Campbell Biology Chapter 54 Review: Community Ecology

54.1 How species help or harm each other

pp. 1194–1200

On the AP exam? Yes

Topic 8.5 tests competition, predation and the three symbioses, labeled with +, − and 0, plus niche partitioning. You don't need the terms fundamental and realized niche, character displacement, the mimicry types or facilitation, but they make good practice in explaining how selection (Topic 7.2) shapes species.

In the course: Topic 8.5 Community Ecology, Topic 7.2 Natural Selection (notes, videos and more questions)

Key points

A community is every population of every species sharing an area closely enough to affect one another. Ecologists label each interaction by what it does to each partner's survival and reproduction: + if it helps, − if it harms, 0 if there's no measurable effect.
Interspecific competition (−/−) happens when two species both need a resource that's in short supply. If two species have exactly the same niche, the slightly better competitor eventually wipes the other out locally. This is competitive exclusion.
A niche is the whole set of resources and conditions a species uses. The fundamental niche is everything it could use with no competitors around; the realized niche is the smaller slice it actually uses when they are there. Similar species coexist by resource (niche) partitioning: using different places, times or food types.
Where two related species overlap, natural selection can push their traits further apart than where each lives alone. This pattern, character displacement, is indirect evidence that competition shaped them.
Predation and herbivory (+/−) drive adaptations on both sides. Selection favors predators with sharp senses, speed, stealth, venom or lures, and prey that escape, hide, find safety in numbers, or carry armor and toxins. Camouflage (cryptic coloration) hides prey, while bold warning (aposematic) colors advertise a defense. In Batesian mimicry a harmless species copies a harmful one; in Müllerian mimicry several harmful species share one look. Plants fight back with toxic or bad-tasting chemicals, thorns and spines.
Symbiosis means two species living in close, lasting contact. Parasitism (+/−) feeds a parasite at its host's expense; some parasites even change host behavior to reach their next host. Mutualism (+/+) helps both, and can be obligate (one partner can't live alone) or facultative. Commensalism (+/0) helps one and leaves the other unaffected, which is hard to prove.
Facilitation (+/+ or 0/+) is one species making conditions better for another without the two being physically bound together. Plants do this for one another a lot, for example a shrub whose shade keeps the seedlings beneath it from drying out.

Key terms (15)

interspecific competition: A contest between individuals of different species for the same limited resource, like water, light or prey. It lowers the success of both (−/−).
competitive exclusion: The rule that two species with exactly the same niche can't share a place forever: the better competitor eventually eliminates the other locally.
ecological niche: A species' whole way of life: the resources it uses, the conditions it tolerates and when and where it is active.
resource partitioning: Similar species splitting up a resource by using it in different places, at different times or in different forms, so they can live side by side. AP calls it niche partitioning.
fundamental niche: The full range of conditions and resources a species could use if no competitors were present.
realized niche: The narrower set of conditions and resources a species really uses once competitors and other species are around.
character displacement: When two related species differ more in their traits where they live together than where they live apart, usually because selection reduced their competition.
herbivory: An animal feeding on the tissues of a plant or seaweed (+/−). The plant often survives, but it's harmed.
aposematic coloration: Bright, bold colors or patterns that warn predators an animal is toxic, foul-tasting or dangerous.
Batesian mimicry: A harmless species that has evolved to look like a harmful one, so predators that avoid the harmful species avoid it too.
Müllerian mimicry: Two or more harmful or bad-tasting species that have evolved to look alike, so predators learn one shared warning.
parasitism: A symbiosis where one organism (the parasite) feeds on or in another (the host) and harms it (+/−).
mutualism: A symbiosis that benefits both species (+/+), like flowers and their pollinators.
commensalism: A relationship that helps one species while the other is neither helped nor harmed (+/0).
facilitation: A positive effect of one species on another (+/+ or 0/+) that doesn't require the two to live in close physical contact.

Check yourself: 54.1 How species help or harm each other

4 questions on 54.1 How species help or harm each other. Pick an answer to see if you got it, and why.

Question 1 of 4

Question 2 of 4

A seed-eating beetle from overseas became established in farm fields where a native beetle had long eaten the same weed seeds. Both beetles take the same seed sizes at the same times, and seeds limit both populations. Twenty years later, the native beetle is gone from fields the newcomer reached but is still common in nearby hedgerows, where it feeds mostly on seeds of a shrub the newcomer ignores. Which idea best explains this pattern?

