IB Environmental systems and societies 2.5D Community and Productivity in Succession Notes

What Shapes the Community That Develops During Succession

1. The community that develops during succession is not random.
2. It is shaped by abiotic factors (climate, soil, topography, disturbance) and biotic factors (grazing, predation, competition).
3. Together these decide which species establish, survive, and dominate, and so what the climax community looks like.

Abiotic Factors Set the Physical Limits

1. Climate
   1. Temperature, rainfall, and seasonality set the pace of succession and the final community type.
   2. Warm, wet climates drive fast succession to forest; cold or arid climates slow it to tundra or shrubland.
2. Bedrock and soil
   1. Parent rock and soil set nutrient supply, pH, drainage, and texture.
   2. Rich soils speed succession toward forest; thin, acidic, or nutrient-poor soils slow it and favour heath or shrubland.
   3. Waterlogged, poorly drained ground limits oxygen to roots and forms wetlands.
3. Geomorphology
   1. Steep slopes erode and hold little soil, supporting only hardy shallow-rooted plants; flat ground builds deep soil and dense forest.
   2. Aspect matters: in the Northern Hemisphere, south-facing slopes are warmer and drier, north-facing slopes cooler and moister.
4. Fire and extreme weather
   1. Fires, storms, and floods reset succession by clearing vegetation and releasing nutrients.
   2. In fire-adapted systems, periodic fire maintains biodiversity by stopping a few species dominating.

Biotic Factors Steer the Trajectory

1. Living interactions, especially grazing and predation, can reshape the path of succession.
2. This is often top-down regulation, where consumers higher in the food chain control the levels below.
3. Herbivores (primary consumers)
   1. Selective feeding shapes vegetation, and overgrazing prevents tree regeneration, holding the system as grassland.
   2. Moderate grazing can raise diversity by stopping any one plant dominating.
   3. In African savannas, elephants topple trees and eat saplings, keeping the land as open grassland.
4. Predators (higher trophic levels)
   1. By controlling herbivore numbers, predators let vegetation and forests recover, a trophic cascade.

Human Activity Redirects Succession

1. Human activity often interrupts or redirects succession, changing the community that develops.
2. Deforestation, grazing, mining, and urbanisation simplify ecosystems and cut biodiversity.
3. When people hold a system below its natural climax, the result is a plagioclimax, covered in Climax communities and plagioclimax.
4. Restoration ecology can restart natural succession by replanting native species and rebuilding soil.

Productivity Changes as Succession Proceeds

1. As succession proceeds, gross primary productivity (GPP), net primary productivity (NPP), and respiration (R) all change.
2. The key measure is the production to respiration ratio (P:R), which falls from above 1 toward 1 as the system matures.

Early stages (pioneer community)

1. GPP is low in absolute terms because conditions are harsh, soil is thin, and producers are few.
2. But respiration is also very low, so a large share of the energy fixed is kept as new growth.
3. This gives a high NPP:GPP ratio and P:R greater than 1, so biomass builds up quickly.

Mid stages

1. As soil develops and diversity rises, GPP climbs sharply with more leaf area and more producers.
2. NPP is at its highest here, though its rate of increase slows as respiration from producers, consumers, and decomposers grows.
3. Species diversity and community complexity also rise, often peaking in mid to late succession.

Late stages (climax community)

1. The ecosystem reaches dynamic equilibrium, where energy captured balances energy used.
2. GPP stays high thanks to abundant producers such as mature trees, but respiration is now just as high because of the large biomass and many consumers.
3. As a result NPP approaches zero and P:R settles near 1, so biomass stops increasing.

The P:R Trajectory in One Line

Early succession: P:R greater than 1, so biomass builds up. Climax: P:R about 1, so biomass stays steady.