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Biology: Ecology and Sustainability - Food Chains, Food Webs, Predation, Competition, and Extinction

Grade 8IB

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

🔑Concepts

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Food Chains and Trophic Levels: A food chain represents the linear flow of energy. Organisms are categorized into trophic levels: Producers (Autotrophs), Primary Consumers (Herbivores), Secondary Consumers (Carnivores), and Tertiary Consumers (Apex Predators).

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Food Webs: In ecosystems, most organisms eat more than one type of food. A food web consists of interconnected food chains, demonstrating the complex feeding relationships and interdependence within an ecosystem.

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Energy Transfer (The 10%10\% Rule): Only approximately 10%10\% of the energy from one trophic level is transferred to the next. The remaining 90%90\% is lost as heat through metabolic processes like respiration, movement, and excretion.

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Predation: A biological interaction where one organism, the predator, kills and eats another organism, its prey. This helps regulate population sizes in an ecosystem.

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Competition: This occurs when organisms strive for the same limited resources, such as food, water, or territory. It can be intraspecific (between members of the same species) or interspecific (between different species).

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Sustainability and Extinction: Sustainability refers to maintaining the balance of an ecosystem so it remains productive over time. Extinction is the permanent disappearance of a species, often caused by habitat loss, overexploitation, or climate change.

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

📐Formulae

Energy Transfer Efficiency=(Energy at higher trophic levelEnergy at lower trophic level)×100\text{Energy Transfer Efficiency} = \left( \frac{\text{Energy at higher trophic level}}{\text{Energy at lower trophic level}} \right) \times 100

Net Primary Productivity (NPP)=Gross Primary Productivity (GPP)−Respiration (R)\text{Net Primary Productivity (NPP)} = \text{Gross Primary Productivity (GPP)} - \text{Respiration (R)}

En=En−1×0.10 (where En is energy at level n)E_{n} = E_{n-1} \times 0.10 \text{ (where } E_{n} \text{ is energy at level } n \text{)}

💡Examples

Problem 1:

In a grassland ecosystem, the producers (grasses) contain 25,000 kJ25,000 \text{ kJ} of energy. Calculate the energy available to the tertiary consumers (e.g., hawks), assuming a 10%10\% energy transfer efficiency at each step.

Solution:

Producer: 25,000 kJPrimary Consumer: 25,000×0.10=2,500 kJSecondary Consumer: 2,500×0.10=250 kJTertiary Consumer: 250×0.10=25 kJ\begin{array}{r} \text{Producer: } 25,000 \text{ kJ} \\ \text{Primary Consumer: } 25,000 \times 0.10 = 2,500 \text{ kJ} \\ \text{Secondary Consumer: } 2,500 \times 0.10 = 250 \text{ kJ} \\ \text{Tertiary Consumer: } 250 \times 0.10 = 25 \text{ kJ} \end{array}

Explanation:

Starting from the producer, we multiply by 0.100.10 (or 10%10\%) for every level we move up. Moving three steps up (Primary, Secondary, Tertiary) results in 25 kJ25 \text{ kJ} reaching the hawk.

Problem 2:

A population of 500500 rabbits lives in a field. If a disease reduces the population by 45%45\%, and then predators kill another 2020 rabbits, calculate the final population.

Solution:

Initial population: 500Loss due to disease: 500×0.45=225Remaining after disease: 500−225=275Final population: 275−20=255\begin{array}{r} \text{Initial population: } 500 \\ \text{Loss due to disease: } 500 \times 0.45 = 225 \\ \text{Remaining after disease: } 500 - 225 = 275 \\ \text{Final population: } 275 - 20 = 255 \end{array}

Explanation:

First, we calculate the 45%45\% loss (225225 individuals). Subtracting this from the initial 500500 gives 275275. Finally, we subtract the 2020 lost to predation to find the surviving population of 255255.