L13 modern coexistence theory 1 Flashcards
What is Modern Coexistence Theory?
A framework explaining how multiple species share environments by identifying mechanisms (e.g., niche differences) that stabilize coexistence.
How does Neutral Theory differ from Modern Coexistence Theory?
Neutral Theory assumes ecological equivalence—all species have identical birth, death, dispersal, and fecundity rates—so community dynamics are driven purely by stochastic drift.
What is a limiting resource?
A resource consumed by organisms whose depletion reduces others’ growth (e.g., soil water, nitrates for plants; prey for animals).
What does the Competitive Exclusion Principle state?
Two species sharing a single limiting resource cannot stably coexist if one species is strictly superior at depleting it.
In R* theory, which species wins competition?
The species that reduces the resource to the lowest equilibrium level while sustaining its population; the other species goes extinct.
What is niche differentiation?
Variation in ecological conditions (e.g., soil types, microhabitats) allowing species to specialize on different resources or conditions, avoiding direct competition.
How is each species’ niche defined?
By the range of conditions and resources under which it maintains positive population growth.
What are the Lotka–Volterra competition equations?
dN₁/dt = r₁N₁(1 – (N₁ + α₁₂N₂)/K₁), dN₂/dt = r₂N₂(1 – (N₂ + α₂₁N₁)/K₂).
What do competition coefficients (αᵢⱼ) represent?
The per-capita effect of species j on the growth of species i in the Lotka–Volterra model.
Under what condition do the Lotka–Volterra equations predict stable coexistence?
When intraspecific competition is stronger than interspecific competition (αᵢᵢ > αᵢⱼ for i≠j).
What is ecological equivalence in Neutral Theory?
The assumption that all species have identical demographic parameters (birth, death, dispersal, fecundity), so no species has a deterministic advantage.
What is recruitment limitation?
Limited dispersal and stochastic arrival of propagules (e.g., seeds, larvae) that prevent deterministic dominance and introduce demographic randomness.
Describe the zero-sum lottery model mechanics.
A fixed number of “sites” are always filled; when an adult dies, all survivors produce equal propagules, and one is drawn at random to fill the vacant site.
How does drift drive community dynamics in the neutral model?
With no deterministic growth differences, stochastic birth–death events (drift) cause random fluctuations in species abundances, leading to local extinctions and turnover.
What key concepts summarize the lecture’s core ideas?
1) Limiting resources & competitive exclusion; 2) Niche differentiation & competition coefficients; 3) Neutral Theory’s ecological equivalence & recruitment limitation; 4) Zero-sum lottery as a stochastic coexistence model.
In Hubbell’s neutral model, how is per capita growth related to species’ abundance?
Independent—higher death probability for common species is exactly offset by higher recruitment chances, so per capita growth is abundance-independent.
What does the logistic growth model describe?
Per capita growth rate: 1/N dN/dt = r(1 – N/K), where r is max per capita growth at low density and K is the carrying capacity.
What empirical evidence demonstrates density dependence?
Serengeti wildebeest data show per capita growth falls as population size rises, preventing unbounded growth.
How does neutral theory enforce community-level regulation?
By fixing the total number of “sites” (zero-sum), replacing species-specific K with a global constraint that caps total individuals.
Describe the illustrative neutral simulation setup and algorithm.
28 fixed sites, 3 species (7, 7, 14); for each of 800 events: randomly kill one individual, then refill the vacant site via an unbiased propagule lottery; repeat.
What key insight comes from varying simulation outcomes?
Only demographic stochasticity (drift) matters—runs differ wildly, with no deterministic pull toward coexistence or exclusion.
Why does neutral drift eventually lead to monodominance?
Random birth–death fluctuations cause species extinctions until one remains; the winner is unpredictable, though large community size slows the process.
How do speciation and immigration balance drift-driven extinctions?
New species arrivals (via speciation or dispersal) counteract random losses, and long adult lifespans slow turnover, sustaining diversity.
What are Species Abundance Distributions (SADs)?
Patterns showing many rare and few common species, visualized as abundance histograms or rank–abundance plots (rank vs. log abundance).
What is MacArthur’s broken stick model?
