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Species interactions, ecological niches, and community structure
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Community: All populations of different species in an area
Both species harmed
Intraspecific: Within same species Interspecific: Between different species
Competitive exclusion principle:
Resource partitioning:
Predator benefits, prey harmed
Predator adaptations:
Define the following types of species interactions: competition, predation, herbivory, parasitism, mutualism, and commensalism. Give an example of each.
Species Interactions (Effects on species: +/0/-)
COMPETITION (-/-) • Both species harmed • Compete for limited resources • Example: Two bird species competing for nesting sites • Lions and hyenas competing for prey
PREDATION (+/-) • Predator benefits, prey harmed • One organism kills and eats another • Example: Lion (predator) eating zebra (prey) • Hawk eating mouse
HERBIVORY (+/-) • Herbivore benefits, plant harmed • Animal eats plant tissue • Example: Caterpillar eating leaves • Deer browsing on shrubs
PARASITISM (+/-) • Parasite benefits, host harmed • Parasite lives on/in host, harms but usually doesn't kill • Example: Tapeworm in human intestine • Ticks on deer, fleas on dogs
| Section | Format | Questions | Time | Weight | Calculator |
|---|---|---|---|---|---|
| Multiple Choice | MCQ | 60 | 90 min | 50% | 🚫 |
| Free Response (Long) | FRQ | 2 | 50 min | 30% | 🚫 |
| Free Response (Short) | FRQ | 4 | 40 min | 20% | 🚫 |
Avoid these 3 frequent errors
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Prey defenses:
Herbivore benefits, plant harmed
Plant defenses:
Close long-term relationship
Mutualism (+ / +):
Commensalism (+ / 0):
Parasitism (+ / -):
Niche: Total of organism's use of biotic and abiotic resources
Fundamental niche: Potential niche (no competition) Realized niche: Actual niche (with competition)
Species richness: Number of species Species diversity: Richness + evenness (relative abundance)
Dominant species:
Keystone species:
Ecological succession: Change in species composition over time
Starts with no soil:
Starts with soil:
Climax community:
Intermediate disturbance hypothesis:
MUTUALISM (+/+) • Both species benefit • Cooperation between species • Example: Bees pollinating flowers (bee gets nectar, plant gets pollinated) • Nitrogen-fixing bacteria in legume root nodules • Mycorrhizae (fungi + plant roots)
COMMENSALISM (+/0) • One species benefits, other unaffected • Example: Barnacles on whale (barnacles get transport, whale unaffected) • Birds nesting in trees • Cattle egrets following cattle (eat insects stirred up)
Note: Actual interactions can be complex and may shift between categories depending on context.
Explain the competitive exclusion principle (Gause's principle). How can two competing species coexist in the same area?
Competitive Exclusion Principle: "Two species competing for the exact same limiting resource cannot coexist indefinitely - the superior competitor will eventually exclude the other."
Gause's Experiments (1930s): • Studied Paramecium species • P. aurelia and P. caudatum compete for same bacteria • Grown separately: both thrive • Grown together: P. aurelia outcompetes and excludes P. caudatum • Complete competitive exclusion occurs
Key Concept: • Complete niche overlap → competitive exclusion • One species will be driven to local extinction • The "better" competitor wins
How Can Competing Species Coexist?
RESOURCE PARTITIONING (Niche Differentiation) • Species divide resources • Use different resources or same resource differently • Reduces competition
Example: Warbler birds in spruce trees • Different species feed at different heights • Different parts of tree • Different prey sizes • Minimal niche overlap
SPATIAL SEPARATION • Occupy different habitats/microhabitats • Physical separation reduces competition
Example: Barnacle species • Chthamalus (top of intertidal zone) • Balanus (lower intertidal zone) • Different tolerance to desiccation and competition
TEMPORAL SEPARATION • Active at different times • Different seasons, times of day
Example: Desert rodents • Some species nocturnal, others diurnal • Feed at different times
CHARACTER DISPLACEMENT • Evolution of differences where species overlap • Divergent evolution reduces competition
Example: Darwin's finches • Beak sizes diverge on islands where species overlap • More similar on islands where isolated • Resource partitioning evolves
PREDATION OR DISTURBANCE • Predators reduce dominant competitor • Prevents competitive exclusion • Intermediate disturbance hypothesis
Example: Starfish removing mussels • Without starfish: mussels dominate, exclude others • With starfish: diversity maintained
Fundamental vs. Realized Niche: • Fundamental niche: Full range of conditions/resources species COULD use • Realized niche: Actual range species DOES use (reduced by competition) • Competition restricts species to realized niche
Key Insight: Competition is a major force shaping community structure and driving evolutionary change!
What is a keystone species? Describe Robert Paine's classic sea star experiment and explain why it demonstrates the keystone species concept.
