LESSON 2.
ECOSYSTEMS AND HOW THEY WORK
1. Humans and Energy Flow
a.
Energy Concept
Energy is a very mysterious concept. It is
involved in every single event in the universe from
humans to photons. Energy is everywhere. Energy never
appears from or disappears into nothing. Energy can
always be accounted for.
From the viewpoint about energy the earth is an
open system.
For life to exist, the earth must
constantly receive inputs of energy from the sun and
make outputs of heat energy, which are passed to outer
space.
Energy from the sun maintains all of the life
processes in the earth ecosystem (Sutton & Harmon,
1973).
Solar energy is radiated toward the earth, but
the atmosphere keeps some solar radiation from reaching
the earth. Only about 50 percent of the sunlight
reaching
the
earth’s
upper
atmosphere
actually
continues to the earth’s surface. Heat from the earth
is constantly rising and passing to outer space (Sutton
& Harmon, 1973).
Figure 1 shows the energy input to the earth’s
surface. As shown, over one-third of the solar energy
reaching the earth’s atmosphere is reflected back into
space by clouds and atmospheric dust, or reflecting
surfaces on earth (snow, oceans, sand). Another 14
percent or so of the solar energy never reaches the
surface of the earth. It is absorbed by gases as it
penetrates
into
the
earth’s
atmosphere.
Of
the
remaining 50 percent, about 25 percent comes directly
to the earth’s surface. The other 25 percent is first
scattered by clouds or dust but eventually it is
radiated to the earth by clouds and particles in the
atmosphere.
SOLAR RADIATION REACHING
EARTH’S ATMOSPHERE
(100%)
30%
Reflection
from clouds
Atmosphere
7%
Absorption in atmosphere
14%
Diffuse scattering
Solar Radiation
reaching the ground
25%
Radiation from sky
25%
Reflection from
ground
Total radiation coming
to earth directly or by
or by radiation from
the sky
51%
4%
EARTH SURFACE
HEATS GROUND
Figure 1.
b.
Energy inputs to earth’s surface at Midday
(Sutton and Harmon, 1973)
Energy Flow
Individual ecosystems, whether we are dealing
with the tropical rain forest or a small pond, are
sustained by a flow of energy through them. The main
source of this energy is sunlight or solar radiation.
Light energy enters the ecosystem when it is absorbed
by plants (Figure 2). It is then passed through the
ecosystem as food plants for animals that consume each
other. Energy absorbed and transferred through the
ecosystem in this way is eventually converted into heat.
This heat, which is exactly equivalent to the solar
input, finally leaves the ecosystem and is lost in
space (O’Hare, 1988)
Energy
input
Ecosystem
Energy
output
Solar
Energy used in
Food production
Energy lost in
energy
photosynthesis
and consumption
respiration
Figure 2.
HEAT
Energy inputs and outputs in an ecosystem
(O’Hare, 1988)
All energy processes are controlled by two very
general laws – the laws of thermodynamics, which give
the relationships between the different forms of energy.
The first law of thermodynamics states that energy can
be transformed from one form to another but can never
be created or destroyed.
The first law tells us that
we can’t get something from nothing. Although the
amount of energy in various forms may change, the sun
in all forms remains constant. The second law of
thermodynamics
states
that
each
time
energy
is
transformed, it tends to go from a more organized and
concentrated form to a less organized and more
dispersed form. The ecological implication of the
second law of thermodynamics is that, the transfer of
energy from one use to another is never very efficient.
In every transfer some energy becomes so disorganized
or dispersed that it is no longer useful (Sutton and
Harmon, 1973).
The two laws of thermodynamics imply that it is
possible to account for all energy that occurs in the
ecological systems and that as energy flows through
ecological systems, less and less of it is available to
do work at each successive step.
As shown in Figure 3, the energy from the sun is
not destroyed as it flows through the earth’s ecosystem.
Rather, it is degraded from a more concentrated form of
energy capable of driving reactions and performing work
into the most diffuse kind of energy – heat. In other
words, the first law insists that the total amount of
energy in the universe remains constant while the
second law insists that concentrated and usable energy
continually diminishes (Sutton and Harmon, 1973).
Solar Energy
(radiated to the earth as sunlight)
Biosphere
Photosynthesis
Solar energy is converted to
chemical energy (glucose and
other high energy compounds)
Respiration
Chemical energy is used to
do work in the cells of
the organisms
Degraded Waste Energy
(radiated into space as heat)
Figure 3.
c.
Energy flow and transformation in the
biosphere (Sutton and Harmon, 1973)
Food Chain, Food Web and Trophic Levels
A food chain is the transfer of energy and
material through a series of organisms as each one is
fed by the next (Nebel, 1990). It is also a series of
feeding relationships between organisms that shows who
eats whom (Sutton and Harmon, 1973).
