Community Ecology

6.1 Introduction to Community Ecology (Expanded)

Definition: Community ecology is the study of how populations of different species interact with one another within a defined geographic area, and how these interactions shape the composition, structure, and function of the biological community.

Key Concepts:

• Community (Biological Community): All populations of different species living and interacting in a common habitat at the same time.

• Assemblage: A subset of a community (e.g., the bird assemblage, the plant assemblage).

• Ecotone: A transitional zone between two communities (e.g., forest edge to grassland). Ecotones often have high biodiversity (edge effect).

Levels of Organization (Hierarchy):

• Individual → Population → Community → Ecosystem → Biome → Biosphere

Core Questions in Community Ecology:

1. What species live together?

2. Why do they live together (or not)?

3. How do interactions determine abundance and distribution?

4. How does the community change over time (succession)?

5. What happens when a species is removed or added?

Importance:

• Explains how ecosystems function (energy flow, nutrient cycling).

• Provides basis for conservation biology and ecosystem management.

• Helps predict effects of species invasions or extinctions.

• Informs restoration ecology (rebuilding damaged communities).

6.2 Species Interactions (Expanded)

Definition: Species interactions are the direct and indirect relationships between different species that share a community. These interactions can be positive (+), negative (−), or neutral (0) for each participant.

Detailed Interaction Matrix:

Expanded Details on Major Interactions:

• Competition (Detailed):

  • Intraspecific: Within same species (strongest competition, same exact needs).

  • Interspecific: Between different species.

  • Competitive Exclusion Principle: Two species cannot coexist indefinitely on the same limiting resource. One will outcompete the other unless they partition resources.

  • Example (Classic Experiment): Gause's Paramecium species. When grown alone, both thrive. When grown together, P. aurelia outcompetes and eliminates P. caudatum.

• Predation (Detailed):

  • True predator: Kills prey immediately (lion, hawk).

  • Grazzer: Consumes parts of many prey without killing (cow, caterpillar).

  • Predator-Prey Cycles: Classic Lotka-Volterra model shows linked oscillations.

  • Adaptations: Camouflage (cryptic coloration), warning coloration (aposematism), mimicry (Batesian: harmless looks harmful; Müllerian: two harmful look alike).

• Parasitism (Detailed):

  • Ectoparasite: Lives on host surface (tick, flea, mite).

  • Endoparasite: Lives inside host (tapeworm, malaria protozoan).

  • Parasitoidism (Bridge to predation): Insect lays eggs on/in host; larvae consume and kill host (e.g., ichneumon wasps).

  • Brood Parasitism: Cuckoo lays eggs in other bird's nest; host raises cuckoo chick.

• Mutualism (Types):

  • Obligate: Both species cannot survive without each other (lichens = fungus + alga).

  • Facultative: Both benefit but can survive independently (pollination by generalist bees).

  • Trophic Mutualism: Both receive food/nutrients (mycorrhizae: fungus + plant roots).

  • Defensive Mutualism: One receives protection, other receives food (ants + acacia tree).

  • Dispersive Mutualism: One receives food, other receives seed dispersal (birds eating fruit).

• Commensalism (Examples):

  • Epiphytes (orchids) growing on tree branches — get light/support, tree unaffected.

  • Cattle egrets following grazing cattle — cattle disturb insects, egrets eat them.

  • Remora fish attached to shark — remora gets transport/food scraps, shark unaffected.

• Amensalism (Often overlooked):

  • Penicillium fungus producing penicillin — kills bacteria (harmful to bacteria, neutral to fungus).

  • Walnut tree (Juglans) releasing juglone toxin — kills nearby plants.

6.3 Ecological Niche (Expanded)

Definition: The ecological niche is the total of a species' use of biotic and abiotic resources in its environment — its "profession" or "way of life" including what it eats, where it lives, when it is active, and how it reproduces.

G. E. Hutchinson's Niche Concept (1957):

• The niche is an n-dimensional hypervolume where each dimension represents an environmental condition or resource needed by the species (temperature, humidity, pH, prey size, nesting sites, etc.).

Fundamental vs. Realized Niche:

Example: A barnacle species Chthamalus can live in deep water (fundamental niche) but is eliminated by competition from another barnacle Balanus; its realized niche is only shallow water.

Niche Partitioning (Resource Partitioning): Process by which competing species evolve differences to coexist.

• Example (Warbler birds): Five species of warblers feed on same spruce tree but at different zones (top, middle, bottom, inner, outer branches) — different realized niches.

• Example (Root depth): Grass and trees can coexist because grass uses shallow water, trees use deep water.

Niche Width/Breadth:

• Generalist: Broad niche, many resources, adaptable (e.g., raccoon, rat, dandelion).

• Specialist: Narrow niche, specific resources, vulnerable to change (e.g., panda eating only bamboo, koala eating only eucalyptus).

