Chapter 9 - From populations to community (CHAPTER) Flashcards
Abundance and Distribution
Determined by a combination of abiotic (conditions, resources) and biotic (competition, mutualism, predation, parasitism) factors.
Context is key
Populations must be viewed within the context of the whole community and patchy environments.
Data goes beyond numbers
Satisfactory studies estimate age classes, sexes, and size groups, not just total numbers.
Correlations vs. Causation
Correlations with external factors (food, weather) can help predict but don’t prove causal relationships; mechanisms are needed for proof.
Fluctuation and Stability
Populations can show overall stability with underlying complexity (irruptions, temporary trends).
Determination of Abundance
Reflects all factors affecting a population, density-dependent or independent.
Regulation of Abundance
Focuses on density-dependent processes (competition, predation, parasitism) that keep populations within limits.
k-value Analysis (Key Factor Analysis)
Identifies key phases in a life cycle by measuring mortality (k-value) in each phase.
- Calculated from life tables.
- Helps determine factors influencing mortality fluctuations and regulation.
Density Dependence
Mortality greatest when density is highest; indicates a potential role in regulation.
Limitations of k-value Analysis
More complex alternatives exist.
Migration Importance
Can be vital in determining and/or regulating abundance.
Patchy Populations
Abundance determined by size and distance of habitable sites and species’ dispersal distance.
Metapopulation
A collection of subpopulations with a realistic chance of extinction and recolonization.
- Emphasis on colonization and extinction of subpopulations, not just local birth/death.
- Can persist stably through a balance of extinctions and recolonizations of subpopulations.
Theory of Island Biogeography (MacArthur & Wilson)
Introduced balance of extinction and colonization on islands.
Levins’s Model
Formalized metapopulation concept with dynamics at individual and patch levels.
Metapopulation Dynamics
Fraction of occupied patches (p(t)) changes based on colonization and extinction rates.
Types of Metapopulations
Continuum from nearly identical subpopulations to those with some effectively stable.
Source and Sink Networks
Some networks (sources) provide colonizers for others (sinks), preventing extinction.
Transient Behaviour
Metapopulation dynamics can be far from equilibrium.
Distinguishing True Metapopulations
Recolonization by dispersal (not just germination from seed banks) is crucial.
Plant Metapopulations
Exist, and molecular methods can help track dispersal and colonization.
Disturbances and Gaps
Common and lead to local extinctions, creating opportunities for colonization.
Founder-Controlled Communities
All species are good colonists and equal competitors; species richness maintained by competitive lotteries.
- Priority effects can occur where the first colonizer holds the gap.
Dominance-Controlled Communities
Some species are competitively superior; succession is more predictable.
- Early species are good colonizers and fast growers.
- Later species tolerate lower resources and outcompete early species.
- Succession involves an increase then decrease in species number.
Primary Succession
Colonization of newly exposed substrates with no remaining seeds or spores.
Secondary Succession
Re-establishment after disturbance where seeds and spores remain.
Chronosequences
Spatial community gradients can represent temporal succession stages.
Old-Field Succession
Succession on abandoned farmland with predictable stages.
- Early succession plants (fugitives) prioritize rapid growth and dispersal.
- Later succession plants are slower growing with longer lifespans and heavier seeds.
Animal Succession Follows Plants
Changes in animal communities often reflect changes in plant communities.
Climax Community
A relatively stable end stage of succession; may not always be reached due to further disturbances.
Patch Dynamics
Succession occurs in a mosaic of patches at different stages, influenced by patch size and shape.
Conservation and Succession
Maintaining specific successional stages may be crucial for endangered species.
Complex Interactions
Species exist within a web of interactions (predator-prey, parasite-host, grazer-plant, competitors) across trophic levels.
Direct vs. Indirect Effects
Species can affect others directly or indirectly through pathways in the food web.
Trophic Cascade
A predator’s effect cascades down trophic levels, e.g., predators reduce herbivores, increasing plants.
- Can occur across multiple trophic levels.
- Strength can depend on food web complexity and nutrient availability.
Top-Down Control
Higher trophic levels control lower levels (e.g., predators controlling herbivores).
Bottom-Up Control
Lower trophic levels influence higher levels (e.g., resources limiting consumers).
Combination of Controls
Top-down and bottom-up effects often interact.
“Why is the World Green?” Debate
Top-down view: Predators keep herbivores in check, allowing plants to accumulate biomass.
Bottom-up view: Plant defenses and limited palatable resources also restrict herbivore populations.
Meta-Analyses
Suggest predator manipulation has a significant negative effect on herbivores and a positive trophic cascade on plants. Fertilization shows bottom-up effects but less consistently across trophic levels.
Stability Definitions
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- Resilience
- Resistance
- Fragile stability
- Robust stability
Resilience
Speed of return to former state after disturbance.
Resistance
Ability to withstand change in the face of disturbance.
Fragile Stability
Stable to small disturbances, unstable to large ones.
Robust Stability
Remains roughly the same despite large disturbances.
Keystone Species
Impact on community composition is disproportionately large relative to its abundance; removal leads to significant changes.
Complexity-Stability Debate
Historically thought increased complexity leads to increased stability.
Mathematical models often suggest increased complexity decreases the resilience of individual populations.
Models suggest increased complexity (especially richness) can increase the stability of aggregate community properties (biomass, productivity) due to statistical averaging.
Compartmentalization
Tendency for subsets of species to interact more strongly within themselves than with other species.
- More compartmentalized food webs may be more resilient to perturbation.
- Compartmentalization can arise from habitat differences, morphological constraints, and trophic level similarities.
Empirical Evidence
Population stability may weakly decrease with richness.
Aggregate community stability (biomass, respiration) often increases with richness.
Community composition may be a better predictor of stability than overall richness.
Environment and Stability
Dynamically fragile complex communities may persist in stable environments, while robust simple communities are favoured in variable environments.
Human Perturbations
May have the most profound effects on dynamically fragile, complex communities in stable environments.
