Energy flow
A field of sunflowers captures solar energy throughout the summer. If you could measure how much of that energy ends up converted into the flesh of a hawk that hunts in that field, you would be surprised at how little there is: for every 1000 units of energy that enter the plants, only one reaches the hawk. Understanding why so much energy is lost along the way is the key to understanding how an ecosystem works.
Producers and primary production
Two identical plots of maize capture the same solar radiation, but the one that grows through warm nights accumulates $15\ \%$ less mass by the end of the summer because its cells respire at a faster rate. How do we measure the difference between all the chemical energy that a plant synthesises and the energy that is actually left available as new plant tissue?
Define gross primary production and net primary production, and explain what part of the energy fixed by producers is actually available to herbivores and what part is…
Define gross primary production and net primary production, and explain what part of the energy fixed by producers is actually available to herbivores and what part is used by the plants themselves.
Grasslands fix $12\,000\ \text{kJ/m}^2$ per year as gross primary production, but the plants use $60\ \%$ of it in their own respiration. Calculate the net primary…
Grasslands fix $12\,000\ \text{kJ/m}^2$ per year as gross primary production, but the plants use $60\ \%$ of it in their own respiration. Calculate the net primary production available to the rest of the ecosystem and explain what the respired fraction is used for.
Trophic levels and ecological efficiency
In a meadow, producers fix $20\,000\ \text{kcal/m}^2$ per year and herbivores assimilate $2\,000\ \text{kcal/m}^2$ over the same period. Calculate the ecological…
In a meadow, producers fix $20\,000\ \text{kcal/m}^2$ per year and herbivores assimilate $2\,000\ \text{kcal/m}^2$ over the same period. Calculate the ecological efficiency of this transfer, expressed as a percentage.
An aquatic ecosystem shows these annual energy values per level: producers, $50\,000\ \text{kJ}$; herbivores, $4\,800\ \text{kJ}$; carnivores, $450\ \text{kJ}$.…
An aquatic ecosystem shows these annual energy values per level: producers, $50\,000\ \text{kJ}$; herbivores, $4\,800\ \text{kJ}$; carnivores, $450\ \text{kJ}$. Calculate the efficiency of each transfer and comment on whether they comply with the $10\ \%$ rule.
The following annual data are known for a steppe ecosystem: producers, $10\,000\ \text{kJ/m}^2$; herbivores, $1\,200\ \text{kJ/m}^2$; carnivores, $60\ \text{kJ/m}^2$.…
The following annual data are known for a steppe ecosystem: producers, $10\,000\ \text{kJ/m}^2$; herbivores, $1\,200\ \text{kJ/m}^2$; carnivores, $60\ \text{kJ/m}^2$. Calculate the efficiency of each transfer and explain why the second is much lower, considering the digestibility of the diet and the carnivores' expenditure on activity.
The energy pyramid
Explain, using the concept of ecological efficiency, why ecosystem food chains rarely have more than four or five levels.
Explain, using the concept of ecological efficiency, why ecosystem food chains rarely have more than four or five levels.
Look at the energy pyramid in the diagram: a) Identify which trophic level each tier (A, B, C, and D) occupies. b) Calculate the missing value at level D ($?$) assuming…
Look at the energy pyramid in the diagram:
- a) Identify which trophic level each tier (A, B, C, and D) occupies.
- b) Calculate the missing value at level D ($?$) assuming that all transfers have the same efficiency as the first one.
Pyramids of biomass and of numbers
In a sample of open-sea water, $4\ \text{g/m}^3$ of single-celled phytoplankton and $12\ \text{g/m}^3$ of herbivorous zooplankton that feeds directly on it are recorded. How can such a small quantity of producers support, at any given moment, a mass of consumers three times greater without becoming extinct?
Differentiate between ecological pyramids of energy, biomass, and numbers. Explain in which situation a biomass pyramid can appear inverted (give a marine example) and…
Differentiate between ecological pyramids of energy, biomass, and numbers. Explain in which situation a biomass pyramid can appear inverted (give a marine example) and why an energy pyramid is never inverted.
In a holm oak forest, a single holm oak can feed thousands of caterpillars, and these in turn dozens of tits. Describe in words the resulting pyramid of numbers, explain…
In a holm oak forest, a single holm oak can feed thousands of caterpillars, and these in turn dozens of tits. Describe in words the resulting pyramid of numbers, explain why it appears "inverted" at its base, and discuss whether the biomass pyramid of that same holm oak forest would also be inverted.
You can do it on paper and send a photo with your answer.
Compare the biomass pyramid of a grassland with that of an oceanic phytoplankton zone: a) explain why the oceanic pyramid can be inverted at a given instant whereas the…
Compare the biomass pyramid of a grassland with that of an oceanic phytoplankton zone: a) explain why the oceanic pyramid can be inverted at a given instant whereas the grassland one is not; b) discuss which property of phytoplankton (its enormous reproduction rate) makes this possible; c) explain why the energy pyramid of that same ocean, in contrast, is always upright.
Open flow of energy versus the cycling of matter
Explain, using the concepts of useful energy and dissipated heat, why energy passes through the ecosystem in one direction only (an open flow) whereas matter can…
Explain, using the concepts of useful energy and dissipated heat, why energy passes through the ecosystem in one direction only (an open flow) whereas matter can circulate over and over again (a closed cycle).
Assuming an ecological efficiency of $10\ \%$ in each transfer, calculate how many kilograms of producers are needed to generate $1\ \text{kg}$ of biomass of a predator…
Assuming an ecological efficiency of $10\ \%$ in each transfer, calculate how many kilograms of producers are needed to generate $1\ \text{kg}$ of biomass of a predator situated at the fourth trophic level of a chain. Comment on what the result implies for the conservation of large predators.
A crop field fixes $20\,000\ \text{kJ/m}^2$ per year. a) Assuming an efficiency of $10\ \%$ per transfer, calculate how many $\text{kJ}$ remain as chicken meat after one…
A crop field fixes $20\,000\ \text{kJ/m}^2$ per year. a) Assuming an efficiency of $10\ \%$ per transfer, calculate how many $\text{kJ}$ remain as chicken meat after one transfer (from crop to chicken) and what area is needed to obtain $2\,000\ \text{kJ}$ of that meat. b) Calculate the area needed to obtain those same $2\,000\ \text{kJ}$ by eating the crop directly. c) Comment on what the comparison implies for feeding the world population.
Back to the beginning: The hawk receives only $1$ in every $1000$ units of energy captured by the sunflowers because it sits at the fourth trophic level (sunflower $\to$ insect or rodent $\to$ small bird $\to$ hawk): applying the $10\ \%$ rule at each successive transfer, $1000 \cdot 0{,}10 \cdot 0{,}10 \cdot 0{,}10 = 1$ unit.