🌳 Community Ecology and Interactions

Community

Community: All populations of different species in an area

Species Interactions

1. Competition (- / -)

Both species harmed

Intraspecific: Within same species Interspecific: Between different species

Competitive exclusion principle:

• Two species can't occupy same niche
• One outcompetes the other
• One goes extinct or evolves

Resource partitioning:

• Species divide resources
• Reduces competition
• Allows coexistence
• Example: Warbler species feed at different tree heights

2. Predation (+ / -)

Predator benefits, prey harmed

Predator adaptations:

• Speed, strength, claws, teeth
• Camouflage (cryptic coloration)
• Acute senses

Prey defenses:

• Camouflage: blend with environment
• Warning coloration (aposematic): bright colors signal toxicity
• Mimicry:
  • Batesian: harmless mimics harmful
  • Müllerian: multiple harmful species look alike

3. Herbivory (+ / -)

Herbivore benefits, plant harmed

Plant defenses:

• Thorns, spines
• Chemical defenses (toxins, tannins)
• Tough leaves

4. Symbiosis

Close long-term relationship

Mutualism (+ / +):

• Both benefit
• Examples:
  • Pollinators and flowers
  • Nitrogen-fixing bacteria and legumes
  • Mycorrhizae (fungi + plant roots)
  • Lichens (fungus + algae/cyanobacteria)

Commensalism (+ / 0):

• One benefits, other unaffected
• Examples:
  • Barnacles on whales
  • Cattle egrets and cattle
  • Remoras on sharks

Parasitism (+ / -):

• Parasite benefits, host harmed
• Examples:
  • Tapeworms, ticks, fleas
  • Mistletoe (plant parasite)

Ecological Niche

Niche: Total of organism's use of biotic and abiotic resources

• "Ecological role" or "profession"
• Includes: where it lives, what it eats, when it's active

Fundamental niche: Potential niche (no competition) Realized niche: Actual niche (with competition)

Community Structure

Species richness: Number of species Species diversity: Richness + evenness (relative abundance)

Dominant species:

• Most abundant/biomass
• Greatest influence

Keystone species:

• Disproportionate effect relative to abundance
• Removal drastically changes community
• Examples: sea otters, wolves, beavers

Succession

Ecological succession: Change in species composition over time

Primary Succession

Starts with no soil:

• Bare rock (lava, glacier retreat)
• Pioneer species: lichens, mosses
• Slow soil formation
• Grasses → shrubs → trees
• Takes centuries

Secondary Succession

Starts with soil:

• After disturbance (fire, farming, flood)
• Faster than primary
• Seeds/roots already present
• Grasses → shrubs → trees
• Takes decades

Climax community:

• Stable end point
• Mature forest (usually)
• High biodiversity

Disturbance

Intermediate disturbance hypothesis:

• Moderate disturbance → highest diversity
• Too frequent → only early successional species
• Too rare → competitive exclusion

Key Concepts

1. Competition: Resource partitioning allows coexistence
2. Predation: Predator-prey arms race (adaptations)
3. Mutualism: Both species benefit (+/+)
4. Parasitism: Parasite benefits, host harmed (+/-)
5. Keystone species: Disproportionate effect on community
6. Primary succession: No soil (bare rock → climax)
7. Secondary succession: Soil present (faster recovery)