define the terms ecosystem and niche

Learning Objectives (aligned with Cambridge International AS & A Level Biology 9700)

• Define ecosystem and niche and explain their significance for biodiversity. (Key concept: Organisms in their environment)
• Calculate species richness, evenness and diversity indices (Shannon‑Wiener, Simpson’s). (Key concept: Observation & experiment)
• Explain the role of keystone species, ecosystem engineers and genetic diversity in maintaining ecosystems. (Key concept: Natural selection & evolution)
• Identify major threats to biodiversity and evaluate the effectiveness of conservation strategies (in‑situ & ex‑situ). (Key concept: Human impact on the environment)
• Describe the taxonomic hierarchy, binomial nomenclature and modern phylogenetic methods. (Key concept: Cells as the unit of life – molecular basis of classification)
• Link ecosystems and niches to energy flow, nutrient cycles, population dynamics and evolutionary processes (AO2/AO3). (Key concepts: Energy flow, biochemical processes, DNA, natural selection)

Master‑Topic Checklist – Coverage of Syllabus 9700

Key Definitions

Ecosystem

An ecosystem is a self‑sustaining functional unit that includes:

• Biotic components: populations and communities of organisms.
• Abiotic components: climate, soil, water, light, nutrients, and physical factors.
• Interactions: energy flow (sun → producers → consumers → decomposers) and nutrient cycling (e.g., carbon, nitrogen, phosphorus).

Niche

A niche is the “profession” or role of a species (or population) within an ecosystem. It comprises:

• Range of abiotic conditions tolerated (temperature, moisture, pH).
• Resources used (food, shelter, nesting sites).
• Behavioural patterns (diurnal/nocturnal activity, foraging strategy).
• Biotic interactions (predation, competition, mutualism, parasitism).

Comparison: Ecosystem vs. Niche

Link to Cambridge Key Concepts (six overarching ideas)

Biodiversity Concepts (Topic 18)

1. Species Richness & Evenness

• Species richness (S): total number of different species in a defined area.
• Evenness (E): how equally individuals are distributed among the species present.

2. Diversity Indices

Worked Example (AO2)

Community A: 4 species, abundances 40, 30, 20, 10 (N = 100).

1. pᵢ = 0.40, 0.30, 0.20, 0.10.
2. H′ = –[(0.40 ln 0.40)+(0.30 ln 0.30)+(0.20 ln 0.20)+(0.10 ln 0.10)] ≈ 1.28.
3. Evenness E = H′ / ln 4 = 1.28 / 1.386 ≈ 0.92 (high evenness).

3. Population Genetics (Extended)

• Allele frequency (p, q): proportion of each allele in a population.
• Hardy–Weinberg equation: p² + 2pq + q² = 1 (genotype frequencies).
• Heterozygosity (H): probability that two randomly chosen alleles are different; H = 2pq.
• Example calculation: In a beetle population, 64% are homozygous dominant (AA). p² = 0.64 → p = 0.8, q = 0.2. Expected heterozygotes (2pq) = 0.32 (32%).

4. Evolution & Biodiversity

• Sources of variation: mutation, recombination, gene flow.
• Natural selection: differential survival/reproduction based on phenotype; leads to changes in allele frequencies.
• Speciation mechanisms:
  • Allopatric – geographic isolation (e.g., Darwin’s finches).
  • Sympatric – ecological or polyploidy‑driven (e.g., cichlid radiations).
• Adaptive radiation: rapid evolution of multiple species from a common ancestor, each occupying a distinct niche (e.g., Hawaiian honeycreepers).
• Link to niche: New niches created by ecosystem engineers or environmental change can drive adaptive radiation.

5. Keystone Species & Ecosystem Engineers (Evaluation Required)

• Keystone species: Disproportionately large impact relative to abundance.
  • Mechanism: top‑down control of prey populations, cascade effects on vegetation.
  • Example: Sea otters → control sea‑urchins → protect kelp forests.
• Ecosystem engineers: Modify physical environment, creating or destroying habitats.
  • Mechanism: physical alteration (dams, burrows) that changes water flow, sedimentation, or nutrient availability.
  • Example: Beavers build dams → create ponds → increase habitat heterogeneity, support amphibians and water‑birds.

Major Threats to Biodiversity

Case‑Study Worksheet – Climate‑Change‑Driven Niche Shifts

Task: Using the supplied distribution maps for the European red‑winged grasshopper (Trimerotropis cristata) and the common toad (Bufo bufo), answer the following (AO3):

1. Predict how a 2 °C rise in average summer temperature will alter the grasshopper’s niche breadth and geographic range.
2. Discuss the likely indirect effects on the toad’s niche, considering prey availability and breeding pond temperature.
3. Evaluate two management actions (e.g., assisted migration, creation of climate‑refugia corridors) and justify which would be most effective for maintaining community stability.

