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Biology: Ecology and Sustainability - Energy Flow and Nutrient Cycles in Ecosystems

Grade 8IB

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

🔑Concepts

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Energy enters most ecosystems as sunlight and is converted into chemical energy by producers through the process of photosynthesis: 6CO2+6H2O→lightC6H12O6+6O26CO_{2} + 6H_{2}O \xrightarrow{light} C_{6}H_{12}O_{6} + 6O_{2}.

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Trophic Levels: These represent the feeding positions in a food chain. Energy flows from Producers (T1T_{1}) to Primary Consumers (T2T_{2}), then to Secondary Consumers (T3T_{3}), and finally to Tertiary Consumers (T4T_{4}).

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The 10% Rule: Only approximately 10%10\% of the energy available at one trophic level is transferred to the next level. The remaining 90%90\% is lost as heat through metabolic processes like respiration or as waste.

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Biomass: This refers to the total mass of living organisms in a given area or volume. In a healthy ecosystem, the pyramid of biomass usually tapers upwards because energy loss limits the number of organisms at higher levels.

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Carbon Cycle: Carbon is cycled through the biosphere via photosynthesis, respiration (C6H12O6+6O2→6CO2+6H2O+EnergyC_{6}H_{12}O_{6} + 6O_{2} \rightarrow 6CO_{2} + 6H_{2}O + Energy), combustion of fossil fuels, and decomposition.

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Nitrogen Cycle: Nitrogen gas (N2N_{2}) in the atmosphere is converted into usable forms (nitrates) by nitrogen-fixing bacteria, which are then taken up by plants to build proteins.

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Bioaccumulation and Biomagnification: Bioaccumulation is the buildup of toxins within a single organism over time, while biomagnification is the increase in concentration of toxins (like mercury or DDT) as they move up the food chain.

📐Formulae

6CO2+6H2O+light energy→C6H12O6+6O26CO_{2} + 6H_{2}O + \text{light energy} \rightarrow C_{6}H_{12}O_{6} + 6O_{2}

C6H12O6+6O2→6CO2+6H2O+ATP (Energy)C_{6}H_{12}O_{6} + 6O_{2} \rightarrow 6CO_{2} + 6H_{2}O + \text{ATP (Energy)}

En+1=En×0.10E_{n+1} = E_{n} \times 0.10

Ecological Efficiency=Energy at current levelEnergy at previous level×100%\text{Ecological Efficiency} = \frac{\text{Energy at current level}}{\text{Energy at previous level}} \times 100\%

💡Examples

Problem 1:

In a grassland ecosystem, the net primary productivity (energy stored by producers) is 25,000 kJ/m2/year25,000\text{ kJ/m}^{2}\text{/year}. Calculate the energy available to the tertiary consumers assuming the 10%10\% rule applies at each step.

Solution:

Producers (T1):25,000 kJPrimary Consumers (T2):25,000×0.10=2,500 kJSecondary Consumers (T3):2,500×0.10=250 kJTertiary Consumers (T4):250×0.10=25 kJ\begin{array}{r} \text{Producers (T1)}: 25,000\text{ kJ} \\ \text{Primary Consumers (T2)}: 25,000 \times 0.10 = 2,500\text{ kJ} \\ \text{Secondary Consumers (T3)}: 2,500 \times 0.10 = 250\text{ kJ} \\ \text{Tertiary Consumers (T4)}: 250 \times 0.10 = 25\text{ kJ} \end{array}

Explanation:

According to the 10%10\% rule, each trophic level receives only one-tenth of the energy from the level below it. We multiply by 0.100.10 for each jump from T1T_{1} to T4T_{4}.

Problem 2:

A pesticide is sprayed on a field. A grasshopper eats the grass, a frog eats the grasshopper, and a hawk eats the frog. If the concentration of pesticide in the grass is 0.5 ppm0.5\text{ ppm}, and it increases by a factor of 1010 at each trophic level due to biomagnification, what is the concentration in the hawk?

Solution:

Grass (Producer):0.5 ppmGrasshopper (Primary):0.5×10=5.0 ppmFrog (Secondary):5.0×10=50.0 ppmHawk (Tertiary):50.0×10=500.0 ppm\begin{array}{r} \text{Grass (Producer)}: 0.5\text{ ppm} \\ \text{Grasshopper (Primary)}: 0.5 \times 10 = 5.0\text{ ppm} \\ \text{Frog (Secondary)}: 5.0 \times 10 = 50.0\text{ ppm} \\ \text{Hawk (Tertiary)}: 50.0 \times 10 = 500.0\text{ ppm} \end{array}

Explanation:

Biomagnification causes the concentration of non-biodegradable substances to increase as they move up the food chain because higher-level consumers must eat many lower-level organisms to obtain enough energy.