Welcome to Topic 10.3: Changes in Ecosystems!
In this chapter, we are going to explore the dynamic nature of our world. If you look at a forest today, it might seem like it has always been there, but it likely started as a patch of bare ground or rock. Ecosystems are constantly evolving and changing through a process called succession. We will also look at the different factors—some living, some non-living—that decide how many organisms can live in a certain area. Don't worry if this seems like a lot to take in; we’ll break it down step-by-step!
1. Succession: How Ecosystems Grow Up
Think of succession as the "life story" of an ecosystem. It is the process by which the types of species living in an area change over time. There are two main types you need to know: Primary Succession and Secondary Succession.
Primary Succession: Starting from Scratch
This happens in an area that has never been colonised before. Imagine a brand-new volcanic island or a retreating glacier that leaves behind bare rock. There is no soil, just rock.
Step-by-Step Process:
- Colonisation: The first organisms to arrive are called pioneer species (like lichens or algae). They are the "tough guys" of the plant world because they can survive on bare rock with very little water.
- Weathering: As pioneer species grow, they break up the rock. When they die, they are decomposed by microorganisms, adding organic matter (humus) to the rock fragments.
- Soil Formation: This creates a basic soil. Now that there is soil, small plants like mosses and ferns can grow.
- Increasing Complexity: As these larger plants die, the soil becomes deeper and richer in nutrients. This allows bigger plants like grasses, then shrubs, and finally trees to move in.
- The Climax Community: Eventually, the ecosystem reaches a stable state called the climax community. This is often a woodland or forest that won't change much unless a major event happens.
Secondary Succession: The Comeback
This happens when an existing ecosystem is cleared (e.g., by a forest fire or a farmer clearing a field), but the soil remains. Because the soil is already there, the process is much faster than primary succession.
Analogy: The New House
Imagine Primary Succession is like building a house from the ground up—you have to lay the foundation and the bricks first. Secondary Succession is like moving into a house that’s already built but needs redecorating—the "foundation" (the soil) is already there!
Quick Review: Key Terms
- Seral Stage: Each individual stage of change during succession.
- Pioneer Species: The first species to colonise a bare area.
- Climax Community: The final, stable, and self-sustaining stage of succession.
Did you know? Different climates lead to different climax communities. In a very cold, dry area, the climax community might be a tundra of mosses and lichens rather than a giant oak forest!
Key Takeaway: Succession is the directional change in an ecosystem over time, leading from a bare environment to a complex, stable climax community.
2. Factors Affecting Population Size
The number of individuals of a species (the population size) isn't random. It is controlled by two main types of factors: Abiotic and Biotic.
Abiotic Factors (The Non-Living Stuff)
These are the physical and chemical parts of the environment. If these aren't right, a population cannot grow. Examples include:
- Light intensity: Plants need light for photosynthesis. No light = no food = small population.
- Temperature: Every organism has an optimum temperature. If it's too hot or cold, enzymes work slower, and energy is spent just trying to stay alive.
- Water availability: All life needs water!
- Soil pH: This affects which plants can grow and the availability of minerals.
Biotic Factors (The Living Stuff)
These are interactions between living organisms. As a population grows, these factors become more important.
- Competition: Organisms fight for the same resources (food, space, mates).
- Intraspecific competition: Between members of the same species.
- Interspecific competition: Between members of different species.
- Predation: If the number of predators increases, the number of prey usually decreases.
- Disease: In crowded populations, pathogens (like bacteria or viruses) spread faster, which can cause the population size to crash.
Common Mistake to Avoid: Students often think that a "stable" population size means the number never changes. Actually, it usually fluctuates (wiggles up and down) around a level called the carrying capacity—the maximum population size that the environment can sustain.
Key Takeaway: Population size is a balance. Abiotic factors often determine if a species can live there at all, while biotic factors often determine the final "size" of the population.
3. Core Practical 16: Investigating Abiotic Factors
In this practical, you investigate how one abiotic factor (like light intensity or soil moisture) affects the distribution (where they are) or morphology (what they look like) of a species.
Example Method:
- Place a transect (a long measuring tape) across an area where an abiotic factor changes (e.g., from the shade of a tree out into a sunny field).
- Place quadrats at regular intervals along the tape.
- In each quadrat, count the number of individuals of your chosen species (e.g., daisies) or measure their leaf size (morphology).
- Use an appropriate tool to measure the abiotic factor in each quadrat (e.g., a light meter).
- Analysis: Use a statistical test like Spearman’s Rank Correlation Coefficient to see if there is a significant link between the abiotic factor and the species.
Memory Aid for Abiotic vs. Biotic:
Remember that "A" often means "without."
Bio = Life.
Biotic = Living.
A-biotic = Without life (Non-living).
Key Takeaway: We use transects and quadrats to scientifically prove that changes in the environment (abiotic factors) cause changes in the living organisms (biotic community).
Chapter Summary
- Succession is the process of an ecosystem developing from bare ground to a climax community.
- Colonisation by pioneer species is the first step in primary succession.
- Abiotic factors (non-living) and biotic factors (living) interact to determine the population size of species.
- The carrying capacity is the maximum population size an ecosystem can support.
- We can measure these effects using fieldwork techniques like transects and quadrats.