Conservation Strategies (Evaluation of Effectiveness)

IUCN Red List Categories (AO3)

• Extinct (EX)
• Extinct in the Wild (EW)
• Critically Endangered (CR)
• Endangered (EN)
• Vulnerable (VU)
• Near Threatened (NT)
• Least Concern (LC)
• Data Deficient (DD)
• Not Evaluated (NE)

Classification Systems (Taxonomy) – Connection to Biodiversity

• Hierarchical ranks: Domain → Kingdom → Phylum → Class → Order → Family → Genus → Species.
• Binomial nomenclature: Genus (capitalised) + specific epithet (lowercase), e.g., Panthera leo.
• Cladistics: Classification based on shared derived characters (synapomorphies); produces phylogenetic trees that illustrate evolutionary relationships.
• Molecular phylogenetics: Uses DNA/RNA sequences (e.g., cytochrome b, 16S rRNA, COI) to infer relationships; essential for modern biodiversity assessments and for identifying cryptic species.

Linking Ecosystem & Niche to Other Syllabus Topics

• Energy flow: Photosynthesis (A‑level) provides the primary energy input; the position of a species in a food chain reflects its niche (primary consumer, secondary consumer, etc.).
• Nutrient cycles: Decomposers occupy a niche that recycles organic matter, linking to biochemical processes (carbon, nitrogen cycles).
• Population dynamics: Carrying capacity (K) of an ecosystem is set by the amount of usable resources within niches; density‑dependent factors (disease, competition) act on specific niches.
• Evolution: Niche differentiation reduces competition, allowing adaptive radiation; keystone species can create new niches (e.g., beaver dams forming pond habitats).
• Genetics: Genetic diversity within a niche determines adaptive potential; population genetics calculations (Hardy‑Weinberg) are used to monitor genetic health of threatened populations.

Why These Concepts Matter (Exam‑Focused)

• Understanding ecosystems enables you to predict the consequences of environmental change – a frequent AO2/3 scenario.
• Grasping niche theory explains coexistence, competition and the mechanisms behind speciation – essential for high‑level essay questions.
• Quantitative biodiversity tools (indices, Hardy‑Weinberg) provide the data‑analysis skills required for AO2 marks.
• Evaluating threats and conservation measures demonstrates higher‑order thinking (AO3) and is a core component of the Cambridge exam.
• Taxonomic and phylogenetic knowledge links molecular biology to ecology, satisfying the cross‑topic integration expected at A‑level.

Suggested Diagrams for Revision

1. Energy‑flow diagram of a temperate forest (sun → producers → herbivores → carnivores → decomposers) with parallel arrows for carbon and nutrient cycling.
2. Side‑panel “Woodpecker niche”: habitat (deciduous trees), food (insect larvae), activity (diurnal), interactions (competition with other insectivores, mutualism with fungi).
3. Phylogenetic tree highlighting a keystone species (e.g., sea otter) within Mammalia, with annotations of derived traits.
4. Bar chart comparing species richness and Shannon index for three habitats (pond, grassland, urban park).
5. Hardy‑Weinberg diagram showing genotype frequencies (p², 2pq, q²) and how selection shifts them over generations.

Sample Exam‑Style Questions (AO2/AO3)

1. Define the terms ecosystem and niche. (2 marks)
2. Explain how a beaver acts as an ecosystem engineer and discuss two consequences for biodiversity. (6 marks)
3. A community of five species has the following abundances: 25, 25, 20, 20, 10. Calculate the Shannon–Wiener index and interpret the result in terms of species evenness. (5 marks)
4. Analyse the likely impact of invasive zebra mussels on a freshwater lake ecosystem, referring to both trophic structure and nutrient cycling. (8 marks)
5. Compare in‑situ and ex‑situ conservation, giving one advantage and one limitation of each approach. (4 marks)
6. Using Hardy‑Weinberg, a population of butterflies has 36 % homozygous dominant (AA). Calculate the expected frequency of heterozygotes (Aa) and discuss what a deviation from this value might indicate about the population. (6 marks)

Practical / Analytical Activities (AO2)

• Field survey: Record species abundance in a local pond, calculate species richness, evenness and Shannon index, then write a brief evaluation of factors influencing the observed diversity (e.g., water chemistry, surrounding land use).
• Laboratory: Perform DNA barcoding on unknown insect specimens using the COI gene; construct a simple cladogram and discuss how molecular data can reveal cryptic species.
• Data analysis: Interpret a supplied predator–prey graph (Lotka‑Volterra) and explain how niche overlap can generate population oscillations.
• Genetics exercise: Given allele frequencies for a threatened plant, calculate heterozygosity and suggest a management action to maintain genetic diversity.

Key Take‑away Summary

• An ecosystem is the whole network of biotic and abiotic interactions; a niche is the specific role of a species within that network.
• Biodiversity is quantified by species richness, evenness and diversity indices; population genetics provides a quantitative measure of genetic diversity.
• Keystone species and ecosystem engineers have outsized effects on community structure and ecosystem processes.
• Major threats include habitat loss, over‑exploitation, invasive species, pollution and climate change – each with distinct ecological consequences.
• Conservation combines protected areas, habitat restoration, legislation, captive breeding and community involvement; effectiveness must be evaluated against ecological and socio‑economic criteria.
• Taxonomic hierarchy, binomial nomenclature and molecular phylogenetics give the framework for comparing species and assessing evolutionary relationships.
• All these concepts interlink with energy flow, nutrient cycles, population dynamics and evolutionary processes – essential for high‑level exam answers and for understanding real‑world biodiversity challenges.