Campbell Biology Chapter 55 Review: Ecosystems and Restoration Ecology

55.1 Energy flows, matter cycles

pp. 1219–1220

On the AP exam? Yes

This is the heart of Topic 8.2: energy moves one way through trophic levels and leaves as heat, while atoms are reused. Topic 3.3 covers why living systems need a constant energy supply.

In the course: Topic 8.2 Energy Flow Through Ecosystems, Topic 3.3 Cellular Energy (notes, videos and more questions)

Key points

An ecosystem is every organism living in a place together with the nonliving things they interact with, such as water, soil, air and light. It can be as small as a rotting stump or as big as the whole biosphere.
Two processes run every ecosystem. Energy moves through it in one direction, and elements like carbon and nitrogen go round and round between living things and the environment.
The first law of thermodynamics says energy is never made or destroyed, only changed in form. So ecologists can keep an energy budget: the light a plant intercepts equals what it stores in organic molecules plus what it reflects or turns into heat.
The second law says every energy conversion loses some energy as heat. Heat can't be turned back into chemical energy by organisms, so it radiates away to space. That's why an ecosystem needs a steady new supply of energy, which for most ecosystems is sunlight.
Atoms are conserved as well, and unlike energy they get used again and again. A forest or lake does pick up some elements from outside (in rain, dust, weathered rock or nitrogen fixation) and lose some (as gases or in draining water), but those trickles are usually tiny next to what circulates inside it. When an element leaves faster than it arrives, it eventually becomes what holds growth back.
Producers (autotrophs) support every other trophic level. Most use light, but chemoautotrophic bacteria and archaea get energy from inorganic chemicals in places with no light. Plant-eaters are primary consumers. Meat-eaters that feed on plant-eaters are secondary consumers, and meat-eaters that feed on those are tertiary consumers.
Detritivores, also called decomposers, live on detritus: dead bodies, shed parts and wastes from every level. Bacteria and fungi in this group turn that organic matter back into simple inorganic nutrients, which producers can take up again, so the cycle of matter keeps going.

Key terms (14)

ecosystem: All the organisms in an area plus the nonliving parts of their surroundings, such as soil, water and air, that they exchange energy and matter with.
energy flow: The one-way movement of energy through an ecosystem: captured by producers, passed along as food, and finally lost as heat.
chemical cycling: The reuse of elements like carbon and nitrogen as they move back and forth between organisms and the air, water and soil.
first law of thermodynamics: Energy can change form or move, but the total amount never goes up or down. It lets ecologists add up an energy budget.
second law of thermodynamics: Every energy conversion turns some energy into heat that organisms can't use, so no transfer is 100% efficient.
conservation of mass: Atoms aren't created or destroyed in ordinary processes, so you can track how much of an element enters, leaves or stays in an ecosystem.
primary producer: An autotroph that makes its own organic molecules from inorganic ones, using light or chemical energy. Plants, algae and cyanobacteria are examples.
chemoautotroph: A producer, usually a bacterium or archaean, that gets its energy by oxidizing inorganic chemicals such as hydrogen sulfide instead of using light.
primary consumer: An herbivore: an animal or other heterotroph that eats producers.
secondary consumer: A carnivore that eats primary consumers (herbivores).
tertiary consumer: A carnivore that eats secondary consumers, putting it at least three feeding steps above the producers.
detritus: Dead organic matter: fallen leaves, dead bodies, wood, feces and other wastes.
detritivore (decomposer): An organism that gets its energy from detritus. Bacteria and fungi in this group break dead matter down and return nutrients to the soil, water and air.
trophic level: A feeding step in an ecosystem, set by where an organism gets its energy: producers, primary consumers, secondary consumers and so on.

Check yourself: 55.1 Energy flows, matter cycles

4 questions on 55.1 Energy flows, matter cycles. Pick an answer to see if you got it, and why.

