Ecological Succession: Dynamics of Ecosystem Change

TL;DR

This topic covers how ecosystems change over time, focusing on energy flow and organism interactions. You'll learn about food chains, how energy moves and decreases at different trophic levels, and the orderly progression of species in ecological succession. We'll also look at methods for measuring populations and different environmental perspectives.

1. The Mental Model

Imagine an ecosystem like a play with changing actors and energy flowing through it. The "actors" (organisms) interact, eat, and are eaten, constantly moving energy. These interactions drive long-term changes, much like a story unfolds over time.

2. The Core Material

Trophic Levels and the Food Chain

You're looking at trophic levels, which describe how food chains work. A trophic level is a position an organism occupies in a food chain. Producers (like plants) form the base, converting sunlight into energy. Primary consumers eat producers, secondary consumers eat primary consumers, and so on. Energy flows from one level to the next.

Entropy and Energy in Food Chains

As energy moves up the food chain, something important happens: energy decreases. This relates to entropy, which is a measure of randomness or disorder. The laws of thermodynamics explain this. The second law states that in any energy transfer, some useful energy is lost, usually as heat, increasing the system's entropy. This means randomness increases through each level, and energy decreases as you go up the food chain.

Gross (Secondary) Productivity

Gross (secondary) productivity refers to the total energy assimilated by heterotrophic organisms (consumers) from their food source. It's the total energy consumed and absorbed, before any is used for respiration or lost as waste.

Symbiosis

Symbiosis describes close, long-term interactions between different biological species. These relationships can be mutually beneficial, harmful to one and beneficial to another, or beneficial to one without affecting the other.

Calculating Trophic Efficiency (Energy Efficiency)

Trophic efficiency (or energy efficiency) tells you how much energy from one trophic level is transferred to the next. It's usually a small percentage, often around 10%. To calculate it, you'd divide the energy at a higher trophic level by the energy at the level below it, then multiply by 100 to get a percentage.

Pyramid of Biomass

A pyramid of biomass is a graphical representation showing the total mass of organisms at each trophic level in an ecosystem. Typically, the biomass decreases significantly at higher trophic levels, reflecting the energy loss.

Ecological Succession and Zonation

Succession is the process of change in the species structure of an ecological community over time. You need to understand how to explain its stages.

graph TD
    A["Bare Ground or Disturbed Area"] --> B["Pioneer Species (e.g., lichens, mosses)"]
    B --> C["Early Successional Species (e.g., grasses, small herbs)"]
    C --> D["Mid-Successional Species (e.g., shrubs, fast-growing trees)"]
    D --> E["Late Successional Species (e.g., shade-tolerant trees)"]
    E --> F["Climax Community (stable, mature ecosystem)"]

The process of succession generally follows these stages:

1. Pioneer Stage: Begins with bare ground or after a severe disturbance. Pioneer species are the first to colonize, like lichens or mosses, which can survive harsh conditions and begin to form soil.
2. Intermediate Stage: As soil develops and conditions improve, pioneer species are replaced by fast-growing grasses, herbs, and shrubs. These create more shade and add organic matter to the soil.
3. Climax Stage: Eventually, a stable and mature community known as the climax community develops. This stage is characterized by larger, slower-growing, shade-tolerant trees and a diverse array of species. The community is in equilibrium with the environment.

Zonation refers to the distribution of plants or animals in specific zones, often in response to an environmental gradient like altitude, depth, or moisture. This isn't succession itself, but rather how different communities exist side-by-side based on conditions.

Environmental Value Systems (EVS)

Environmental Value Systems (EVS) are worldviews or paradigms that shape the way individuals and groups perceive and evaluate environmental issues. They influence decisions and actions related to environmental management. You need to identify at least four different features and explain them in detail. Some common features include:

1. Anthropocentric EVS: This value system places humans at the center, seeing them as the most important element on Earth. Environmental protection is valued mainly because it benefits humanity (e.g., providing resources, clean air). Nature has instrumental value.
2. Ecological/Biocentric EVS: This system values all life forms and ecosystems equally, rather than prioritizing humans. It emphasizes the intrinsic value of nature, meaning nature has value for its own sake, not just for human use.
3. Technocentric EVS: This worldview believes that technology and human ingenuity can solve environmental problems. It often promotes sustainable development through technological innovation and management. Resources are seen as plentiful, and solutions come from science.
4. Deep Ecology: An extreme form of biocentric EVS, deep ecology sees humans as just one strand in the web of life, not superior. It advocates for radical changes in human behavior and culture to reduce human impact and respect the inherent worth of all living things.

How to Measure Population of Organisms

Measuring populations of organisms often involves sampling techniques. For mobile organisms, methods might include:

• Capture-Recapture (Mark-Release-Recapture): Organisms are captured, marked, released, and then another sample is captured. The proportion of marked individuals in the second sample helps estimate the total population size.
• Transects: A line is established across an area, and organisms found along the line or within a certain distance from it are counted.
• Quadrats: Square frames are randomly placed in an area, and the organisms within are counted. This is more suitable for stationary or slow-moving organisms.

3. Worked Example

Let's calculate trophic efficiency.

Suppose:
- The producer level (grasses) in a field contains 10,000 kJ of available energy.
- The primary consumer level (deer) that eats the grasses contains 1,000 kJ of available energy.

To calculate the trophic efficiency from producers to primary consumers:

Trophic Efficiency = (Energy at Higher Trophic Level / Energy at Lower Trophic Level) * 100%
Trophic Efficiency = (1,000 kJ / 10,000 kJ) * 100%
Trophic Efficiency = 0.10 * 100%
Trophic Efficiency = 10%

This means only 10% of the energy from the grasses was transferred to the deer. The remaining 90% was lost, mostly as heat due to metabolic processes (entropy increasing).

4. Key Takeaways

• Energy flows through trophic levels in a food chain, decreasing significantly with each transfer.
• The laws of thermodynamics explain energy loss and the increase in entropy (randomness) at higher trophic levels.
• Ecological succession describes the predictable changes in an ecosystem's species over time, moving from pioneer to climax communities.
• Environmental Value Systems (EVS) like anthropocentric or biocentric perspectives shape how people view and interact with the environment.
• Trophic efficiency quantifies the energy transfer between trophic levels, typically a low percentage due to energy loss.
• Pyramids of biomass visually represent the decreasing mass of organisms at successive trophic levels.

Common mistakes you should avoid:
- Confusing gross (secondary) productivity with the energy actually incorporated into biomass.
- Forgetting that entropy increases as energy decreases up the food chain.
- Mixing up the stages of succession or calling zonation a type of succession.
- Not being able to give specific features and explanations for different EVS types.

5. Now Try It

Spend 15 minutes explaining, in your own words, how the concept of entropy directly influences the structure of a pyramid of biomass and why you rarely see more than 4-5 trophic levels in most ecosystems. What success looks like is clearly linking energy loss (due to entropy) to the decreasing amount of biomass possible at higher levels.