Welcome to Ecosystems Over Time!
Hi there! Welcome to one of the most fascinating parts of your Geography course. Have you ever wondered why a patch of bare ground eventually turns into a forest if left alone? Or why some grasslands never seem to grow trees? In this chapter, we are going to explore how ecosystems aren't just "stuck" in one place—they are constantly moving, growing, and changing through a process called succession.
Don't worry if this seems a bit scientific at first. We’ll break it down into simple steps and use plenty of real-world examples to help you master it!
1. What is Vegetation Succession?
Think of succession like a movie cast that changes over time. At the start of the movie, you have a few characters, but as the story progresses, new ones arrive, and the old ones leave until you reach the final scene. In Geography, succession is the process by which the types of plants and animals in an area change over time until a stable community is formed.
Key Terms to Know:
• Sere: This is the entire sequence of stages an ecosystem goes through.
• Seral Stage: One individual "step" or stage in that sequence.
• Pioneer Species: The very first plants to arrive in a bare area. They have to be tough!
Quick Review:
Vegetation doesn't just appear all at once. It develops in stages (seres), starting with pioneer species and ending with a stable climax community.
Did you know? Lichens are often the first pioneers on bare rock. They aren't just one plant; they are a partnership between a fungus and an algae working together!
2. The Two Main Types of Succession
There are two ways nature starts its "story." It depends on whether the land is starting from total scratch or has had a "reset."
A. Primary Succession (Prisere)
This happens on brand-new land where there is no soil. Imagine a cooling lava flow or a newly formed sand dune. Because there is no soil, it is very hard for plants to grow.
The Step-by-Step Process:
1. Bare Rock: No life exists.
2. Pioneer Stage: Lichens and mosses arrive. They break down the rock into tiny bits and, when they die, they add organic matter (humus).
3. Soil Formation: Now there is a thin layer of soil. Small grasses and ferns can grow.
4. Progression: As the soil gets thicker and richer, larger plants like shrubs and small trees (like Birch) move in.
5. Climax: Eventually, large, slow-growing trees like Oak or Beech take over.
B. Secondary Succession (Subsere)
This happens when an existing ecosystem is destroyed (e.g., by a forest fire or a farmer abandoning a field), but the soil remains.
Why is it different? Because the soil is already there, it's like starting a race at the halfway point. It happens much faster than primary succession.
Memory Tip: Remember Primary = Pioneers on bare rock. Secondary = Soil is already there.
Key Takeaway:
Primary succession starts from zero (no soil), while secondary succession starts from a "disturbed" state where soil already exists.
3. The End Goal: The Climatic Climax
If you leave an ecosystem alone for hundreds of years, it eventually reaches a point of balance where it doesn't change much anymore. This is called the Climatic Climax Community.
This final stage is determined by the climate of the area. For example:
• In the UK, the climatic climax is usually Deciduous Oak Woodland.
• In very cold areas, it might be Tundra (mosses and small shrubs).
• In hot, wet areas, it is the Tropical Rainforest.
Common Mistake to Avoid: Many students think the "climax" is always a big forest. That's not true! If the climate is too dry or too cold for trees, the "climax" might just be grass or small bushes.
4. When Humans Intervene: Plagioclimax
Sometimes, a climax community is never reached because humans "stop the clock." This creates a plagioclimax.
Imagine you have a lawn. If you never mowed it, it would eventually become a forest. By mowing it every week, you are arresting (stopping) succession. You are keeping it in a permanent "grass stage."
How do humans create a plagioclimax?
• Grazing: Sheep and cattle eat the young tree saplings, preventing a forest from growing.
• Burning: People burn heather moorlands to encourage new growth for birds like Grouse.
• Deforestation: Cutting down trees to keep land open for farming.
Analogy: A plagioclimax is like hitting the "Pause" button on a YouTube video. The video (succession) wants to play until the end, but human activity keeps it stuck on one frame.
Key Takeaway:
A plagioclimax is an ecosystem that is kept in a specific state by human activity, preventing it from reaching its natural climatic climax.
5. Measuring Diversity Over Time
As succession moves forward, the ecosystem usually becomes more complex.
What changes?
1. Soil Depth: It gets deeper and contains more nutrients.
2. Biodiversity: At first, there are only a few species. In the middle stages, diversity is usually highest. In the final climax stage, it might actually drop slightly as a few dominant species (like big Oak trees) shade out everything else!
3. Biomass: The total weight of living matter increases as plants get bigger.
In your exams, you might see Simpson's Index of Diversity mentioned to measure this. The formula looks like this:
\( D = 1 - \left( \frac{\sum n(n-1)}{N(N-1)} \right) \)
Where:
• \( n \) = the total number of organisms of a particular species.
• \( N \) = the total number of organisms of all species.
Don't panic! You usually just need to know that a higher number (closer to 1) means the ecosystem is more diverse and healthy.
Summary Checklist
• Can you define succession and sere?
• Do you know the difference between primary and secondary succession?
• Can you explain why pioneer species are so important?
• Do you understand that the climate determines the final "climax" stage?
• Can you give an example of how humans create a plagioclimax?
Great job! You've just covered the core concepts of how ecosystems change over time. Keep reviewing these terms, and you'll be an expert in no time!