What is the primary difference between a food chain and a food web?

A food chain is a simple, linear model showing one path of energy transfer. A food web is a network of many interconnected food chains that more accurately represents the complex multiple feeding relationships in an ecosystem.

Why can a pyramid of numbers be 'inverted' while a pyramid of biomass cannot?

A pyramid of numbers can be inverted if a few large producers support many small consumers, creating a narrow base. However, biomass always decreases at higher levels due to energy loss and waste, meaning the base must always be the widest level.

How do arrows in a food web differ from a general drawing of predation?

In a food web, arrows represent energy transfer and must point from the prey to the predator. In a casual drawing, someone might point an arrow from the predator to show it attacking, but in ecology, that would incorrectly imply the prey is eating the predator.

What is the most common mistake made when drawing arrows in a food chain?

The most common mistake is drawing arrows pointing from the predator toward the prey. Arrows must always point from the organism being eaten to the organism consuming it to correctly represent the flow of chemical energy.

Why is it incorrect to state that a population will 'die out' if its predator is removed in a food web?

Ecosystems are complex and usually reach a new equilibrium. While the prey population will likely increase significantly, it is more accurate to say their numbers will fluctuate or be limited by food competition rather than assuming the entire system collapses.

What mistake occurs if you ignore the '10% rule' when drawing a pyramid of energy?

Ignoring energy loss would lead to drawing a rectangle rather than a pyramid. Since energy is lost as heat and waste at each level, the bars must become significantly narrower as you move up the trophic levels to reflect the actual energy available.

What is the definition of 'Biomass'?

Biomass is the total mass of living material at a specific level of a food chain or within a certain area. It represents the organic material that stores the chemical energy passed from one organism to another in a food chain.

What does a 'Food Web' represent?

A food web is a graphical model consisting of multiple interconnected food chains. it shows the diverse feeding relationships and energy pathways that exist simultaneously within a single ecosystem.

What is the fundamental role of 'Producers' in an energy pyramid?

Producers, such as plants, capture light energy and convert it into chemical energy through photosynthesis. They form the base of all ecological pyramids and provide the energy that sustains every consumer at higher trophic levels.

Why do food chains rarely extend beyond five trophic levels?

Energy is lost at each level due to heat from respiration, movement, and undigested waste. By the time energy reaches the fourth or fifth level, there is usually too little remaining to support the metabolic needs of another population of consumers.

Food chains, Food webs & Pyramids | Pearson Edexcel IGCSE Biology

4. Ecology & the Environment

Food chains, Food webs & Pyramids

Summary

This knowledge module explains the linear and complex models used to represent feeding relationships in an ecosystem, as well as the quantitative pyramids that illustrate how biomass and energy dissipate as they move through trophic levels.

1. Definition & Core Concepts

Food Chains and Energy Transfer: A food chain is a linear diagram used to show how energy flows between organisms when they feed on one another. The arrows in the chain represent the transfer of chemical energy, pointing from the organism that is eaten toward the organism that consumes it. All valid food chains must begin with a producer, which is the source of energy for the entire ecosystem.
Interconnected Food Webs: In natural environments, feeding relationships are far more complex than a single line, leading to the formation of food webs. These networks are made of many interconnected food chains that demonstrate how most organisms consume more than one type of prey. Within a food web, an organism may hold different trophic levels depending on which specific pathway is being observed.
Producers as the Energy Gateway: Producers, such as plants and algae, form the first trophic level of every chain because they convert light energy into chemical energy. This chemical energy is stored in the form of glucose and other carbon compounds, which are then passed on to primary consumers when they eat the plant tissues.

2. Underlying Principles

Thermodynamic Loss of Energy: At each trophic level, a significant amount of energy is lost to the environment as heat through metabolic processes like respiration and physical movement. Because of this inefficiency, typically only about 10% of the energy available at one level is converted into new biomass at the next. This rapid decrease in available energy is the primary reason why most food chains are limited to four or five levels.
Biomass and Stored Potential: Biomass is the total mass of living matter at a specific trophic level, and it directly represents the chemical energy stored in biological tissues. In most stable ecosystems, the total biomass decreases at higher trophic levels because energy is lost as waste or used for survival, meaning there is less potential to build new tissue at the top.
Population Interdependence: Populations within a food web are mutually dependent, meaning a change in one species can trigger significant effects elsewhere in the web. For example, if a primary consumer population decreases, the producer population may increase due to reduced grazing pressure, while the secondary consumer population might decrease due to a lack of available food.

Graphical comparison of two pyramid types: A pyramid of numbers shows an unconventional shape where a single producer supports many insects, while a pyramid of biomass always maintains a strictly tapered pyramid shape.

3. Methods & Techniques

4. Key Distinctions

5. Exam Strategy & Tips

6. Common Pitfalls & Misconceptions