Lecture 5: Maintenance of Diversity (Species Coexistence)

1. Productivity and Its Role in Biodiversity

  • Primary Production: The creation of organic compounds from CO₂ by primary producers (e.g., plants and algae), which is the base of the food chain and supports all higher trophic levels.

    • Measured as grams of dry weight per area per time.

    • Net Primary Productivity (NPP): Carbon fixed through photosynthesis minus carbon lost via respiration.

  • Influence of Productivity:

    • Higher Productivity → More biomass, resources, and species richness.

    • Hypothesis: High productivity leads to high species richness, especially in the tropics.

2. Species Interactions and Coexistence

  • Density-Dependent Predation:

    • Janzen-Connell Hypothesis: Focuses on how density-dependent predation by insects and rodents affects seed distribution. Seeds close to the parent tree suffer more predation, reducing competition among seedlings and promoting species diversity.

    • Frequency-Dependent Selection: Common species experience more predation, while rare species are overlooked, maintaining diversity by limiting any one species from becoming dominant.

  • Niche Theory and Partitioning:

    • Niche: The range of conditions allowing a species to survive and reproduce, including access to space, food, water, and other resources.

    • Competition:

      • Direct (Interference) Competition: Directly competing for a resource.

      • Indirect (Exploitative) Competition: Competing by consuming shared resources, reducing their availability.

    • Competitive Exclusion Principle: Two species cannot occupy the same niche simultaneously. If coexisting, species do so by niche differentiation.

    • Facultative vs. Obligate Niche Partitioning:

      • Facultative: Resource partitioning happens only in the presence of competitors.

      • Obligate: Evolutionary response to prolonged competition, often resulting in specialized resource use.

3. The Role of Chance in Species Coexistence

  • Island Biogeography Theory (IBT):

    • Developed by MacArthur and Wilson, IBT explains how island size and distance affect species richness.

    • Species-Area Effect: Larger islands or areas support more species due to greater resources and reduced extinction rates.

    • Distance Effect: Islands closer to the mainland have higher immigration rates, supporting more species.

    • Implications for Conservation:

      • Larger, closer reserves are more effective at preserving biodiversity.

  • Hubbell’s Neutral Theory of Biodiversity:

    • Extends beyond islands, viewing all ecosystems as metacommunities connected by immigration and emigration.

    • Community Drift: Similar to genetic drift; common species become more abundant while rare species decline over time.

    • Predictions of Species Richness:

      • High connectivity (immigration), large population sizes, and high speciation rates support high species richness.

    • Tropical Biodiversity: Higher species richness in the tropics may be due to larger community sizes, higher productivity, better connectivity, and higher speciation rates.

4. Conservation Implications

  • Preserving Biodiversity:

    • Effective conservation should focus on maintaining ecological and evolutionary processes that support biodiversity, including:

      • Species Interactions (e.g., predation and competition),

      • Habitat Area (larger areas support more species),

      • Connectivity (facilitates gene flow and species dispersal),

      • Population Size (reduces extinction risk).

5. Summary and Conservation Activity

  • Summary of Key Concepts:

    • Energy availability contributes to biodiversity by increasing resources.

    • Interactions like density-dependent predation and niche partitioning support species coexistence.

    • Chance events, area effects, and connectivity also play crucial roles in maintaining biodiversity.