Lecture notes
Topic 2.4 ESS revision
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- AQA
All the revision notes you need to know for Ess topic 2.4. Well organised and has practice exam questions. Combined class notes with notes from a revision guide. Used for IB ESS exams.
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THE TRICELLULAR MODEL OF ATMOSPHERIC CIRCULATION SUCCESSIONThe distribution of biomes can be explained by the tricellular model of atmospheric circulation,
which shows how differences in pressure and corresponding wind lead to differences in KEY DEFINITION
precipitation of different latitudes. The model explains why rainfall is high at the equator and at Succession- the orderly process of change over time in a community.
60o north and south.
KEY DEFINITION Changes in the community of organisms frequently cause changes in the physical environment
Latitude- The angular distance from the equator (north or south of it) as measured from the that allow another community to become established and replace the former through
center of the Earth (usually in degrees) competition. Often, but not inevitably, the later communities in such a sequence or sere are
more complex than those that appear earlier.
The formation of an ecosystem from, for example, bare rock is called primary succession:
Pioneer species arrive (e.g. lichens, mosses, bacteria)
As pioneers die, soil is created.
New species of plant arrive that need soil to survive- these displace pioneer
species.
Growth of plants causes changes in the environment (e.g. light, wind, moisture).
Growth of roots enables soil to be retained; nutrients and water in the soil
increase.
Nitrogen- fixation plants arrive, adding nitrates to the soil.
Soil depth increases further, allowing shrubs and other plants to arrive.
Animal species arrive as species of plant they rely on become established.
A climax community is established.
The tricellular model is made up of the polar cell, the Ferrel cell in mid-latitudes and the Hadley During succession, the patterns of energy flow, gross and net productivity, diversity, and
cell in the tropics. mineral cycling change over time. Through the succession, greater habitat diversity leads to
greater species and genetic diversity.
Downward air movement creates high pressure. Upward air movement creates low pressure and
cooling air that leads to increased cloud formation and precipitation. Succession in areas that already have soil is called secondary succession.
The tricellular model of atmospheric circulation explains the distribution of precipitation and Greater habitat diversity leads to greater species and genetic diversity
temperature and how they influence structure and relative productivity of different terrestrial Complex (i.e. climax) ecosystems have a wider variety of nutrient and energy
biomes. pathways, which provide stability.
The Hadley cell controls weather over the tropics, where the air is warm and unstable.
High levels of insolation at the equator heat up the air. Hot air rises, creating the Hadley
cell.
As hot air rises, it cools and condenses, forming large clouds that lead to the heavy rainfall
that is characteristic of tropical rainforest. The pressure at the equator is low because air is
rising.
The air from the Hadley cell cools as it travels away from the equator, where it meets air
from the Ferrel cell. The cooled air descends at 30o north and south of the equator.
Pressure here is high. The air is dry, and so it is in these locations that the desert biome is
found.
Air either returns to the equator at ground level or travels towards the poles as warm
winds. Where the warm air travelling north and south hits the colder polar winds, at
approximately 60o north and south of the equator, it rises because it is less dense. This
creates an area of low pressure. Comparison of pioneer and climax communities:
As the air rises, it cools and condenses, forming clouds. Precipitation results, so this is
where temperate biomes are found. Pioneer communities Climax communities
Contain the first organisms to colonise a The end- point of ecological succession
EXAM PRACTISE new environment
-Explain how different limiting factors will determine productivity in two contrasting biomes. Dominated by r- strategists Dominated by K-strategists
-Explain why productivity in tundra is low Simple in structure, with low diversity Complex in structure, with high diversity
-Tropical rainforest has a mean NPP of 2.20 kg m-2 yr-1, and tundra 0.14 kg m-2 yr-1. When NPP Tolerate harsh conditions, for example Characteristics determined by climate and
strong light/ low nutrient levels soil
per kg biomass per year is calculated, tropical rainforest has a value of 0.049, and tundra a
Example: community of lichens covering Example: mature rainforest ecosystem
value of 0.233. Compare and explain these data.
bare rock
There is no one climax community, but rather a set of alternative stable states for a given
THE EFFECT OF CLIMATE CHANGE ON BIOMES ecosystem. These depend on the climatic factors, the properties of the local soil and a range of
random events that can occur over time.
The distribution of biomes is dependent on climate. Changes in patterns of rainfall associated
with climate change, and alternations in land and sea surface temperature, will lead to shifts in KEY DEFINITIONS
the distribution of biomes. Sere- The set of communities that succeeds another over the course of succession at a
given location.
Pioneer community- The first stage of an ecological succession that contains
species able to withstand difficult conditions.
Climax community- A community of organisms that is more or less stable, and that is in
equilibrium with natural environmental conditions such as climate. It is the end point of
ecological succession.
If you are asked to give an example of a succession, include organisms from the pioneer
community, the climax community and the seral stages in between.
Examples of succession, from pioneer community to climax community:
Aquatic plants (e.g. water lilies) reeds low woodland species (e.g. willow)
and lichens grasses and herbs shrubs(e.g. birch) woodland
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