Question 1 of 4

A kelp blade uses sunlight to build a glucose molecule. A sea urchin eats the kelp, and a sea otter later eats the urchin. Which statement best describes what eventually happens to the energy and to the carbon atoms that were in that glucose?

Question 2 of 4

A sealed glass jar holds moss, moist soil, soil bacteria and a few small snails. Only light and heat can pass through the glass. On a sunny windowsill the community lasts for years, but in a dark closet everything in it dies within a few months. Which explanation accounts for both results?

Question 3 of 4

In a marsh, snails graze on algae, and small sunfish eat the snails. A heron eats both snails and sunfish. Which trophic level or levels does the heron occupy?

Question 4 of 4Calculator allowed

Ecologists measure yearly nitrogen flows for one hectare of grassland: 3.0 kg arrives dissolved in rain, 1.5 kg is added by nitrogen-fixing bacteria, 4.0 kg leaves in stream water and 2.5 kg escapes to the air as gases. If these rates continue, what is the most likely long-term result?

0 of 4 answered

55.2 Primary production and what limits it

pp. 1220–1225

On the AP exam? Yes

Gross and net primary productivity and how light, water and nutrients limit producers are Topic 8.2; nutrient runoff causing algal blooms is Topic 8.7. Net ecosystem production and satellite methods are useful background, not required terms.

In the course: Topic 8.2 Energy Flow Through Ecosystems, Topic 8.7 Disruptions in Ecosystems, Topic 3.4 Photosynthesis (notes, videos and more questions)

Key points

Primary production is the rate at which producers turn light (or, for chemoautotrophs, chemical energy) into chemical energy in organic molecules. Every consumer gets its energy secondhand from this, so it sets the energy budget for the whole ecosystem.
Only a tiny fraction of the sunlight reaching Earth ends up in food. A lot falls on bare ground, open water or ice, and chlorophyll and other pigments can't use every wavelength. Even for light that does fall on plants and algae, roughly 1% of its visible energy gets captured in sugars and other organic molecules.
Gross primary production (GPP) is all the energy producers capture. Producers burn part of it in their own cellular respiration (R). What's left, net primary production, is NPP = GPP − R. Typically NPP comes to roughly half of GPP, and it's the energy actually available to consumers.
NPP is a rate (new biomass per m² per year), not the amount of plant material standing there. An old forest holds an enormous mass of wood, yet a marsh whose plants die back and regrow every season can add more new biomass in a year.
Net ecosystem production subtracts the respiration of every organism, not just producers, from GPP. If it's positive, the ecosystem is storing carbon; if it's negative, it's releasing carbon even though its plants may still be growing. Ecologists estimate it from the net flow of CO₂ or O₂ in or out.
In lakes and oceans, light drops off quickly with depth, but nutrients usually limit production more. Nitrogen or phosphorus is most often the limiting nutrient, and iron limits some open-ocean regions. Upwelling brings nutrient-rich deep water to the surface, so those zones are highly productive. Runoff of fertilizer or sewage can trigger algal blooms (eutrophication).
On land, temperature and moisture set production at large scales: warm, wet tropical forests are most productive, and dry deserts and cold tundra are least. Within a region, a soil nutrient, usually nitrogen and sometimes phosphorus, is often what's in shortest supply. Extra amounts of a plentiful nutrient won't boost growth; extra of the scarce one will, but only until a different resource becomes the bottleneck. Partnerships with nitrogen-fixing bacteria and mycorrhizal fungi, plus root hairs, help plants gather scarce nutrients.

