Environmental Systems and Society Guide Part 1

Topic 1: Systems and models

1.1. 1: Systems

Why Environmental Systems?

SYSTEM: an assemblage of parts and their relationship forming a functioning

entirety or whole.

During the 1970’s, British chemist James Lovelock and American biologist Lynn

Margulis came up with the GAIA HYPOTHESIS: That the world acts like a single

biological being made up of many individual and interconnected units ( A SYSTEM ).

Gaia was the Greek Earth goddess

figure 1. A systematic view of the Earth’s biological and chemical components

The Components

The Earth’s systems comprise interactions between the living ( Biotic ) and non-

living ( Abiotic ) constituent parts. As in any system these interactions involve

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INPUTS, PROCESSING of the inputs to

create OUPUTS.

Even if we look at the starting point of

all food chains on Earth, photosynthesis

and conversion of light energy to stored

chemical energy in the leaf, this to can

be viewed as a system component

within a bigger system.

So Photosynthesis comprises inputs, a

process and outputs

But photosynthesis is also a component in a larger system. A food chain the initial

light energy gets processed and converted into chemical energy (food) that is

passed along the system.

Yet if you take each of the organisms in the diagram above and place them in

individual plant pots or cages at a zoo and the system breaks down: the

interactions between the components are what make the system not the

components themselves


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1.1. 2: Types of System

Systems can be thought of as fitting into one of three types: Open (exchange

matter and energy with its surroundings), Closed and Isolated

Open Systems: exchange matter

and energy with its surroundings.

Most systems are open, including

ecosystems. In forest ecosystems

plants fix energy from light entering

the system during photosynthesis.

Nitrogen is fixed by soil bacteria.

Herbivores that live within the forest

canopy may graze in adjacent

ecosystems such as a grassland, but

when they return they enrich the soil

with feces. After a forest fire top soil

may be removed by wind and rain.

Mineral nutrients are dissolved out of

the soil and transported in ground water to streams and rivers.

Open system models can even be applied to the remotest oceanic island - energy

and mater is exchanged with the atmosphere, surrounding oceans and even

migratory birds.

It is important to remember that if we are thinking in the terms of systems, then

each component of a system is

surrounded by a larger environment.

A single tree ( a system in its own

right ) within a forest system

exchanges energy and material with

the surrounding forest.

Closed Systems: exchange energy

but not matter.

Closed systems are extremely rare in

nature.


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No natural closed systems exist on Earth but the planet itself can be thought of as

an “almost” closed system.

Light energy in large amounts enters the Earth’s system and some is eventually

returned to space a long wave radiation (heat).

Biosphere 2 was a human attempt to create a habitable Closed system on Earth.

build in Arizona at the end of the 1980’s Biosphere 2 was intended to explore the

use of closed biospheres in space colonization. Two major “missions” were

conducted but both run into problems. The Biosphere never managed to produce

enough food to adequately

sustain the the participants and

at times oxygen levels became

dangerously low and needed

augmenting.

Isolated Systems: An isolated

system exchanges neither

matter nor energy.

These do not exist naturally.

Though it is possible to think of

the entire Universe as an

isolated system.


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1.1. 3: Energy in Systems

Energy in all systems is subject to the Laws of Thermodynamics.

According to the First Law of Thermodynamics: Energy is neither created or

destroyed. What this really means is that the total energy in any system including

the entire universe is constant all that can happen is that the form the energy takes

changes. This first law is often called the law of conservation of energy.

In the food chain above the energy enters the system as light energy, during

photosynthesis it gets converted to stored chemical energy (glucose). It is the

stored chemical energy that is passed along as food. No new energy is created it is

just passed along.

Even if we look at the sunlight falling on Earth not all of it is used for

photosynthesis.

30% is reflected, around 50% is converted to heat,

and most of the rest powers the hydrological cycle -

rain, evaporation, wind, etc. Less than 1% of

incoming light is use for photosynthesis.

The Second Law of Thermodynamics states that

the entropy of an isolated system not in equilibrium

will tend to increase over time. what this really

means is that the energy conversions are never

100% efficient: When energy is transformed into

work, some energy is always dissipated as waste

heat.


