A community that builds itself up: During a succession, five things change over time: energy flow, productivity, species diversity, soil depth and nutrient cycling. From the first pioneers to the climax community, most of them rise.
The points to remember
- Species diversity rises: more species, more niches, more even numbers.
- Soil gets deeper, with more humus, water and nutrients.
- Nutrient cycling speeds up as decomposers multiply.
- Gross productivity and biomass rise towards the climax.
- Energy flow: food chains get longer and food webs more branched.
- Habitat variety grows: layers of plants, shade, leaf litter, dead wood.
Real example: the 1980 eruption of Mount St Helens, USA, buried the land north of the volcano under pumice and ash. Forty years on, lupins, grasses, willows and young firs grow there, soil is forming, and elk, frogs and birds have returned.
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At first only a few hardy pioneer species can survive. Each stage makes the place suit more species, because it creates new niches.
The points to remember
- More habitats and niches: plants of different heights, shade, litter and dead wood.
- New species arrive (carried by wind, water or animals) once conditions suit them.
- Productivity rises, so there is more food and energy for more species.
- Nutrient cycling develops, so the soil supports more kinds of plants.
- Numbers become more even: richness and evenness both rise.
- Near the climax, diversity may level off or dip as a few long-lived species dominate.
Real example: in North Carolina, USA, fields left after farming grow crabgrass and horseweed in the first year, then broomsedge grass, pine forest after about 10-20 years and oak-hickory forest after about 100 years. Bird species increase at almost every stage.
Using a species table: Quote numbers: the stage with the most species, and whether their numbers are even. Then give a reason, such as more niches or higher productivity.
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Pioneers start on bare rock, sand or ash with no real soil. Every stage adds to it, so the soil gets deeper and richer in humus and nutrients.
The points to remember
- Pioneers (lichens, mosses, bacteria, fungi) help weather rock into small particles.
- Dead organisms add humus, so the soil gets deeper stage by stage.
- Humus holds more water and nutrients, so the soil gets more fertile.
- Decomposers multiply and recycle nutrients faster.
- Roots hold soil against erosion and break up more rock.
- Earthworms and burrowers mix and aerate the soil; animal droppings add organic matter.
- Plant cover shades the soil, so less water evaporates.
Remember it as: Break, add, hold, recycle: rock breaks, humus adds, roots hold, decomposers recycle.
Real example: at Glacier Bay, Alaska, the ice has retreated about 100 km since about 1750. On the land freed longest ago, alder added nitrogen, the soil pH fell from about 8 to about 5, and spruce and hemlock forest now grows on deep soil.
Say how, not just that: 'The soil improves' is not enough. Name the organism and what it does: roots stop erosion, earthworms aerate the soil, decomposers recycle nutrients.
Productivity is how fast new biomass is made. gross productivity (GP) is the total; net productivity (NP) is what is left after respiration.
The points to remember
- Pioneer stage: poor soil and few plants, so GP is low; little respiration, so NP is a big share of GP.
- Middle stages: more plants, so GP rises; consumers arrive, so secondary productivity rises and NP slows.
- Climax: GP is highest, but respiration uses almost all of it, so NP is near zero: biomass stops growing.
- GP rises because soil fertility and nutrient cycling improve.
- Energy flow: more trophic levels, longer food chains, more branched food webs.
- More energy and biomass at each level; decomposers become more important.
Remember it as: Young communities grow fast; old ones break even.
Real example: forests regrowing on abandoned farmland in Latin America regained about 120 tonnes of biomass per hectare in their first 20 years, taking up carbon about 11 times faster than old-growth forest, where growth and respiration are nearly balanced.
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Not everything goes up. As plants grow taller they change the conditions beneath them, so some abiotic factors fall.
The points to remember
- Light at ground level falls as the tree canopy closes.
- Temperature range gets smaller: shade keeps days cooler.
- Humidity rises as trees transpire water.
- Soil moisture can fall: trees take up more water than grasses.
- Soil nutrients can fall in a secondary succession, as growing trees lock them up.
- Biotic changes: shrubs then trees replace grasses; forest animals and more predators arrive.
Wet beaver meadow
- Bright, full sun
- Wet soil, rich in nitrogen
- Grasses and sedges
Forest decades later
- Shade, smaller temperature range
- Drier soil, less nitrogen
- Trees, forest mammals
Real example: in Algonquin Park, Canada, when beavers leave, their ponds drain into grassy beaver meadows. Over decades, shrubs and then forest spread back in, and the soil becomes shadier, drier and lower in nitrogen.
Name the change: Not just 'the vegetation changes': say it changes from grasses to shrubs to trees. And a meadow is dry land, so a change in water cloudiness does not apply there.
How this comes up: Paper 2, Section B (b): explain how succession changes productivity [7].
On farmland abandoned in the eastern USA, grasses are replaced by shrubs, then pines, then oak-hickory forest over about 150 years.
Explain how the process of succession leads to changes in the productivity of the community.
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