

Biology Unit 4 Chapter 7 – Ecological Succession

change in community over time;
change is due to species present
Explain what is meant by succession (2)
stable community with few changes to populations
Explain what is meant by a climax community (1)
increased interspecific competition;
for light
Heather plants are small shrubs. Heather plants are the dominant species in the climax community of some moorlands. The structure and shape of a heather plant changes as it ages. This results in changes in the species composition of the community. A large area of moorland was burnt leaving bare ground. The table shows four stages of succession in this area.
Explain why the number of other plant species decreases between 4 and 12 years after burning. (2)
fewer leaves;
less photosynthesis to produce new biomass;
competition with other species for nitrates
The rate at which a heather plant produced new biomass was measured in g per kg of heather plant per year. This rate decreased as the plant aged. Use the information in
the table to explain why. (3)
reduction in insect predators from ponds, because ponds kept shallow;
reduction in animals that prefer pH outside 5-7, because lime added

(Refer to exam q) Attempts are being made to conserve the natterjack toad, which breeds in ponds. A number of recommendations have been made about how to do this. Some of these are shown in the table, together with a reason for each one.

Explain two ways in which these recommendations could reduce populations of animals other than common toads. (2)

conservation measures tends to stop this;
keep communities the same;
adding lime stops pH change
Most of the UK populations of natterjack toads are found on sand dune systems. The communities of plants and animals on such systems usually change over time as a result of succession. Explain how the natterjack conservation recommendations could affect succession in a sand dune system. (3)
Volcanoes erupting and depositing lava;
Lakes or ponds being created by land subsidising
Identify two events that may lead to colonosation of an inhospitable environment. (2)
A species that can colonoise an inhospitable environment
What is meant by a pioneer species? (1)
the ability to fix nitrogen from the atmosphere as there are few nutrients;
tolerance to extreme conditions;
rapid germination of seeds;
the ability to photosynthesise
What features suit pioneer species to colonisation? (4)
The abiotic environment becomes less hostile, e.g. soil froms with plentiful nutrients;
This leads to a greater number of habitats;
This leads to increased biodiversity as different species occupy these habitats
How does succession lead to more complex food webs? (3)
The main abiotic factor
What is the climax community determined by? (1)
The pioneer species photosynthesise and fix nitrogen;
These die and form a soil with nutrients;
Further colonisers can survive in this soil as environment is less hostile;
More habitats and food sources are available;
Increased biodiversity
Describe how changes in the environment lead to increased biodiversity during succession. (5)
Nitrogen is fixed into compounds such as proteins and amino acids;
by the nitrogen-fixing bacteria of the pioneer species;
when pioneer species die decomposers break them down into ammonium ions;
which are then broken down into nitrates and nitrites;
by nitrifying bacteria
Using your knowledge of the nitrogen cycle, explain how nitrogen from the atmosphere becomes incorporated into the soil, causing its level to increase. (5)
A method of maintaining ecosystems and the living organisms that occupy them
What is meant by conservation? (1)
Ethical reasons;
Economic reasons – maintains biodiversity. Species possess many genes that may be useful in the future
What are the main reasons for conservation? (2)
Ulva lactuca

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(Refer to June 2013 paper) Algae are photosynthesising organisms. Some algae grow on rocky shores. A scientist investigated succession involving different species of algae. He placed concrete blocks on a rocky shore. At regular intervals over 2 years, he recorded the percentage cover of algal species on the blocks. His results are shown in the graph.

Name the pioneer species. (1)

Too many to count
The scientist used percentage cover rather than frequency to record the abundance of algae present. Suggest why. (1)
It is flat whereas rocks normally aren’t
Some scientists reviewing this investigation were concerned about the validity of the results because of the use of concrete blocks.
Suggest one reason why these scientists were concerned about using concrete blocks
for the growth of algae. (1)
Pioneer species increases then decreases;
The species makes the conditions less hostile;
Pioneer species outcompeted by new competitor;
G. coulteri increases and other species decrease
Use the results of this investigation to describe and explain the process of succession. (4)
Decrease in percentage cover of bare ground linked to more species;
Change in diversity due to more soil;
Increase in depth of soil as plants die
(Refer to Jan 2013) A student investigated an area of moorland where succession was occurring. She used quadrats to measure the percentage cover of plant species, bare ground and surface water every 10 metres along a transect. She also recorded the depth of soil at
each quadrat. Her results are shown in the table.Explain how these data suggest that succession has occurred from points A to E along the transect. (3)
Greater variety of food;
Greater variety of niches
The diversity of animal species is higher at E than A. Explain why. (2)
Plants change the environment;
Less hostile habitat
The species that are present change during succession. Explain why. (2)
Only plants which can photosynthesise with less light remain
The pine trees in the forest have leaves all year. Explain how this results in a low species diversity of plants in the forest. (1)
Stops sand shifting;
Makes conditions less hostile
Marram grass is a pioneer species that grows on sand dunes. It has long roots and a vertically growing stem that grows up through the sand. Sand dunes are easily damaged by visitors and are blown by the wind. Planting
marram grass is useful in helping sand dune ecosystems to recover from damage.
Use your knowledge of succession to explain how. (2)
Colonisation by pioneer species;
Change in environment caused by organisms present;
Enables other species to survive;
Change in biodiversity;
Less hostile environment;
Climax community
Succession occurs in natural ecosystems. Describe and explain how succession occurs. (5)
Increase in dead organisms;
Leading to increase in nitrification
(Refer to June 2010) Ecologists investigated succession in some abandoned crop fields. The data that they
collected are shown in the graph. The curves show the trends that occurred over a period of 60 years.Explain the change in soil nitrate concentration shown on the graph. (increases) (2)
Bare soil temperatures fluctuate;
More bare soil at start of succession
The pioneer plants had different characteristics from the plants that colonised the fields
after 50 years.
The pioneer plants had seeds that germinate better when the temperature fluctuates. Explain the advantage of this to these pioneer plants. (2)
Plant will survive when overshadowed by taller plants
Explain the advantage to a plant that colonises after 50 years of having a high rate of photosynthesis at low light intensities. (1)
Grassland consists of small plants;
Will be outcompeted by woody plants;
So these woody plants must be removed
Conservation of grassland habitats involves management of succession. Use the data in the graph to explain why. (2)
Species X has high rate of photosynthesis at high light intensity;
Species X grow fastest at high light intensities;
Will outcompete other species Y/Z

(Refer to Jan 2010 paper) The graph shows the effects of light intensity on the rate of photosynthesis of three species of tree, X, Y and Z. Each of these species occurs at a different stage in succession.

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Species X is the first tree to become established in the succession. Use the graph to explain why it is likely to become established earlier in the succession than Y or Z. (3)

Produces shade;
Species Z grows best in low light intensity
Species X may change the environment so that it becomes more suitable for species Z.
Use the graph to explain why. (2)

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Biology Unit 4 Chapter 7 - Ecological Succession
change in community over time; change is due to species present
Explain what is meant by succession (2)
stable community with few changes to populations
2018-10-20 00:26:11
Biology Unit 4 Chapter 7 - Ecological Succession
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