12.4 Chapter 12 Summary

Ecosystem Structure and Components

An ecosystem is defined as a community of living organisms (biotic factors) and their complex interactions with their nonliving physical environment (abiotic factors), such as latitude, rainfall, temperature, and topography.

Ecosystems are broadly categorized into three types: freshwater, ocean water (the most common, covering over 70% of Earth’s surface), and terrestrial (grouped into biomes like forests and deserts).

In ecology, an ecosystem’s stability is measured by two parameters:

• Resistance: The ability of an ecosystem to remain in equilibrium despite disturbances (e.g., severe weather).
• Resilience: The speed at which an ecosystem recovers and returns to its equilibrium state after being disturbed.

Trophic Levels and Food Webs

The movement of energy and nutrients in an ecosystem is described by trophic levels:

1. Primary Producers (Autotrophs): Organisms that synthesize their own food from inorganic sources. These are the base of the food chain. Most use photosynthesis (photoautotrophs, like plants and algae), while others use chemosynthesis (chemoautotrophs, like certain bacteria in deep-sea vents).
2. Primary Consumers (Herbivores): Organisms that eat the primary producers.
3. Secondary Consumers (Carnivores): Organisms that eat the primary consumers.
4. Tertiary Consumers (Apex Consumers): Carnivores that eat other carnivores.

This flow is represented by:

• Food Chain: A simple, linear sequence showing a single path of energy transfer (e.g., grass $\to$ deer $\to$ wolf).
• Food Web: A more accurate, holistic, and non-linear model that shows all the complex, interconnected feeding relationships among multiple species in an ecosystem.

Energy Flow and the Ten Percent Rule

Energy transfer through an ecosystem is inefficient, a principle governed by the Second Law of Thermodynamics, which results in the loss of energy as metabolic heat at each transfer.

• Net Primary Productivity (NPP): The energy remaining in the primary producers after accounting for their own respiration and heat loss; this is the energy actually available to primary consumers.
• Trophic Level Transfer Efficiency (TLTE): The measurement of energy transfer efficiency between two successive trophic levels.
• The 10 Percent Rule: On average, only about 10% of the energy stored in one trophic level is transferred and incorporated into the biomass of the next successive trophic level. The remaining 90% is lost, primarily as heat.

The immense energy loss at each step is the primary factor that limits the number of trophic levels in an ecosystem, as very little energy remains for higher-level consumers.

Ecological Pyramids and Biomagnification

The structure of ecosystems and the energy transfer between trophic levels can be modeled using ecological pyramids:

• Pyramids of Energy: Always upright (pyramid-shaped) because energy decreases at each successive trophic level, making them the most consistent models of ecosystem structure.
• Pyramids of Biomass and Numbers: Can be inverted in some ecosystems, such as in the English Channel where phytoplankton (producers) have a much lower biomass than zooplankton (consumers) at any given time due to the phytoplankton’s rapid turnover rate.

A critical environmental consequence of inefficient energy transfer is biomagnification (or bioaccumulation). This is the increasing concentration of persistent, non-biodegradable toxic substances (like DDT or heavy metals) in organisms as they move up the food chain, with the apex consumers accumulating the highest and most harmful concentrations.

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“12.4 Chapter 12 Summary” was initially generated by Gemini 2.5 Flash and then modified by Christelle Sabatier. “12.4 Chapter 12 Summary” is licensed under CC-BY-NC 4.0.