Community Ecology and Species Interactions

Gregor Welcomes You!

Welcome, investigators! In Chapter 18 you studied individual populations — now it's time to zoom out and see what happens when populations of different species share the same space. Who eats whom? How do competitors divide resources? Can one species reshape an entire ecosystem? Community ecology reveals the web of interactions that determine the diversity and structure of life in every habitat on Earth. Let's investigate!

Summary

A community is all the species sharing an ecosystem, and their interactions determine who survives, who thrives, and how resources are divided. This chapter systematically covers the full spectrum of interspecific interactions: predation and herbivory as antagonistic forces driving co-evolutionary arms races; competition and the competitive exclusion principle; resource partitioning and the niche as the mechanism by which species coexist; and the mutualistic, commensal, and parasitic relationships that also shape community structure. Keystone species and trophic cascades illustrate disproportionate effects of single species. The chapter closes with ecological succession — primary and secondary — and island biogeography as the quantitative theory explaining species richness on habitat patches.

Concepts Covered

This chapter covers the following 18 concepts from the learning graph:

Interspecific Interactions

Community Ecology
Interspecific Interactions
Predation
Herbivory
Competition
Competitive Exclusion Principle
Resource Partitioning
Ecological Niches
Mutualism
Commensalism
Parasitism

Community Structure and Dynamics

Keystone Species
Trophic Cascades
Ecological Succession
Primary Succession
Secondary Succession
Climax Community
Island Biogeography

Prerequisites

This chapter builds on concepts from:

Chapter 18: Population Ecology and Life History

Community Ecology

Community ecology studies how populations of different species interact and how these interactions shape the structure, diversity, and function of biological communities. A community is defined as all the populations of different species living and interacting in the same area.

Interspecific Interactions

Interspecific interactions are relationships between individuals of different species. These interactions can be classified by their effects on each species:

Interaction	Effect on Species 1	Effect on Species 2	Example
Predation	+ (predator gains energy)	− (prey loses life)	Lion and zebra
Herbivory	+ (herbivore gains energy)	− (plant loses tissue)	Deer and wildflowers
Competition	− (reduced access to resources)	− (reduced access to resources)	Two bird species competing for nest sites
Mutualism	+ (benefit)	+ (benefit)	Bee and flower
Commensalism	+ (benefit)	0 (no effect)	Barnacle on whale
Parasitism	+ (parasite gains nutrition)	− (host loses resources/health)	Tapeworm in intestine

Antagonistic Interactions

Predation

Predation is the interaction in which one organism (the predator) kills and eats another (the prey). Predation is a powerful selective force that drives the evolution of:

Prey defenses:

Camouflage (cryptic coloration) — blending with the background
Warning coloration (aposematic coloration) — bright colors advertising toxicity (e.g., poison dart frogs)
Batesian mimicry — harmless species mimics a toxic one (e.g., viceroy butterfly mimics monarch)
Müllerian mimicry — two or more toxic species resemble each other (shared warning signal)
Mechanical defenses — spines, shells, quills
Chemical defenses — toxins, venom

Predator adaptations:

Acute senses (vision, hearing, smell)
Speed and stealth
Cooperative hunting strategies

Predator and prey populations often show cyclical dynamics — as prey increases, predators increase; as predators increase, prey declines; as prey declines, predators decline. The classic example is the lynx-hare cycle in Canadian boreal forests.

Herbivory

Herbivory is the consumption of plant tissue by animals. Plants have evolved extensive defenses:

Chemical defenses — alkaloids, tannins, terpenoids (many are used as medicines and spices)
Physical defenses — thorns, spines, tough leaves, bark
Induced defenses — some plants increase toxin production after being damaged

Competition

Interspecific Competition

Competition occurs when two or more species use the same limited resource. Competition is harmful (−/−) to both species, because each reduces the other's access to the shared resource.

Competitive Exclusion Principle

The competitive exclusion principle (Gause's principle) states that two species competing for the exact same niche cannot coexist indefinitely in the same habitat — one will eventually outcompete and eliminate the other.

G.F. Gause demonstrated this experimentally (1934) using two Paramecium species grown in the same flask with the same food source: P. aurelia consistently drove P. caudatum to extinction.

Resource Partitioning

In nature, closely related species often coexist because they partition resources — dividing the niche to reduce direct competition.

Resource partitioning can involve:

Spatial partitioning — species feed in different parts of the habitat (e.g., different canopy layers in a forest)
Temporal partitioning — species feed at different times (e.g., hawks hunt by day, owls by night)
Morphological specialization — species evolve different body structures for different food types (e.g., beak shapes in Darwin's finches)

Ecological Niches

An organism's ecological niche is the sum of all the abiotic and biotic conditions in which it lives and reproduces — its "role" in the ecosystem.

Fundamental niche — the full range of conditions under which a species CAN survive (without competition)
Realized niche — the actual conditions under which a species DOES live (narrower, due to competition and other interactions)

Key Insight

The competitive exclusion principle and resource partitioning are two sides of the same coin. If two species have identical niches, one will be excluded. In reality, species coexist because their niches are slightly different — they partition resources. The AP exam often asks students to explain how species with similar needs coexist: the answer is always resource partitioning (spatial, temporal, or morphological).

