L13 modern coexistence theory 1 Flashcards
(51 cards)
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).
