L15 modern coexistence theory and community assembly Flashcards
What is the focus of today’s lecture?
How Modern Coexistence Theory informs Community Assembly.
What does Modern Coexistence Theory (MCT) build on?
The idea that species’ interactions determine coexistence, assuming concepts like stabilizing vs. equalizing mechanisms.
What are the three main steps in the community assembly process?
- Regional Species Pool & Local Patches
- Environmental Filtering
- Formation of the Local Community
What is a Regional Species Pool & Local Patches?
A large pool of potential species and multiple smaller habitats each harboring a subset of those species.
What is Environmental Filtering?
Abiotic conditions acting as a filter—only species whose Fundamental Niche includes those conditions can pass.
How is a Local Community defined in community assembly?
The set of species (alpha diversity) present in a patch after abiotic filtering and biotic interactions.
Define Alpha Diversity.
Species richness within a local patch.
Define Beta Diversity.
Turnover or dissimilarity in species composition between patches.
Define Gamma Diversity.
Total species richness across the entire region (the full Regional Species Pool).
What is the Fundamental Niche?
The set of abiotic conditions under which a species can persist without biotic interactions.
What is the Realized Niche?
The subset of the Fundamental Niche after accounting for biotic interactions (competition, predation, etc.).
What role does Local Competition play in community assembly?
After passing the abiotic filter, species compete biotically; weaker competitors are excluded, narrowing the Realized Niche.
What is Dispersal Limitation?
Chance-based absence of species in patches due to limited arrival, even if conditions are suitable.
What are Priority Effects?
Historical sequence of colonization determining community outcomes when multiple stable states exist (e.g., Lotka–Volterra model scenarios).
How do Deterministic and Stochastic factors differ in community assembly?
Deterministic factors yield predictable outcomes from abiotic/biotic filters; stochastic factors involve chance and history (dispersal events, priority).
In the hypothetical tree community example, how would environmental filtering shape trait distributions?
Co-occurring species at each site should exhibit similar trait values matching local environmental conditions (phenotypic convergence).
What are the five most critical concepts summarized for community assembly?
- Environmental Filtering drives local establishment
- Realized Niches are narrower than Fundamental Niches
- Alpha, Beta, Gamma Diversity quantify local, between-site, and regional richness
- Dispersal Limitation & Priority Effects introduce stochasticity
- The balance of Deterministic vs. Stochastic processes is a central debate
Why move from species presence–absence to trait-based analysis in community assembly?
Traits are the direct targets of environmental and competitive filters, allowing clearer tests of assembly mechanisms.
What is Specific Leaf Area (SLA)?
The ratio of leaf area to dry mass; high SLA means thin, expansive leaves, low SLA means thick, dense leaves.
How does high SLA function ecologically?
It supports high light capture and growth rates where water is plentiful by fueling a strong transpiration stream.
What ecological advantage does low SLA confer?
Reduced water loss in arid or drought-prone environments.
What is the objective of null models and bootstrapping in trait analysis?
To determine if observed within-site trait distributions differ from random expectations.
What is the procedure for trait-based null models?
Calculate observed trait statistic per site, randomly reassign species to sites many times, compute the statistic for each draw, and compare to the observed.
What does trait clustering (observed variance < null variance) indicate?
Evidence of environmental filtering—co‐occurring species share similar traits.
What does trait overdispersion (observed variance > null variance) indicate?
Evidence of niche differentiation—competition drives trait divergence.
How was Kraft et al.’s Amazonian forest case study designed?
A 25 ha plot with ~150,000 mapped trees (~1,100 species), divided into 20×20 m quadrats; traits (e.g., SLA, seed mass, wood density) measured per species; species labels shuffled among trees to build null distributions.
What did Kraft et al. find about SLA and topography?
SLA clustered by terrain: lower SLA on ridge tops and higher SLA in valleys, showing environmental filtering.
How did Kraft et al. detect local competition via seed mass?
Within quadrats, observed seed mass variance exceeded null expectations, indicating overdispersion from competition.
