In a cornfield in Jalisco, the plants capture energy from the Sun throughout the rainy season. If you could measure how much of that energy reaches a hawk that hunts mice in that same field, you would be surprised by how little it is: for every 1,000 units of energy that enter the plants, only about one reaches the hawk. Why is so much energy lost along the way? That is the key to understanding how an ecosystem works.
Producers and primary productivity
Producers are photosynthetic organisms: plants, algae, and phytoplankton. They capture energy from sunlight and store it in organic molecules (sugars). The energy fixed per unit of area and time is called primary productivity. It is measured in kJ/m²·year, kcal/m²·year, or g/m²·year.
But producers also respire: just like you, they break down some of their sugars in cellular respiration to get usable energy. That is why two measures are distinguished.
Why does productivity change from one ecosystem to another?
Photosynthesis needs light, water, and CO₂, and works best at favorable temperatures. That is why productivity varies a great deal:
Ecosystem
Most limiting factor
Productivity
Lacandon Jungle
almost none (plenty of light, water, and warmth)
very high
Coral reef (Mexican Caribbean)
scarce nutrients, but plenty of light
high
Sonoran Desert
water
very low
Open ocean (far from the coast)
nutrients (nitrogen, phosphorus, iron)
low per m²
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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.
A food chain shows a single line of "who eats whom": corn → field mouse → rattlesnake → eagle. But almost no animal eats just one thing. Chains intertwine and form a food web, which describes the real ecosystem better. The arrows show the direction in which energy travels: from the food to the one that eats it.
Decomposers close the web: they turn the remains of animals, leaf litter, and droppings into nutrients that producers absorb again.
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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.
Not all the energy from one level reaches the next. The percentage that does pass on is called ecological transfer efficiency (or ecological efficiency):
Two transfers in a row: $0.10 \times 0.10 = 0.01$, that is, $1\%$ of the original energy.
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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.
If each transfer lets only a small fraction through, what shape does the available energy take across levels? If you stack one floor per trophic level, with the width proportional to the energy, you get a pyramid.
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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.
In a sample from the open ocean, $4\ \text{g/m}^3$ of phytoplankton and $12\ \text{g/m}^3$ of the zooplankton that feeds on it are recorded. How can such a small amount of producers support a mass of consumers three times larger?
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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.
Because the levels are connected, removing one affects the others. If overfishing eliminates the large predatory fish, their prey (smaller fish and herbivores) increase; with more herbivores, they eat more algae and the algae decrease. This chain effect is called a trophic cascade.
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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.
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.
Back to the beginning: the hawk receives only 1 out of every 1,000 units of energy from the plants because it is on the fourth level (corn → insect or mouse → snake or small bird → hawk). With the 10% rule at each transfer: $1{,}000 \times 0.10 \times 0.10 \times 0.10 = 1$ unit.
Summary
Let's practice! 3 medium exercises in this section