Environmental systems: ecosystems, biodiversity, soil and biogeochemical cycles
Domain 10 - Environmental systems and interactions. Questions test energy flow, the structure and formation of soil, biodiversity measures and the human alteration of element cycles.
Ecosystems and energy
- An ecosystem = biotope (physical environment) + biocenosis (community). Energy flows one way; matter cycles.
- GPP is total fixation; NPP = GPP − respiration. Transfer between trophic levels averages about 10 %, so food chains are short.
- Succession runs from pioneer to climax communities; resilience is the capacity to recover, resistance the capacity to absorb change.
- Limiting factors (Liebig) and carrying capacity constrain populations.
Biodiversity
- Three levels: genetic, species, ecosystem. Indices: Shannon (H = −Σ pi ln pi) and Simpson, plus species richness and evenness.
- The Mediterranean basin is a global biodiversity hotspot with high endemism; the Valencian Community holds sites such as L'Albufera, La Devesa del Saler, the Columbretes Islands and the Font Roja.
- Conservation instruments: Natura 2000 (habitats and birds directives), protected areas, connectivity corridors.
Soil
- Profile horizons: O, A, E, B, C, R. Texture (sand, silt, clay triangle), structure, pH, organic matter, cation exchange capacity, salinity.
- Formation factors (Jenny): climate, organisms, relief, parent material, time. Mediterranean soils include calcisols, cambisols, fluvisols and regosols.
- Soil erosion by water is the main degradation process; the USLE/RUSLE framework multiplies rainfall erosivity, erodibility, slope length and steepness, cover and support practice. Arid southeast Spain is at high desertification risk.
Biogeochemical cycles
- Carbon: photosynthesis, respiration, fossil-fuel combustion, ocean and land sinks.
- Nitrogen: fixation, nitrification, denitrification; the Haber–Bosch process roughly doubled reactive nitrogen.
- Phosphorus has no gaseous phase and is sediment-bound, so it is often limiting; excess causes eutrophication.
Worked example
A food chain with 10 % transfer: to support 10 kg of top predator you need about 100 kg of herbivores and about 1,000 kg of producers. If a pollutant biomagnifies, its concentration in the predator can be ten times that in its prey per level, which is why top predators such as raptors accumulated DDT.
Traps
- Treating the 10 % rule as exact; ocean biomass pyramids can invert.
- Confusing GPP with NPP.
- Describing soil as inert dirt rather than a living, structured system.
- Assuming biodiversity means species number only, ignoring evenness and endemism.
Environmental and geological field and practical skills
Domain 11 - Field and practical skills. Practical questions give you a map, a cross-section, a data table or a sampling plan and ask for a quantitative reading. Accuracy of units and scale matters more than prose.
Maps and cross-sections
- Scale: 1:25,000 means 1 cm = 250 m; area scales with the square, so 1 cm2 = 0.0625 km2. Measure distances with the bar scale, never by eye.
- Contours: interval constant, spacing inversely proportional to slope; the V points upstream in valleys; closed loops with hachures are depressions.
- Strike and dip: dip is measured perpendicular to strike; apparent dip is always less than true dip. Use the three-point method on a geological map to find strike and dip from three boreholes.
- Cross-sections: project boundaries, honour vertical exaggeration, keep the scale consistent.
Sampling and laboratory work
- Sampling designs: random, systematic (grid), stratified, targeted. Record position (GPS), depth, mass, date, and avoid contamination; use clean tools and appropriate containers.
- Grain-size analysis by sieving and sedimentation; thin sections for mineralogy and texture; XRF for major elements; pH and conductivity meters; titration for carbonates; loss on ignition for organic matter.
Data interpretation
- Check units and significant figures; state the uncertainty; distinguish precision from accuracy.
- Identify trends, outliers and correlations; do not infer causation from correlation.
- GIS layers (topography, geology, land use, flood zones) combine by overlay analysis.
Worked example
On a 1:25,000 map, two points lie 4 cm apart, with elevations 300 m and 500 m. Ground distance = 4 × 250 m = 1,000 m. Gradient = 200 / 1,000 = 0.20, i.e. 20 %; the slope angle is arctan(0.20) ≈ 11°. A 2 cm error in measurement would change the distance by 500 m and halve the gradient, so state the measurement uncertainty.
Traps
- Reporting gradient as an angle or as a percentage without saying which.
- Forgetting that map area scales as the square of the linear scale.
- Confusing apparent dip with true dip.
- Reading contours downslope in valleys, reversing the V rule.
- Ignoring sampling bias when a grid sample misses a thin, economically important unit.