Population size and ecosystems — Eduqas A-Level Biology

Population size and ecosystems is a core topic in WJEC A-Level Biology that explores how populations of organisms change over time and interact within their environments. It covers key concepts such as carrying capacity, limiting factors, and the dynamic nature of ecosystems. Understanding these principles is essential for explaining real-world issues like conservation, pest control, and the impact of climate change on biodiversity.

This topic builds on GCSE knowledge of food chains and nutrient cycles, but delves deeper into quantitative methods like mark-release-recapture and the use of logarithmic scales for population growth. You'll learn to distinguish between exponential and logistic growth, and how density-dependent and density-independent factors regulate population size. Mastering this topic is crucial for tackling exam questions on ecological stability and human impact on ecosystems.

Population ecology is not just theoretical; it has practical applications in managing fisheries, controlling invasive species, and restoring habitats. By the end of this topic, you should be able to interpret population growth curves, calculate population estimates, and explain how abiotic and biotic factors interact to shape ecosystems. This knowledge forms the foundation for more advanced topics like succession and energy flow.

• →Carrying capacity (K): the maximum population size an environment can sustain indefinitely, determined by limiting factors like food, water, and space.
• →Exponential vs. logistic growth: exponential growth occurs when resources are unlimited (J-shaped curve), while logistic growth shows an S-shaped curve as the population approaches carrying capacity.
• →Density-dependent factors (e.g., competition, disease) and density-independent factors (e.g., natural disasters, climate) regulate population size.
• →Mark-release-recapture method: used to estimate population size of mobile organisms; formula: (number marked in first sample × total in second sample) / number marked in second sample.
• →Ecological niche: the role and position of a species within its ecosystem, including its habitat, resource use, and interactions with other species.

• Quantitative treatment of population growth including immigration, emigration, birth and death rates
• Factors affecting population growth such as competition and carrying capacity
• Distinction between density-dependent and density-independent factors
• Energy flow in ecosystems with the sun as the primary source
• Biomass transfer efficiency, gross and net production, and pyramids of biomass
• Principles of primary and secondary succession including pioneer species and climax communities
• Role of bacteria in the nitrogen cycle and the significance of nitrates
• Human impact on nutrient cycles, including eutrophication and algal blooms

• 💡Ensure you can define and apply the terms habitat, community, and ecosystem correctly
• 💡Be prepared to calculate and interpret population growth data
• 💡Practice drawing and labeling diagrams of nutrient cycles
• 💡Understand the link between human activities like ploughing/drainage and their effects on nitrification
• 💡When drawing or interpreting population growth curves, always label the axes (time on x-axis, population size on y-axis) and clearly indicate the carrying capacity (K) on logistic curves. Use a ruler for straight lines and smooth curves for logistic growth.
• 💡For mark-release-recapture calculations, show all working and check your units. Common errors include swapping the numbers in the formula or forgetting to multiply by the total in the second sample. Practice with past paper questions.
• 💡In essays, use specific examples (e.g., rabbit population in Australia, human population growth) to illustrate concepts like exponential growth and limiting factors. This shows application and gains higher marks.

• Confusing gross production with net production
• Failing to distinguish between density-dependent and density-independent factors
• Misunderstanding the role of bacteria in different stages of the nitrogen cycle
• Incorrectly interpreting pyramids of biomass versus pyramids of numbers
• Misconception: Carrying capacity is fixed. Correction: Carrying capacity can change over time due to environmental changes, such as seasonal variations or human activities like deforestation.
• Misconception: Exponential growth can continue indefinitely. Correction: Exponential growth only occurs when resources are unlimited; in reality, limiting factors eventually slow growth, leading to logistic growth.
• Misconception: Mark-release-recapture gives an exact population size. Correction: It provides an estimate, and accuracy depends on assumptions like no migration, equal catchability, and marks not affecting survival.