Models, Systems and Loops

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

A model is a simplified representation of reality.

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

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).

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.

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

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.

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As new evidence and knowledge appear (and values may change), assumptions can become outdated, so models must be revised to stay useful.

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

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

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.

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.

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

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.

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

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.

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

Connections, interactions, and boundaries.

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

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

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

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

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

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

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

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

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).

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

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

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

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

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

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.

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

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

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

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

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

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

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, so the storage stays constant even though flows continue.

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

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).

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.