Ecosystems Over Time: How Nature Changes Itself (Syllabus 3.3.2.4)
Hello Geographers! This chapter is all about dynamic change. Instead of seeing ecosystems as fixed snapshots, we look at them like a movie: constantly evolving, growing, and reacting. This process of change, called succession, is fundamental to understanding how environments develop and, crucially, how human actions can interrupt or redirect these natural paths.
Don't worry if terms like 'Psammosere' sound complicated—they are just fancy names for predictable changes! We will break down the steps, using clear examples.
1. Understanding Ecological Succession
Ecosystems are never truly static. Ecological Succession is the process by which an ecosystem changes over time, usually progressing from simple, pioneer communities to complex, stable ones.
The Process of Change
Think of succession like renovating an old, empty building.
• Start Simple: The first organisms (pioneer species) are like the initial clean-up crew—they can survive in very harsh conditions.
• Making Improvements: As these pioneers die, they add organic matter to the soil, changing the environment and making it suitable for larger, more demanding species. This is called Facilitation.
• The Final Result: Eventually, the ecosystem reaches a mature, stable state called the Climax Community.
Key Concepts in Succession
• Seral Stages: These are the recognizable, distinct communities that replace one another during succession. Each stage is known as a Sere. (E.g., Grassland is a Seral Stage leading towards a forest).
• Pioneer Community: The initial community of organisms that colonize a new or barren habitat. These species are highly adapted to extreme conditions (lack of nutrients, high exposure).
• Climatic Climax: The final, stable ecosystem achieved when the environment is in equilibrium with the prevailing regional climate. It is the community dictated purely by climate controls (temperature and rainfall).
We often categorize succession based on the starting environment:
1. Lithosere: Starts on bare rock surfaces (Think LITH = stone).
2. Halosere: Starts in salty or marine environments (Think HALO = salt circle).
3. Psammosere: Starts on bare sand (Think PSAMMO = sandy).
4. Hydrosere: Starts in freshwater (Think HYDRO = water).
2. Succession Illustrated: The Psammosere (Sand Dune Development)
We will use the Psammosere, or sand dune succession, as our example to understand the step-by-step process from bare sand to a stable climax community.
Step 1: Embryo and Fore Dunes (The Pioneer Stage)
The environment is hostile: high wind, low nutrients, fast drainage, and salinity.
• Pioneer Species: Plants like sea rocket or sand couch start to colonize the mobile sand above the high tide line.
• The Role of Marram Grass: As the dunes grow, Marram grass (Ammophila arenaria) is key. It has deep roots to stabilize the sand and can grow rapidly vertically, trapping more sand as it builds the dune up. This is an excellent example of feedback: the plant helps build the dune, and the dune protects the plant.
Step 2: Yellow Dunes
The dune begins to stabilize.
• Characteristics: The colour is still 'yellow' because vegetation covers less than 50% of the surface. Sand continues to be deposited.
• Soil Changes: Organic matter slowly increases, and rainwater starts leaching out shell fragments, reducing the pH (making the soil slightly more acidic).
Step 3: Grey Dunes (or Fixed Dunes)
This stage marks significant stability and soil development (pedogenesis).
• Characteristics: Vegetation cover is high (often 100%), dominated by plants that cannot tolerate mobile sand, such as heather, gorse, and creeping willow.
• Soil Changes: A proper soil layer (humus) has developed, reducing drainage and increasing nutrient retention.
Step 4: The Climax Community
If left undisturbed, the grey dune will slowly transition to the final community supported by the regional climate.
• In Temperate Western Europe: The Climatic Climax is often deciduous broadleaf woodland (like oak or beech forest).
• Characteristics of the Climatic Climax: This community has maximum biomass, complex food webs (many trophic levels), high biodiversity, and nutrient cycling that is stable and efficient. The energy input (insolation) is balanced by the output (respiration, decomposition).
In many coastal dune systems, you might find 'Dune Slacks'—low-lying areas between dunes where the water table is exposed. This creates a small Hydrosere within the larger Psammosere, demonstrating how succession types can interact!
3. The Effects of Human Activity on Succession
This is where the concept of "Ecosystems Under Stress" becomes very real. Human actions often halt or divert the natural progression towards a climatic climax.
Sub-Climax and Plagioclimax
When human intervention prevents the full development of a climatic climax community, the result is a less mature, maintained community.
4.1. Sub-Climax
A Sub-climax community is an intermediate seral stage that is maintained *just* before the true climax is reached, often due to natural or low-level human factors.
• Example: A fire-resistant pine forest (Pinus species) might persist as a sub-climax even though the climatic climax should be a mixed deciduous forest, simply because the soil structure or specific local drainage conditions favour the pine.
4.2. Plagioclimax (The Human Intervention Stage)
A Plagioclimax community is the most critical concept here. It is a stable community maintained solely by regular human interference (anthropogenic factors). If the interference stops, the ecosystem will resume its journey towards the true climatic climax.
Common Causes of Plagioclimax:
• Grazing: Sheep, cattle, or other livestock prevent saplings and trees from growing past the seedling stage, maintaining a stable grassland or heathland instead of woodland. (e.g., maintaining the open space of the English moorlands).
• Mowing/Cutting: Regular maintenance of parks, gardens, or playing fields prevents succession from moving past the initial grass stage.
• Burning (Controlled or Uncontrolled): Intentional burning (like rotational burning in heather moorlands) clears larger vegetation, releasing nutrients and ensuring the persistence of fire-resistant species.
• Farming (Agricultural Extension): The constant cultivation of a field maintains a monoculture (one crop type) which is a very early, highly unstable stage of succession.
Analogy: Imagine a lawn you mow every week. If you stop mowing, it eventually grows into long grass, then shrubs, and finally maybe a small forest. Your weekly mowing prevents succession from moving beyond the grass stage—that lawn is a plagioclimax.
4. Key Takeaways and Exam Focus
Focus on the Link Between Climate and Ecosystems
Remember that the Climatic Climax is determined by long-term average temperature and precipitation. Understanding this link allows you to predict the climax community for any biome:
• Hot, Wet Climate: Tropical Rainforest Climax.
• Warm, Temperate Climate: Deciduous Woodland Climax.
• Cold, Dry Climate: Tundra/Coniferous Forest Climax.
Common Mistake to Avoid
Do not confuse Climatic Climax (the theoretical, highest level of stability possible) with Plagioclimax (the stable level maintained by human intervention). If a question asks about the effect of grazing, the answer is usually Plagioclimax, not Climatic Climax.
• Succession is the change from pioneer to climax communities.
• Seral Stages (e.g., Embryo, Yellow, Grey Dunes) show this progression.
• The natural end point, governed by regional climate, is the Climatic Climax.
• Human interference (like grazing or burning) creates a stable but unnatural community known as a Plagioclimax, illustrating how ecosystems are put under stress.