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If you examine the food chain again

in terms of the second law then:

when the lion chases the zebra, the

zebra attempts to escape changing

the stored chemical energy in its

cells into useful work. But during its

attempted escape some of the

stored energy is converted to heat and lost from the food chain.

This process can be summarized by a simple diagram showing the energy input and

outputs.

The Second Law can also be thought off as a

simple word equation:

ENERGY = WORK + HEAT (and other wasted

energy)

So what does the term ENTROPY mean?

Entropy refers to the spreading out or dispersal of

energy. Using the above example the energy

spreads out - the useful energy consumed by one level is less than the total energy

at the level below - energy transfer is never 100% efficient.

Depending on the plant their efficiency at converting solar energy to stored sugars

is around 2%. Herbivores on average only use around 10% of the total plant energy

they consume the rest is lost in metabolic processes and a carnivores efficiency is

also only around 10%.

So the carnivores total efficiency in the chain is 0.02 x 0.1 x 0.1 = 0.0002

This means the carnivore only uses 0.02% of incoming solar energy that went into

the grass. The rest of the energy is dispersed into the surrounding environment.


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1.1. 4: Equilibria

Open systems tend to exist in a state of balance: Equilibrium. Equilibrium avoids

sudden changes in a system, though this does not mean that all systems are none

changing. If change exists it tends to exist between limits. We can therefore think

of equilibrium states in two ways STATIC and “STEADY STATE”.

Static Equilibrium is where the components of a system remain constant over a

long period of time.

Possibly the best example of static equilibrium in the environmental system in

which we ourselves have to survive is the oxygen content of the atmosphere.

Around 4 billion years ago there was very little oxygen in the atmosphere. Why?

Our planet was void of life. Then life appeared and importantly photosynthesizing

life, first cyanobacteria (bacteria with chlorophyll) and later plants. Both of which

produce molecular oxygen a a waste product.

As the oxygen levels rose so a new type of organism appeared that could use the

external oxygen in respiration - animals - and so the Oxygen cycle was born.

Eventually over time a balance was achieved in the level of atmospheric oxygen and

for the last 2 billion years, plants and animals have held the oxygen level stable at

21% of the atmosphere.

Steady State equilibria: this is a

much harder concept to define and

there are still arguments for what a

dynamic equilibrium really is. The

best way to think about it is that a

system is in a steady state because

the inputs and outputs that affect it

approximately balance over a long

period of time.

An example of this can be seen in a

classic study of the populations of

Snowshoe Hares and Lynx in Canada. As the population of the Lynx rises the Hare

population falls this is then followed by a fall in the Lynx population which in itself is

followed by a rise in the Hare population etc. etc.


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1.1. 5: Feedback systems

Systems are continually affected by information they have to react to from both

within and outside. Two simplistic examples, if you start to feel cold you can either

put on more clothes or turn the heating up. The sense of cold is information putting

on clothes is the reaction. Secondly if you feel hungry, you have a choice of

reactions that you can take to this “information”

Natural systems act in exactly the same way. The information starts a reaction

which in turn may input more information which may start another reaction. This is

called a Feedback Loop.

Negative Feedback:this tends to damp down, neutralize or counteract any

deviation from an equilibrium, and promotes stability.

Using the example of the Snowshoe Hare / Lynx population cycle presented in the

last section

When Hare the population is high, there is surplus food for the Lynx so their

numbers go up. This puts a pressure on the Hare population as more are eaten and

their numbers fall. Less food for the Lynx so they start to starve and their numbers

fall. Fewer Lynx means fewer hares are eaten and their numbers start to go back

up. And so it continues as a loop.

Positive Feedback amplifies or increases

change; it leads to exponential deviation

away from an equilibrium.

An example of this is the possible

effect that rising global temperature

could have by adding more water vapor to

the atmosphere. Water is a powerful

greenhouse molecule trapping heat in the

atmosphere. If there is a global temperature rise

more water will evaporate trapping more heat making

more water evaporate trapping more heat and on and on. Again a diagram helps

explain the idea.


