Systems

A model is a simplified representation of reality.

Diagram models, mathematical models, physical models, computer models, and written models.

A model is a simplified representation of reality used to understand, explain, or predict how a system works.

Real environmental systems are too complex to study in full. Models help us focus on the most important features so we can test ideas and make predictions.

Simplification is focusing on the important features of a system while leaving out less relevant details.

A food web showing feeding relationships in an ecosystem (e.g., coral reef or pond).

Simpler models are easier to understand and use, but they are usually less accurate and can miss important details.

A trade-off is a balance between competing factors: simpler models are easier to use, but may be less accurate.

An equation predicting population growth, such as N = N0 e^(rt), used to model how population size changes over time.

Examples include diagram models and computer models (also mathematical, physical, and written).

Models rely on assumptions and may miss important information. Results depend on data quality, so predictions can be inaccurate.

A food chain is a model that shows feeding relationships and energy transfer between organisms in an ecosystem.

Always include simplification and purpose: a model simplifies reality to understand, explain, or predict a system.

Name the model and state what it shows (e.g., food chain shows feeding relationships).

As new evidence and knowledge appear (and values may change), assumptions can become outdated, so models must be revised to stay useful.

The boundary decides what is included and excluded. If it is too small, important influences are missed; if it is too large, the system becomes too complex to analyse.

A method of studying how parts of a system are connected and interact, rather than examining parts in isolation.

A system is interacting parts forming a whole.

A system is a group of interacting parts that form a whole, with components, connections, a function, and emergent properties.

Emergent properties: new characteristics arise from interactions between parts.

Studying a lake’s water quality without including upstream farmland can miss fertiliser runoff as the cause of eutrophication.

Characteristics that appear only when parts of a system interact, not in the parts on their own.

The system includes too many variables and interactions, making it hard to identify key drivers or explain cause and effect clearly.

An imaginary line that defines what is included in the system and what is outside it.

Predator-prey cycles: population patterns emerge only when predator and prey interact.

State what you included and excluded, and explain why that boundary is useful for answering the question (focuses on the key influences).

Boundaries affect what factors you include, so they change how you understand the problem and what conclusions you reach.

Ask: Does it include the key inputs, outputs, and interactions that control the system behaviour for this question?

Connections, interactions, and boundaries.

Small scale (e.g., pond), medium scale (e.g., rainforest), large scale (e.g., Earth system).

A closed system exchanges energy with its surroundings but does not exchange matter (matter stays inside and is recycled).

An open system exchanges both matter and energy with its surroundings across the system boundary.

Both matter and energy cross the system boundary (enter and leave).

A pond is an open system: sunlight and rain enter, while water (evaporation/runoff) and organisms/heat can leave.

Energy enters as sunlight and leaves as heat, but almost no matter enters or leaves Earth, so matter is recycled within the system.

Energy crosses the system boundary, but matter stays inside and is recycled.

Earth (at the global scale) is the classic closed system example because matter is retained but energy is exchanged.

Matter is physical stuff with mass (water, nutrients, organisms). Energy is not physical stuff (sunlight, heat) that drives change.

Open: a pond (matter + energy exchange). Closed: Earth (energy exchange, matter retained).

Because you can clearly identify inputs (sunlight, rain, nutrients) and outputs (evaporation, runoff, organisms leaving), showing matter and energy exchange.

They are closed systems at the global scale because the matter (atoms) is recycled within Earth, while energy enters and leaves.

Input: sunlight or rainfall or nutrients. Output: heat loss, oxygen release, runoff water, or organisms leaving.

Open: exchanges matter and energy. Closed: exchanges energy but not matter. Always state what enters and what leaves.

State what crosses the boundary: energy crosses (sunlight in, heat out) but matter does not cross (it stays and is recycled).

Because inputs and outputs happen continuously, so storages and conditions can change over time.

A storage is a place where matter, energy, or information builds up over time (e.g., water in a reservoir, CO2 in the atmosphere).

Boxes represent storages (stocks) where matter, energy, or information accumulates over time.

An input is the thing that moves (e.g., water). An inflow is the process moving it into a storage (e.g., rainfall).

“Rainfall is an inflow.” The water is the input; rainfall is the flow process.

A flow is the movement of matter, energy, or information into or out of a storage, changing the amount stored.

Arrows represent flows moving matter, energy, or information into or out of storages.

Dynamic equilibrium occurs when inflows equal outflows, so the storage stays constant even though flows continue.

An inflow is a flow that enters a storage and increases the amount stored (e.g., rainfall filling a reservoir).

Dynamic equilibrium occurs when inflows equal outflows, keeping the storage constant.

The storage increases because more enters than leaves (e.g., reservoir fills when rainfall exceeds evaporation).

