IB ESS SL cheat sheet

System types: open systems exchange both matter and energy across boundaries; closed systems exchange energy only; isolated systems exchange neither (a theoretical ideal).

Energy flows through ecosystems as a one-way throughput while matter is recycled; this distinction underpins all nutrient-cycling models.

First law of thermodynamics: energy is neither created nor destroyed, only converted between forms; second law: each conversion increases entropy (disorder) and reduces usable energy.

Positive feedback amplifies a change away from equilibrium (e.g., permafrost thaw releasing methane); negative feedback counteracts change and stabilises (e.g., predator-prey regulation).

Resilience is the capacity of a system to return to its previous state after disturbance; tipping points mark thresholds beyond which a new stable state emerges.

Models are simplified representations used to predict system behaviour; their validity depends on assumptions, and they carry inherent uncertainty.

Sustainability is meeting present needs without compromising those of future generations; it integrates environmental, social and economic dimensions (the triple bottom line).

The precautionary principle justifies action to prevent serious or irreversible harm even where scientific certainty is incomplete.

Pollution is classified as point-source (single identifiable origin) or non-point-source (diffuse), and as primary (directly emitted) or secondary (formed in the environment, e.g., tropospheric ozone).

Trophic levels: producers (autotrophs) → primary consumers → secondary consumers → tertiary consumers; decomposers and detritivores act at every level.

Energy transfer between trophic levels is typically only about 10% efficient (range roughly 1–20%), so food chains rarely exceed four or five levels.

Productivity terms: gross primary productivity (GPP) is total energy fixed by photosynthesis; net primary productivity (NPP) = GPP − respiration (R).

Biomass pyramids, energy pyramids and numbers pyramids differ; numbers pyramids can be inverted (e.g., one tree supporting many insects).

Photosynthesis: 6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂; respiration is the reverse and releases energy for metabolism.

The carbon cycle exchanges carbon among atmosphere, biosphere, oceans and lithosphere via photosynthesis, respiration, combustion, decomposition and fossilisation.

The nitrogen cycle depends on fixation (Rhizobium, lightning, Haber-Bosch), nitrification, assimilation, ammonification and denitrification; phosphorus has no gaseous phase and cycles via weathering and runoff.

Ecological succession: primary succession begins on bare substrate with pioneer species (lichens, mosses); secondary succession follows disturbance where soil persists.

Niche concepts: fundamental niche is the full potential range of conditions; realised niche is the narrower portion actually occupied due to competition and predation.

Biodiversity has three components: genetic diversity, species diversity and ecosystem/habitat diversity.

Species diversity is described by richness (number of species) and evenness (relative abundance); Simpson's reciprocal index D = 1/Σ(n/N)² rises with both.

Bottleneck and founder effects reduce genetic diversity in small populations; inbreeding depression lowers fitness.

Threats ranked by IUCN Red List categories: Least Concern, Near Threatened, Vulnerable, Endangered, Critically Endangered, Extinct in the Wild, Extinct.

In-situ conservation protects species within their habitat (reserves, national parks); ex-situ conservation maintains them outside it (zoos, seed banks, botanic gardens).

Edge effects reduce effective habitat area: fragments have proportionally more edge, altering light, wind, humidity and predation pressure.

Keystone species exert disproportionate influence relative to abundance (e.g., sea otter, fig trees); flagship species are charismatic and used to mobilise support; indicator species signal environmental quality.

Habitat corridors connect fragments, enabling gene flow and migration, but can also spread disease and invasive species.

International agreements include CITES (trade in endangered species) and the Convention on Biological Diversity (CBD); protected-area design favours large, round, connected reserves.

Water distribution: about 97% of Earth's water is saline; of freshwater, most is locked in ice and groundwater, with a small fraction in surface and atmospheric stores.

The hydrological cycle fluxes are evaporation, transpiration, condensation, precipitation, infiltration, percolation, runoff and groundwater flow; deforestation and urbanisation increase runoff and reduce infiltration.

Aquifers are recharged slowly; abstraction exceeding recharge causes water-table decline, land subsidence and saltwater intrusion in coastal zones.

Water scarcity is physical (insufficient supply) or economic (lack of infrastructure/funds); water stress is measured per capita availability, often using the Falkenmark thresholds.

Eutrophication sequence: nutrient enrichment (N, P) → algal bloom → shading and death of submerged plants → decomposition depletes dissolved oxygen → hypoxia and fish kills.

Dissolved oxygen is temperature-dependent; warmer water holds less oxygen, and biochemical oxygen demand (BOD) indicates organic pollution load.

Aquaculture produces a rising share of fish for human consumption; concerns include eutrophication from feed and waste, disease transfer to wild stocks and escapees.

Maximum sustainable yield (MSY) is the largest catch that can be taken indefinitely; it roughly corresponds to half the carrying capacity, but real stocks are affected by environmental variability.