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1.1. 6: Transfers and Transformations

Both Material and Energy move or flow through ecosystems. A transfer is when the

flow does not involve a change of form and a transformation is a flow involving a

change of form. Both types of flow use energy, transfers being simpler use less

energy and are therefore more efficient than transformations.

Transfers can involve:

The movement of material through living organisms (carnivores eating other

animals)

The movement of material in a non-living process (water being carried by a stream)

The movement of energy (ocean currents transferring heat)

Transformations can involve:

Matter (glucose converted to starch in plants)

Energy (Light converted to heat by radiating surfaces)

Matter to energy (burning fossil fuels)

Energy to matter (photosynthesis)

1.1. 7: Flows and Storages

Both energy and matter flows (inputs and outputs) through ecosystems but at

times is also stored (stock) within the ecosystem:

The Biogeochemical Cycle illustrates the general flows in an ecosystem.

Energy flows from one compartment to another. E.g. a food chain. But when one

organism eats another organism the energy that moves between them is in the

form of stored chemical energy: Flesh

Energy Flows through an ecosystems in the form of carbon–carbon bonds within

organic compounds. These bonds ae broken during respiration when carbon joins

with oxygen to produce carbon dioxide. Respiration releases enrgy that is either

used by organisms (life processes) or is lost as heat.


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The origin of all the energy in an ecosystem is the sun and the fate of the energy is

eventually to be released as heat

In the diagram the flow of

energy is shown by the red

arrows.

Unlike energy MATTER cycles

through the system as

minerals (blue arrows). Plants

absorb mineral nutrients from

the soil. These nutrients are

combined in to cells.

Consumers eat plants and other consumers egest the minerals they contain and re-

combining them in cells. Eventually decomposers break down dead organic matter

(DOM) and then return the minerals to the soil. These minerals may betaken out of

the soil quickly by plants or can eventually through geological processes become

locked within rocks until erosion eventually returns them to new soil.

The geochemical cycles illustrate the flows

and storage of energy and matter: The carbon

cycle shows the flow of both where as the

other geochemical cycles e.g. nitrogen only

show the flow and storage of matter.

In both cases though the direction of the flow

- producer to consumer, and the magnitude -

loss of material up a food chain, amount of

carbon dioxide moving from respiration and

combustion to the atmosphere, can be

described.


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Topic 2: The ecosystem

2.1. 1: Biotic and Abiotic.

The components of an ecosystem

Ecosystems are made up of the interactions between the living and non-living

components within them.

It is impossible to think of an ecosystem without including these interactions

The living components of an ecosystem are known as the “biotic factors” - living

biological factors that influence the other organsims or environment of an

ecosystem.

This is a lot more than just listing the plants, animals or micro-organisms found in

an ecosystem. It includes the roles played by the organisms.

Biotic factors interact as : Producers, consumers, detrivores, decomposers,

parasite, host, predator, competitor, herbivore, symbiant and pathogen.

A tree in a woodland is a producer providing the

basic unit of energy for the rest of the

ecosystem. But at the same time it competes for

light with other trees and may be the host to

parasitic plants such as mistletoe or

decomposing fungi. During the annual cycle in

the wood, the tree will at times take water and

mineral nutrients from the soil and at others

return nutrients from fallen wood and leaves.

The Physical and Chemical components of an ecosystem are called the “abiotic

factors” and include:


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• The atmosphere

• Climate and water

• Soil structure and chemistry

• Water chemistry

• Seasonality

These factors operate at a broad

scale but within ecosystems

smaller component abiotic factors

also work.

The relative humidity within the

bowl of an oak tree is higher than

that of a woodland as a whole.

This provides the physical and chemical conditions needed for a community of

mosses, lichens and ferns to develop.

In a very simplistic form it is the availability of suitable abiotic environment that

provides the conditions for a distinct biotic community to exist. Importantly

thought, the biotic community can greatly influence and even change the abiotic

one.

Commercial Forestry in parts of Scotland illustrates

this well.