A neutral, purely mathematical model that randomly partitions a “stick” into abundance segments for species, lacking ecological processes.
How did neutral theory reproduce observed SADs?
Hubbell’s model—using only stochastic birth, death, dispersal, and speciation—matched empirical SADs without species-specific parameters, suggesting randomness can drive macroecological patterns.
What is the key flaw in pure Neutral Theory?
It assumes species are identical (fecundity, fitness, dispersal); even tiny fitness differences collapse the model.
In the example simulation, what happens when one species’ fecundity is boosted from 1,000 to 1,500 seeds?
That species steadily outcompetes and excludes the others—deterministic advantage overrides drift.
What are fitness differences?
Density-independent, constant advantages in per capita growth (e.g., higher fecundity or lower mortality) that don’t depend on other species’ abundances.
What are niche differences?
Mechanisms causing intraspecific competition to exceed interspecific competition, generating negative frequency dependence that stabilizes coexistence.
What does MacArthur’s warblers example illustrate?
Warbler species partition tree branches (resource partitioning), so each species limits itself more than others via niche differentiation.
How do niche differences appear in Lotka–Volterra terms?
As αᵢᵢ > αᵢⱼ, meaning self-limitation exceeds interspecific effects.
How is density-dependent fecundity incorporated into the neutral model?
Per capita fecundity fᵢ is made to decline as conspecific density Nᵢ increases, introducing a self-limiting term.
In the modified fecundity function, what do fₘₐₓ and α represent?
fₘₐₓ is the maximum fecundity at low density; α is the strength of the stabilizing niche effect.
What happens to species abundances when density-dependent fecundity is added?
All species persist with stochastic fluctuations bounded away from zero; rare species receive a fecundity “boost,” making extinctions rare.
How does density-dependent fecundity create negative frequency dependence?
As a species becomes rare, its per capita fecundity rises steeply, increasing its chance to rebound.
What is the effect of reintroducing a modest fitness advantage alongside niche effects?
The advantaged species is more abundant on average, but others still persist because niche stabilization counterbalances the advantage.
When does niche stabilization fail to prevent exclusion?
If the fitness disparity is too large (e.g., fecundity boosted to 3,000), niche effects cannot offset the advantage, leading to exclusion.
What balances species dynamics in the presence of both fitness and niche differences?
The trade-off between the magnitude of fitness differences and the strength of stabilizing niche effects.
What are the two axes in Chesson’s coexistence framework?
Horizontal axis: fitness ratio (magnitude of density-independent differences); vertical axis: stabilization strength (niche, negative frequency dependence).
What are equalizing mechanisms?
Processes that reduce fitness differences, moving species toward demographic equivalence (the neutral axis).
What are stabilizing mechanisms?
Processes that increase niche differences, enhancing negative frequency dependence to stabilize coexistence.
How is the coexistence region defined in the fitness–niche space?
It’s the “wedge” where stabilizing niche strength outweighs fitness differences, so both species can invade when rare.
How is niche overlap ρ related to niche difference?
Niche difference = 1 – ρ; larger values mean stronger stabilizing effects.
How is the fitness ratio defined?
As the ratio of species’ density-independent per capita growth parameters, indicating average competitive asymmetry.
What is the invasion criterion for coexistence?
Each species must have a positive per capita growth rate when rare in the presence of the resident species.
What does mutual invasibility mean?
Both species can invade each other’s equilibrium populations (both have positive invasion growth rates), ensuring stable coexistence.
Why are stabilizing and equalizing mechanisms considered orthogonal?
They act on different components—stabilizing on frequency dependence (niche), equalizing on average growth rates (fitness).
How can an annual‐plant model illustrate Chesson’s framework?
By deriving explicit expressions for niche overlap and fitness ratio from germination rates, seed production, and competition coefficients, then plotting them.
Why is Chesson’s framework valuable?
It quantifies exactly how much niche partitioning and fitness parity are needed to sustain diversity, moving beyond simple neutral vs. niche debates.
What do empirical studies need to estimate to apply Chesson’s framework?
Niche overlap (ρ) and fitness ratios for species pairs, then check if they fall within the coexistence region.
