Systems practice questions.

Which observation about an ecosystem is a direct consequence of the second law of thermodynamics?

• Matter is recycled within the ecosystem boundary
• Producers convert light into chemical energy
• Rainfall is stored in soil before being taken up
• Energy is lost as heat at every transfer

Answer: Energy is lost as heat at every transfer

The second law states that energy transfers are never fully efficient, so some energy is always degraded to low-grade heat and the system needs a continuous input from the Sun. Recycling of matter follows from the Earth behaving as a closed system, which is a separate idea.

Warming thaws permafrost, which releases methane, which increases warming and thaws more permafrost. This loop is best described as

• negative feedback, because it stabilises the system
• positive feedback, because it amplifies the change
• negative feedback, because methane is harmful
• not feedback at all, because two different storages are involved

Answer: positive feedback, because it amplifies the change

Positive feedback drives the system further in the direction of the original change; negative feedback would counteract it. Negative does not mean harmful and positive does not mean beneficial, which is the misconception this loop is usually used to expose.

A computer model of a lake predicts a smaller algal bloom than the one actually measured. Which is the most likely reason for the difference?

• Models can only be applied to systems that are already in equilibrium
• The measured data must contain a systematic error
• The model simplifies the lake by omitting some flows
• Models cannot represent storages, only flows

Answer: The model simplifies the lake by omitting some flows

Every model is a simplification, so a flow left out, such as nutrient inflow in groundwater, shows up as a gap between prediction and measurement. The mismatch is normally a reason to refine the model rather than evidence that the field data are wrong.

A sealed glass terrarium standing on a sunlit windowsill exchanges no matter with the room, but sunlight passes into it and heat passes out. In systems terms the terrarium is

• an open system
• a closed system
• an isolated system
• a static equilibrium

Answer: a closed system

A closed system exchanges energy but not matter across its boundary, which is exactly the terrarium and, on a far larger scale, the Earth. An isolated system exchanges neither, and no real ecosystem is one.

Two grassland plots burn in the same fire. Plot A holds twelve plant species and deep soil; Plot B holds three species and thin soil. Plot A regains its former cover within two growing seasons, while Plot B does not. Plot A is best described as having greater

• carrying capacity for grazing livestock
• resilience to this kind of disturbance
• net primary productivity before the fire
• biomass held in its standing vegetation

Answer: resilience to this kind of disturbance

Resilience is the capacity of a system to absorb a disturbance and return towards its former state, and it rises with diversity and with the size of the storages the system can draw on, such as soil and seed banks. Productivity and biomass describe how much the system holds or makes, not how well it recovers.

In a model of a forest carbon system, photosynthesis, litter fall and decomposition are all held constant while respiration by the trees falls. Which storage increases?

• The soil organic carbon storage, because less carbon is respired from the soil surface
• The carbon storage in tree biomass
• The atmospheric carbon storage, because carbon released by the trees is no longer taken back up
• No storage changes, because respiration is a transformation and transformations do not alter storages

Answer: The carbon storage in tree biomass

A storage grows whenever its inputs exceed its outputs. Respiration is an output from the biomass storage, so cutting it while photosynthesis is unchanged leaves more carbon in the trees, and the atmospheric storage falls rather than rises.