Question 3 of 4

Question 4 of 4

A harmless clearwing moth has the same black-and-orange bands as a wasp with a painful sting. Birds that get stung learn to avoid the banded pattern. In which situation would the moth gain the least protection from looking like the wasp?

0 of 4 answered

54.2 Diversity, food webs and species with outsized effects

pp. 1200–1206

On the AP exam? Yes

Species diversity and trophic cascades are Topic 8.5, keystone species and diversity's link to resilience are Topic 8.6, and food chains and the 10% energy pattern are Topic 8.2. The exam uses Simpson's diversity index (on the formula sheet), not the book's Shannon index. You won't be asked to name the food chain length hypotheses or the bottom-up and top-down models.

In the course: Topic 8.5 Community Ecology, Topic 8.6 Biodiversity, Topic 8.2 Energy Flow Through Ecosystems (notes, videos and more questions)

Key points

Species diversity has two parts: species richness (how many species there are) and relative abundance, or evenness (how evenly individuals are spread among them). A community where one species makes up almost everything is less diverse than one with the same species in similar numbers.
AP Biology measures diversity with Simpson's index, $1 - \sum \left(\frac{n}{N}\right)^2$ , where n is the count of one species and N is the total. It runs from 0 (one species) toward 1. The book uses the Shannon index, which you may meet in class; both rise with more species and more evenness.
Experiments that assemble communities with different numbers of species usually find that richer mixes grow more total biomass, hold steadier from one year to the next, bounce back faster after a stress like drought and are harder for invaders to break into, partly because a varied set of species leaves fewer resources unused.
A food chain traces energy from producers to primary, secondary and higher consumers; decomposers feed on every level. Real chains link into food webs, and omnivores feed at more than one level.
Most food chains have five or fewer links. The energetic hypothesis says roughly a tenth of the energy at one level ends up in the next, so little is left for top predators; it predicts longer chains where producers are more productive. The dynamic stability hypothesis says long chains recover too slowly from disturbances. Most evidence favors the energetic hypothesis.
A dominant species is the most common one, or the one with the greatest total biomass. Keystone species are often fairly scarce, yet their role is so central that removing one reshapes the whole community. Ecosystem engineers change the physical habitat itself, for example by building dams, digging burrows or forming reefs.
In the bottom-up model, nutrients set plant growth, which sets herbivore and then predator numbers. In the top-down model (a trophic cascade), predators control herbivores, which control plants, so a change at the top flips each level below in turn. Managers have used top-down effects to clear algae-choked lakes by changing fish populations.

Key terms (15)

species diversity: The variety of species in a community, combining how many species there are with how evenly individuals are spread among them.
species richness: A simple count of how many species a community contains.
relative abundance: Each species' share of all the individuals in a community. Even shares mean high evenness.
Simpson's diversity index: AP's diversity measure: $1 - \sum \left(\frac{n}{N}\right)^2$ . It's near 0 when one species dominates and approaches 1 when many species are evenly represented.
Shannon diversity index: Another diversity measure, used in the book: H = −Σ p ln p, where p is each species' share. Higher H means a more diverse community.
trophic structure: The feeding relationships in a community: who eats whom, and how energy moves between levels.
food chain: A single path of energy transfer, from a producer through a series of consumers.
food web: Many linked food chains showing all the feeding connections in a community.
energetic hypothesis: The idea that food chains are short because roughly a tenth of each level's energy makes it to the level above.
dynamic stability hypothesis: The idea that long food chains are fragile, because top predators recover slowly when disturbances cut the food supply below them.
dominant species: The species with the most individuals or the most biomass in a community. It strongly shapes conditions for the others.
keystone species: A species whose effect on its community is far larger than its numbers would suggest. Removing it can cause a big drop in diversity.
ecosystem engineer: A species that changes the physical environment, like building dams or digging burrows, and so changes which other species can live there.
bottom-up model: The view that a community is controlled from below: nutrients limit producers, which limit herbivores, which limit predators.
top-down model: The view that predators control a community from above, so changes at the top cascade down through each level. Some call it the trophic cascade model.