An outflow is a flow that leaves a storage and decreases the amount stored (e.g., dam release reducing reservoir water).

A buffer is a storage that absorbs sudden changes in flows, slowing system response and creating time delays.

A system boundary is an imaginary line separating the system from its surroundings; choosing it affects what inputs/outputs are included and how useful the model is.

Storages are shown as boxes and flows are shown as arrows; thicker arrows often represent larger flows.

A positive feedback loop where tourism growth generates more income and investment, attracting even more tourism.

Wealth enables investment and influence, producing more wealth, widening the gap unless interrupted.

A condition where inputs and outputs are balanced so the system stays roughly the same over time.

A diagram showing cause-and-effect links between variables, forming feedback loops over time.

A chain where a change causes effects that feed back to influence the original change.

Movement of matter or energy without changing its form.

Negative feedback reduces change and helps stabilise a system.

A mature forest: growth and death balance so overall biomass stays similar.

A positive relationship: the variables change in the same direction.

Start change → chain of effects → show the loop closes → state if reinforcing or balancing.

Creates jobs and income, and can fund infrastructure or conservation.

A change in form, state, or chemical nature of matter or energy.

Body temperature control: too hot → sweating → cooling → back to normal.

It stabilises systems by reducing change and helping maintain equilibrium.

Positive feedback amplifies change; negative feedback counteracts change and stabilises the system.

A time gap between a change and when its effects are seen in the system.

Higher water/energy demand, more waste/pollution, and habitat loss from development.

A negative relationship: the variables change in opposite directions.

Because the output (tourism income/infrastructure) feeds back to increase the input (tourist attraction).

People or processes overcorrect because the system responds slowly, leading to repeated over- and under-shooting.

It amplifies change and can push systems towards tipping points.

Eutrophication: more nutrients → more algae → plant death/decomposition → more available nutrients.

Reinforcing loops amplify change; balancing loops resist change and stabilise the system.

Positive feedback amplifies the original change and pushes the system further from balance.

Use limits such as visitor caps, zoning, pricing/taxes, and protected areas to reduce growth pressure.

Name variables, follow arrows, explain +/− links, and state whether the loop is reinforcing or balancing.

Ice-albedo: ice melts → darker surface → more heat absorbed → more melting.

Predator–prey: prey increases → predators increase → prey decreases → predators decrease.

A threshold where a small change triggers a large, often hard-to-reverse shift to a new equilibrium.

Crossing a tipping point can shift a system into a new equilibrium that may be difficult to reverse.

It reduces biodiversity and biomass storage, weakening buffers and increasing tipping point risk.

Ability to recover from disturbance and keep functioning over time.

Strong negative feedback loops usually support resilience because they counteract change.

Excess nitrates and phosphates from agriculture runoff or sewage discharge.

Resilience is a system’s ability to absorb disturbance and keep functioning (or recover) without collapsing.

Loss of biodiversity, repeated disturbances, removal of storages, or strong human pressures (pollution/deforestation).

More species/roles create redundancy; if one fails, others can replace its function.

It reduces biodiversity and functional redundancy, making ecosystems less able to recover from disturbance.

High biodiversity and large/multiple storages (buffers).

A sudden event that disrupts a system (e.g., fire, flood, disease, pollution).

Rapid growth of algae due to high nutrient levels, often turning water green and reducing light.

Strong positive feedback amplifies change and can reduce resilience by pushing systems toward tipping points.

Loss of diversity, shrinking storages, and strong human pressures (pollution/deforestation/overuse).

They can change in the short term after disturbance but remain stable in the long term.

Decomposition of dead algae/plants uses dissolved oxygen, causing hypoxia and fish kills.

Protect habitats, restore mixed native species, improve soil management, or restore wetlands.

Clear lake + nutrient input → algal bloom → murky, low-oxygen lake state.

Large/multiple storages buffer change and slow system response, reducing collapse risk.

The system settles into a new equilibrium, often difficult to reverse.

Crossing tipping points and shifting to a new equilibrium.

Soil nutrients, forest biomass, water in lakes/reservoirs, or carbon in vegetation.

Nutrients stored in sediments can keep feeding algal growth even after inputs are reduced.

Feedback delays mean impacts appear later, so humans may respond only when collapse is near.

A diverse forest that can regrow after fire and continue functioning.

With weaker buffers and fewer stabilising processes, disturbances push the system past thresholds more easily.

Yes: more nutrients → more algae → more death/decomposition → conditions that can release/retain nutrients, driving more algae.

The system is more likely to cross a tipping point and shift to a new equilibrium.

Actions that increase diversity and storages usually increase resilience.

Reduce pressures, protect diversity, and strengthen storages/buffers to support stabilising feedback.

Human actions can raise or lower resilience by changing biodiversity and storages, affecting tipping point risk.