Keystone Species: A species that has a disproportionately large effect on its community relative to its abundance. Removal causes dramatic changes in community structure.
Paine's Sea Star Experiment (1960s):
Location: Rocky intertidal zone, Pacific Northwest coast
Experimental Design: • Control plot: Sea star (Pisaster ochraceus) present • Experimental plot: Sea star manually removed • Monitored species diversity over time
Results:
CONTROL (with sea star): • High diversity maintained • 15+ species of invertebrates • Mussels present but not dominant • Barnacles, algae, other species abundant
EXPERIMENTAL (without sea star): • Diversity crashed dramatically • Mussels (Mytilus californianus) dominated • Outcompeted and excluded other species • Only 8 species remained • Barnacles, algae mostly eliminated
Mechanism:
Without sea star: • Mussels competitively superior • Overgrow and smother other organisms • Competitive exclusion occurs • Diversity plummets
Why This Demonstrates Keystone Species Concept:
Disproportionate Impact • Sea stars relatively low abundance • But removal causes ecosystem collapse • Impact >> abundance
Community Structure Control • Sea star maintains diversity • Prevents dominance by single species • "Architect" of community
Trophic Cascade • Top-down control • Predator affects entire food web • Effects cascade through trophic levels
Non-Redundant Role • No other species performs same function • Cannot be replaced • Loss has unique consequences
Other Keystone Species Examples:
Sea otters • Eat sea urchins • Prevent urchins from overgrazing kelp • Maintain kelp forest ecosystems
Wolves in Yellowstone • Control elk populations • Prevent overgrazing of riparian vegetation • Affect stream ecology, beaver populations, etc. • Trophic cascade when reintroduced
Elephants in African savanna • Knock down trees • Maintain grassland-woodland mosaic • Create habitat for other species
Prairie dogs • Burrows provide shelter for many species • Grazing affects plant composition • Prey for predators
Beavers • Dam-building creates wetlands • Habitat for many species • "Ecosystem engineers"
Keystone Species vs. Dominant Species: • Dominant: High abundance/biomass, major competitor • Keystone: Low-moderate abundance, disproportionate impact • Can overlap but distinct concepts
Conservation Implications: • Protect keystone species first • Their loss causes cascading effects • Restoration may require keystone species • Not all species equally important to ecosystem function
Key Principle: Community structure depends not just on what species are present, but on their ecological roles and interactions!
Explain primary and secondary succession. How do they differ? What is a climax community?
Ecological Succession: Predictable changes in species composition over time in a community.
PRIMARY SUCCESSION: Starts from bare substrate with NO soil
Where it occurs: • Newly formed volcanic islands • Retreating glaciers (bare rock) • New lava flows • Sand dunes • Rock exposed by landslide
Sequence:
Pioneer Species • Lichens, mosses • Can colonize bare rock • Produce acids that break down rock • Begin soil formation • Very hardy, stress-tolerant
Early Colonizers • Small herbaceous plants • Grasses, ferns • Thin soil layer present • Add organic matter when die
Intermediate Stages • Shrubs, small trees • Deeper soil develops • More nutrients available • Shade-intolerant species
Late Stages • Large trees • Shade-tolerant species • Complex community structure • Approaches climax
Timescale: Hundreds to thousands of years
SECONDARY SUCCESSION: Starts from disturbed area with existing soil
Where it occurs: • Abandoned agricultural fields • After forest fire • After logging/clear-cutting • After hurricane/storm damage • Disturbed but soil remains
Sequence:
Annual Weeds • Fast-growing, opportunistic • Example: Crabgrass, ragweed • First year after abandonment
Perennial Herbs and Grasses • Years 2-3 • Replace annuals • More stable
Shrubs and Small Trees • Years 5-15 • Shade out grasses • Pine, cedar (fast-growing)
Shade-Tolerant Trees • Years 15+ • Oak, hickory, maple • Replace pine forest • Approach climax
Timescale: Decades to ~200 years (faster than primary)
KEY DIFFERENCES:
| Feature | Primary | Secondary |
|---|---|---|
| Starting point | Bare rock | Disturbed soil |
| Soil present? | No | Yes |
| Speed | Very slow | Faster |
| Pioneer species | Lichens | Weeds/grasses |
| Timescale | 1000s years | 10s-100s years |
CLIMAX COMMUNITY: Stable, mature community that represents endpoint of succession
Characteristics: • Species composition relatively stable • In equilibrium with environment • Dominated by late-successional species • High biodiversity (usually) • Complex structure • Efficient nutrient cycling • Slower-growing, longer-lived species
Examples: • Old-growth forest (temperate regions) • Tropical rainforest • Prairie grassland (dry regions)
Modern Understanding: • "Climax" may not be truly stable • Climate change affects climax state • Disturbances reset succession • Multiple stable states possible • "Shifting mosaic" of patches at different stages
Intermediate Disturbance Hypothesis: • Highest diversity at intermediate disturbance levels • Too little disturbance: competitive exclusion by dominant species • Too much disturbance: only early-successional species survive • Intermediate: Mix of early and late species
Mechanisms of Succession:
Facilitation • Early species make environment suitable for later species • Example: Nitrogen fixation by pioneer plants
Tolerance • Later species can tolerate conditions early species create • Example: Shade tolerance
Inhibition • Early species prevent establishment of later species • Later species only arrive when early species die
Key Insight: Succession is predictable but not inevitable - disturbances can reset the process at any stage!