Check yourself: 54.2 Diversity, food webs and species with outsized effects

4 questions on 54.2 Diversity, food webs and species with outsized effects. Pick an answer to see if you got it, and why.

Question 1 of 4Calculator allowed

A student sweeps a meadow with a net and catches 40 grasshoppers, 30 leafhoppers, 20 beetles and 10 crickets. What is Simpson's diversity index, $1 - \sum \left(\frac{n}{N}\right)^2$ , for this sample?

Question 2 of 4Calculator allowed

Four leaf-litter samples each contain 100 small arthropods (invented data). Sample | Individuals of each species P | 60, 10, 10, 10, 10 Q | 25, 25, 25, 25 R | 34, 33, 33 S | 50, 10, 10, 10, 10, 10 Which sample has the highest Simpson's diversity index, and why?

Question 3 of 4

Question 4 of 4

0 of 4 answered

54.3 Disturbance and succession

pp. 1207–1210

On the AP exam? Background

Succession and the intermediate disturbance hypothesis aren't in the current course. What carries over: how well a community resists and recovers from disturbance is resilience (Topic 8.6), and human activities as a major force changing ecosystems is Topic 8.7.

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

Key points

Early ecologists pictured each community settling into one stable 'climax' state set by climate. Most ecologists now use a nonequilibrium view: storms, fires, floods and other disturbances keep communities changing all the time.
A disturbance is anything that knocks organisms out of a community or shifts which resources are on offer: a wildfire, a storm, a flood, a hard freeze, a bulldozer. Its impact depends on both how strong it is and how often it comes.
The intermediate disturbance hypothesis says diversity is highest at moderate disturbance. Too much disturbance removes species that can't tolerate it or that colonize slowly; too little lets the strongest competitors take over. Many studies fit this pattern, but plenty don't, so treat it as a common pattern, not a law.
Some communities depend on disturbance. Certain plants release seeds or sprout only after fire. Where people have stopped frequent, mild ground fires for decades, dead wood and brush pile up, so when a fire finally comes it can burn far hotter than the local plants can survive.
Ecological succession is the gradual change in which species live in a place after a disturbance. Primary succession starts where there's no soil yet, such as fresh lava or rock left by a melting glacier. Microbes, lichens and mosses come first, soil slowly forms, and reaching a forest can take centuries. Secondary succession begins where soil survives, as in an old field or after a fire, and often moves from weeds and grasses to shrubs and then to trees.
Early species can help later ones (facilitation, for example by adding nitrogen or organic matter to the soil), hold them back (inhibition) or have no effect on them (tolerance).
People now cause more disturbance than any natural force, through logging, farming, overgrazing and building roads and cities. These changes are often too severe or too frequent for many species, so diversity usually drops.

Key terms (9)

disturbance: Any event, natural or human-caused, that kills or removes organisms or shifts the supply of resources, such as a wildfire, a flood or logging.
nonequilibrium model: The view that most communities are always changing because disturbances keep interrupting them, rather than settling into one stable state.
climax community: An older idea of a final, stable community that a place would always reach and keep, set mainly by its climate.
intermediate disturbance hypothesis: The idea that species diversity peaks at moderate levels of disturbance, rather than at very low or very high levels.
ecological succession: The gradual, step-by-step replacement of some species by others in an area after a disturbance.
primary succession: Succession that starts on bare ground with no soil, such as new lava or rock scraped clean by a glacier.
secondary succession: Succession that starts where a disturbance removed the community but left the soil, like an abandoned field or a burned forest.
pioneer species: The first hardy organisms, such as lichens, mosses or fast-growing weeds, to colonize a newly disturbed area.
inhibition (in succession): When early colonizers make it harder for later species to establish, so later species take over only when the early ones die or are removed.

Check yourself: 54.3 Disturbance and succession

4 questions on 54.3 Disturbance and succession. Pick an answer to see if you got it, and why.

Question 1 of 4

Which of these events would most likely be followed by primary succession rather than secondary succession?

Question 2 of 4

On a rocky shore, waves flip small boulders several times a year, medium boulders every few years and large boulders almost never. Ecologists counted the species of seaweeds and attached invertebrates on each size (invented data). Boulder size | Times overturned per year (average) | Species per boulder (average) Small | 4.2 | 1.8 Medium | 0.4 | 4.1 Large | 0.02 | 2.3 Which explanation best accounts for this pattern?

