Community Stability: Keystone Species & Ecosystem Modifiers

Lecture Scope & Objectives

• Focus: factors determining community stability and species composition, specifically exploring how species interactions and physical habitat modifications influence the abundance and distribution of other species within an ecosystem.

• Core themes

  • Keystone species: precise definition, underlying mechanisms of their impact, and landmark studies that established the concept.

  • Ecosystem modifiers (also known as ecosystem engineers): fundamental similarities and contrasts to keystone species.

  • Case studies: In-depth examination of Robert Paine’s classic intertidal sea-star experiment, the ecological role of North-American beavers, and a brief survey of other significant taxa.

• Connections to earlier material

  • Competition (minus–minus interactions): how competitive dynamics are influenced by keystone species or ecosystem modifiers.

  • Logistic population growth $\frac{dN}{dt}=rN\left(1-\frac{N}{K}\right)$ and carrying capacity ($K$) concepts: understanding how population growth patterns are altered by key species interactions.

  • Niche theory & Competitive Exclusion Principle: how these principles explain community structure changes when key species are removed or introduced.

Keystone Species: The Concept

• Architectural metaphor

  • Keystone in a freestone arch: the wedge-shaped, central block at the apex of an arch that, despite its relatively small size, is indispensable for distributing compressive forces and locking all other stones in place; its removal causes the entire arch to collapse.

  • Analogy in ecology: certain species exert a community-wide stabilizing force that is vastly disproportionate to their actual biomass or numerical abundance within that community.

• Working definition in ecology

  • Species whose feeding, behavioral, or other ecological activities have an influence on community structure (e.g., species richness, trophic structure, primary productivity) far greater than would be predicted from their total biomass or numerical abundance in the system.

  • Typically carnivores at secondary or tertiary consumer levels (e.g., apex predators); occasionally exceptional herbivores (e.g., elephants) can also act as keystones by preventing competitive exclusion or facilitating habitat diversity.

• Quantitative expectation

  • These species will have a low biomass (small $\sum<em>{i\in ks} B</em>i$, where $B_i$ is the biomass of individual i within the keystone species group) but will exert a high impact on overall species richness ($S$) and ecosystem function.

Robert Paine’s Intertidal Sea-Star Removal Study

• Study design

  • Location: Exposed rocky intertidal pools along the coast of Washington State in the Pacific Northwest, a dynamic environment characterized by strong wave action and tidal fluctuations.

  • Two 8-m stretches (~25 ft long, >6 ft deep) of shoreline were selected:

    • Control site: The natural community was left entirely intact to serve as a baseline.

    • Experimental site: All individuals of the keystone sea star species, Pisaster ochraceus (a voracious predator), were manually removed on a weekly basis to maintain a predator-free environment.

• Temporal scale

  • Long-term monitoring was conducted at $t=0$ (initial state), then continuing at $2, 5, 10, 12$ years, demonstrating the long-term ecological consequences of predator removal.

• Findings

  • Initial diversity at both sites: $S=15$ species, indicating similar starting conditions.

  • Experimental site trajectory: Species richness dramatically declined. From $S=15$, it rapidly dropped to $S\rightarrow 8$ species within 2 years, and further decreased to approximately $S\approx2$ species within 5–12 years. The community became dominated by a single species of barnacle (Balanus glandula) and a single species of mussel (Mytilus californianus).

  • Control site: Species richness remained stable at $S\approx 15$ species throughout the 12-year study, demonstrating the ongoing role of the sea star.

• Mechanistic interpretation

  • Sea stars, as generalist predators, prey broadly on competitively dominant species like mussels and barnacles, along with other invertebrates. This predation keeps their populations well below their carrying capacities ($K_i$).

  • When the predator is absent, these competitively superior prey populations undergo competitive release. Their numbers rise unimpeded toward their respective carrying capacities ($K_i$), leading to intense inter- and intra-specific competition for limited resources (e.g., space on rocks, light).

  • Competitive Exclusion Principle: Species with highly overlapping or identical niches cannot coexist indefinitely if resources are limited. Eventually, the competitively superior species (mussels and barnacles in this case) outcompete and displace the weaker competitors.

