Topic 4.6: Human Impacts on Energy Flow
and Biogeochemical Cycles
Part of the Local Ecosystems Module
Spotlight Biology Preliminary Text Chapter 4
Authors: D. Heffernan, J. Bastina, B. Grieve, K.
Humphreys, A. Sartor
Science Press 2002
Human impacts on Energy Flow and
Biogeochemical Cycles
The natural processes and systems on Earth are balanced and
designed to sustain life. With the growing human population
and the advance of technology, we are having an impact on
these balanced systems.
Human impacts on The Water Cycle
We have recently discovered that water often cycles more
locally and rainfall patterns are often strongly influenced by
living organisms. It used to be believed that the water cycle
was this giant physical phenomenon that affects life but is itself
little affected by life.
Human impacts on The Water Cycle
For example, trees in a tropical rainforest return a great deal of
water to the atmosphere through transpiration from their
leaves. Much of this moisture feeds the heavy local rainstorms
that keep the forest well watered.
So removing large areas of rainforest interrupts this cycle and
can cause major, long lasting changes to local climate.
Human impacts on The Water Cycle
We use a huge amount of fresh water for a range of daily
activities which includes general use at home, irrigation of
gardens and crops, mining, steel production and other
industrial processes. These are all non-natural uses of water.
Human impacts on The Water Cycle
Much of Central Australia’s current water supply is pumped
from the underground Great Artesian Basin. This water is
being used faster than it’s being replaced. How will this affect
future generations?
Human impacts on The Water Cycle
In some parts of the world, water shortages have the potential
to provoke wars between nations. As we continue to alter the
face of the biosphere we MUST understand how our changes to
the environment may affect the water cycle.
Human impacts on The Carbon Cycle
Over the last two centuries, humans began to alter the carbon
cycle in two ways:
1. Cut down some of the world’s great forests, particularly
the tropical rainforests and many trees destroyed were then
2. Burning large quantities of stored carbon: fossil fuels like
coal, oil and gas.
Human impacts on The Carbon Cycle
Cutting down trees destroys living tissue that fixes atmospheric
carbon dioxide, and then burning them immediately returns the
carbon dioxide they contain to the atmosphere
Human impacts on The Carbon Cycle
Removing stores of carbon in the form of coal, oil and gas and
burning them also returns carbon dioxide back into the
atmosphere. This unbalances the cycle because more carbon
dioxide is being out into the atmosphere than is being removed.
Human impacts on The Carbon Cycle
These two forms of combustion has returned enough carbon
dioxide to the atmosphere to raise its atmospheric
concentrations measurably.
Human impacts on The Carbon Cycle
We need carbon dioxide in the atmosphere. This gas traps heat
and keeps the Earth’s temperature stable. We call this natural
balance the greenhouse effect.
Human impacts on The Carbon Cycle
When humans increased the levels of carbon dioxide in the
atmosphere, more radiation is trapped which is leading to a
steady rise in global temperature. This is called the enhanced
greenhouse effect.
Human impacts on The Carbon Cycle
World temperatures are going up but no one knows if this is
because of the enhanced greenhouse effect or natural climatic
cycles. A steady rise in temperature has the potential to affect
the biosphere in a number of ways.
 Changes to climates (drought/flooding)
 Increased storms
 Sea level rise
 Melting of the polar ice caps
Human impacts on The Nitrogen Cycle
The nitrogen cycle is being adversely affected by us as well.
The amount of nitrogen fixed by natural means, such as
bacteria and lightning, is now about the same as that produced
by humans. Therefore we are upsetting the natural balance.
Human impacts on The Nitrogen Cycle
There are two main ways we add nitrogen to our environment:
1. Cars (when cars burn fossil fuels, the high temperatures
inside modern engines produce large amounts of nitrogen
oxides. When it rains, these nitrogen oxides dissolve to
form nitrates which end up in our soils, lakes and oceans)
2. Fertilisers (farmers use enormous amounts of nitrogen
fertilisers in their farms. This eventually makes it into our
water systems as well)
Human impacts on The Nitrogen Cycle
As a result of excess fertilisers in our water ways is called
eutrophication, this causes rapid growth of microscopic
algae called algae blooms. How do you think this would this
affect an ecosystem?
Human impacts on The Nitrogen Cycle
When the algae die,
the bacteria that break
them down
(decomposition) use
most of the oxygen in
the water in this
process. This produces
smelly discoloured
water. Not to mention
this water is also toxic
to humans and
Energy Flow and Distribution
We also interfere with the flow
of energy through the
ecosystem. More and more of
the energy naturally trapped by
photosynthesis is being removed
for human use and less is left for
the natural community. (What
does this mean?)
Energy Flow and Distribution
Clearing land for agriculture is a way of redirecting energy
from the sun for human use rather than for the natural
community. With less energy in its biomass, fewer organisms
can be maintained in the natural environment
Energy Flow and Distribution
Burning fossil fuels and wood
releases carbon dioxide into the
atmosphere. This increase in
carbon dioxide traps more heat
in the atmosphere and interferes
with the energy flow.
What causes the greenhouse effect and how does this differ
from the enhanced greenhouse effect?
2. Describe the two main ways humans take nitrogen from
the atmosphere and add it to the soil.
3. Complete Activity 4.6.1 Cycle Diagrams Handout

Ecosystems Topic 4: Trophic Levels