Worksheet 2.

5: Krakatau
Succession in action
You are going to look at the relationships between ecosystem stability, diversity, succession and habitat, using the
specific example of succession on Krakatau (Figure 1). Krakatau erupted with a force 10 times the magnitude of
Mt St Helens and 2000 times the force of a nuclear explosion. The sound of the explosion was heard 2700 miles
away.

Figure 1 Krakatau is part of Indonesia, a band of islands on the equator.

Read about the physical geology of Krakatau and the ecology after the eruption. Also, view this presentation
about ecosystems, featuring definitions and key terms.
Note: Press the ‘full’ tab (bottom right of PowerPoint slide) after accessing the PowerPoint presentation.

Questions
1 In which year did Krakatau erupt? [1 mark]
2 Which islands were formed following the eruption? [1 mark]
3 Was there any life left on the islands? [1 mark]
4 What type of succession occurred on the islands? [1 mark]
5 What were the first organisms to arrive on the islands following the eruption? What is the name for
this type of organism (in terms of ecological succession)? [2 marks]
6 What changes did these pioneers make to the islands? [1 mark]
7 What organisms came next? Why were they able to establish themselves on the islands? [2 marks]
8 How did new species arrive on the island? (HINT: three ways) [1 mark]
9 Why was this overall succession able to take place rapidly? [2 marks]
10 What were the third and fourth stages of the succession? [2 marks]

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11 What made it possible for large trees to appear on the islands? [1 mark]
12 How did diversity change through the succession? Why did diversity change in this way? [2 marks]
13 A complex ecosystem, with its variety of nutrient and energy pathways, provides stability. What is
the most complex ecosystem now on Krakatau? [1 mark]
14 Human activities often simplify ecosystems, rendering them unstable. Which human activities modify
succession? What is plagioclimax? [2 marks]

Pages 114–22 of the textbook contain further information about succession and about how human activities can
modify succession (plagioclimax).

Succession on Krakatau
Studies of colonization of the volcanic island of Krakatau, after the massive eruption in 1883, show that tropical
rainforest ecosystems are capable of recovery from even extreme damage, given sufficient time. After the initial
eruption, no living thing remained on what was left of the island, but today Krakatau is covered by tropical forest.
On the islands left after the eruption, there are now over 400 species of vascular plants, thousands of species of
arthropods, over 30 species of birds, 18 species of land molluscs, 17 species of bats and 9 reptiles.
Organisms can colonize isolated land using several mechanisms:
 by air – flying (birds, insects) or passive transportation (lightweight seeds or spores)
 by sea – swimming or floating on a log
 by animal – travelling (or hitchhiking) inside or attached to animals that swim or fly (plant seeds and animal
larvae).

Jump dispersal: long-distance dispersal to remote areas by one or a few individuals

This dispersal mechanism explains widely distributed species in geographically isolated areas. It can be used to
explain the aerial spread of plants, insects and microbial organisms over huge distances. The colonization of the
Galápagos Islands by finches from mainland South America is an example of this type of dispersal.

Diffusion: slower than jump dispersal, diffusion involves populations, rather than individuals
Diffusion describes the spread of species at the edge of their ranges into new areas; it often follows jump
dispersal events.

Secular migration: dispersal over geological timescales (thousands to millions of years)

This is diffusion taking place so slowly that the diffusing species undergoes evolutionary change during the
process. It includes the diversification and spread of flowering plants, and the evolution of South American llamas
and vicuñas that are descended from the now extinct North American members of the camel family that migrated
during the Pliocene.

Primary succession
The formation of an ecosystem from bare rock (as on Krakatau) is called a primary succession. The succession
always follows the same sequence with the arrival of different organisms in turn (Figure 2).

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 Pioneer species arrive (e.g. lichens, algae, bacteria) and colonize a bare or disturbed site. As these organisms
die, soil is created.
 Growth in plants causes changes in the physical environment (e.g. light, moisture).
 New species of plants arrive that need soil to survive. They displace existing pioneer plants because their
seedlings are better able to become established in the changed environment.
 The growth of roots enables soil to be retained and not washed away.
 Newly arriving species alter the physical conditions (e.g. increased shade; more minerals and nutrients in the
soil as plants die and decay, and nitrogen-fixing plants arrive), allowing other species to become established.
 Animals come in with or after the plants they need for food.
 Eventually, a climax community that is more or less stable is established.
 Disturbances start the process of succession again.

Figure 2 Succession on Krakatau began with blue–green bacteria and then ferns (the pioneer species) followed by grasses and
shrubs. Next came mixed woodland (smaller trees) and finally the climax community of tropical rainforest (including tall
hardwood trees).

Soil depth, moisture, and species diversity increase through the succession, reaching their maximum in the climax
community (the last stage of the succession). Greater habitat diversity leads to greater species and genetic
diversity.

Secondary succession
When succession occurs in an area that already has soil, it is called a secondary succession. Human activities, such
as forest clearance (logging or burning), can cause this type of succession, providing no further disturbance
occurs. Succession can be modified (i.e. halted at an early stage) through activities such as grazing, where climax
species (e.g. trees) are not allowed to become established.
The set of communities that succeed one another over the course of succession at a given location is called a
sere.

© Pearson Education Ltd 2015.

For more information about the Pearson Baccalaureate series please visit www.pearsonbacconline.com