Question 3 of 4

After a forest is clear-cut, a dense thicket of ferns covers the ground. Tree seedlings that sprout under the ferns mostly die in the shade, and trees become common only in patches where the ferns have died back. Which process does this describe?

Question 4 of 4

In a heathland in southwestern Australia, seeds of several shrub species can lie dormant in the soil for decades and germinate mainly after contact with chemicals in smoke. Surveyors compared these shrubs in two similar reserves (invented data). Reserve | Years since last fire | Young plants per 100 m² | Adult plants per 100 m² P | 4 | 85 | 30 Q | 60 | 1 | 9 Which explanation best fits these data?

0 of 4 answered

54.4 Latitude, area and islands

pp. 1211–1213

On the AP exam? Not tested

The latitude pattern, species-area curves and the island equilibrium model aren't in the current course. They're useful background on why biodiversity (Topic 8.6) is spread unevenly and how speciation (Topic 7.10) adds species over time, and they come up in class tests and conservation discussions.

In the course: Topic 8.6 Biodiversity, Topic 7.10 Speciation (notes, videos and more questions)

Key points

For most groups of plants and animals, species richness is highest in the tropics and drops toward the poles.
Two explanations work together. Evolutionary history: tropical communities are older and weren't repeatedly wiped clean by ice ages, and their year-round growing seasons speed up the pace at which new species form. Climate: warm, sunny, wet places can support more life, and ecologists often track that mix of energy and water with evapotranspiration, the total water moving into the air from soil and plants.
The species-area relationship: the bigger the area, the more species it holds, mostly because a larger area contains more kinds of habitat. Ecologists use it to predict how many species could be lost as habitat shrinks.
'Islands' aren't just land in the ocean. A pond, a cave, a city park or a patch of prairie among cornfields is a habitat island too.
The island equilibrium model says an island's species number settles where the rate of new species arriving equals the rate of resident species dying out. Arrivals slow as the island fills up, and extinctions speed up as more species compete.
Large islands have higher immigration (a bigger target) and lower extinction (bigger populations, more habitats), so they hold more species. Islands near a mainland also hold more, because colonists arrive often and newcomers keep small populations from disappearing. The equilibrium is dynamic: the count stays steady while the species list keeps changing.
Real islands don't always behave this neatly. Over thousands of years new species evolve on them and big storms reshuffle them, so the model fits best over short time spans. Conservation planners still lean on it when deciding how big and how connected protected areas should be, and when estimating losses from shrinking habitat.

Key terms (8)

latitudinal gradient: The broad pattern of species richness being highest near the equator and falling toward the poles.
evapotranspiration: Water returned to the air by evaporation from soil and surfaces plus transpiration from plants. High values signal lots of heat, light and water.
species-area curve: A graph showing that larger areas hold more species, all else being equal.
habitat island: Any patch of suitable habitat surrounded by unsuitable habitat, like a pond, a cave or a city park.
island equilibrium model: MacArthur and Wilson's model: an island's species number levels off where new species arrive about as fast as resident species disappear.
immigration rate (island model): How quickly species not yet on an island arrive and settle. It's higher for large islands and islands near a source.
extinction rate (island model): How quickly species already on an island die out there. It's higher on small islands with small populations.
species turnover: The ongoing replacement of some species by others, even while the total number of species stays about the same.

Check yourself: 54.4 Latitude, area and islands

4 questions on 54.4 Latitude, area and islands. Pick an answer to see if you got it, and why.

Question 1 of 4

Ecologists removed every insect and spider from three small mangrove islands of about the same size and then tracked how many species recolonized each one (invented data). Island | Distance from shore (m) | Species before removal | Species one year later E1 | 2 | 43 | 44 E2 | 160 | 25 | 22 E3 | 530 | 21 | 15 According to the island equilibrium model, what best explains why the farthest island had the fewest species both before and after?

Question 2 of 4Calculator allowed

Ecologists often describe the species-area relationship as $S = cA^{z}$ , where S is the number of species, A is the area, and c and z are constants. For one group of forest plants, z = 0.25. If a forest is cut down to 1/16 of its original area, about what fraction of its original plant species does this relationship predict the remaining forest can support in the long run?

