Productivity During Succession (HL) (DP IB Environmental Systems & Societies (ESS))

Productivity During Succession

• Productivity in ecosystems is the rate at which energy is produced and stored, measured as gross productivity (GP) and net productivity (NP)

  • Gross productivity is the total amount of energy captured by producers (like plants) through photosynthesis

  • Net productivity is the energy that remains after respiration by the producers, which is available for growth and consumption by other organisms

Early stages of succession (pioneer community)

• In the early stages of succession, gross productivity is low due to:

  • Harsh environmental conditions (e.g. bare rock or poor soil)

  • Low nutrient availability (limits the growth of producers)

  • Low density of producers:

    • Pioneer species like mosses and lichens are the first to colonise these areas

    • There are fewer plants or algae to carry out photosynthesis

    • The conditions are not yet favourable for large-scale plant growth

• Net productivity is relatively high and increases quickly at this stage because:

  • Most energy produced through photosynthesis goes toward growth and biomass accumulation

  • There is less cellular respiration taking place, as the ecosystem has fewer organisms, especially consumers

Mid-stages of succession

• As succession progresses, conditions improve:

  • Soil formation occurs, allowing larger plants like shrubs and trees to establish

  • More nutrients become available, and the number of producers increases

• Gross productivity rises:

  • More plants are present to carry out photosynthesis

  • Plant diversity increases, which increases the efficiency of energy capture

• Net productivity still increases but the rate of increase slows and will eventually reach a peak

• The rate of biomass accumulation slows because:

  • Respiration increases as more organisms, including herbivores and decomposers, become part of the system

  • There is more competition for resources like light, water, and nutrients, slowing the growth of new biomass

Late stages of succession (climax community)

• In the final stages of succession:

  • The ecosystem reaches a stable state with a mature community of species e.g. large trees in a mature forest or diverse plant species in grasslands

  • Gross productivity may be high, as the large number of producers continue to photosynthesise

  • However, net productivity decreases and approaches zero because:

    • Most of the energy produced through photosynthesis is used up by cellular respiration of both producers and consumers

    • Biomass no longer accumulates significantly; instead, it cycles between organisms

  

  

Comparison of Pioneer and Climax Communities