Fishery collapse indicators include declining catch per unit effort and shrinking mean size/age of landed fish; management tools include quotas, closed seasons and marine protected areas.

Soil texture is determined by proportions of sand, silt and clay; the soil texture triangle classifies them, and loam (balanced mixture) is generally most fertile.

Soil profile horizons: O (organic litter), A (topsoil, humus-rich), E (elution/leaching), B (illuviation/accumulation), C (weathered parent material), R (bedrock).

Soil formation depends on parent material, climate, organisms, topography and time (the CLORPT factors).

Leaching removes soluble nutrients downward; laterisation (tropical) concentrates iron and aluminium oxides, leaving nutrient-poor soils when cleared.

Soil degradation processes: erosion by wind and water, salinisation from irrigation in arid areas, compaction by machinery, and loss of organic matter.

Food production systems are classified as subsistence, commercial, intensive or extensive; intensive systems raise yields per area but increase inputs and externalities.

The green revolution increased yields via high-yielding varieties, irrigation, fertilisers and pesticides, with costs including water depletion, salinisation and reduced agro-biodiversity.

Terrestrial food production accounts for a large share of land use and greenhouse-gas emissions (methane from ruminants and paddy rice; nitrous oxide from fertiliser).

Food security depends on availability, access, utilisation and stability; food waste occurs at harvest, post-harvest, retail and consumption stages.

Atmospheric layers by altitude: troposphere (weather, temperature decreases with height), stratosphere (ozone layer, temperature rises with height), mesosphere, thermosphere.

The greenhouse effect is natural: greenhouse gases absorb outgoing long-wave radiation and re-emit it, warming the surface; without it Earth's mean surface temperature would be far lower.

Major greenhouse gases by contribution to anthropogenic forcing: CO₂, methane (CH₄), nitrous oxide (N₂O) and halocarbons; methane has a much higher global warming potential than CO₂ per molecule.

Photochemical smog forms when nitrogen oxides and volatile organic compounds react in sunlight, producing tropospheric ozone; temperature inversions trap pollutants near the ground.

Stratospheric ozone is destroyed by chlorine and bromine radicals from CFCs and halons; the Montreal Protocol phased out these substances.

Acid deposition results from SO₂ and NOₓ emissions forming sulfuric and nitric acids; it acidifies soils and lakes and damages forests and buildings.

Particulate matter (PM₁₀, PM₂.₅) affects health; indoor air pollution from biomass burning is a major cause of respiratory disease in low-income regions.

El Niño–Southern Oscillation alters global precipitation patterns: El Niño brings drought to Australia and Indonesia and wetter conditions to parts of the Americas.

Radiative forcing is the change in energy balance (W/m²) caused by a factor; positive forcing warms, negative forcing cools.

Climate feedbacks: ice-albedo feedback is positive (melting ice exposes darker surfaces, absorbing more radiation); increased cloud cover and carbon uptake by oceans are partly negative.

Mitigation reduces causes (emissions cuts, carbon sinks); adaptation reduces vulnerability to impacts (sea defences, drought-resistant crops, early warning systems).

Energy sources: renewable (solar, wind, hydro, tidal, geothermal, biomass) versus non-renewable (coal, oil, gas, nuclear fuel); the distinction is about replenishment rate, not cleanliness.

Energy security involves availability, affordability and reliability of supply; energy density and intermittency are key comparison criteria for sources.

Carbon footprint is total greenhouse-gas emissions expressed as CO₂ equivalent, calculated over a defined life cycle or activity boundary.

Nuclear power has low operational CO₂ emissions but raises issues of waste disposal, accident risk and decommissioning cost.

International climate governance: UNFCCC (1992), Kyoto Protocol (1997, binding targets for developed countries) and the Paris Agreement (2015, nationally determined contributions aiming to hold warming well below 2 °C).

Carbon trading and carbon taxes are market-based instruments that put a price on emissions; cap-and-trade sets an absolute emissions limit.

Human population growth is described by the demographic transition model: high birth and death rates → falling death rates → falling birth rates → low, stable rates.

Population momentum means that even after fertility falls to replacement level, population continues to grow because of a large cohort of young people.

Age–sex population pyramids indicate growth (broad base), stability (straight sides) or decline (narrow base).

The IPAT equation: Impact = Population × Affluence × Technology, used to decompose drivers of environmental pressure.

Ecological footprint measures the area of biologically productive land and water needed to supply a population's resources and absorb its wastes, expressed in global hectares.

Carrying capacity is the maximum population an environment can support sustainably; it can be extended by technology or degraded by resource depletion.

The Kuznets curve hypothesis suggests environmental degradation rises then falls with income per capita, but evidence is mixed and depends on pollutant type.

Resource classification: renewable versus non-renewable, and recyclable versus non-recyclable; sustainable yield means harvest rate does not exceed regeneration rate.

The waste hierarchy ranks prevention, reuse, recycling, recovery (energy) and disposal; circular economy designs out waste by keeping materials in use.