Food webs and energy flow: flashcards, quiz and summary

Flashcards

1. 1Why does energy flow one way through an ecosystem while matter cycles?

   Energy enters as sunlight, passes through organisms as food, and is lost as heat at each step, so it cannot be reused. Matter such as carbon and nitrogen is passed between living things and the environment and is reused.

2. 2What roles do producers, consumers and decomposers play in an ecosystem?

   Producers make their own food, mostly by photosynthesis. Consumers eat other organisms, and herbivores, carnivores and omnivores differ by what they eat. Decomposers, mainly bacteria and fungi, break down dead matter and wastes and return nutrients to the soil and water.

3. 3How does the 10 percent rule explain why food chains are short?

   Only about 10 percent of the energy at one trophic level is stored in the next. The rest is used for respiration and activity, lost as heat, or never eaten. Each higher level therefore has far less energy available, so there is little energy left to support many levels or many top predators.

4. 4How is a food web different from a food chain, and why does it add stability?

   A food chain is one path of energy. A food web links many chains because most species eat and are eaten by several others. With more connections, consumers can switch to other food sources if one food source drops, which makes the ecosystem more stable.

5. 5What are the five steps of the nitrogen cycle, and what does each one do?

   Nitrogen fixation turns N2 into ammonia or ammonium. Nitrification turns ammonium into nitrite and then nitrate. Assimilation is when plants take up nitrate or ammonium to build proteins and DNA. Ammonification turns wastes and dead bodies back into ammonium. Denitrification turns nitrate back into N2 gas in low-oxygen soil.

6. 6How do photosynthesis, respiration and burning fuels connect in the carbon cycle?

   Photosynthesis removes CO2 from the air and builds it into sugars. Cellular respiration by all organisms, including decomposers, returns CO2 to the air. Burning fossil fuels and forests releases carbon that was stored for a long time, adding CO2 faster than the normal cycle removes it.

7. 7Why is the energy pyramid always upright, while the biomass and numbers pyramids can be inverted?

   Each level receives and stores less energy than the level below it, so the energy pyramid must narrow upward. Biomass depends on how fast organisms reproduce, and numbers depend on how many small organisms one large organism can support, so those pyramids can invert.

8. 8What makes a keystone species different from other species in an ecosystem?

   A keystone species has an effect on its ecosystem that is far larger than its population size suggests. Removing it changes the whole ecosystem, as when sea otters are removed and urchins overgraze kelp forests.

9. 9What is carrying capacity, and how does it shape population growth?

   Carrying capacity (K) is the largest population an environment can support over the long term. With unlimited resources, populations grow exponentially in a J-shaped curve. Real populations slow down and level off near K, which gives an S-shaped logistic curve.

10. 10How do density-dependent and density-independent limits on populations differ?

    Density-dependent limits, such as competition, disease and predation, hit crowded populations harder. Density-independent limits, such as storms, droughts and fires, affect a population regardless of its size.

Practice quiz

Pick an answer to check it.

1. 1If producers store 10,000 kcal, about how much energy reaches the secondary consumers under the 10 percent rule?

   1. 10 kcal
   2. 100 kcal
   3. 5,000 kcal
   4. 1,000 kcal
   Show the answer

   B. 100 kcal Each step passes about 10 percent. 10,000 kcal becomes about 1,000 kcal in primary consumers, and about 100 kcal reaches the secondary consumers.

2. 2Which statement about energy and matter in an ecosystem is correct?

   1. Energy cycles back to producers as heat
   2. Matter flows through the ecosystem and leaves as heat
   3. Carbon cycles, while energy flows one way and is lost as heat
   4. Energy and carbon are both recycled equally
   Show the answer

   C. Carbon cycles, while energy flows one way and is lost as heat Matter such as carbon cycles and is reused. Energy enters as sunlight, moves through food, and leaves as heat, so it is not recycled.

3. 3Which pyramids can be inverted in some situations?

   1. Only the pyramid of numbers
   2. Biomass and numbers pyramids, but not the energy pyramid
   3. Only the energy pyramid
   4. Only the biomass pyramid
   Show the answer

   B. Biomass and numbers pyramids, but not the energy pyramid The energy pyramid is always upright because each level has less energy than the one below. Biomass and numbers pyramids can invert, such as in some oceans or when one tree feeds thousands of insects.