  • Predator-mediated coexistence: The presence of sea stars actively maintains high species richness by preventing one or a few dominant competitors from monopolizing resources and excluding other species.

Mechanistic Links to Population Theory

• Logistic curve reminder: Population competition pressures accelerate significantly as a population approaches its carrying capacity, particularly near the inflection point, often approximated at $N = \frac{K}{2}$, where growth rate is maximal but resource limitation begins to exert strong effects.

• Keystone predator function: A keystone predator effectively keeps each prey’s population size ($N$) in a region where $N \ll K$. This prevents populations from reaching densities where interspecific competition becomes severe, leading to minimal overlap for shared resources and negligible competition coefficients ($\alpha_{ij}$) in the Lotka–Volterra interspecific competition models.

• Community-level “aggregate carrying capacity” concept: By suppressing dominant competitors, keystone predators facilitate a more equitable partitioning of the total resource base. Instead of resources being monopolized by a few high-density species, they are distributed among many low-density species, supporting higher overall biodiversity.

Formal Definition Recap

• Keystone species criteria:

  1. Biomass proportion: Their total biomass is a very small fraction ($\ll$) of the total community biomass.

  2. Impact on community: Their removal or significant reduction causes a marked and often rapid drop in species richness and/or evenness across multiple trophic levels.

  3. Trophic position: Usually top predators or broad omnivores. Plant keystones are rare because, at the base of the trophic energy pyramid, their biomass typically comprises a very large proportion of the ecosystem's total biomass, which contradicts the low-biomass criterion of a keystone species. Exceptions exist for plants that create complex habitats.

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

• Similarities with keystones

  • Both types of species, through their presence or absence, significantly alter the diversity and composition of ecological communities.

• Key differences

  • Mode of action: Ecosystem modifiers primarily modify the physical habitat structure (e.g., creating burrows, felling trees, building dams), rather than exerting top-down trophic control through predation.

  • Trophic level & biomass: Often herbivores or detritivores, and are not necessarily characterized by low biomass relative to their impact; their impact is due to their physical transformations.

  • Community outcome: While keystone species often prevent ecological collapse or major richness reduction, ecosystem modifiers tend to shift which species dominate or introduce entirely new community types, rather than solely preventing a collapse of total richness. They create new niches.

Beaver (Castor canadensis) Case Study

Historical context

• Pre-1600s population estimates: Ranged widely from $60\times10^6$ to $400\times10^6$ individuals across North America, indicating their widespread historical ecological influence.

• Over-trapping: The fur trade led to massive declines, with, for example, approximately $8\times10^4$ skins/year being exported from New York alone during 1630–1640. This, combined with extensive habitat loss due to agricultural expansion, led to their near extinction by 1900 in many areas.

• Modern rebound: Conservation efforts and reintroductions have led to a recovery, with current populations estimated at $6\times10^6$ to $12\times10^6$ individuals, though this still represents less than 10% of their original population size.

Feeding preferences & dam building

• Diet: Primarily bark, leaves, and twigs of fast-growing deciduous (hardwood pioneer) trees such as willow, aspen, birch, and maple. They generally avoid conifers due to their high lignin content, which makes them less digestible.

• Behaviour sequence:

  1. Select stream section: Beavers choose suitable stream sections, often those with moderate flow and available woody vegetation.

  2. Fell trees & construct dam: They fells trees using their incisors and construct intricate stick-and-mud dams. These dams slow water flow and create impoundments.

  3. Pond formation & food access: As water backs up, a pond forms, flooding new areas and making previously inaccessible trees at the new edge available as a food source.

  4. Build central lodge: A large lodge, typically constructed from sticks, mud, and rocks, is built within the pond with an underwater entrance. This provides a secure refuge from terrestrial predators and a warm, insulated environment during winter.

  5. Family units & expansion: Beaver colonies usually consist of 4–6 individuals (a parental pair and their offspring). Older offspring may either expand existing ponds by building secondary dams or disperse to establish new territories.

Aquatic impacts

• Hydrology:

  • Conversion: Transform swift, lotic (flowing) stream habitats into a series of interconnected, lentic (still or slow-moving) ponds, creating a