52 part 2

Community Structure Definition: Key components used to describe a community.
Component 1: Species Composition: A complete list of all species present in a community.
Component 2: Species Richness: The total number of different species in the community.
Component 3: Species Evenness: The relative abundance of each individual species within the community.
Comparative Examples: Sun-grown vs. Shade-grown Coffee:
  - Community 1 (Low Evenness):
    - Species Richness: $6$
    - Species Diversity Index: $1.350$
    - Evenness: Low (dominated by specific species like 'A' and 'B').
  - Community 2 (High Evenness):
    - Species Richness: $6$
    - Species Diversity Index: $1.794$
    - Evenness: High (distribution across species like 'C', 'D', 'E', and 'F' is more balanced).
  - Community 3:
    - Species Richness: $5$
    - Species Diversity Index: $1.610$
    - Evenness: High.

Experimental Site: Cedar Creek Ecosystem Science Reserve.
Diversity and Productivity: Experiments test if species diversity is important by measuring plant community outcomes.
Key Experimental Results:
  - Higher-diversity plant communities are generally more productive.
  - They produce higher amounts of biomass (the total mass of all organisms) per year.
  - They exhibit higher stability in productivity from year to year.
  - They are better able to withstand and recover from environmental stresses.
Diversity-Stability Hypothesis: This hypothesis posits that higher biodiversity leads to greater ecosystem stability.
Data Visual:
- A graph of Plant biomass ( $g/m^2$ ) shows a positive correlation as the number of species (species richness) increases from $0$ to $35$ .

Definition: Keystone species are those that have a much greater impact on the community than would be expected based simply on their abundance or biomass.
Ecological Role: They exert strong control on a community due to their pivotal ecological roles, involving both direct and indirect effects.
Impact of Absence: Without keystone species, large-scale changes occur in the community structure.
Case Study: P. ochraceus (Sea Star):
  - With Keystone Predator: Community diversity remains stable ( $15$ to $20$ species present).
  - Without Keystone Predator: Community diversity falls drastically ( $5$ or fewer species) due to the loss of top-down regulation.
  - Timeline Data: Observations from $1963$ to $1973$ showed a sharp decline in species richness immediately following the removal of the keystone predator.

Definition: Disturbance is any disruption to a community that changes the distribution of living or nonliving resources.
Examples: Forest fires, floods, disease epidemics, logging, and tillage.
Impact Factors: A disturbance's impact is determined by three factors:
  1. Type of disturbance.
  2. Frequency of disturbance.
  3. Severity of disturbance.
Intermediate Disturbance Hypothesis:
  - Moderate (intermediate) levels of disturbance foster higher levels of diversity than either high or low levels.
  - High Disturbance: Leads to low diversity because nothing but the fastest-growing species can persist; slow-growing species are excluded.
  - Low Disturbance: Leads to low diversity because competitively dominant species exclude less competitive ones (Competitive Exclusion).
Data Graphing: Plots show the Number of Taxa vs. an Index of disturbance intensity (log scale) ranging from $0.9$ to $2.0$ , with a peak in taxa at the intermediate points (approx. $1.1$ to $1.4$ ).

Primary Correlates: Species richness is generally correlated with two abiotic variables:
1. Geographic area occupied by the community.
2. Latitude of the community.
Latitude Trends: Species richness is highest in the tropics and generally declines in a gradient moving toward the poles.
Latitudinal Gradient Factors:
1. Evolutionary History: Ice ages "reset" the evolutionary stage in temperate regions. During the Last Glacial Maximum (approx. $20,000$ years ago), $20\%$ of the Earth was covered in ice. The tropics never experienced direct ice age effects, allowing longer uninterrupted periods for diversification.
2. Climate: Higher speciation is driven by climate factors like more sun and rain, leading to higher carrying capacity.
Processes Influencing Richness (Pyron & Wiens, 2013):
  - Speciation: Higher in the tropics.
  - Extinction: Higher in temperate regions.
  - Dispersal: More dispersal into the tropics.
Geographic Area (Species-Area Curve):
- Quantifies that a larger geographic area will contain more species, assuming other factors are equal.
- Reason: Larger areas provide a greater diversity of habitats and microhabitats both locally and globally.

Model Definition: The number of species on an island depends on island size, distance from the mainland, and the balance between immigration and extinction.
Function of Presence: Both immigration and extinction rates are functions of the species already existing on the island.
MacArthur and Wilson’s Theory Predictions:
  1. Species richness is higher on larger islands compared to smaller ones.
  2. Species richness is higher on nearshore (close) islands compared to remote islands.
  - Result: Lowest species richness is found on small, remote islands; highest on large, close islands.
Florida Keys Mangrove Experiment:
  - Researchers counted arthropod species on six small mangrove islands.
  - Findings confirmed richness increased with island size and proximity to the mainland.
  - After fumigating four islands, the number of species eventually returned to pre-fumigation values, demonstrating the ongoing balance of the model.

Food Chains: Focus on one specific pathway of energy flow.
- Grazing Food Chain: Network of herbivores (primary consumers) and the organisms that eat them.
- Decomposer Food Chain: Species that consume dead remains (detritus). Includes primary decomposers like bacteria, archaea, and earthworms at the second trophic level.
Energy vs. Nutrients:
- Energy: Flows through ecosystems, dissipating as heat. It enters as sunlight and ultimately exits as heat.
- Nutrients: Cycle through ecosystems, constantly flowing among organisms and the abiotic environment.
Trophic Levels:
  1. Level 1: Primary Producers (e.g., Live maple leaves or dead maple leaves).
  2. Level 2: Primary Decomposers or Consumers (e.g., Crickets, Bacteria, Earthworms).
  3. Level 3: Secondary Consumers (e.g., Robins).
  4. Level 4: Tertiary Consumers (e.g., Cooper’s hawk).
  5. Level 5: Quaternary Consumers.
The 10% Rule: In the transfer of energy up trophic levels, the efficiency of biomass transfer is only approximately $10 rac{1}{2}$ . This explains why there is less biomass and fewer individuals at the top of a food chain.
Food Webs: Complex, embedded networks of food chains that summarize energy flows and document complex trophic interactions in ecosystems.