PYTR Class

Learning Objectives

1. Outline influences on the type of community that develops in a succession
2. Summarise the changes in net productivity and gross productivity in a community undergoing succession
3. Outline r- and K-strategist species
4. Outline challenges to the concept of a climax community
5. Outline the term plagioclimax

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Part 1: Influences on the type of community that develops in a succession

Influenced by bottom-up and top-down factors

• Bottom-Up Factors:
  • Pressures restricting resource availability for lower trophic levels (e.g., producers) reduce resource supply, limiting the abundance of organisms at higher trophic levels.
  • Effect of soil and producers, shaping ecosystem development. Factors are:
    • Steep Slopes: Limit soil formation.
    • Poor Drainage: Leads to waterlogging.
    • Parent Rock: Can create extreme soil conditions (e.g., ultra-basic soils).
  • Soil’s Role: Varies in mineral content and structure, influencing plant communities.

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• Top-Down Factors:
  • A higher trophic level shapes the community structure of a lower trophic level through predation.
  • Effect of higher trophic levels and cascade down the food chain.
    • Primary Consumers: Influence plant populations.
    • Higher Trophic Levels: Regulate lower levels, impacting succession.
  • Impact on Succession: Living organisms shape the structure of the final community.

Part 2: Changes in net productivity and gross productivity in a community undergoing succession

• In the early stages of succession:
  • Gross Primary Productivity (GPP) is low, but Net Primary Productivity (NPP) is high due to low overall cellular respiration rates.
  • This relationship is represented by the production-to-respiration ratio (P : R).

• P : R ratio dynamics:
  • P : R = 1 → Production equals cellular respiration, resulting in a steady-state community.
  • P : R > 1 → Biomass accumulates.
  • P : R < 1 → Biomass is depleted.
• In later succession stages:
  • Increased consumer populations lead to higher community respiration rates.
  • Gross productivity remains high in a climax community, but net productivity approaches zero (NPP = GPP − R).
  • The P : R ratio approaches 1 as respiration balances productivity.

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• Comparing food production systems to natural ecosystems:
  • Intensive crop production differs from a natural climax community.
  • Both fields and woodlands start with low productivity, increasing as biomass accumulates.
  • Farmers prevent P : R from reaching 1, as high respiration would negate productivity gains.
  • Crops like wheat are harvested before P : R = 1 to maximize yield.
  • In food production, herbivore populations are controlled to increase net productivity.
  • In natural woodlands, consumer populations rise, balancing productivity with respiration, leading to a climax community where P : R = 1.

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Part 3: K-strategist and r-strategist Species [Recall]

• Natural selection may favour individuals with high reproductive rates and rapid development (r-species) over those with lower reproductive rates but better competitive ability (K-species).
• In predictable environments (e.g., climax communities where resources do not fluctuate), there is little advantage to rapid growth.
  • Natural selection favours species that maximize use of natural resources.
  • These species produce fewer young with a higher probability of survival.
  • K-species tend to have long life spans, large body sizes, and develop slowly.
• In disturbed habitats with rapidly changing conditions (e.g., pioneer communities), r-species are favoured.
  • R-species can respond quickly, develop rapidly, and reproduce early.
  • This leads to a high rate of productivity.
  • Colonizer species often have high dispersal abilities to reach disturbed areas.

r-Strategist Species

r-strategists are species that focus on high reproductive rates and rapid population growth in unpredictable environments.

Characteristics:

• Short lifespan
• High reproductive rate (many offspring per reproductive cycle)
• Minimal parental care (offspring are left to survive on their own)
• Small body size
• Rapid maturation (short time to reach reproductive age)
• High mortality rate (many offspring die before reaching adulthood)
• Boom-and-bust population cycles (population size fluctuates dramatically)
• Efficient colonisers of new or disturbed environments

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K-Strategist Species

K-strategists focus on long-term survival, stable populations, and competition in environments with limited resources.

Characteristics:

• Long lifespan
• Low reproductive rate (few offspring per reproductive cycle)
• High parental care (offspring receive protection and nurturing)
• Larger body size
• Slow maturation (longer time to reach reproductive age)
• Low mortality rate (higher survival of offspring)
• Stable population size (close to carrying capacity, K)
• Strong competitors in stable environments

Part 4: Climax Community and Plagioclimax

Challenges to the Climax Community Concept

• The idea of a single, definitive climax community is debated.
• Uncertainty exists about which ecosystems would naturally develop without human influence.
• Traditionally, it was believed that closed-canopy forests would dominate Central and Western Europe wherever trees could grow.
• The Vera wood–pasture hypothesis, proposed by Frans W.M. Vera, suggests that large herbivores (e.g., cattle, horses) would have created a dynamic landscape of alternating forests and open areas.
  • In this hypothesis, herbivore activity leads to alternative stable states rather than a single climax community.
  • Herbivore Influence: Large herbivores play a key role in shaping plant communities.
  • No Fixed Endpoint: Ecosystem development is not destined for a single, stable state.
  • Dynamic Mosaic: Grazing maintains a mix of open grasslands and scattered trees, preventing full forest cover.

Plagioclimax

• Climatic and soil (edaphic) factors determine the nature of a climax community.
• Human disturbances can interrupt succession, leading to a different stable state rather than the natural climax community. This process is known as plagioclimax.
• Examples of causes leading to plagioclimax:
  • Footpath erosion from continuous trampling.
  • Sand dune degradation caused by walkers destroying vegetation.
  • Human activities such as agriculture, hunting, forest clearance, burning, and grazing alter succession and prevent the original climax community from forming.
• An ecosystem’s ability to survive change depends on its diversity and resilience.

Exercises

Exercise 1 – Multiple Choice Questions

Answer for the MCQs

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Exercise 2 – Written Answer Questions