A pond as a system
Nineteen components of a pond, each one measured. Work out what each one is: a storage inside the pond, a flow crossing the boundary in or out, or a flow between storages within it. The diagram draws itself as you go, and it draws to scale, so the sizes are the numbers rather than your judgement of them.
Flows within the pond
- 1Photosynthesis 800 kg/year
- 2Herbivores eating plants and algae 200 kg/year
- 3Carnivores eating herbivores 50 kg/year
- 4Death and sedimentation 600 kg/year
- 5Decomposition releasing nutrients 550 kg/year
Box area is directly proportional to mass. Arrow width uses the square root of the flow rate, so that the 2 kg/year arrow is still visible next to the 800 kg/year one: that compromise is a limitation of the model (ESS 1.2.13), which is why every arrow also carries its figure.
Components
19 leftAquatic plants500 kg
Algae40 kg
Herbivores (snails, tadpoles, small fish)15 kg
Carnivores (large fish, dragonfly larvae)5 kg
Decomposers (bacteria and fungi)100 kg
Sediment (dead organic matter at the bottom)2,000 kg
Solar energyunlimited
Falling leaves300 kg/year
Rain with nutrients50 kg/year
Bird droppings10 kg/year
Heat from respirationnot quantified
Overflow water with nutrients40 kg/year
Adult insects flying away8 kg/year
Birds catching fish2 kg/year
Photosynthesis800 kg/year
Herbivores eating plants and algae200 kg/year
Carnivores eating herbivores50 kg/year
Death and sedimentation600 kg/year
Decomposition releasing nutrients550 kg/year
Storages
0/6Inside the pond, drawn as boxes
Inputs
0/4Cross the boundary going in
Outputs
0/4Cross the boundary going out
Flows within
0/5Between storages, inside the boundary