4. 4Which nitrogen cycle step turns N2 gas into ammonia or ammonium?

   1. Denitrification
   2. Ammonification
   3. Nitrification
   4. Nitrogen fixation
   Show the answer

   D. Nitrogen fixation Nitrogen fixation, done by bacteria in soil and legume root nodules, converts atmospheric N2 into ammonia or ammonium. Denitrification does the reverse, and ammonification returns wastes to ammonium.

5. 5In the sea otter example, what happens after otters are removed from a kelp forest?

   1. Urchins overgraze the kelp, and species that depend on kelp decline
   2. Hawks replace the otters as the main predator of urchins
   3. Sea urchins decline because they have fewer predators
   4. Kelp forests grow faster without predators
   Show the answer

   A. Urchins overgraze the kelp, and species that depend on kelp decline Otters keep urchin numbers low. Without them, urchins overgraze the kelp forest, which harms the species that rely on kelp.

6. 6Which of these is a density-dependent limit on a population?

   1. A drought
   2. A forest fire
   3. A severe storm
   4. A disease that spreads faster through a crowded population
   Show the answer

   D. A disease that spreads faster through a crowded population Density-dependent limits such as disease, competition and predation hit crowded populations harder. Droughts, fires and storms affect populations regardless of their size.

Test traps

What teachers like to test on this, and the mistakes students make.

• Saying energy cycles like carbon. Energy flows in one direction and leaves as heat, while carbon and nitrogen are reused.
• Reversing the 10 percent figure. About 10 percent is passed on, and about 90 percent is not.
• Drawing food chain arrows from the eater to the food. Arrows point from the food to the eater, the direction energy moves.
• Claiming all ecosystem pyramids must be upright. Only the energy pyramid must be upright. Biomass and numbers pyramids can be inverted.
• Mixing up the nitrogen cycle steps, such as calling denitrification the step that fixes nitrogen or confusing nitrification with fixation.
• Confusing density-dependent limits with density-independent limits. Competition and disease depend on crowding, while storms and fires do not.

Summary

• Energy moves through an ecosystem in one direction: sunlight enters, passes through organisms as food, and leaves as heat.
• Matter such as carbon and nitrogen cycles and is reused, unlike energy.
• Producers make their own food by photosynthesis, consumers eat other organisms, and decomposers break down dead matter and return nutrients to the soil and water.
• About 10 percent of the energy at one trophic level is stored in the next level, which limits how many levels a food chain can have.
• A food web links many food chains, and more connections usually make an ecosystem more stable.
• Energy pyramids are always upright, while biomass and numbers pyramids can be inverted in some cases.
• Keystone species have an effect far larger than their numbers suggest, and removing one can reshape the whole ecosystem.
• Carrying capacity is the largest population an environment can support long term, and real populations level off near it in an S-shaped curve.

Key terms

Biotic factor

A living part of an ecosystem, such as plants, animals, fungi or bacteria.

Abiotic factor

A nonliving part of an ecosystem, such as sunlight, temperature, water, soil or pH.

Producer (autotroph)

An organism that makes its own food, mostly by photosynthesis. Examples are plants, algae and cyanobacteria.

Consumer (heterotroph)

An organism that eats other organisms. Herbivores eat producers, carnivores eat animals, and omnivores eat both.

Decomposer

An organism, mainly bacteria and fungi, that breaks down dead matter and wastes and returns nutrients to soil and water.

Trophic level

A step in a food chain, such as producers, primary consumers, secondary consumers or tertiary consumers.

Food web

A network of many linked food chains. It is more realistic than a single chain because most species eat, and are eaten by, several others.

10 percent rule

The standard teaching figure that about 10 percent of the energy at one level is stored in the next level. Real values vary from roughly 5 to 20 percent.

Keystone species

A species that has a far larger effect on its ecosystem than its numbers suggest. Removing it reshapes the ecosystem.

Carrying capacity (K)

The largest population an environment can support over the long term.

Nitrogen fixation

The conversion of atmospheric N2 into ammonia or ammonium by bacteria in soil and legume root nodules.

Biomass pyramid

A diagram showing the total mass of organisms at each trophic level. It is usually upright but can be inverted in some oceans.

How to study it

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