welcome to one of the most vital chapters in your A2 Biology course! In this section, we move beyond looking at individual cells and start looking at the "big picture." We will explore how energy from the sun is captured by plants, how it flows through different organisms, and how entire landscapes change over time through a process called succession. Understanding these concepts is essential for grasping how our planet maintains balance and how we can protect biodiversity.

1. The Language of Ecology

Before we dive into the complex processes, we need to make sure we are using the right terminology. Ecology has a very specific "vocabulary" that helps biologists describe the natural world accurately.

  • Habitat: The specific place where an individual organism lives (e.g., a pond, a rocky shore, or a specific type of tree).
  • Population: A group of organisms of the same species living in the same area at the same time.
  • Community: All the populations of different species living and interacting in the same habitat.
  • Ecosystem: A dynamic system that includes the community (biotic factors) and the non-living environment (abiotic factors) interacting as a unit.
  • Niche: The specific role an organism plays within its ecosystem. This includes what it eats, where it lives, and how it behaves.

Top Tip: Think of a habitat as an organism's "address" and its niche as its "job." Two species can live in the same habitat, but they cannot occupy the exact same niche for long because they will compete until one outcompetes the other!

Biotic vs. Abiotic Factors

An ecosystem is shaped by two main types of factors:

  1. Biotic Factors: These are the living parts of the environment that affect organisms, such as predation, competition (for food or mates), and disease.
  2. Abiotic Factors: These are the non-living, physical, and chemical parts of the environment, such as temperature, light intensity, soil pH, and water availability.

2. Energy and Productivity

All life requires energy. In most ecosystems, this energy enters as sunlight and is "fixed" into organic molecules by producers (plants and algae) through photosynthesis. (For a deep dive into how plants capture this light, see the chapter on Photosynthesis and Chloroplasts).

Primary Productivity

Biologists use specific terms to measure how much energy plants are actually capturing and storing:

  • Gross Primary Productivity (GPP): This is the total amount of chemical energy (organic matter) produced by photosynthesis in a given area over a given time. Think of this as a plant's "gross salary" before any taxes are taken out.
  • Plant Respiration (R): Plants are living things; they need to use some of the energy they make to stay alive (for active transport, growth, and repair). This energy is lost as heat.
  • Net Primary Productivity (NPP): This is the energy that remains after the plant has met its own respiratory needs. This is the energy available to the next trophic level (the consumers).

The relationship between these three is vital for your exams. You must remember this formula:

\(NPP = GPP - R\)

Quick Review: If a forest has a \(GPP\) of \(20,000\text{ kJ m}^{-2}\text{ yr}^{-1}\) and the plants use \(12,000\text{ kJ m}^{-2}\text{ yr}^{-1}\) for respiration, the \(NPP\) is \(8,000\text{ kJ m}^{-2}\text{ yr}^{-1}\). This \(8,000\) is the only energy available to be passed on to herbivores.

Efficiency of Energy Transfer

Energy transfer between trophic levels (e.g., from plant to herbivore) is surprisingly inefficient. Usually, only about \(10\%\) of the energy is passed on. Why? Because energy is lost through:

  • Respiration: Lost as heat during movement or metabolic processes.
  • Egestion: Undigested food (like cellulose) passed out as faeces.
  • Excretion: Metabolic waste like urea in urine.
  • Uneaten parts: Bones, roots, or woody stems that are not consumed.

3. Succession: How Ecosystems Change

Ecosystems are not static; they change over time. Succession is the process by which the species composition of a community changes over time.

The Stages of Succession

  1. Colonisation: It starts with a harsh, empty environment (like bare rock or sand).
  2. Pioneer Species: Only very hardy species can survive here. These are called pioneer species (e.g., lichens or mosses). They are adapted to survive extreme temperatures and lack of soil.
  3. Soil Formation: As pioneer species die and decompose, they add organic matter (humus) to the ground. This, along with the weathering of rock, begins to form soil.
  4. Changing the Environment: The presence of soil makes the environment less harsh. Now, larger plants like grasses and small shrubs can grow. These new species often outcompete the pioneer species.
  5. Increasing Complexity: As the soil becomes deeper and richer in nutrients, larger trees can grow. Biodiversity increases as more habitats and food sources become available for animals.
  6. Climax Community: Eventually, the ecosystem reaches a stable state called the climax community. This is often a woodland or forest. It remains stable unless the environment changes significantly.

Don't worry if this seems tricky: Just remember that at each stage, the organisms change the abiotic factors (like soil depth and nutrient levels), making the environment less harsh and more suitable for new, more complex species.

4. Core Practical 11: Investigating Ecosystems

In Unit 4, you are required to know how to study a habitat in the field. This usually involves using quadrats and transects.

Key Techniques:

  • Random Sampling: Use a grid and a random number generator to place quadrats. This avoids bias. Use this when you want to compare two distinct areas.
  • Systematic Sampling (Transects): Place quadrats at regular intervals along a line (a tape measure). This is used to study how species distribution changes across an environmental gradient (e.g., moving away from the shore or up a hill).
  • Measuring Abiotic Factors: You should measure things like light intensity (using a light meter), soil pH, or temperature alongside your plant data to see how they correlate.

Summary of Quantitative Methods:

  • Percentage Cover: Estimating how much of the quadrat area is covered by a specific species.
  • Species Density: Counting the actual number of individuals of a species per unit area.
  • Species Richness: The number of different species present in a community.

5. Key Takeaways Table

Concept Key Point to Remember
NPP Formula \(NPP = GPP - R\)
Pioneer Species The first organisms to colonise a harsh environment; they start soil formation.
Climax Community The final, stable stage of succession (e.g., an oak forest).
Energy Loss Most energy is lost as heat from respiration or via undigested material.
Abiotic Factors Non-living factors like pH and light that determine which species can survive.

Note on Speciation: While ecosystems focus on the environment, remember that populations can evolve into new species through allopatric (geographical) or sympatric (behavioural/genetic) isolation. This is covered more deeply in the context of evolution and climate change in other Unit 4 chapters.

Congratulations! You've covered the essentials of Ecosystems, Productivity, and Succession. Keep practicing those NPP calculations and remember the steps of succession—they are frequent favorites in the exam!