Key terms (13)

primary production: The amount of light (or chemical) energy that producers convert into chemical energy in organic molecules over a set time.
gross primary production (GPP): All the energy producers capture by photosynthesis or chemosynthesis in a set time, before they use any of it.
net primary production (NPP): GPP minus the energy producers use in their own respiration (NPP = GPP − R). It's the energy stored in new producer biomass that consumers can eat.
standing crop: The total mass of producers present at one moment. It's an amount, unlike NPP, which is a rate.
biomass: The dry mass of living (or recently living) material in an area, often used to measure stored energy.
net ecosystem production (NEP): GPP minus the respiration of every organism in the ecosystem. Positive means the ecosystem is gaining carbon; negative means it's losing carbon.
limiting nutrient: The nutrient in shortest supply compared with what producers need, so adding it increases growth and adding others doesn't.
photic zone: The upper layer of a lake or ocean where enough light reaches for photosynthesis.
upwelling: The rise of cold, nutrient-rich deep water to the ocean surface, which feeds very high production.
eutrophication: Overloading a body of water with nutrients, often from fertilizer or sewage, which fuels algal blooms and can later strip oxygen from the water.
algal bloom: A sudden population explosion of algae or cyanobacteria, usually set off by extra nitrogen or phosphorus.
evapotranspiration: The total water a landscape sends into the air by evaporation from soil and water plus transpiration from plants. It rises with warmth and water supply.
mycorrhizae: Partnerships between plant roots and fungi. The fungi help the plant absorb phosphorus and other scarce minerals and get sugars in return.

Check yourself: 55.2 Primary production and what limits it

4 questions on 55.2 Primary production and what limits it. Pick an answer to see if you got it, and why.

Question 1 of 4Calculator allowed

Pond water starts with 7.6 mg/L of dissolved O₂. Some is sealed in a clear bottle and some in a foil-wrapped bottle, and both are hung in the pond for 24 hours. Afterward, the clear bottle holds 9.1 mg/L O₂ and the dark bottle holds 6.4 mg/L. What is the gross primary production in the water, in mg O₂/L per day?

Question 2 of 4Calculator allowed

Question 3 of 4

Water from a mountain lake is divided into flasks. Some get added nutrients, and algal growth is measured as chlorophyll after 10 days (invented data). Nutrient added | Chlorophyll (µg/L) None | 2.1 Nitrogen | 2.3 Phosphorus | 6.8 Nitrogen + phosphorus | 14.5 Which conclusion best fits the data?

Question 4 of 4Calculator allowed

To estimate aboveground NPP in a grassland, a researcher clips and dries all the plants in sample plots. At the start of the growing season the plots hold 120 g/m² of plant material, and at the end they hold 780 g/m². Grasshoppers ate an estimated 90 g/m² during the season. What is the best estimate of aboveground NPP for the season?

0 of 4 answered

55.3 Why only about 10% moves up each level

pp. 1225–1227

On the AP exam? Yes

Topic 8.2 tests energy loss between trophic levels, the roughly 10% rule and energy pyramids, and why endotherms need more food than ectotherms. The term "production efficiency" isn't required, but tracking where a consumer's energy goes is fair game.

In the course: Topic 8.2 Energy Flow Through Ecosystems (notes, videos and more questions)

Key points

Secondary production is the new biomass consumers build from their food. Plant-eaters never eat most of what plants grow, and a good share of what they do swallow passes through undigested, so decomposers, not herbivores, end up using most of the plant growth in an ecosystem.
Energy a consumer eats has three fates. Some passes out undigested in feces. Of what is absorbed (assimilated), much is burned in cellular respiration and lost as heat. Only the rest becomes growth and offspring, and only that part can feed the next trophic level.
Production efficiency is the share of assimilated energy that becomes new biomass: net secondary production ÷ assimilation × 100. It's tiny for endotherms (roughly 1–3% for birds and mammals), because holding body temperature high and steady takes so much fuel. Ectotherms do much better: around 10% for fishes and often 40% or more for insects and microbes.
Trophic efficiency is the share of one level's production that becomes production at the next level. It's usually around 10% (roughly 5–20%) and is always lower than production efficiency, because it also counts food that is never eaten.
Losses multiply. With 10% passed along at each step, secondary consumers get about 1% of NPP and tertiary consumers about 0.1%. That's why food chains rarely have more than four or five levels and why top predators are few and especially at risk of extinction.
An energy (production) pyramid always narrows going up. A biomass pyramid usually does too, but in some lakes and oceans it's upside down: phytoplankton reproduce and get eaten so quickly that a small standing crop supports a bigger mass of zooplankton. A pyramid of numbers can also be inverted, as when one tree feeds thousands of insects.
Eating lower on the food chain gets more food energy from the same land, so how much meat people eat strongly affects how many people Earth can feed.

