Community Stability: Keystone Species & Ecosystem Modifiers

Keystone Species

  • Keystone species: organisms whose ecological impact on community structure and diversity is disproportionately large relative to their low biomass.
  • Concept metaphor: the keystone in a freestanding stone arch.
    • Keystone pushes laterally on adjacent stones → distributes forces → entire arch remains intact.
    • Remove keystone → arch collapses.
  • Mechanistic rationale
    • Feeding activities of a few predators suppress prey abundances.
    • Populations remain well below carrying capacity KK, preventing intense interspecific and intraspecific competition.
    • Result: greater species richness and stable community composition.
  • Typical traits
    • Usually carnivores (secondary or tertiary consumers).
    • Low total biomass compared with total community biomass.
    • Broad diet spanning multiple prey species.

Robert T. Paine’s Intertidal Sea-Star Experiment (Landmark Study)

  • Study system: rocky intertidal pools (≈8 m long × 2 m deep; ~25 ft × 6 ft).
  • Design
    • Control site: sea stars (Pisaster) left intact.
    • Experimental site: weekly manual removal of every sea star for many years.
  • Temporal scale of monitoring: 2, 5, 10, 12 years.
  • Findings on species richness
    • Control: remained ≈15 species throughout.
    • Experimental: declined from 15 → 8 species (2 yrs) → eventually only 2 species (5 yrs) — one barnacle + one mussel.
  • Interpretation
    • Sea stars preyed on dominant competitors (barnacles & mussels), keeping their numbers low.
    • Without the predator, competitive exclusion unfolded: species with identical/overlapping niches cannot coexist indefinitely; superior competitors displaced others.
    • Demonstrated that a predator with small biomass could “hold together” a diverse community.
Competitive Exclusion Refresher
  • Interaction type: competition = (−,−)(-,-) relationship; both competitors incur costs.
  • Principle: two species with identical niches cannot coexist; one outcompetes the other.
  • Logistic growth context: population growth described by dNdt=rN(1−NK)\frac{dN}{dt}=rN\left(1-\frac{N}{K}\right).
    • Competition effects become strong near the inflection point (mid-K).
    • Predation keeps N≪KN \ll K ⇒ competition minimized.

Definition & Diagnostic Criteria for Keystone Species

  • Definition: A species that, despite low biomass, exerts a large influence on community structure and species richness.
  • Biomass criterion: individual species’ mass ≪ total community biomass.
  • Trophic level: commonly top predators; herbivorous keystones are rare because plant biomass must exceed herbivore biomass due to trophic energy loss.

Ecosystem Modifiers (a.k.a. Ecosystem Engineers)

  • Similarity to keystone species: presence/absence alters diversity and community composition.
  • Distinction: they physically modify habitat, thereby determining which guilds can live there.
  • Effect: With modifier present → one suite of species; without → entirely different assemblage.

Case Study: Beavers as Ecosystem Modifiers

  • Taxonomy: largest North-American rodent.
  • Historical abundance: 60 – 400 million pre-European; plummeted to near extinction (~1900) due to trapping.
    • Example record: 80 000 pelts/yr (1630-1640, New York).
    • Modern estimate: 6–12 million (<10 % of original).
  • Diet & tree preferences
    • Consume leaves, twigs, bark of hardwoods; avoid conifers (high lignin).
    • Favor pioneer species (willow, aspen) that grow in high-light riparian zones.
  • Dam building sequence
    • Fell trees → construct stick/mud dams across streams.
    • Creates backed-up pond; enlarges via further tree removal.
    • Build central lodge (partially below water) for predator refuge & winter insulation.
  • Aquatic consequences
    • Converts lotic (fast-flowing) stream to lentic pond.
    • Inundation increases soil-water contact → enhanced sediment & organic matter retention.
    • Community shift: stream taxa (e.g.
      mayflies) → pond taxa (e.g.
      dragonflies, filtering clams).
    • Biomass: pond = 2–5× equivalent stream volume.
  • Terrestrial consequences
    • Up to 1 metric ton of wood cut per colony per year.
    • Canopy opening → increased sunlight → proliferation of pioneer plants.
    • Soil nitrogen rises; attracts herbivores; faunal waste further fertilizes site.
  • Post-beaver succession
    • If lodge/dam persists & wood decomposes → soil accrues → beaver meadow with high fertility.
    • If acidic, waterlogged conditions dominate → development of bog (thick peat mat).

Additional Examples

  • Keystone Species
    • Alewife (Great Lakes): controls zooplankton; removal alters fish community.
    • Western coyote: predator whose absence can trigger trophic cascade and system collapse.
  • Ecosystem Modifiers
    • Alaskan moose: create wallows, altering hydrology & vegetation.
    • American alligator: excavates gator holes; shapes wetland mosaics.
    • Prairie dogs: burrowing aerates soil & influences plant communities.
    • Elephants: knock down trees, convert woodland to savanna.

Broader Implications for Community Stability & Biodiversity

  • Not every community contains a keystone or modifier, but many more exist than currently recognized.
  • Discovery usually occurs via removal experiments (intentional or accidental) revealing cascading effects.
  • Management considerations
    • Conservation of low-biomass, high-impact species is critical.
    • Ecosystem restoration may require re-establishing both keystone predators and physical engineers.
  • Upcoming lectures: (1) Energy flow in ecosystems, (2) Human population growth – concluding Unit 3.