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Ecology and Earth Science - Food Chains and Food Webs

Grade 7IB

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Producers (Autotrophs) are organisms that capture solar energy to create glucose via photosynthesis, represented by the chemical equation: 6CO2+6H2O+light→C6H12O6+6O26CO_2 + 6H_2O + \text{light} \rightarrow C_6H_{12}O_6 + 6O_2.

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Consumers (Heterotrophs) obtain energy by consuming other organisms. They are classified as Primary (herbivores), Secondary (carnivores eating herbivores), or Tertiary (top predators).

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Trophic Levels represent the position an organism occupies in a food chain. Energy flows from T1T_1 (producers) to TnT_n (top consumers).

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The 10% Rule states that only approximately 10%10\% of the energy from one trophic level is transferred to the next; the remaining 90%90\% is lost as heat or used for metabolic processes like cellular respiration: C6H12O6+6O2→6CO2+6H2O+ATPC_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{ATP}.

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Food Webs are interconnected food chains that show the complex feeding relationships in an ecosystem, highlighting how one organism can occupy multiple trophic levels.

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Decomposers (e.g., fungi and bacteria) break down dead organic matter, recycling essential nutrients like Nitrogen (NN) and Phosphorus (PP) back into the soil.

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Biomagnification is the process where the concentration of toxic substances (such as HgHg or DDTDDT) increases as it moves up the food chain.

📐Formulae

En+1=En×0.10E_{n+1} = E_n \times 0.10

6CO2+6H2O→lightC6H12O6+6O26CO_2 + 6H_2O \xrightarrow{\text{light}} C_6H_{12}O_6 + 6O_2

Trophic Efficiency=Energy at Trophic Level nEnergy at Trophic Level n−1×100\text{Trophic Efficiency} = \frac{\text{Energy at Trophic Level } n}{\text{Energy at Trophic Level } n-1} \times 100

💡Examples

Problem 1:

In a local grassland ecosystem, the total energy produced by the grass (producers) is calculated to be 8000 kJ8000\text{ kJ}. Using the 10% rule, calculate the energy available to the snakes (secondary consumers) in the food chain: Grass→Grasshopper→Snake\text{Grass} \rightarrow \text{Grasshopper} \rightarrow \text{Snake}.

Solution:

80 kJ80\text{ kJ}

Explanation:

First, calculate the energy for the primary consumer (grasshopper): 8000 kJ×0.10=800 kJ8000\text{ kJ} \times 0.10 = 800\text{ kJ}. Next, calculate the energy for the secondary consumer (snake): 800 kJ×0.10=80 kJ800\text{ kJ} \times 0.10 = 80\text{ kJ}.

Problem 2:

Identify the products of photosynthesis that become the primary energy source for the rest of the food chain.

Solution:

C6H12O6C_6H_{12}O_6 (Glucose) and O2O_2 (Oxygen).

Explanation:

Plants convert CO2CO_2 and H2OH_2O into C6H12O6C_6H_{12}O_6, which stores chemical energy in its bonds, and release 6O26O_2 as a byproduct which is used by consumers for respiration.