Key terms (10)

secondary production: The new biomass consumers build from what they eat over a set time, counted as stored chemical energy.
assimilation: The food energy a consumer actually absorbs, which is everything it eats minus what passes out in feces.
net secondary production: The part of a consumer's assimilated energy stored as growth and offspring, which is all that the next trophic level can eat.
production efficiency: Net secondary production divided by assimilated energy, as a percentage. It's low in endotherms, which spend a lot of energy staying warm.
trophic efficiency: The percentage of production at one trophic level that becomes production at the next one, usually about 10%.
energy pyramid (pyramid of net production): A diagram with a tier for each trophic level, sized by its production. It always narrows toward the top.
biomass pyramid: A diagram with tiers sized by the standing crop (dry mass) at each trophic level. It usually narrows upward but can be inverted.
turnover time: How long it takes a level to replace its standing crop: standing crop divided by production. Phytoplankton have a very short one.
top predator: A consumer at the highest trophic level in a food web. Because so little energy reaches it, its populations are small.
endotherm: An animal, such as a bird or mammal, that keeps its body warm mainly with heat from its own metabolism, at a high energy cost.

Check yourself: 55.3 Why only about 10% moves up each level

4 questions on 55.3 Why only about 10% moves up each level. Pick an answer to see if you got it, and why.

Question 1 of 4Calculator allowed

Over a month, a young perch eats zooplankton containing 550 kJ of energy, and 150 kJ of that passes out in its feces. The perch's production efficiency (net secondary production ÷ assimilated energy × 100) is 10%. How much energy did it release in cellular respiration that month?

Question 2 of 4

A shrew and a cricket each assimilate 100 J of food energy. Which prediction is most likely?

Question 3 of 4Calculator allowed

Hawks in a grassland are tertiary consumers, and their population adds about 2 kJ of new biomass per m² per year. If trophic efficiency is about 10% at every step, roughly what NPP must the grassland's producers have to support them?

Question 4 of 4

Researchers sampling a clear lake in midsummer find a larger dry mass of zooplankton than of phytoplankton per square meter. How can a smaller mass of producers support a larger mass of consumers?

0 of 4 answered

55.4 The water, carbon, nitrogen and phosphorus cycles

pp. 1227–1232

On the AP exam? Yes

Topic 8.2 tests the four cycles: know the main reservoirs, the key processes and why phosphorus has no gas phase. Topic 1.2 covers which molecules need nitrogen and phosphorus. The four-box reservoir model and specific forest studies aren't required.

In the course: Topic 8.2 Energy Flow Through Ecosystems, Topic 1.2 Elements of Life, Topic 8.7 Disruptions in Ecosystems (notes, videos and more questions)

