Energy flow
A field of barley in County Kildare captures solar energy throughout the summer. If you could measure how much of that energy ends up converted into the flesh of a barn owl hunting along the field margin, you would be surprised at how little there is: for every 1,000 units of energy that enter the plants, only one reaches the owl. Understanding why so much energy is lost along the way is the key to understanding how an ecosystem works.
Producers and primary production
Producers (plants, algae and phytoplankton) are autotrophs: they make their own food by photosynthesis, which you met in 5th Year. This short extension looks at how much of that energy is left for other organisms.
What limits NPP?
NPP depends on the abiotic factors that limit photosynthesis: light, water, temperature and mineral nutrients. In an Irish oak woodland, light reaching the forest floor and the length of the growing season set the limit. In the open ocean, light is plentiful at the surface but nitrates and phosphates are scarce, so phytoplankton cannot make full use of it.
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.
A holm oak forest and an open ocean area far from the coast have very different net primary productivities. Explain what net primary productivity is and explain the…
A holm oak forest and an open ocean area far from the coast have very different net primary productivities. Explain what net primary productivity is and explain the abiotic factors that limit it in each of these two ecosystems.
Trophic levels and ecological efficiency
Define trophic level and construct a four-link food chain typical of a Mediterranean holm oak forest, indicating which level each link belongs to and the type of feeding…
Define trophic level and construct a four-link food chain typical of a Mediterranean holm oak forest, indicating which level each link belongs to and the type of feeding of each organism.
Classify these organisms from a dehesa (a Mediterranean wooded pasture) into producers, consumers (indicating their trophic level) and decomposers: holm oak, rabbit,…
Classify these organisms from a dehesa (a Mediterranean wooded pasture) into producers, consumers (indicating their trophic level) and decomposers: holm oak, rabbit, fox, vulture, earthworm and mushroom. Justify each classification.
Distinguish between a food chain and a food web, and explain why ecosystems with highly interconnected webs withstand the disappearance of a species better than those…
Distinguish between a food chain and a food web, and explain why ecosystems with highly interconnected webs withstand the disappearance of a species better than those with simple, linear chains.
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.
Energy loss and the energy pyramid
The energy pyramid
Because of the losses above, the energy pyramid is never inverted: each tier is always smaller than the one below it.
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.
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.
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?
The biomass of a level is the total dry mass of living organisms in it. In most land ecosystems it falls at each level up the chain, because energy is lost at each transfer.
Comparing the two pyramids. A pyramid of numbers counts individuals; a pyramid of biomass measures their mass. Numbers ignore size: one oak and thousands of aphids each count by individual, so the shape can be inverted. Biomass takes size into account, so it usually gives a more realistic picture on land. Both have limitations: they show a single moment, so seasonal changes (for example, insects peaking in summer) alter the shape, and neither shows how fast the organisms reproduce.
Open flow of energy versus the cycling of matter
Decomposers, cascades and links between ecosystems
Decomposers (bacteria and fungi) release the energy in dead remains from every level as heat and return nutrients to the soil or water. Without them, nutrients would stay locked in dead matter, producers would run short and every level would decline.
A trophic cascade is a chain of effects triggered by changing one level. Removing a top predator lets its prey increase, and the prey then overgraze the level below. Adding a predator does the reverse.
Some animals move matter and energy between ecosystems. Seabirds feed at sea and deposit guano on land, so they act as vectors carrying nutrients from marine to terrestrial systems.
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).
Seabird colonies accumulate guano on the islands where they breed. Explain what flow of matter and energy this guano represents between the marine and the terrestrial…
Seabird colonies accumulate guano on the islands where they breed. Explain what flow of matter and energy this guano represents between the marine and the terrestrial ecosystems, and what role the birds play as "vectors" connecting both systems.
Overfishing eliminates large predatory fish from a coastal area. Predict what will happen to the immediately lower trophic levels and explain how this 'trophic cascade'…
Overfishing eliminates large predatory fish from a coastal area. Predict what will happen to the immediately lower trophic levels and explain how this 'trophic cascade' can end up affecting even the seabed algae.
These annual flows are measured in a lake: producers, $8\,000\ \text{kJ/m}^2$; herbivores, $900\ \text{kJ/m}^2$; carnivores, $110\ \text{kJ/m}^2$; in addition,…
These annual flows are measured in a lake: producers, $8\,000\ \text{kJ/m}^2$; herbivores, $900\ \text{kJ/m}^2$; carnivores, $110\ \text{kJ/m}^2$; in addition, decomposers receive $3\,000\ \text{kJ/m}^2$ of remains from all levels. a) Calculate the efficiency of the two transfers between levels. b) Explain what role decomposers play in the lake's energy balance. c) Predict what would happen to the nutrient cycle and to the trophic levels if decomposers disappeared.
Trout are introduced into a high-mountain lake that had no fish and hosted abundant amphibians and large planktonic crustaceans. a) Predict what will happen to the…
Trout are introduced into a high-mountain lake that had no fish and hosted abundant amphibians and large planktonic crustaceans. a) Predict what will happen to the amphibian and zooplankton populations. b) Explain how the change in zooplankton can affect the phytoplankton and the clarity of the water (a trophic cascade). c) Argue what should be done from an environmental management point of view and why these recreational introductions were common practice for decades.
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.
The phrase "eat from low trophic levels" is used as environmental advice. Analyse it scientifically: what basis it has in terms of energy flow, what nuances should be…
The phrase "eat from low trophic levels" is used as environmental advice. Analyse it scientifically: what basis it has in terms of energy flow, what nuances should be added (fishing for low-level but overexploited species, livestock grazing on dehesas unsuitable for cultivation) and what other factors, besides trophic level, determine the impact of a food.
Back to the beginning: The barn owl receives only 1 in every 1,000 units of energy captured by the barley because it sits at the fourth trophic level (barley $\to$ insect $\to$ shrew $\to$ barn owl, a chain in which the shrew eats insects): applying the 10% rule at each successive transfer, $1{,}000 \times 0.10 \times 0.10 \times 0.10 = 1$ unit.