Positive Interactions

Mutualism

Mutualism (+/+) benefits both interacting species.

Examples:

Pollination — bees get nectar, flowers get pollinated
Mycorrhizae — fungi provide minerals to plant roots, plants provide sugars to fungi
Nitrogen fixation — Rhizobium bacteria fix \(\ce{N2}\) for legume roots, legumes provide sugars to bacteria
Coral-zooxanthellae — photosynthetic algae provide nutrients to coral, coral provides shelter

Commensalism

Commensalism (+/0) benefits one species with no significant effect on the other.

Examples:

Barnacles attached to a whale (barnacle gets transport; whale is unaffected)
Cattle egrets following grazing cattle (egrets catch insects disturbed by cattle)
Epiphytic orchids growing on tree branches (orchid gets access to light; tree is unaffected)

Parasitism

Parasitism (+/−) benefits the parasite at the expense of the host.

Ectoparasites — live on the host's surface (ticks, fleas, lice)
Endoparasites — live inside the host (tapeworms, Plasmodium malaria parasite)
Parasites typically do not kill their hosts (unlike predators) — a dead host is useless

Diagram: Species Interactions Web

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Species Interactions Web — Specification

Type: Graph model (vis-network)
sim-id: species-interactions
Library: vis-network
Status: Specified

Learning objective: Students will be able to classify (Bloom's L4: Analyze) species interactions as predation, competition, mutualism, commensalism, or parasitism, and explain (Bloom's L2: Understand) the effect (+, −, 0) on each species.

Instructional Rationale: A network graph where species are nodes and interactions are labeled, color-coded edges gives students a visual map of community structure. Clicking edges reveals detailed descriptions and effect symbols.

Canvas: 780 × 480 px, responsive.

Layout: Force-directed graph with ~12 species nodes and ~15 interaction edges.

Nodes (species): Lion, Zebra, Grass, Bee, Wildflower, Clownfish, Sea anemone, Tick, Deer, Oak tree, Mycorrhizal fungus, Barnacle, Whale

Edges (interactions): - Lion → Zebra: predation (+/−) - Zebra → Grass: herbivory (+/−) - Bee ↔ Wildflower: mutualism (+/+) - Clownfish ↔ Sea anemone: mutualism (+/+) - Tick → Deer: parasitism (+/−) - Oak tree ↔ Mycorrhizal fungus: mutualism (+/+) - Barnacle → Whale: commensalism (+/0) - Lion ↔ Cheetah: competition (−/−) (add Cheetah node)

Interaction: - Hover over edge: tooltip showing interaction type, description, and +/−/0 effects - Click node: highlight all its interactions - "Quiz Mode": edges are unlabeled; student must classify each interaction type - Drag nodes to rearrange

Colors: Predation edges: red. Competition: orange. Mutualism: green. Commensalism: blue. Parasitism: purple.

Responsive design: Graph recalculates layout on resize.

Community Structure and Dynamics

Keystone Species

A keystone species has a disproportionately large effect on community structure relative to its abundance. Removing a keystone species triggers cascading changes throughout the community.

Classic example: Sea otters in Pacific kelp forests

Sea otters eat sea urchins
Without otters, urchin populations explode and overgraze kelp
Without kelp, the entire kelp forest ecosystem collapses (fish, invertebrates, seabirds lose habitat)

Trophic Cascades

A trophic cascade is an indirect effect that ripples through multiple trophic levels when a top predator is added or removed.

Example: Yellowstone wolves

Wolves reintroduced to Yellowstone (1995)
Wolf predation reduced elk populations and changed elk behavior (elk avoided riverbanks)
Riparian vegetation recovered (willows, aspens)
Streambank erosion decreased, beaver populations returned
Fish and bird diversity increased

This is a top-down trophic cascade — changes at the top predator level cascade down through herbivores to primary producers.

Gregor's Tip

Trophic cascades are a favorite AP exam free-response topic. To earn full marks, describe the mechanism at each level — don't just say "wolves affect plants." Say "wolves reduce elk populations AND change elk grazing behavior, which allows riparian vegetation to recover." Show the chain of cause and effect through at least three trophic levels.

Ecological Succession

Ecological succession is the gradual, directional change in the species composition of a community over time.

Primary Succession

Primary succession occurs on newly exposed surfaces with no prior soil — bare rock, cooled lava, glacial retreat, new volcanic islands.

Stages:

Pioneer species colonize bare rock — lichens and mosses break down rock, begin soil formation
Small plants (grasses, ferns) establish in thin soil
Shrubs and small trees arrive as soil deepens
Larger trees eventually dominate

Primary succession can take hundreds to thousands of years because soil must be created from scratch.

Secondary Succession

Secondary succession occurs on land where an existing community has been disturbed but soil remains intact — after fire, farming, logging, or flooding.

Proceeds much faster than primary succession because soil, seeds, and root systems already exist
Pioneer species are typically fast-growing, sun-loving annuals and grasses
Gradually replaced by shrubs, then shade-tolerant trees

Climax Community

A climax community is the relatively stable, mature community that persists at the end of succession until the next major disturbance.