Energy flows through the biosphere in a sequence
of steps from one organism to another. The energy from
the sun which is first assembled and stored by producers
through the process of photosynthesis is transferred
from one organism to another with the rearrangement of
chemical compounds at each step. Producers are eaten by
a series of consumers. Ultimately any energy fixed by
producers or accumulated by consumers and not used by
them is released by decomposers. At each step, some of
the energy becomes heat that escapes the system. A
diagram of a food chain is shown in Figure 4.
Energy enters the ecosystem as solar radiation
Ecosystem
Producers
(plants)
Consumers
(Herbivores)
Consumers
(carnivores)
Decomposers
Energy leaves
the ecosystems
as heat
Figure 4.
A food chain (Sutton and Harmon, 1973)
Food
chains
are
rarely
isolated
sequences.
Several food chains usually interweave to form a food
web,
a
relatively
complex
series
of
feeding
relationships (Sutton and Harmon, 1973) which may
comprise a group of food chains (Figure 5).
These basic levels, producers and various levels
of consumers or decomposers are called trophic levels.
Trophic levels are feeding levels (Nebel, 1990).
These may refer to the number of steps the organisms
are away from primary production (Sutton & Harmon,
1973).
All producers belong to trophic level one.
All primary consumers, whether feeding on living or
dead
producers,
belong
to
trophic
level
two.
Organisms feeding on the primary consumers belong to
the third trophic level and so on (Nebel, 1990).
A
diagram comparing the food chain, a food web and
trophic levels is shown in Figure 6.
d.
Humans and Energy Flow
Humans are able to modify greatly the flow of
energy in an ecosystem. In many human regulated
ecosystems, energy flow is altered so that yields are
as high as possible. Yield refers to the rate at which
an ecosystem produces useful products.
One objective
of management is to increase yield by channeling as
much net production to the product in question as is
possible.
Thus, a forester may be interested in wood
yield, a farmer in the yield of rice, potatoes, pork or
beef, and a fisherman in fish yield (O’Hare, 1988).
The yield of certain desired products can be
enhanced by: 1) ensuring good growth conditions, e.g.
adding fertilizer and water using good seed; 2)
preventing loss of yield by consumption and competition,
e.g. reduction of grain yield by disease attack,
consumption by birds and small mammals, or weed
invasion.
Here as some of the human implications to the
energy flow (Tivy and O’Hare, 1981)
1.
Humans divert through themselves an amount of
solar energy flow out of all proportion to their
number.
3rd Trophic
Level
All Primary
Carnivores
2nd Trophic
Level
All Herbivores
1st Trophic
Level
All
Producers
Figure 6. Three ways of representing the
nutrients and energy. Each single pathway from
to the top is a food chain. All interconnected
are the food web. The basic feeding levels are
levels (Nebel, 1990).
2.
3.
transfer of
the bottom
food chains
the trophic
The effect of humans on net primary productivity
is generally to reduce the productivity of wild
system and to increase that of cultivated system.
Humans exploit the biosphere by increasing their
crop yield and shortening their food chains.
4.
5.
6.
7.
8.
9.
10.
11.
12.
13.
Increased crop yields are achieved by plant
breeding and energy subsidation of the cropping
system.
The energy efficiency of food production from
arable and livestock farming as well as fishing
generally
decreases
as
energy
subsidation
increases.
Even in the shortest and most efficient of food
chains, loss of energy may be high.
There are four main crops taken out of the
biosphere: arable food, fodder and industrial
crop plants; domestic animals; fish and wood.
The methods of cropping vary in terms of length
of food chain, rate of energy flow, and energy
intensity.
Energy intensification in cropping has been
accompanied by a substitution of fossil fuels for
human and animal power.
Crop and livestock production is generally
characterized
by
comparatively
short-route,
short-run energy flow, and wide variations in
energy intensity.
Increasing demand for animal protein is met only
by very intensive energy systems.
The lowest energy efficiencies are generally
associated with intensive commercial fishing.
Forestry in comparison to farming and fishing is
a short route, very long–run system with a
relatively low level of energy subsidation.
In
forest
ecosystem,
there
is
a
varying
relationship between crop maturity and economic
return
Activity 2.
Answer the questions concisely and perform
asked in order to proceed to the next lesson.
what
are
1.
What is energy?
2.
Describe the two laws of thermodynamics.
3.
What are the ecological implications of these two laws
of thermodynamics?
4.
What is a food chain?
5.
Give a typical example of a food chain and a food web
in an agroecosystem. Diagram your examples.
6.
Give other human implications to the energy flow?
A food web?