Patterns in Community Structure

Overview of the discussion on community structure.
Initial focus on small-scale patterns, comparing single sites to areas up to half a continent.
Transition to examining patterns on a global scale.

Definition: There is a unimodal relationship between diversity (or richness) and productivity.
As primary productivity increases:
- Richness/diversity first increases.
- Hits a maximum level and then decreases.

This pattern has been observed in nearly all taxonomic groups.
The consistent narrative:
- With increasing resource abundance, richness/diversity rises initially, then declines.
Explanation for Initial Increase:
- There are limiting resources, thus when resources are more abundant, additional species can be supported.

The decline phase raises questions:
- An asymptotic response suggests continuous addition of species until saturation.
Possible Explanations for Decrease:
  1. Competition Limits Richness:
     - At moderate resource levels, superior competitors struggle to monopolize resources but gain dominance at higher levels.
  2. Transient Maxima:
     - Another resource becomes limiting, leading to peaks and valleys in species richness.
  3. Other Processes:
     - Other ecological or environmental processes may play a role in determining richness.
Conclusion: There is no singular explanation; likely a combination of factors influences different communities.

Communities are not necessarily stable; they can change temporally.
Key Factor: Disturbance.
  - Definition: A temporary change in environmental conditions causing pronounced ecosystem changes.
  - Challenges in application: What is a disturbance for one organism may not be for another.
  - Example: Fire in grasslands.
  - True disturbances eliminate some community members, creating niches for other species.

The frequency of disturbance impacts community richness/diversity significantly.
The hypothesized relationship is unimodal:
- Richness/diversity is low both when disturbances are frequent and rare.
- Highest diversity observed with moderate disturbance frequency.
Intermediate Disturbance Hypothesis:
- This principle also applies to disturbances based on the area impacted.

High disturbance frequency allows only species that can withstand rapid fluctuations to survive:
  - Examples include disturbance-resistant species like certain grasses adapted to fire.
  - These species may also be proficient dispersers.
  - The emphasis is on r-selected species that reproduce quickly but are less competitive.

Conversely, when disturbances are rare to nonexistent:
  - Richness/diversity is also low.
  - Competitive exclusion occurs where a few superior competitors dominate.
  - These species are typically not tolerant of disturbances and are eliminated by them.
  - Commonly seen in k-selected species, which are late to colonize but can outcompete others once established.

Moderate disturbances allow for a mix of species:
- Disturbance-tolerant species remain alongside disturbance-intolerant species.
- Additionally, competitively subordinate species can coexist.

Scenario Example: Application of herbicide leads to community changes over time:
  - Initial dominance by ragweed and annual sunflower (seed bank durability > 100 years).
  - Following disturbance, swift colonization by good dispersers like butterfly milkweed.
  - Over time, slower dispersers like big bluestem become part of the community.
  - Eventually, dominant species include big bluestem, indiangrass, and switchgrass.
  - Peak diversity typically occurs approximately 4 years post-disturbance.