Key points

Sunlight arrives fresh every day, but Earth's supply of each element is fixed, so life depends on reusing atoms. Cycles that run through both living things and the nonliving world are called biogeochemical cycles. Some steps, like respiration, excretion and decay, move atoms within days or years; others, like rock weathering or the burial of carbon in coal and oil, take thousands to millions of years, unless people speed them up by burning fossil fuels.
Elements with gas forms (carbon, oxygen, nitrogen and sulfur) cycle globally through the air. Phosphorus, potassium and calcium have no common gas form, so on land they cycle mostly locally between soil and organisms, carried farther only by water or dust.
Water: nearly all of it (about 97%) sits in the oceans. The sun's heat lifts water into the air, it forms clouds and falls as rain or snow, and on land plants pump a huge volume back up through their leaves. Rivers and groundwater eventually carry the rest back to the sea.
Carbon: producers take in CO₂ for photosynthesis, and the respiration of all organisms returns roughly as much each year, so the two nearly balance. Burning coal, oil, gas and forests tips that balance by adding extra CO₂. Most of Earth's carbon is locked in rocks such as limestone, where it stays for very long periods.
Nitrogen: N₂ is about 78% of the air, but most organisms can't use it. Nitrogen-fixing bacteria turn N₂ into ammonia/ammonium (lightning and fertilizer factories fix some too). Decomposers release ammonium from dead matter (ammonification), nitrifying bacteria turn ammonium into nitrite and then nitrate, plants absorb ammonium and nitrate, and denitrifying bacteria in low-oxygen soil return nitrogen to the air as gases. Animals get nitrogen only from organic food.
Phosphorus: rock slowly breaks down and releases phosphate (PO₄³⁻), which producers absorb and build into DNA, RNA, ATP and phospholipids. Phosphate clings to soil particles, so it tends to stay near where it was released, and since it forms no common gas, only a little ever travels through the air, on dust or salt spray.
How fast nutrients cycle depends mostly on decomposition, which is fastest when it's warm and moist with plenty of oxygen. Where decay is quick, as in warm, wet tropical forests, few nutrients sit in the soil because most are tied up in living plants. Cold or waterlogged places such as peat bogs build up undecayed matter. Living plants also soak up water and nutrients, so stripping away vegetation lets more of both wash out.

Key terms (13)

biogeochemical cycle: The path an element takes as it moves between living things and the air, water, soil and rock, and back again.
reservoir: A place where an element is stored for a time, such as the atmosphere, the ocean, living biomass, soil or rock.
transpiration: Loss of water vapor from plant leaves through their stomata. It moves a large share of water from soil to air on land.
nitrogen fixation: Converting N₂ gas into forms organisms can use. Nitrogen-fixing bacteria make ammonia (NH₃), which becomes ammonium (NH₄⁺); lightning and fertilizer factories fix some nitrogen too.
ammonification: Decomposers breaking down proteins and other organic nitrogen compounds and releasing ammonium into the soil.
nitrification: Soil bacteria turning ammonium into nitrite and then nitrate (NO₃⁻), a form plants absorb easily.
denitrification: Bacteria in low-oxygen soil or water using nitrate in place of oxygen and releasing nitrogen gases, which return nitrogen to the air.
nitrogen assimilation: Plants absorbing ammonium or nitrate and building the nitrogen into amino acids, nucleotides and other organic molecules.
phosphate: The ion PO₄³⁻, the main form in which producers take up phosphorus.
weathering: The slow breakdown of rock by water, air and living things, which releases minerals such as phosphate and calcium into soil.
decomposition: The breakdown of dead organic matter by bacteria, fungi and other detritivores, which releases inorganic nutrients and CO₂.
leaching: The loss of dissolved nutrients from soil as water drains through it.
isotope tracer: A rare or radioactive isotope, such as ¹⁵N or ¹³C, added to an ecosystem so scientists can follow where an element goes.

Check yourself: 55.4 The water, carbon, nitrogen and phosphorus cycles

4 questions on 55.4 The water, carbon, nitrogen and phosphorus cycles. Pick an answer to see if you got it, and why.

Question 1 of 4

A farmer treats a field with a chemical that blocks nitrifying bacteria but doesn't harm other soil microbes. Over the next few weeks, which change in the soil is most likely?

Question 2 of 4

Nitrogen gas makes up about 78% of the air a rabbit breathes. Where does the nitrogen in the rabbit's proteins and DNA come from?

Question 3 of 4

On an old volcanic island far from any continent, soils slowly lose phosphorus to runoff over thousands of years. New phosphorus arrives mostly as windblown dust and in the droppings of seabirds that feed at sea. Why can't the island's ecosystem draw phosphorus from the air, the way nitrogen-fixing bacteria draw nitrogen from it?

Question 4 of 4

Researchers expose grassland plots for one hour to CO₂ made with the heavy isotope ¹³C, then follow where the label goes. Which observation would best show that soil microbes were feeding on carbon the grass had just fixed?

