PPT 15

Community Ecology

• Trophic relationships and succession

  • Metacommunity: extends the metapopulation concept to the community level involving multiple species.

Species Diversity

• Composed of two factors:

  • Species richness: Number of different species in a specific area.

  • Relative abundance: Measurement of species evenness; number of individuals per species.

Forest Communities

• Two forest communities:

  • Composed of 100 individuals, each containing 4 species.

  • Same species richness but different relative abundances.

Example of Species Diversity

• Flower Species Counts:

  • Field 1:

    • Daisy: 300

    • Dandelion: 335

    • Buttercup: 365

    • Total: 1000

  • Field 2:

    • Species richness (3 species) remains constant.

    • Total individuals also 1000, yet Field 1 shows greater evenness and thus is more diverse.

Primary Productivity

• Definition: Amount of light energy converted into organic compounds by autotrophs over time.

• Limiting Factors:

  • Aquatic ecosystems: limited by light and nutrients.

  • Terrestrial ecosystems: limited by temperature and moisture.

Secondary Production

• Amount of chemical energy in consumer food converted into biomass during a certain timeframe.

Primary Production Metrics

• Gross Primary Production (GPP): Total primary production in a specific ecosystem.

• Net Primary Production (NPP): Primary production remaining after cellular respiration.

Food Chains and Food Webs

• Organisms organized along food chains/trophic levels.

• Energy Transfer:

  • Ultimate energy source: Sun.

  • Carbon source: CO2 (from atmosphere or dissolved forms).

Production Efficiency

• Measurement of energy transfer efficiency through trophic levels.

• Consumption Insights:

  • Herbivores consume only a fraction of primary production (1/6th globally).

  • Majority of ecosystem production goes to detritivores (bacteria and fungi).

Measuring Production Efficiency

• Efficiency = Net secondary production × 100% / Assimilation of primary production.

• Example Calculation:

  • Caterpillar consumes 100 J:

    • 67 J for respiration.

    • 33 J for secondary production.

  • Efficiency = 33 J / (33 J + 67 J) = 33%.

Production Efficiency Across Species

• Insects and microorganisms: 40% or more.

• Fishes: 10% or more.

• Birds and mammals: 1-3%.

Trophic Efficiency

• Percentage of production transferred between trophic levels (usually ~10%).

• Reasons for losses in transfer include respiration, feces, and unconsumed material.

The 10% Rule

• Energy transfer between trophic levels averages 10%; 90% loss due to inefficiencies.

• Limitations on the number of trophic levels due to availability of biomass.

Food Webs

• All species within a community interconnected through food chains.

• Changes in species abundance affect community dynamics drastically (e.g., Sea otter and urchin populations).

Top-Down Control

• Predation is a key factor in community organization.

  • Trophic Cascade: Primary producers increase in response to predator control of herbivores.

Disturbance in Communities

• Definition: Events that change a community by removing organisms or altering resources.

• Types and Levels of Disturbance:

  • High Disturbance: High intensity/frequency.

  • Low Disturbance: Low intensity/frequency.

Intermediate Disturbance Hypothesis

• Moderate disturbance levels increase species diversity more than low or high levels.

Succession

• Definition: Orderly transition in species composition after disturbance.

  • Can occur through primary succession (no previous community) and secondary succession (existing community disrupted).

Pioneer Species

• First organisms to colonize a new habitat (e.g., lichens, mosses).

• Modify environment making it suitable for subsequent species.

Gradual Changes in Succession

• Increases in species diversity and food web complexity.

• Older communities support more species and biomass.

• Achieving dynamic equilibrium: a predictable set of species in the habitat.

Biomagnification

• Accumulation of toxins in organisms, resulting in higher concentrations in top-level carnivores.

Biogeochemical Cycles: The Carbon Cycle

• Carbon as the universal "currency" for energy flow in ecosystems.

• Importance of organic molecules and photosynthesis in fixing inorganic carbon (e.g., CO2).

Carbon Reservoirs

• Major sources: sedimentary rocks, fossil carbon.

• Active carbon reservoirs primarily in oceans and organisms.

Key Processes Affecting Carbon

1. Photosynthesis - CO2 uptake.

2. Respiration - returns CO2 to atmosphere.

3. Burial of dead organisms leading to fossil fuels.

4. Human activities significantly influence carbon reservoirs (e.g., fossil fuel use).

Human Activity and Global Change

• Transition into the "Anthropocene" era due to human impact on ecosystems.

• Major stressors:

  1. Habitat loss.

  2. Nutrient enrichment.

  3. Pollution.

  4. Ozone depletion.

  5. Climate change.

Solutions to Environmental Issues

• Promote renewable energy sources.

• Educate and empower communities, particularly women.

• Encourage sustainable agricultural practices and reduced meat consumption to mitigate farming impacts.

• Focus on biodiversity conservation and mitigating deforestation.