Until the 1970’s large areas of Scottish Blanket Bog

was viewed beyond the reach of commercial forest

operations. It was to wet for Sitka spruce the

predominant cash wood crop to grow and too

expensive to drain.

Then it was discovered that if a “nurse” crop of

Lodgepole pine was planted ahead of the Sitka,

even though the pines would eventually die in the

very wet conditions, they would dry the soil enough

to allow Sitka to take hold.

Along with this the drying of the area and closing in of the canopy with trees

planted tightly in rows would prevent continued growth and accumulation of

sphagnum moss. This in turn aided the drying process.


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Many different abiotic factors an animal or plant species and also interact and

change with time themselves.

E.g. temperature is dependent upon:

solar radiation, wind speed, time of year, time of day, altitude and aspect.

Temperature affects water loss from organisms and respiration, and for plants the

rate of photosynthesis. Changes in temperature affect relative humidity and

evaporation from water bodies and soils.

It is the abiotic conditions in an environment which ultimately give rise to the biotic community present. This is illustrated below with examples of six different ecosystems, including an ecosystem found on the surface of some rocks, each of which is the result of the initial controlling abiotic factors which operate.

Alpine Grassland Acidic Heath Temperate Deciduous Forest

Mediterranean Maquis or Chaparral

Lichen rock face ecosystem

Sand dune system


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2.1. 2: Trophic Levels

Trophic level:The position that an organism occupies in a food chain, or a group of

organisms in a community that occupy the same position in food chains.

It is possible to classify the way organisms obtain energy into two categories.

Producers or Autotrophs: These manufacture their own

food from simple inorganic substances (plants)

Consumers or Heterotrophs: Feed on autotrophs or

other heterotrophs to obtain energy (herbivores,

carnivores, omnivores, detrivores and decomposers

But within the consumers their is a feeding hierarchy of

feeding

Plants capture the suns energy and convert it to

glucose, herbivores eat plants and carnivores eat

herbivores - different feeding levels (Greek for food is

trophe)

Trophic level 1 - producer

Trophic level 2 - herbivore (primary consumers)

Trophic level 3 - carnivore (secondary consumers)

Trophic level 4 - carnivore (tertiary consumer)

The first trophic level, the autotrophs supports the energy requirements of all the

other trophic levels above.


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2.1. 3: Food chains and Food webs

Ecosystems have an hierarchy of feeding relationships (trophic levels) that

determine the pathway of energy flow in the ecosystem. The energy flow in the

ecosystem can be illustrated as a Food chain.

It is possible to construct food chains for an entire

ecosystem, but this starts to create a problem.

The food chains below are form a European Oak

Woodland. In fact they are based on real food

chains at Wytham Wood in Oxford

.

In the four different food chains only ten species are listed and some of them are in

more than one food chain. If we continued to list all the species in the wood and

their interactions in every food chain the list would run for many pages.

Food chains only illustrate a direct feeding relationship between one organism and

another in a single hierarchy. The reality though is very different. The diet of almost

all consumers is not limited to a single food species. So a single species can appear

in more than one food chain.

A further limitation of representing feeding relationships by food chains is when a

species feeds at more than one trophic level. Voles are omnivores and as well as

eating insects they also eat plants. We would then have to list all the food chains


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again that contained voles but moving them to the second trophic level rather than

the third in a shorter food chain.

The reality is that there is a complex network of interrelated food chains which

create a food web.

2.1. 4: Ecological pyramids

Pyramids of number

A bar diagram that indicates the relative numbers of organisms at each trophic level

in a food chain. The length of each bar gives a measure of the relative numbers.

Pyramids begin with producers, usually the greatest number at the bottom

decreasing upwards.

Advantages

This is a simple easy method of giving an overview and is good at comparing

changes in population numbers with time or season.

Disadvantages

All organisms are included

regardless of their size,

therefore a system say based

on an oak tree would be

inverted (have a small bottom

and bet larger as it goes up trophic levels). Also they do not allow for juvenilles or

immature forms. Numbers can be to great to represent accurately.


