Primary and secondary production

Chapter 6: Community and Ecosystem

Primary Production and Energy Flow

Outline

  • Introduction

  • Terrestrial Primary Production

    • Evapotranspiration

  • Aquatic Primary Production

  • Consumer Influences

  • Trophic Levels/Dynamics

Introduction

  • Primary Production:

    • The fixation of energy by autotrophs in an ecosystem (conversion of inorganic energy into organic energy).

  • Rate of Primary Production:

    • Amount of energy fixed over a certain period.

  • Gross Primary Production (GPP):

    • Total energy fixed by autotrophs.

  • Net Primary Production (NPP):

    • Energy leftover after autotrophs have met metabolic needs (NPP = GPP - Rautotrophs).

Primary Productivity

  • Definition:

    • Fixation of energy by autotrophs.

  • GPP vs. NPP:

    • GPP: Total energy fixed.

    • NPP: Energy remaining after autotroph needs are met.

Actual Evapotranspiration (AET)

  • Influences on Primary Production:

    • Rosenzweig estimated the influence of moisture and temperature on primary production rates by correlating annual net primary production with annual actual evapotranspiration (AET, measured in mm/year).

  • AET Contribution:

    • Total water evaporating/transpiring from a landscape yearly.

  • Patterns of Terrestrial Primary Production:

    • AET affected by temperature and precipitation.

    • Highest PP in warm ecosystems with high precipitation; low PP in cold, arid environments (deserts, tundras).

Aquatic Primary Production

Algal Biomass and Production

  • Phosphorus concentration impacts algal biomass.

  • Higher biomass leads to increased rates of primary production.

Consumer Influence on Primary Production

  • Trophic Cascade:

    • Consumers affect PP rates through trophic cascades.

    • Bottom-Up Controls:

      • Influences from abiotic factors (light, nutrients).

    • Top-Down Controls:

      • Predation influences on herbivores.

Trophic Cascade Dynamics

  • Animals control biomass of lower trophic levels.

    • Carnivores regulate herbivore populations, benefiting plants.

  • Bottom-Up Control:

    • Primary production limited by abiotic factors.

  • Top-Down Control:

    • Primary production regulated by herbivore predation.

Trophic Levels

  • Ecological Efficiency:

    • Percentage of energy transferred to the next trophic level (Lindeman, 1942).

  • Trophic Structure:

    • Fourth (Tertiary Consumers)

    • Third (Secondary Consumers)

    • Second (Primary Consumers)

    • First (Primary Producers)

    • Energy transfer decreases at each level (typically ~10%).

Energy Flow in Trophic Levels

  • Energy flow demonstrated:

    • Sun energy input → Producers (10,000 kcal) → Primary Consumers (1,000 kcal) → Secondary Consumers (100 kcal) → Tertiary Consumers (10 kcal).

  • Energy Degradation:

    • Due to cellular respiration, incomplete consumption, and digestion.

Conclusion

  • The health and dynamics of ecosystems depend on understanding primary production, consumer influence, and energy flow through trophic levels.