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?

1. 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

2. 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

3. TEMPORAL SEPARATION • Active at different times • Different seasons, times of day

   Example: Desert rodents • Some species nocturnal, others diurnal • Feed at different times

4. 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

5. 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!

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:

1. Sea stars are top predators
2. Preferentially eat mussels (competitive dominant)
3. Prevent mussels from monopolizing space
4. Create openings for other species
5. Maintain diversity through predation

Without sea star: • Mussels competitively superior • Overgrow and smother other organisms • Competitive exclusion occurs • Diversity plummets

Why This Demonstrates Keystone Species Concept:

1. Disproportionate Impact • Sea stars relatively low abundance • But removal causes ecosystem collapse • Impact >> abundance

2. Community Structure Control • Sea star maintains diversity • Prevents dominance by single species • "Architect" of community

3. Trophic Cascade • Top-down control • Predator affects entire food web • Effects cascade through trophic levels

4. Non-Redundant Role • No other species performs same function • Cannot be replaced • Loss has unique consequences

Other Keystone Species Examples:

1. Sea otters • Eat sea urchins • Prevent urchins from overgrazing kelp • Maintain kelp forest ecosystems

2. Wolves in Yellowstone • Control elk populations • Prevent overgrazing of riparian vegetation • Affect stream ecology, beaver populations, etc. • Trophic cascade when reintroduced

3. Elephants in African savanna • Knock down trees • Maintain grassland-woodland mosaic • Create habitat for other species

4. Prairie dogs • Burrows provide shelter for many species • Grazing affects plant composition • Prey for predators

5. 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!

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:

1. Pioneer Species • Lichens, mosses • Can colonize bare rock • Produce acids that break down rock • Begin soil formation • Very hardy, stress-tolerant

2. Early Colonizers • Small herbaceous plants • Grasses, ferns • Thin soil layer present • Add organic matter when die

3. Intermediate Stages • Shrubs, small trees • Deeper soil develops • More nutrients available • Shade-intolerant species

4. 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:

1. Annual Weeds • Fast-growing, opportunistic • Example: Crabgrass, ragweed • First year after abandonment

2. Perennial Herbs and Grasses • Years 2-3 • Replace annuals • More stable

3. Shrubs and Small Trees • Years 5-15 • Shade out grasses • Pine, cedar (fast-growing)

4. Shade-Tolerant Trees • Years 15+ • Oak, hickory, maple • Replace pine forest • Approach climax

Timescale: Decades to ~200 years (faster than primary)

KEY DIFFERENCES:

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:

1. Facilitation • Early species make environment suitable for later species • Example: Nitrogen fixation by pioneer plants

2. Tolerance • Later species can tolerate conditions early species create • Example: Shade tolerance

3. 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!

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 | High| /
| /
| /
| /
Low |_/_ Low Medium High Disturbance

Classic Examples:

1. 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

2. 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

3. TROPICAL RAINFOREST • Low: Shade-tolerant canopy dominants • Intermediate: Tree falls create light gaps • Gap colonizers + canopy trees coexist • High: Frequent clearing → secondary growth only

4. 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

5. 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:

1. Not universal • Some systems show different patterns • Depends on type of disturbance • Context-dependent

2. Scale matters • Frequency and extent of disturbance • Size of disturbed patches • Regional vs. local effects

3. Type of diversity • Species richness vs. evenness • Functional diversity • Genetic diversity

4. Multiple stable states • Some systems have alternative stable states • Disturbance can flip between states

Conservation and Management Implications:

1. Prescribed burns • Maintain fire-adapted ecosystems • Prevent fuel buildup • Promote diversity

2. Rotational grazing • Prevents overgrazing • Mimics natural herbivore patterns • Maintains grassland diversity

3. Selective logging • Creates gaps without clear-cutting • Mimics natural tree fall • Maintains forest diversity

4. Dam removal/flow management • Restore natural flood regimes • Maintain river diversity

5. 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.