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.

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

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

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

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

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

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

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

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

As new evidence and knowledge appear (and values may change), assumptions can become outdated, so models must be revised to stay useful.

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).

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

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 system is interacting parts forming a whole.

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

Emergent properties: new characteristics arise from interactions between parts.

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

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.

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

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

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

Connections, interactions, and boundaries.

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

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

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

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

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.

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

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.

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.

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

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

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

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.

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.

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

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

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

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

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.

1 storage, 2 inflows, 3 outflows, 4 whether it is in equilibrium or changing.

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

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.

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

Movement of matter or energy without changing its form.

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

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

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

Negative feedback reduces change and helps stabilise a system.

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

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

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

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

It stabilises systems by reducing change and helping maintain equilibrium.

A negative relationship: the variables change in opposite directions.

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

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

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

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

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

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

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

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

It amplifies change and can push systems towards tipping points.

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

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

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

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

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

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

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

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

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

Ability to recover from disturbance and keep functioning over time.

Excess nitrates and phosphates from agriculture runoff or sewage discharge.

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

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.

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

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

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

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

High biodiversity and large/multiple storages (buffers).

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

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

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

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

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

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

Crossing tipping points and shifting to a new equilibrium.

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

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.

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

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

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.

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

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

Actions that increase diversity and storages usually increase resilience.

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