Ecological community struc Flashcards
What are ecological communities, and how do they change over time?
- = Groups of potentially interacting organisms.
- Do not evolve in the strict sense but change in composition, diversity, and structure.
- Assembly Processes: Governed by local or regional processes. These processes influence emergent ecological and evolutionary patterns.
What is the competitive exclusion principle, and how was it studied?
- Two species with similar niches cannot coexist; one will outcompete the other.
- Joseph Grinnell (1904) studied chestnut-backed chickadee distributions in North America
- Natural history perspective, observing niche-based competition.
- Observations form the basis for theories on the role of competition in structuring ecological communities.
How did MacArthur explain warbler coexistence?
Observed niche partitioning in warblers feeding on evergreen trees.
Although diets overlap, behavioral differences (feeding on different parts of the tree) reduce competition.
What is limiting similarity, and how does Hutchinson’s ratio relate?
- There is a minimum niche difference required for coexistence.
- Hutchinson’s Ratio: Co-occurring species often differ in size by ~1.3, indicating consistent ecological separation.
- While there’s no ‘golden ratio’, there is partitioning
- lead to Diamond’s controversial proposal of community assembly rules…
What are community assembly rules?
Diamond observed that certain species combinations are “forbidden” = checkerboard distributions
- Example: Pacific Island Kingfishers (Genus Todiramphus), where each island is occupied by only one species, never both (due to competition over similar resources)
How does phylogeny help explain community structure?
- Phylogenetic Clustering (more related than expected by chance): Habitat filtering groups closely related species.
- Phylogenetic Overdispersion (less related than expected by chance): Competition spreads distantly related species.
- Niche conservatism means related species retain similar niches over time.
What is the role of null models in studying community structure?
- Randomization Models compare observed patterns against random expectations.
- Measure phylogenetic distance metric
→ if observed value falls outside random distribution then its likely NOT due to chance:
Less (phlogenetic distance) = clustering (habitat filtering)
More (distance) = overdispersion (competition) - Limitations: Cannot infer underlying ecological/evolutionary processes
What is the DAMOCLES model, and how does it improve community analysis?
- Dynamic Assembly Model of Colonisation, Local Extinction, and Speciation
- Simulates species colonization, extinction, and persistence over time.
- Neutral-like assumptions: No explicit competition or niche filtering, only stochastic processes.
- Baseline for comparison: Helps test if real ecosystems deviate from random assembly.
DAMOCLES provides a null expectation for biodiversity patterns, revealing when non-random processes shape communities.
What does community structure in Manu National Park reveal?
Random null model vs dynamic null model
High Extinction Rates
- Community structure matches the random null model (species are randomly distributed).
- Reason: Species go extinct quickly, so there’s less time for competition or specific interactions to shape distribution.
Low Extinction Rates
- Community structure shows overdispersion (species are evenly spaced).
- Species persist longer, and historical factors like speciation, extinction, and colonization influence distribution, not just competition.
Misinterpretation: Using only the random null model could wrongly suggest competition when community patterns reflect historical processes.
What are the +/- of null models in testing the causes of community structure
+ Powerful tool to test hypotheses on the causes of community structure
+ But, limited so must consider the fundamental evolutionary processes that determine species distribtion
Describe a habitat we would expect to see phylogenetic clustering in.
In harsh environments (e.g. deserts) species that share adaptations to extreme conditions (e.g., water conservation traits in cacti and succulents) may be closely related
Describe a habitat we would expect to see phylogenetic overdispersion in.
In tropical rainforests, coexisting tree species may be phylogenetically distant because competition for light, water, and nutrients favors species with distinct functional traits.
Give an example of when DAMOCLES is used?
🌴 Island Biogeography & Community Assembly
- Use DAMOCLES to predict species richness on islands over time.
- The model assumes only colonization and extinction (no competition or niche differences).
- If real island diversity exceeds DAMOCLES predictions, it suggests ecological interactions (e.g., competition, adaptation) influence species assembly.
🔍 Example DAMOCLES has been applied to Caribbean lizards to test if their diversity is shaped by random processes or competition-driven extinction.
DAMOCLES helps distinguish between stochastic colonization/extinction and non-random ecological processes.
Why use random null models if we know ecology drives speciation and extinction?
- Detects Non-Random Processes: If real data differs from a null model, it suggests ecological forces (e.g., competition, filtering) are shaping biodiversity.
- Sets a Baseline for Comparison: Null models show what patterns would look like under randomness, helping identify when additional processes are at play.
- Distinguishes Between Hypotheses: Helps separate random extinction from competition-driven exclusion or other evolutionary mechanisms.
Null models test whether ecological processes are truly needed to explain biodiversity patterns, preventing overinterpretation