What is the intermediate disturbance hypothesis? Explain how disturbance affects species diversity with specific examples.
Intermediate Disturbance Hypothesis: Species diversity is highest at intermediate levels of disturbance frequency and intensity.
Proposed by: Joseph Connell (1978)
Core Idea:
• Too little disturbance → competitive exclusion
• Too much disturbance → only tolerant species survive
• Intermediate disturbance → maximum diversity
Mechanism:
LOW Disturbance: • Competitive dominant species take over • Exclude inferior competitors • Community reaches climax state • Lower diversity (competitive exclusion) • Predictable succession to equilibrium
Example: Mature forest without disturbance • Shade-tolerant trees dominate • Exclude light-requiring species • Few pioneer species remain
INTERMEDIATE Disturbance: • Creates gaps/openings periodically • Prevents competitive exclusion • Resets succession in patches • Mix of early, mid, and late-successional species • MAXIMUM diversity • Shifting mosaic of different-aged patches
Example: Forest with periodic small fires • Opens canopy in some areas • Allows pioneer species to establish • Old-growth patches persist • Many successional stages present simultaneously
HIGH Disturbance: • Frequent, severe disturbance • Only stress-tolerant, fast-colonizing species survive • Cannot reach late-successional stages • Lower diversity (only pioneers) • Continuous early succession
Example: Frequently plowed agricultural field • Only annual weeds survive • Perennials and woody plants cannot establish • Low diversity
Graphical Representation:
Diversity
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High| /
| /
| /
| /
Low |_/_
Low Medium High
Disturbance
Classic Examples:
CORAL REEFS (Connell's original study) • Low disturbance: Fast-growing corals dominate • Intermediate: Mix of fast and slow-growing species • High disturbance (hurricanes): Only hardy species • Greatest diversity at intermediate
ROCKY INTERTIDAL ZONES • Low disturbance: Mussels monopolize space • Intermediate: Sea star predation creates openings • High disturbance (extreme waves): Few species tolerate • Diversity peaks with moderate predation/wave action
TROPICAL RAINFOREST • Low: Shade-tolerant canopy dominants • Intermediate: Tree falls create light gaps • Gap colonizers + canopy trees coexist • High: Frequent clearing → secondary growth only
GRASSLANDS • Low: Woody plants invade, convert to shrubland • Intermediate: Fire maintains grassland, prevents woody takeover • Diverse mix of grass species • High: Too frequent fire → only fire-tolerant grasses
STREAMS AND RIVERS • Low flow: Siltation, algae overgrowth • Intermediate: Periodic floods reset, create diverse habitats • High: Frequent flooding → only attached organisms
Types of Disturbance:
Natural: • Fire • Storms (hurricanes, tornadoes) • Floods • Landslides • Volcanic eruptions • Disease outbreaks • Herbivory/predation • Tree falls
Human-caused: • Logging • Agriculture • Development • Pollution • Altered fire regimes • Climate change
Critiques and Refinements:
Not universal • Some systems show different patterns • Depends on type of disturbance • Context-dependent
Scale matters • Frequency and extent of disturbance • Size of disturbed patches • Regional vs. local effects
Type of diversity • Species richness vs. evenness • Functional diversity • Genetic diversity
Multiple stable states • Some systems have alternative stable states • Disturbance can flip between states
Conservation and Management Implications:
Prescribed burns • Maintain fire-adapted ecosystems • Prevent fuel buildup • Promote diversity
Rotational grazing • Prevents overgrazing • Mimics natural herbivore patterns • Maintains grassland diversity
Selective logging • Creates gaps without clear-cutting • Mimics natural tree fall • Maintains forest diversity
Dam removal/flow management • Restore natural flood regimes • Maintain river diversity
Avoid both extremes • Complete protection can reduce diversity (no disturbance) • Excessive disturbance destroys ecosystems • Aim for intermediate
Key Principle: Disturbance is not always bad! Moderate levels of disturbance can actually INCREASE biodiversity by preventing competitive exclusion and creating heterogeneity.