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Pyramids of biomass

As pyramids of number but

uses dry mass of all

organisms at each trophic

level.

Advantages

Overcomes the problems of pyramids of number.

Disadvantages

Only uses samples from populations, so it is impossible to measure biomass

exactly.also the time of the year that biomass is measured affects the result.

Pyramids of energy

The bars are drawn in proportion to the total energy utilized at each trophic level.

Also the productivity of producers in a given area measured for a standard time,

and the proportion utilized by consumers can be calculated.

Advantages

Most accurate system shows the actual energy transferred and allows for rate of

production.

Disadvantages

It is very difficult and complex to to collect energy data.

Why use ecological pyramids.

Ecological pyramids allow you to examine easily energy transfers and losses. They

give an idea of what feed s on what and what organisms exist at the different

trophic levels. They also help to demonstrate that ecosystems are unified

systems,that they are in balance.


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2.1. 5: Pyramids and Ecosystem Function

Bioaccumulation

Story of Minamata Bay.

Minamata is a small factory town in Japan, dominated by one factory, The Chisso

Factory. Chisso make petrochemical based substances from fertilizer to plastics.

Between 1932 and 1968 Chisso dumped an estimated 27 tons of mercury into

Minamata Bay.

Beginning in the 1950’s, thousands of people started to suffer from mercury

poisoning.

What had happened?

Some bacteria can change mercury to a modified form called methylmercury.

Methylmercury is easily absorbed into the bodies of small organisms such as

shrimp. When the shrimp are eaten by fish, the methylmercury enter the fish. The

methylmercury does not break down easily and can stay in the fish bodies for a

long time. As the fish eat more and more shrimp, the amount of methylmercury

increases. The same increase in concentration happens when people then eat the

fish. fish are a major part of the diet of people around Minamata bay. This process

is known as bioaccumulation.

There is a slow magnitude build up along the food chain: Very many bacteria

absorb very small amounts of mercury - many shrimp eat a lot of bacteria building

up the mercury concentration - lots of fish eat lots of shrimp again building up the

concentration and finally a small number of humans at the top of the food chain

eventually eat a lot of fish and absorb high levels of methylmercury.

The end of the food chain

It is the often the highest trophic level in a food chain that is the most susceptible

alterations in the environment. Another example of the effects of toxins on a food

chain was DDT (a pesticide) and Peregrine folcans in Britain in the 1950’s and 60’s.

Follow this link to find out more PEREGRINES IN YORKSHIRE

The top of the food chain is always vulnerable to the effects of changes further

down the chain. Top carnivores often have a limited diet so a change in their food

prey has a knock on effect. Their population numbers are low because of the fall in


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efficiency alone a food chain, therefore their ability to withstand negative influences

is more limited than species lower in the food chain with larger populations.

2.1. 7: Population interactions

COMPETITION

1. All the organisms in any ecosystem have some effect on every other organism in

that ecosystem.

2. Also any resource in any ecosystem exists only in a limited supply.

When these two conditions apply jointly, competition takes place.

In a seagull colony on an oceanic outcrop, as the population

grows, so the pressure for good nesting sites increases. This

can affect the number of eggs that each female can

successfully hatch, and so affects the birth rate of the

population as a whole. This sort of interaction is called a

Density Dependent factor - the effect is depends on the

population density ( low density small effect, high density

large effect). This mainly associated with pressure for food,

nutrients or space.

Competition between members of the same species is INTRASPECIFIC

COMPETITION.

When the numbers of a population are small, there is little real

competition between individuals for resources. Provided the

numbers are not too small for individuals to find mates,

population growth will be high.

As the population grows, so does the competition between

individuals for the same resources until eventually the carry

capacity of the ecosystem is reached. In this situation, often the

stronger individuals claim the larger share of the resources.

Some species deal with intraspecific competition by being

territorial. An individual or pair hold an area and fend off rivals.

Individuals that are the most successful reproductively will hold

the biggest territory and hence have access to more resources.


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Small population

Large population



Intraspecific competition tends

stabilize populations dependent

upon the controlling resources.

