IB Biology Topic D4.2.13: Changes Occurring During Primary Succession Notes

Primary succession rebuilds an ecosystem from bare rock into a stable community

Primary succession begins in lifeless places such as bare rock, sand, or clay left by glaciers or volcanic eruptions.

1. The community changes through four broad stages as soil deepens and conditions improve.
2. Pioneer species (bacteria, lichens, and mosses) are the first colonisers of bare rock.
   1. They break rock down into smaller particles and add organic matter as they die, forming the first thin soil.

1. Herbs and grasses establish once a little soil is present.
   1. Their roots anchor the soil against erosion and improve water retention.
   2. As they die they add more organic matter, enriching the soil further.

1. Shrubs and small trees take root as the soil deepens.
   1. They provide shade, creating microhabitats for shade-tolerant species.
   2. Their fallen leaves and branches decompose, enhancing nutrient cycling.
2. A climax community is the stable, mature endpoint of succession, covered in detail in D4.2.15.
   1. Its exact form depends on the climate, so it may be forest, grassland, or desert shrubland rather than always a forest.

Key changes during primary succession point toward greater complexity

1. Plant size increases, progressing from small lichens to towering trees that provide habitat and modify light, moisture, and soil.
2. Primary production rises as more plant biomass captures more sunlight through photosynthesis.
3. Species diversity increases as new plants, animals, and microorganisms find niches to colonise.
4. Food webs become more complex, gaining more trophic levels and interdependent relationships.
5. Nutrient cycling becomes more efficient as decomposers return nutrients to the soil and support further growth.