0 of 4 answered

55.5 Restoring damaged ecosystems

pp. 1232–1235

On the AP exam? Background

Restoration ecology isn't part of the current course, and you won't be asked about bioremediation or biological augmentation by name. It's good background for human disruptions (Topic 8.7), why diverse ecosystems recover better (Topic 8.6) and mutualisms (Topic 8.5).

In the course: Topic 8.7 Disruptions in Ecosystems, Topic 8.6 Biodiversity, Topic 8.5 Community Ecology (notes, videos and more questions)

Key points

Left alone, many damaged places slowly recover through succession. But severe harm, such as land stripped bare, soil poisoned by metals or made salty, or farmland drained of nutrients, can leave a site struggling for hundreds of years.
Restoration ecologists try to kick-start or hurry that recovery. Their work rests on the hope that harm can be partly reversed, balanced by the knowledge that ecosystems have limits, so they hunt for the one or two problems that most slow recovery and tackle those.
Sometimes the physical setting has to be repaired before anything can grow: filling drainage ditches so a drained wetland floods again, rebuilding eroded dunes, or reshaping gullied slopes so rain soaks in instead of carving them deeper.
Bioremediation uses living things, usually bacteria, fungi or plants, to remove or neutralize pollution. Some plants soak up heavy metals and can then be cut and carted off, taking the metal with them. Some microbes digest oil or other toxins, and some change dissolved metals into solid forms that stay put instead of spreading.
Biological augmentation uses living things to supply something a damaged system is missing. Nitrogen-fixing plants can build up nitrogen in poor soil, and adding the right root fungi can help native plants take hold.
Plants alone don't make a working ecosystem. Animals carry pollen and seeds and keep plants in check by grazing, so restorers may bring species back or connect the site to wild areas with strips of habitat they can travel along.
Because no two sites are alike, many restorers use adaptive management: they test a few approaches side by side, keep track of the results and shift toward what succeeds. The final goal is an ecosystem that works as much like the original, undamaged one as possible.

Key terms (9)

restoration ecology: The field that uses ecology to help damaged ecosystems recover toward a more natural state.
ecological succession: The gradual change in which species live in an area after a disturbance, as one community replaces another.
bioremediation: Using organisms such as bacteria, fungi or plants to remove, break down or lock up pollutants.
biological augmentation: Using organisms to add something a damaged ecosystem lacks, such as nitrogen from nitrogen-fixing plants.
hyperaccumulator: A plant that pulls unusually large amounts of a metal like nickel or zinc from soil into its tissues, which makes it useful for cleanup.
adaptive management: Testing several management methods, watching the results and changing course based on what works.
habitat corridor: A strip of habitat that connects separated patches so animals and plants can move between them.
resilience: How well an ecosystem bounces back after a disturbance.
topsoil: The dark upper layer of soil, rich in organic matter, nutrients and soil organisms, that plants rely on most.

Check yourself: 55.5 Restoring damaged ecosystems

4 questions on 55.5 Restoring damaged ecosystems. Pick an answer to see if you got it, and why.

Question 1 of 4

On land polluted with lead from an old smelter, restorers grow Indian mustard, a plant that moves lead from the soil into its leaves and stems. Why must the plants be cut and hauled away at the end of each growing season?

Question 2 of 4

A volcanic mudflow buries a valley under fresh ash and rock with almost no nitrogen. Before planting other native species, restorers plant alder trees, whose root nodules house nitrogen-fixing bacteria. What is the main purpose of the alders?

Question 3 of 4

After an oil spill, the beach sand holds bacteria that can break down oil, but cleanup is slow. Workers spread nitrogen and phosphorus fertilizer on the oiled sand, and oil breakdown speeds up. What best explains this result?

Question 4 of 4

Managers restoring a burned shrubland aren't sure whether seeding native plants, covering the ground with mulch, or both will work best. Which plan best fits adaptive management?

0 of 4 answered