It produces something called

logistic growth. The graph

illustrates this for a colony of

yeast grown in a constant but

limited supply of nutrient.

During the first few days the

colony grows slowly as it starts

to multiply (lag phase) then it

starts to grow very rapidly as the multiplying colony has a plentiful nutrient supply

(exponential phase). Eventually the population size stabilizes as only a set number

of yeast cells can exploit the limited resources (stationary phase). Anymore yeast

cells and there is not enough food to go around.

Competition does not only occur between individuals of the same species.

Individuals of different species could be competing for the same resource.

This is INTERSPECIFIC COMPETITION.

Interspecific competition may result in a balance, in which

both species share the resource. The other outcome is

that one species may totally out compete the other, this is

the principal of competitive exclusion. An example of both

of these outcomes can be seen in a garden that has

become overrun by weeds. A number of weed species

coexist together, but often the original domestic plants

have been totally excluded.

In a woodland light is a limiting resource. Plant species

that can not get enough light will die out in a woodland.

This is especially true of small flowering plants on the

woodland floor that are not only shaded out by trees but

by shrubs and bushes as well. Beech trees have very closely overlapping leaves,

resulting in an almost bare woodland floor.


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But even in beech woods flowers manage to grow in the spring.

Carpets of Snowdrops, Primroses and Bluebells an integral part

of all Northern European deciduous woodlands in the spring. The

key to these species success is that the grow, flower and

reproduce before the shrub and tree species burst into leaf. They

avoid competing directly with species that would out compete

them for light by completing the stages of their yearly cycle that

require the most energy and therefore the greatest

photosynthesis when competition is less.

The amount of competition depends on how much each species need for the

resource overlaps:

Interspecific competition may

result in a balance, in which both

species share the resource.

But with the population size of

each species reduced compared

to without competition

The other outcome is that one

species may totally out compete the other.

This is the principal of competitive exclusion


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2.4. 1: Biomes

What is a biome?

A collection of ecosystems sharing similar climatic conditions, eg tundra, tropical

rainforest, desert

How many biomes are there?

Opinion differs slightly on the number of biomes, but it is possible to group biomes

into six major types with sub divisions in each type.

Freshwater

Marine

Desert

Forest

Grassland

Tundra


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If we only consider the terrestrial biomes we can split the major groups up again:

Deserts - Hot and Cold

Forests - Tropical, Temperate and Boreal (Taiga)

Grasslands - Tropical or Savannah and Temperate

Tundra - Arctic and Alpine

2.4. 2: Why are Biomes where they are?

Ecologist Robert Whittaker plotted records of

annual precipitation against annual

temperature for locations around the planet

and then grouped them within the biomes

generally found in those places. The result

was the graph on the left. This helps to

illustrate that biomes that form anywhere in

the world are mainly the result of the

combination of rainfall and temperature

found in those areas. The graph also

illustrates that the concept of biome may not

be as precise and clear cut as it at first

appears. Marginal areas exist where a

continuum of climatic conditions can give

rise to a gradient of ecosystems as one

biome is slowly replaced by another. Large

areas of Boreal forest in

Northern Europe slowly change

as you move South into areas

that support predominantly

Deciduous forest.

This is because the climatic

conditions across the planet are

not distinct but themselves

show a gradients. From the

equators out both North and

South temperature drops until

the permanently frozen regions


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At the Poles the suns energyis spread over a large area

At the Equatorthe suns energyis spread over asmall area



at the poles is encountered. The result

of incoming solar radiation being

spread over greater and greater area

with the curvature of the Earth.

The Earth also tilts at an angle of

23.5˚, creating summer and winter in

each hemisphere. During the Northern

Winter almost no solar energy reaches

the high arctic. This again reduces

productivity at the poles

The maximum incoming solar radiation at the equator gives rise to high

temperatures which in turn lead to maximum evaporation of water from the large

expanses of ocean found here. As the moisture laden warm air at the equator rises

in the atmosphere it the water condenses out as clouds and falls back to Earth as

exceptionally high rainfall. the rainfall which when combined with high temperatures

and maximum sunlight creates the perfect conditions for maximum plant growth.