Question 3 of 4

Cool forests grow on the tops of isolated mountains in a desert region. Forest-dwelling mammals can't cross the hot, dry lowlands between the peaks. Treating the peaks as habitat islands, which prediction follows?

Question 4 of 4

Which observation would best support the idea that the time available for species to accumulate helps explain why the tropics are so species-rich?

0 of 4 answered

54.5 Pathogens in communities

pp. 1213–1215

On the AP exam? Background

Topic 8.7 covers introduced diseases as a disruption to ecosystems (Dutch elm disease and potato blight are its examples), and parasites and hosts fit Topic 8.5. Zoonotic diseases, vectors and the book's flu and forest-disease case studies aren't tested. The book's disease figures are from 2010; updates are noted here.

In the course: Topic 8.7 Disruptions in Ecosystems, Topic 8.5 Community Ecology, Topic 7.2 Natural Selection (notes, videos and more questions)

Key points

Pathogens are disease-causing agents: bacteria, fungi, protists and viruses, plus viroids (infectious RNA molecules) and prions (misfolded, infectious proteins). Ecologists now see them as major forces shaping communities.
Pathogens tend to hit hardest when people carry them somewhere new: the local hosts have never faced them, so natural selection hasn't had the chance to spread resistance. Dutch elm disease, chytrid fungus in amphibians (linked to declines in hundreds of amphibian species) and white-nose syndrome in North American bats all show this.
When a pathogen kills a dominant or habitat-building species, the effects ripple outward. Species that relied on it for food or shelter decline, others move in, and diversity can fall. After a wasting disease wiped out most sunflower sea stars along the Pacific coast starting in 2013, the sea urchins they ate multiplied and helped strip away kelp forests.
Shipping, the plant trade and air travel now carry pathogens between continents in days or weeks.
Zoonotic pathogens pass from other animals to people, either directly or through a vector, an organism such as a tick or mosquito that carries the pathogen between hosts. Most emerging human diseases are zoonotic.
Community ecology helps control disease: finding which reservoir species keep a pathogen going, following migratory animals that may carry it, and managing vectors. The book notes that H5N1 bird flu hadn't reached the Americas by 2010; it arrived in North America in late 2021 and by 2024 was even infecting U.S. dairy cattle.

Key terms (8)

pathogen: Anything that causes disease: a bacterium, virus, fungus, protist, viroid or prion.
viroid: A tiny, naked loop of RNA, with no protein coat, that can infect plants and cause disease.
prion: A misfolded protein that causes normal copies of the same protein to misfold, leading to diseases such as mad cow disease.
emerging disease: A disease that is new to a population, or that is quickly increasing in how often it occurs or where it's found.
zoonotic pathogen: A pathogen that spreads from other animals to humans.
vector: An organism, often a biting insect or tick, that carries a pathogen from one host to another.
reservoir host: A species that carries a pathogen over the long term, often without getting very sick, and keeps it circulating in nature.
introduced pathogen: A pathogen carried by people into a region outside its native range, where hosts often have no resistance.

Check yourself: 54.5 Pathogens in communities

4 questions on 54.5 Pathogens in communities. Pick an answer to see if you got it, and why.

Question 1 of 4

A fungus native to East Asia has recently spread across Europe. Researchers infected seedlings of two related ash tree species with it in a greenhouse (invented data). Seedling source | Seedlings with only small leaf spots (%) | Seedlings killed (%) East Asian ash | 88 | 3 European ash | 21 | 64 What best explains the difference?

Question 2 of 4

A virus circulates among wild songbirds, which carry it in their blood for several days. Mosquitoes pick it up when they bite infected birds and pass it on when they later bite other birds, horses or people. Horses and people rarely have enough virus in their blood to infect another mosquito. What role do the mosquitoes play?

Question 3 of 4

In a mountain stream, frog tadpoles graze algae from rocks, and several snake species eat mostly frogs. A fungal disease then kills most frogs in the region within a few years. Which pair of changes would most likely follow?

Question 4 of 4

Which of these diseases is caused by a zoonotic pathogen?

0 of 4 answered