Exercises: Energy flow

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.

Explain, using the concept of ecological efficiency, why ecosystem food chains rarely have more than four or five levels.

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.

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.

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.

In an ecosystem, producers fix $15\,000\ \text{kJ/m}^2$ per year, herbivores assimilate $1\,500\ \text{kJ/m}^2$ and carnivores $120\ \text{kJ/m}^2$ over the same period. Calculate the ecological efficiency of each transfer and comment on whether they fit the general rule.

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.

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.

Look at the energy pyramid in the diagram:

1. a) Identify which trophic level each tier (A, B, C, and D) occupies.
2. b) Calculate the missing value at level D ($?$) assuming that all transfers have the same efficiency as the first one.

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.

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).

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.

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.

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.

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.

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.

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 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 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 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.