Niche Overlap: Degree to which species share resources. High overlap → intense competition or extinction.

Niche Differentiation/Partitioning: Evolutionary adaptation that reduces competition, allowing coexistence.

6.4 Community Structure (Expanded)

Definition: Community structure refers to the composition, organization, and pattern of species within a community, including who is there, how many there are, and how they relate to each other.

Key Components of Structure:

1. Species Richness: Number of different species in a community.

• Latitudinal Gradient: Richness highest near equator, lowest near poles.

  • Tropical rainforest: ~500 tree species per hectare.

  • Temperate forest: ~20–30 tree species per hectare.

  • Boreal forest: ~5–10 tree species per hectare.

• Area Effect: Larger areas have more species (Species-Area Relationship: $S=c{A}^z$).

2. Species Evenness (Equitability): Relative abundance of each species.

• High evenness = all species have similar numbers (e.g., 10 species each with 100 individuals).

• Low evenness = one dominant species, many rare (e.g., Species A = 950, rest total = 50).

3. Species Diversity Indices (Combining richness + evenness):

• Shannon-Wiener Index (H'): $H^{\mathrm{\prime }}=-\sum (p_i\ln p_i)$ where $p_i$ = proportion of species i. Higher value = more diverse.

• Simpson's Index: Probability two randomly chosen individuals are different species. 0 to 1; higher = diverse.

4. Dominant Species: Species with highest abundance or biomass that exert strong influence.

• Example: Oak trees in temperate forest; krill in Southern Ocean.

5. Keystone Species: Species with disproportionately large effect on community structure relative to its abundance.

• Classic Example (Robert Paine's experiment): Sea stars (Pisaster) eat mussels. Remove sea stars → mussels overgrow and outcompete 15 other species → species richness drops from 15 to 5.

• Other examples:

  • Sea otters → control sea urchins → protect kelp forests.

  • African elephants → knock down trees → maintain grasslands.

  • Wolves in Yellowstone → control elk → allow willow/aspen regrowth → beavers and songbirds return.

• Identifying keystone species: Keystone index = (impact / abundance). Value > 1 means keystone.

6. Foundation Species (Ecosystem Engineers): Species that create or modify habitats for others.

• Corals → build reef structures providing habitat for 25% of marine species.

• Beavers → build dams creating wetlands.

• Trees in forest → create shade, leaf litter, nesting sites.

7. Trophic Structure: Hierarchical feeding relationships.

• Trophic Levels: Producers → Primary consumers (herbivores) → Secondary consumers (carnivores) → Tertiary consumers → Apex predators.

• Food chain: Linear path (grass → rabbit → fox → wolf).

• Food web: Interconnected food chains (more realistic).

  • Connectance: Proportion of possible feeding links realized.

  • Food webs with higher connectance are more stable.

• Trophic Cascade: When top predators suppress herbivores, allowing plants to thrive.

  • Example: Yellowstone wolves reintroduced (1995) → elk decrease & behavior change → willow regrows → beavers return.

8. Guilds: Groups of species that use the same resources in similar ways (not necessarily related).

• Example: Nectar-feeding birds (hummingbirds, sunbirds, honeyeaters) from different continents fill same guild.

6.5 Succession (Expanded)

Definition: Ecological succession is the orderly, predictable process of change in species composition and community structure over time, following a disturbance or creation of new habitat.

Types of Succession:

Stages of Succession (Detailed):

Key Concepts in Succession:

• Facilitation: Early species modify environment to make it suitable for later species (lichens break rock → soil forms → mosses → grasses).

• Inhibition: Early species actively hinder later species (some plants release toxins to prevent competitors).

• Tolerance: Later species simply tolerate conditions created by early species, not dependent on them.

Climax Community Debate:

• Traditional view (Clements): Succession leads to a single, stable climax determined by climate.

• Modern view (Gleason, Pickett): Communities are individualistic; multiple possible endpoints (climax pattern hypothesis); disturbances natural part of dynamics.

Example data: Mount St. Helens eruption (1980). Primary succession on barren pumice plain. After 40 years: some lupine plants, few trees, still early stage. Recovery will take centuries.

Mechanisms of Succession:

• Dispersal (seeds arriving)

• Colonization (establishing)

• Facilitation (making easier for others)

• Competition (outcompeting earlier species)

• Disturbance (resetting process)

6.6 Biodiversity in Communities (Expanded)

Definition: Biodiversity (biological diversity) is the variety of life at all levels of biological organization, from genes to ecosystems.

Three Main Levels of Biodiversity:

Other Dimensions of Biodiversity:

• Phylogenetic diversity: Evolutionary relationships; preserving distinct lineages (e.g., coelacanth, ginkgo tree).

• Functional diversity: Range of ecological roles (different pollination strategies, different rooting depths).