What ontogenetic shift did Kraft et al. observe?
Saplings’ trait distributions matched random expectation, while adults showed stronger trait structuring as competition thinned similar individuals.
What is phylogenetic clustering and what does it imply?
Co‐occurring species are more closely related than expected, implying environmental filtering on conserved traits.
What is phylogenetic overdispersion and what does it imply?
Co‐occurring species are more distantly related than expected, implying competition driving divergence in conserved traits.
Why integrate abiotic, biotic, and evolutionary perspectives in community assembly?
Combining filters (abiotic/biotic) with phylogenetic context yields a richer, multiscale understanding of how communities form.
Why incorporate phylogeny into community assembly analyses?
Phylogeny reveals evolutionary conservation of traits, helping infer whether filtering or competition drives community patterns.
What was the design of the Florida oak community case study?
Seventeen oak species in Florida, grouped into red, white, and live oak clades; measured each species’ mean soil moisture preference and mapped them on a phylogeny.
What pattern of soil moisture preference emerged among Florida oaks?
Closely related species often occupied contrasting moisture niches, showing phylogenetic overdispersion.
What does phylogenetic overdispersion of a trait indicate?
Divergent adaptation within clades and niche partitioning driving diversification rather than conserved trait filtering.
How was field co-occurrence analyzed in the oak study?
By calculating pairwise co-occurrence probabilities and comparing against phylogenetic distances among species.
What did the oak co-occurrence analysis reveal within each clade?
Specialists on dry, intermediate, and wet niches evolved in each clade, and co-occurrence was higher among phylogenetically distant specialists.
What is a key critique about defining the Regional Species Pool in trait studies?
Arbitrary spatial boundaries can bias results, making ‘regional pool’ delineations appear random.
How does spatial scale sensitivity affect trait-based community assembly findings?
The size of sampling units (e.g., quadrat area) can change whether filtering or competition signals are detected.
Why is separating environmental filtering from competition challenging?
Observed trait clustering may conflate abiotic filtering with competitive exclusion if both produce similar patterns.
What limitation does experimental realism impose on trait-filter studies?
Only about 15% of studies properly isolate filtering and competition, making causal inference weak.
How does Modern Coexistence Theory (MCT) refine classical trait-pattern predictions?
By partitioning coexistence into niche differences (stabilizing) and fitness differences (equalizing), tailoring expectations per trait.
Under MCT, what pattern is expected for traits linked to fitness differences?
Phenotypic convergence (clustering) reduces fitness disparities and promotes coexistence.
Under MCT, what pattern is expected for traits linked to niche differences?
Phenotypic overdispersion (divergence) maximizes niche separation and promotes coexistence.
How do classical vs. MCT perspectives differ on interpreting competition signals?
Classical: competition always yields overdispersion; MCT: competition may yield clustering for fitness traits and overdispersion for niche traits.
Describe the algal competition experiment using MCT.
Monoculture growth assays for fitness, mutual invasion trials for invasibility, across 8 freshwater green algae species.
Which MCT metrics predicted coexistence in the algal experiment?
Niche differences (invasion-rate divergence) predicted coexistence; fitness differences did not.
What was the phylogenetic signal outcome in the algal MCT study?
No phylogenetic signal for coexistence, niche, or fitness differences—key traits were evolutionarily labile.
Why is classical trait-clustering vs. overdispersion theory considered overly simplistic?
It ignores trait-specific roles in fitness vs. niche axes and evolutionary lability, leading to mixed or misleading signals.
What outstanding challenges remain for community assembly research?
Defining regional pools and scales, linking field trait patterns to coexistence mechanisms, and integrating variable phylogenetic signals.
How does trait phylogenetic signal affect assembly predictions?
Conserved traits yield clear clustering/overdispersion interpretations, while labile traits weaken phylogenetic inference.
Why must co-occurrence analyses account for clade-level niche partitioning?
Because closely related species may diverge ecologically within clades, masking true filtering or competition drivers.