The composition of the climax community depends on the region's climate, soil, and available species pool
Modern ecologists recognize that many communities are in a state of continuous change and that true "climax" stability is rare — disturbances continually reset different patches

Diagram: Ecological Succession Timeline

View Fullscreen

Ecological Succession Timeline — Specification

Type: MicroSim (p5.js)
sim-id: ecological-succession
Library: p5.js
Status: Specified

Learning objective: Students will be able to compare (Bloom's L4: Analyze) primary and secondary succession and sequence (Bloom's L1: Remember) the stages from pioneer community to climax community.

Instructional Rationale: An animated landscape that transforms through succession stages — with species appearing and disappearing — makes the temporal scale and gradual nature of succession visually compelling.

Canvas: 780 × 440 px, responsive.

Layout:

Top: Toggle — "Primary Succession" / "Secondary Succession"
Center: Landscape illustration showing terrain, soil depth, and vegetation at each stage
5 stages (slides or smooth animation)
Primary: bare rock → lichens/mosses → grasses → shrubs → forest
Secondary: disturbed field → grasses/annuals → shrubs → young forest → mature forest
Bottom: Timeline bar with stage labels and approximate time scales
Right panel: Species list for current stage with brief descriptions

Interaction: - Click or drag along the timeline to advance through stages - "Compare" mode: primary and secondary shown side by side, aligned by time axis (showing secondary is faster) - Hover over any plant/animal icon: species name and role in succession - Soil depth indicator bar that grows through stages (primary) or stays constant (secondary)

Colors: Bare rock: gray. Lichens/mosses: light green. Grasses: yellow-green. Shrubs: medium green. Forest: dark green. Soil: brown gradient increasing in depth.

Responsive design: Landscape and timeline scale with container; comparison mode stacks vertically on narrow screens.

Island Biogeography

The theory of island biogeography (MacArthur and Wilson, 1967) explains the number of species on an island (or any isolated habitat patch) as a balance between two opposing rates:

Immigration rate — the rate at which new species arrive (decreases as species richness increases, because fewer "new" species remain to colonize)
Extinction rate — the rate at which established species go locally extinct (increases as species richness increases, due to competition and limited resources)

Equilibrium is reached when immigration rate = extinction rate. At this point, species richness is stable (though the specific species may turn over).

Key predictions:

Factor	Effect on species richness
Larger islands	More species (lower extinction rate, more habitats)
Closer to mainland	More species (higher immigration rate)
Smaller islands	Fewer species
Farther from mainland	Fewer species

This theory extends beyond literal islands to any isolated habitat: mountaintops, lakes, forest fragments surrounded by farmland, and even national parks.

You've Got This!

You have now explored the full web of community ecology — from individual species interactions to the grand patterns of succession and island biogeography. These concepts connect directly to conservation biology in Chapter 20, where you will see how habitat fragmentation, invasive species, and climate change threaten the community dynamics you have studied here. One more chapter to go!

Key Takeaways

Interspecific interactions include predation (+/−), herbivory (+/−), competition (−/−), mutualism (+/+), commensalism (+/0), and parasitism (+/−).
The competitive exclusion principle states that two species with identical niches cannot coexist. Resource partitioning (spatial, temporal, morphological) allows coexistence by reducing niche overlap.
An organism's ecological niche encompasses all abiotic and biotic conditions it needs. The realized niche is smaller than the fundamental niche due to competition.
Keystone species have disproportionate effects on community structure. Trophic cascades transmit the effects of adding or removing top predators through multiple trophic levels.
Primary succession starts on bare substrate (no soil); secondary succession starts on disturbed land with intact soil. Both proceed toward a climax community.
Island biogeography predicts species richness as a balance between immigration and extinction rates, influenced by island size and distance from the mainland.

AP Practice: Test Your Understanding

Question 1: Two species of warblers both eat insects in the same spruce trees but feed at different heights. What concept explains their coexistence?

Answer: Resource partitioning (specifically, spatial partitioning). By feeding at different heights in the canopy, the two species reduce their niche overlap and avoid competitive exclusion. This was famously studied by Robert MacArthur with Cape May, Blackburnian, and other warbler species.

Question 2: Wolves are removed from a grassland ecosystem. Predict the effects on elk, grasses, and songbird populations, explaining the trophic cascade.

Answer: Without wolves (top predators), elk populations increase (reduced predation). More elk means increased grazing on grasses and shrubs → grassland and riparian vegetation decline. Declining vegetation reduces habitat and food for songbirds, so songbird populations decrease. This is a top-down trophic cascade: predator removal → herbivore increase → producer decrease → secondary effects on other species.

Question 3: Two islands are the same distance from the mainland, but Island A is 10 times larger than Island B. According to island biogeography theory, which will have more species? Explain.

Answer: Island A (the larger island) will have more species. Larger islands have lower extinction rates (more resources, more habitat diversity, larger population sizes) while immigration rates are similar (same distance from mainland). The equilibrium species richness is therefore higher on the larger island.