Unit 3: Stability of Communities — Keystone Species & Ecosystem Modifiers - Video 3

Keystone Species

  • Concept & Origin
    • Term coined by Robert Paine (often miss-spelled “Payne”) after his intertidal‐zone research.
    • Metaphor: the top “keystone” brick in a mortar-less stone arch; once removed, the entire arch collapses. Similarly, a keystone species holds a community together.
  • Core Definition
    • A species whose biomass is disproportionately low relative to its community-wide impact on species diversity/richness.
    • Typically carnivores (secondary/tertiary consumers); rarely plants because plants generally represent large community biomass.
  • Robert Paine’s Sea-Star Experiment
    • Study area: Two 8-m stretches of Washington-state rocky intertidal zone, ≈2 m deep tide pools (~25 ft × 6 ft).
    • Design:
    • Control pools: sea stars (Pisaster spp.) left undisturbed.
    • Experimental pools: weekly manual removal of every sea star for 2, 5, 10, and 12 yr follow-ups.
    • Results:
    • Control: species richness stable at ~15 spp.
    • Experimental: species drop from 15 → 8 (2 yr), then to 2 spp. (5 yr: 1 mussel + 1 barnacle).
    • Mechanism:
    • Sea stars prey broadly, keeping each prey’s population well below carrying capacity, KK, preventing intense inter- & intra-specific competition.
    • Without predators, prey populations grow logistically dNdt=rN(1−NK)\frac{dN}{dt}=rN\left(1-\frac{N}{K}\right), quickly filling niche space; competitive exclusion eliminates weaker competitors.
  • Ecological Significance
    • Maintains species richness by suppressing competitive exclusion.
    • Demonstrates community-level carrying capacity concept (aggregate of all competing species relying on the same resources).
  • General Traits of Keystone Species
    • Low numerical abundance/biomass.
    • Broad diet breadth (often generalist predators).
    • Cascading, community-wide effects when removed.

Competitive Exclusion & Niche Context

  • Competition = minus–minus (both competitors pay a cost).
  • Gause’s Competitive Exclusion Principle: two species with identical niches cannot coexist indefinitely; one outcompetes the other.
  • Keystone predation suppresses densities such that niches do not completely overlap at carrying capacity, preventing exclusion.

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

  • Definition & Distinction from Keystone Species
    • Species that physically alter the environment, thereby changing which suites of organisms can inhabit an area.
    • Presence/absence shifts community composition type, not necessarily total richness.
    • Both keystones & modifiers increase diversity, but via different mechanisms:
    • Keystone → trophic interactions (biological control).
    • Modifier → habitat transformation (physical/chemical modification).
Beaver (Castor canadensis) — Flagship Ecosystem Modifier
  • Historical Abundance
    • Pre-European estimates: 60–400 million across N. America (tundra → Mexican deserts).
    • Fur trade (1600s–1900): e.g., 80 000 trapped in one decade in New York.
    • By 1900: near extinction; modern rebound to 6–12 million (<10 % of original).
  • Dietary Preferences
    • Leaves, twigs, bark of hardwoods; dislike spruce/conifers.
    • Favor willows, aspens (shade-intolerant pioneer spp.).
  • Dam Construction & Immediate Effects
    • Collection of logs, branches, mud → dam → pond backs up.
    • Progressive enlargement as more trees felled; steep hillsides eventually halt lateral water escape.
    • Lodge built within pond; underwater entrance → thermal refuge & predator avoidance.
  • Hydrological & Aquatic Impacts
    • Converts lotic (stream) → lentic (pond) system.
    • “Stepping-lake” profile: series of ponds along stream continuum.
    • Slower flow → sediment retention, ↑ organic matter, ↓ turbidity downstream.
    • Warmer, well-lit water → plankton bloom, productivity boost; total biomass in pond 2–5× that of equivalent-volume stream.
    • Acid-neutralizing capacity ↑ via CaCO₃ leaching where pond edges contact upland soils.
    • Macroinvertebrate diversity & abundance ↑ → trophic upsurge attracting predators (fish, amphibians, birds, mammals).
  • Terrestrial & Vegetative Impacts
    • Up to 1 metric ton of wood cut per year per colony.
    • Canopy opening → sunlight on soil → rapid pioneer growth (willow/aspen).
    • New shoots richer in nitrogen → herbivore attraction; herbivore waste further fertilizes soil.
  • Successional Outcomes When Beavers Leave
    • Short-lived dams break → sediment-filled beaver meadow (fertile, sun-lit).
    • Long-lived, intact dams may persist or progress to bogs (acidic, partially decomposed peat mats; e.g., cranberry bogs) if decomposition lags.
  • Broader Ecological Role
    • Template for understanding how physical habitat alteration drives community turnover.
Other Notable Ecosystem Modifiers
  • Alaskan moose — wallows create ephemeral wetlands.
  • American alligator — “gator holes” concentrate water, refuge for fish & amphibians.
  • Prairie dogs — vegetation clipping forms short-grass patches, aerates soil.
  • African/Asian elephants — tree felling → grassland pockets in savanna/forest mosaics.

Additional Keystone Examples

  • Alewife (Alosa pseudoharengus) — small fish dramatically restructuring zooplankton & native fish assemblages in Great Lakes.
  • Western coyote (Canis latrans) — top-down control; removal can precipitate mesopredator release, rodent outbreaks, potential plant-community collapse.

Ethical & Conservation Implications

  • Because keystone species often have low biomass, they can be overlooked until removal triggers drastic collapse; underscores need for precautionary management.
  • Ecosystem modifiers illustrate how habitat engineering species amplify heterogeneity; restoration plans frequently re-introduce such engineers (e.g., beaver re-wilding).
  • Detecting keystones/modifiers generally requires manipulative or natural-experiment evidence (e.g., removal, extirpation, or reintroduction).

Connections to Previous & Upcoming Topics

  • Builds on earlier discussions of competition, niche theory, and logistic growth.
  • Sets stage for forthcoming lectures on:
    1. Energy flow through ecosystems (food-web energetics).
    2. Human population growth dynamics (Unit 3 conclusion).