Ecology and Environment: Ecosystems, Populations and Human Impact
The ecology and environment domain of the Basque PAU Biology exam tests energy flow, population growth, biodiversity and human effects on ecosystems. You should interpret pyramids and food webs, calculate population size, and evaluate conservation and impact data.
Ecosystem structure and energy
- Producers fix energy; consumers transfer it; decomposers recycle nutrients.
- Energy flow is unidirectional and inefficient; the 10% rule is an approximation, not a law.
- Pyramids of energy are always upright; pyramids of biomass or numbers can be inverted in some ecosystems.
- Biogeochemical cycles include carbon, nitrogen, phosphorus and water cycles.
Population dynamics
- Exponential growth occurs with unlimited resources; logistic growth levels off at carrying capacity K.
- Density-dependent factors include competition, disease and predation; density-independent factors include fires, droughts and storms.
- Mark–recapture estimate: N = (marked × recaptured total) ÷ marked in recapture.
Biodiversity and human impact
- Biodiversity includes genetic, species and ecosystem diversity; species richness and evenness both matter.
- Habitat loss, invasive species, pollution, overexploitation and climate change are major threats.
- Conservation includes protected areas, restoration, ex situ measures and sustainable resource use.
Worked example
Eighty animals are marked and released. A later sample of 100 contains 20 marked individuals. The estimated population is N = (80 × 100) ÷ 20 = 400. The estimate assumes a closed population, random mixing of marked animals, no mark loss and equal catchability. If marked animals avoid traps, the estimate is biased.
Traps
- The 10% rule is a teaching approximation; real ecological efficiencies vary.
- An inverted biomass pyramid is possible when producers turn over very quickly.
- Carrying capacity is not fixed; it can change with conditions.
- High species richness does not guarantee high evenness.
- A correlation between human activity and biodiversity loss is not automatically causation.