The Biosphere and Soils: Ecosystems, Biodiversity, Soil Formation and Degradation
This domain links living communities to their physical substrate. The exam reviews trophic structure, biodiversity patterns and the five soil-forming factors, then applies them to Mediterranean and island degradation problems.
Ecosystem structure
An ecosystem is a biotope (physical environment) plus a biocenosis (living community). Energy flows one way through producers, consumers and decomposers, with roughly 10% transfer between trophic levels; matter cycles (carbon, nitrogen, phosphorus). Biodiversity operates at genetic, species and ecosystem levels.
Mediterranean and island biodiversity
The Mediterranean basin is a biodiversity hotspot with high endemism. In the Balearics, insularity produces endemic species and subspecies, and Posidonia oceanica meadows are a key marine ecosystem: they stabilise sediment, oxygenate water, act as a carbon sink and produce beach-cast banquettes.
Soil formation
Soil develops from the interaction of parent material, climate, organisms, relief and time. Profile horizons: O (litter), A (humus-rich topsoil), E (eluvial), B (illuvial accumulation), C (weathered parent material), R (bedrock). Key properties: texture (sand/silt/clay), structure, pH, organic matter, cation exchange capacity and porosity.
Mediterranean soil types
- Terra rossa: red clay-rich soil formed on limestone through decalcification and iron-oxide accumulation.
- Rendzina: shallow dark soil directly on limestone.
- Brown calcareous soils, alluvial soils in valleys, and saline soils in coastal flats.
Degradation
Water erosion after vegetation loss, salinisation from saline irrigation, compaction, contamination, urban sealing and desertification. Traditional dry-stone terraces (marges) in the Serra de Tramuntana retain soil and slow runoff; abandonment leads to terrace collapse and accelerated erosion.
Worked example
Terraced olive groves on a steep Tramuntana slope are abandoned. Walls fail, soil washes off within a few seasons, and runoff peaks downstream increase. The chain is: loss of maintenance → terrace collapse → loss of soil and infiltration → higher runoff coefficient → greater flood hazard. Reasoning must link the land-use change to a soil process and then to a hydrological consequence.
Traps that cost marks
- Confusing humus (decomposed organic matter) with litter.
- Claiming clay soils are impermeable; they drain slowly but store water.
- Confusing erosion (transport) with weathering (breakdown in place).
- Assuming all island endemics are plants; many are invertebrates and reptiles.