Succession

Question 1

… This is due to a natural process called succession

Answer 1

The plants and animals in an environment gradually change over long periods of time- and the environment changes too.

Question 2

Succession may arise as a result of:

Answer 2

A glacier retreating and depositing rock;
Sand dunes forming by wind or sea;
Volcanoes depositing lava;
Silt and mud being deposited by rivers;
Lakes or ponds being created by land subsidence.

Question 3

The diagram below shows what may happen:

Answer 3

Question 4

What is meant by succession?

Answer 4

A change in a community over time due to a changed environment
A change in a community over time due to changed abiotic factors
A change in a community over time due to a changed species

Question 5

Succession: Pioneer Species
Stage 1:

Answer 5

Stage 1: Very few species can live on bare rock since it stores little water and has few available nutrients.
The first colonisers are usually lichens, a mutualistic relationship between an alga and a fungus.
The alga photosynthesises and makes organic compounds, while the fungus absorbs water and minerals and clings to the rock.
Mosses are able grow on top of the lichens.
The colonisers are able to erode the rock and change the abiotic conditions and start to form a very thin soil.; this allows other species to colonise.

Question 6

The first stage of this type of succession is…

Answer 6

the colonisation of an inhospitable environment by organisms called pioneer species

Question 7

Pioneer species often have features that suit them to colonisation eg

Answer 7

Tolerance to extreme conditions
Rapid germination of seeds on arrival as they do not require a period of dormancy
Production of vast quantities of wind dispersed seeds or spores so that they can easily reach isolated situations such as volcanic islands.
The ability to photosynthesise, as light is normally available but other food is not .

Question 8

Pioneer species

Answer 8

Other early Pioneer species such as grasses and ferns can grow in the thin soil and their roots help accelerate soil formation.
They have a larger photosynthetic area, so they grow faster, so they make more detritus, so they form better soil, which holds more water further changing the environment

Question 9

Pioneer species change the abiotic conditions –

Answer 9

they die and microorganisms decompose the dead organic material (humus) which forms a basic soil.

This makes conditions less hostile eg the basic soil helps to retain water, so new organisms with different adaptations can move in and grow.

The new organisms then die and are decomposed, adding more organic matter, making the soil deeper and richer in minerals such as nitrates.

Larger plants like shrubs can start to grow in the deeper soil, which retains even more water and contains more nutrients.

Some new species may change the environment so that it becomes less suitable for the previous species eg sand sedge stabilises the sand through the growth of underground stems. This makes the conditions less suitable for marram grass which needs constant reburial by sand in order to stay healthy

Question 10

Succession: Herbaceous Plants
Stage 2:

Answer 10

Herbaceous plants such as dandelions (‘weeds’) have small wind-dispersed seeds and rapid growth, so they become established before larger plants.
Leguminous plants also grow which help fix nitrogen and increase soil fertility

Question 11

Succession: Shrubs
Stage 3:

Answer 11

Stage 3: Larger plants (shrubs) such as bramble, gorse, hawthorn, broom and rhododendron can now grow in the good, deeper soil.
These grow faster and so out-compete the slower growing pioneers by creating less suitable conditions.

Question 12

Succession: Trees
Stage 4:

Answer 12

Stage 4: Trees grow slowly, but eventually shade and out-compete the shrubs, which are replaced by shade tolerant forest floor species.
A complex food web is now established with many trophic levels and interactions.
This is called the climax community.

Question 13

As succession goes on, the ecosystem becomes more complex

Answer 13

New species move in alongside existing species, which means that biodiversity increases. Plants create more habitats for animals, then abiotic conditions become less hostile and the amount of biomass increases.
Eventually these changes result in a climax community – the ecosystem supporting the largest and most complex community of plants and animals it can

Question 14

What is meant by a climax community?

Answer 14

It is a stable, final community with no further succession

Question 15

Scientists investigated the changes in the different types of plant species growing on a disused football pitch. Eventually the grass became replaced by a woodland community.
Which of the following explain how this happened?

Answer 15

Mowing the pitch prevented a climax community forming
When mowing stopped competition between species started
Changes in the community lead to changed abiotic conditions

Question 16

Secondary Succession

Answer 16

Secondary Succession

This happens on land that’s been cleared of all plants, but where the soil remains.

Eg after a forest fire or where the forest has been cut down by humans.

The established community of species is usually destroyed but without too much disturbance of the soil.

The process of secondary succession is very similar to primary succession but because there is already a soil layer, secondary succession starts at a later stage.

Question 17

What is the difference between primary and secondary succession?

Answer 17

Primary succession starts with bare rock or sand, such as behind a retreating glacier, after a volcanic eruption, on a new sand dune, from erosion and weathering of a mountain, etc.
Primary succession can take thousands of years to reach a climax community.
Secondary succession starts with soil, but no (or only a few) species following certain natural disasters such as ploughing, grazing, fires, flooding, etc.
Soil already has been developed (therefore no pioneer species are needed), soil nutrients are present, seeds of the whole sere are often present and can germinate immediately to give a mixed climax community of trees, grasses etc.
Secondary succession can take just tens of years to reach a climax community.

Question 18

How can humans affect succession?

Answer 18

Human activities have a big effect on natural succession, often meaning a natural climax community is not reached.
Common landscapes today like farmland, grassland, moorland and gardens are all maintained at pre-climax stages by constant human interventions, e.g. ploughing, weeding, herbicides, burning, crop planting and grazing animals.
These are examples of an artificial climax, or plagioclimax.

Question 19

Which of the following are ways farming practices can prevent the formation of climax communities such as woodland?

Answer 19

Regular grazing

Using herbicides
Ploughing or burning