Environmental Systems, Biodiversity and Ecological Interactions
This domain tests ecosystem structure, population dynamics, energy and matter flows, and the factors that generate and maintain biodiversity. PAU questions usually give data - a survivorship curve, a trophic table, a growth curve - and ask for interpretation plus a management consequence.
Ecosystem structure and energy
Producers, consumers and decomposers are linked by trophic levels. Energy transfer is inefficient, roughly 10% per step, so energy pyramids are steep and top predators are scarce; biomass pyramids can invert in aquatic systems where phytoplankton are consumed rapidly. Matter, unlike energy, cycles: carbon, nitrogen, phosphorus and water pass through reservoirs and fluxes. The carbon cycle involves photosynthesis, respiration, combustion and carbonate deposition; the nitrogen cycle requires fixation, nitrification and denitrification; phosphorus cycles slowly and mostly sedimentarily, so it often limits primary production.
Population dynamics
Exponential growth (dN/dt = rN) applies to unlimited resources; logistic growth (dN/dt = rN(K−N)/K) introduces the carrying capacity K. r-strategists produce many small offspring (insects, weeds); K-strategists invest in few, well-protected offspring (large mammals). Regulating factors are density-dependent (competition, disease, predation) or density-independent (fires, storms, drought).
Biodiversity and interactions
Interactions: competition, predation, mutualism, parasitism, herbivory. Diversity indices count richness and evenness; alpha diversity is local, beta between habitats, gamma across a region. Diversity is highest with habitat heterogeneity, intermediate disturbance and connectivity, and lowest on isolated, stressed or homogeneous sites.
Mediterranean ecosystems of Catalonia
Sclerophyllous forest (alzina, Quercus ilex), degraded variants (brolla, garriga), riverbank forests, wetlands such as the Aiguamolls de l'Empordà and the Ebro Delta, and the Montseny Biosphere Reserve with its beech–fir–heath altitudinal zonation. Invasive species (crayfish, tiger mosquito) alter food webs; fire regimes and fragmentation are the main structural threats.
Worked example
In a coastal lagoon, nitrate from farmland enters the water. Reasoning: nitrogen becomes the limiting nutrient, so it stops limiting phytoplankton; biomass increases, light is attenuated and submerged macrophytes decline; decomposition of dead algae consumes oxygen, BOD rises and fish mortality follows. Conclusion: eutrophication, driven by a diffuse agricultural source. The correct response is nutrient reduction at source, plus riparian buffers, not simply removing the visible algae.
Traps that cost marks
Saying energy cycles (it flows one way); confusing richness with evenness; describing K as a fixed number rather than environment-dependent; treating all disturbance as negative.