• Taxonomic diversity: Classification-based measures.

Patterns of Biodiversity:

Factors Affecting Biodiversity:

Importance of Biodiversity (Ecosystem Services):

1. Provisioning services: Food, clean water, timber, medicine (~25% of modern drugs derived from plants).

2. Regulating services: Climate regulation (forests store carbon), water purification (wetlands), pollination (75% of crops need animal pollinators), pest control.

3. Supporting services: Nutrient cycling, soil formation, primary production.

4. Cultural services: Recreation, tourism, aesthetic, spiritual.

Biodiversity-Productivity Relationship: Experiments show that communities with more species produce more biomass and are more productive (ecosystem functioning increases with diversity).

6.7 Stability and Disturbance (Expanded)

Definition: Ecological stability is the ability of a community to maintain its structure and function in the face of disturbance, or to return to that state after disturbance.

Components of Stability:

Types of Disturbance:

Intermediate Disturbance Hypothesis (Connell, 1978):

• Graph: Diversity vs. disturbance frequency = hump-shaped curve.

• Low disturbance: Competitive exclusion reduces diversity (dominant species takeover).

• Intermediate disturbance: Highest diversity (disturbance prevents exclusion but not so severe as to eliminate species).

• High disturbance: Only disturbance-tolerant species survive (low diversity).

• Example: Coral reefs. Rare storms → one coral dominates. Moderate storms → many corals coexist. Frequent hurricanes → only rubble species survive.

Fire Ecology (Case Study):

• Many ecosystems are fire-adapted (chaparral, savanna, boreal forest, Australian eucalyptus).

• Pyrophytes: Plants that require or tolerate fire.

  • Serotinous cones (pines): Open only with heat.

  • Thick bark (giant sequoia): Protects from fire.

  • Fireweed: Colonizes burned areas.

• Fire suppression (e.g., Yellowstone, California) leads to fuel accumulation → catastrophic megafires.

Community Stability vs. Diversity Relationship (Debate):

• Traditional view (MacArthur, Elton): More diverse = more stable (more redundant pathways).

• Modern view: Relationship is complex. Sometimes diversity increases stability (more species = more likely one will tolerate disturbance). Sometimes diversity decreases stability (complex webs more fragile to extinction).

• Current consensus: Biodiversity enhances resilience and productivity but not necessarily constancy.

Human Impacts on Community Stability (Expanded):

Case Study: Yellowstone Wolves Reintroduction (Trophic Cascade)

• 1926: Wolves eliminated from Yellowstone.

• 1926–1995: Elk population exploded → overgrazed willows and aspens → no beavers (no dam wood), no songbirds (no nesting).

• 1995: Wolves reintroduced.

• 1995–2020: Elk reduced and behavior changed (avoid risky areas) → willows recovered along streams → beavers returned → songbirds increased.

• Key lesson: Keystone predator restores community stability.

📊 Quick Revision Table (Expanded)

Summary / Key Takeaways (Expanded)

1. Community ecology studies the interactions among different species in a shared environment and how those interactions shape community structure and dynamics.

2. Species interactions range from antagonistic (competition −−, predation +−, parasitism +−) to beneficial (mutualism ++) to neutral (commensalism +0, neutralism 00, amensalism −0). These interactions define the ecological roles of species.

3. The ecological niche is a species' complete "way of life" (Hutchinson's n-dimensional hypervolume). Fundamental niche (potential) is larger than realized niche (actual) due to competition.

4. Community structure includes species richness (# species), evenness (relative abundance), diversity indices, dominant species, keystone species (disproportionate impact), foundation species (ecosystem engineers), and trophic structure (food webs).

5. Succession is predictable change over time. Primary succession starts without soil (lichens → mosses → grasses → shrubs → trees → climax). Secondary succession starts with soil (weeds → grasses → shrubs → trees → climax). Mechanisms include facilitation, inhibition, and tolerance.

6. Biodiversity exists at genetic, species, and ecosystem levels. Patterns include latitudinal gradients (higher near equator), elevation gradients (mid-elevation peaks), area effects (larger = more species), and the intermediate disturbance hypothesis (highest diversity at moderate disturbance).

7. Stability has multiple components: resistance (staying unchanged), resilience (recovering quickly), constancy, and persistence. Disturbances (fire, flood, human activity) shape communities. The intermediate disturbance hypothesis predicts highest diversity at moderate disturbance frequencies.

8. Keystone species (like sea otters, wolves, sea stars) have disproportionate effects on community structure. Their removal can trigger trophic cascades and diversity loss.

9. Human activities (deforestation, climate change, overfishing, invasive species, fragmentation) are now major disturbances, generally reducing community stability and biodiversity.

10. Applications: This knowledge is used in conservation biology (protecting keystone species), restoration ecology (succession management), ecosystem management, and predicting climate change impacts.