Primary succession at Higher Level: The same ideas as SL, with a different example: Glacier Bay in Alaska, where the ice has left 250 years of new ground in a row. At HL, name pioneers as r-strategists and the climax species as K-strategists.
Practise this as you read
- Trace the sere at Glacier Bay, stage by stage.
- Link each stage to what it changes for the next.
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Starting from nothing: Primary succession begins on a newly formed substratum. There is no soil and no living community yet, so everything must arrive and build up from scratch.
The points to remember
- Primary succession starts on new ground with no soil and no community living there before.
- New volcanic rock: lava and ash from an eruption.
- Moraines: rock and gravel left behind as a glacier retreats.
- Wind-blown sand (new dunes) and waterborne silt (new mudflats where rivers meet the sea).
- It is very slow: soil must be built first, so it takes hundreds to thousands of years.
Remember it as: New ground, no soil, no seeds: start from zero.
Real example: since the 1750s the ice in Glacier Bay, Alaska, has melted back about 100 km. It left bare rock and moraine, with no soil. Ground near the ice is newest, ground at the mouth of the bay the oldest.
Primary or secondary?: Ask one question: was there soil? New lava, fresh moraine or new dunes: primary succession. A burnt forest or an old field keeps its soil: that is secondary succession.
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The first colonisers are pioneer species. Together they form the pioneer community.
What makes a pioneer
- Pioneer species are the first to colonise bare ground: lichens, mosses, algae, hardy grasses.
- They tolerate harsh conditions: heat, cold, drought, wind and almost no nutrients.
- Their tiny spores and seeds are carried in by wind, birds or the sea.
- They are r-strategists: many offspring, mature early, spread and colonise fast.
- Some fix nitrogen (mountain avens, clover), adding nutrients to the poor ground.
- Lichens release acids that slowly break down the rock.
Remember it as: Tough, tiny seeds, fast, first.
Real example: at Glacier Bay, mats of mountain avens cover the new moraine. Bacteria in its roots fix nitrogen, so the poor gravel slowly gains the nutrient every later plant needs.
Arrive, survive, spread: Calling pioneers r-strategists is credited, but give the reason: many offspring, early maturity, fast spread, suited to harsh ground. A trait with no link to colonising, such as 'short life', is not enough.
On bare rock, the first job is making soil. A primary succession on rock is called a lithosere. The ground in front of a retreating glacier shows every step.
How the pioneer community develops
- Weathering: frost (freeze-thaw), heat and rain break the bare rock into small mineral grains.
- Pioneers arrive: wind, birds and animals bring in spores and seeds.
- The pioneers establish and spread fast over the bare ground.
- When they die they decompose, adding organic matter and nutrients: a thin soil forms.
- Their roots bind the soil, so less is washed or blown away (less erosion).
- As the soil gets deeper, more plants can survive, so plant cover rises.
Remember it as: Break the rock, bring the seeds, rot the dead, hold the soil.
Real example: at Glacier Bay the gravel holds almost no nitrogen at first. Soil nitrogen rises steeply once nitrogen-fixing alder arrives, and only then can spruce grow well.
Pioneer community means the first stages: A question on how the pioneer community develops wants weathering, arrival, decomposition and roots. Writing about the climax forest does not answer it and gets no credit.
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A succession is a chain of communities. Each one is a seral stage. The last is the climax community.
The seral stages
- The sequence: pioneers, then grasses and herbs, then shrubs, then trees, then the climax community.
- Each seral stage changes the conditions (deeper soil, more shade), so the next stage outcompetes it.
- The climax community is stable: it stays unless it is disturbed.
- It has the most biodiversity and the most complex food webs, and is in equilibrium (inputs about equal outputs).
- K-strategists dominate it: long-lived, few offspring, each with a high chance of survival.
- The climax depends on the climate: rainforest in the tropics, oak woodland where it is temperate, grassland where it is dry.
Real example: at Glacier Bay the sere runs from mountain avens to alder, then Sitka spruce. After more than 200 years, spruce and hemlock forest forms the climax, in deep, stable soil.
Use the words: Name the stages with the right terms: pioneer community, seral stages, climax community. Then say what each stage changes that lets the next one in.
How this comes up: Paper 2, Section B (a): outline, with a named example, how primary succession develops [4].
Retreating glaciers and new volcanic eruptions create bare ground in many parts of the world.
With reference to a named example, outline how primary succession develops on newly formed ground.
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