The result equatorial or tropical rainforest.

This rapidly rising warm air sucks in air from both Southern and Northern latitudes

along the planets surface. In the atmosphere the still warm but now dry air moves

away from the equator. These two air currents set up an atmospheric cell with

descending warm dry air at around 30° north and south. This leads to the

establishment of desert biomes at these latitudes.


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23.5˚Tilt

Southern HemisphereSummer

Northern HemisphereWinter



2.5. 3: Energy flow through the ecosystems

The sun produces immense amounts of

energy in the form of electromagnetic

radiation. This is a broad spectrum from X

rays to radio waves, though most exists

in the ultraviolet, visible light and infrared

radiation bands. Almost half of the sun’s

total radiation is visible light.

The distance of the earths orbit around

the sun is fairly constant, around

1.5×10km, and the amount of energy

reaching the outer atmosphere is within 5% of a constant energy quantity of 1400

J/m2/s, this is the Earths solar constant.

The second law states that the efficiency of

energy conversion to useful work is never

perfect: when energy changes from one form,

some of the energy is not available to do

useful work in the system. It leaves the

system mainly as useless heat. This is called

entropy. This is true for all energy changes

even those involving the living organisms.

Only a very small part of the total sunlight

reaching the Earths surface is ever

transformed into energy used by living

organisms. A study of an Illinois cornfield

suggested only 1.6% was actually used by the

corn. Some of that energy becomes stored

chemical energy (sugars, fats and proteins)

within the plants, some is used to maintain life processes and ultimately is

dissipated as heat during respiration. The stored energy can then either be passed

on to consumers as food or die and enter the detritus food chain. Consumers again

are inefficient processors of the energy consumed with respiration taking place and

some losses immediately as faeces and so the process of flow, storages and losses

continues up the food chain.


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Boxes and Circles represent Storage and Arrow thickness represents magnitude of flow



Organisms that gain energy from inorganic

sources and fix it as energy rich organic

molecules are called Autotrophs. Most are

plants obtaining their energy directly from

light, carbon dioxide and water:

Photosynthetic autotrophs.

Some bacteria obtain energy directly from

inorganic chemicals: chemosynthetic

autotrophs.

Organisms that utilize energy rich organic

molecules, edible food, for their energy supply

are termed Heterotrophs. The hetrotrophs can

be split into two kinds, consumers that obtain

their from living organisms and decomposers

that obtain theirs from dead organisms or

from organic material dispersed in the

environment

Autotrophs and particularly plants are the

base unit of all stored energy in any

ecosystem. Light energy is converted into

chemical energy by photosynthesis within the

cells of plants. Pigments in plant cells of which

chlorophyll is the most important catalyse this

reaction


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Cyclamen in an Alpine forest



2.5. 4: Transfers and Transformations - Global Cycles

The biogeochemical cycles.

Movement of nutrients and energy through the ecosystem is quite different.

Energy travels from the sun, through food webs and is eventual lost to space as

heat.

Nutrients are recycled and reused. (Via the decomposer food chain)

Organisms die and are decomposed

Nutrients are released

Eventually become parts of living things again, when they are taken up by plants

These are the BIOGEOCHEMICAL CYCLES

The Carbon cycle.

The balance between

Photosynthesis, Respiration

and incorporation into the

lithosphere

Carbon is an essential

element in living systems,

providing the chemical

framework to form

molecules that make up

living organisms. Carbon

makes up around 0.03% of

the atmosphere as carbon dioxide, and is present in the Oceans as carbonate and

bicarbonates and in rocks such as limestone and coal.

Carbon cycles between living (biotic) and non-living (abiotic) chemical cycles:

carbon is fixed by photosynthesis and released back to the atmosphere through

respiration. Carbon is also released back to the atmosphere through combustion,

including fossil fuels and biomass.


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http://www.sciencebitz.com/wp-content/uploads/2007/11/carbon-diagrams.jpg



Carbon can remain locked in either cycle for long periods of time. ie in the wood of

trees or as coal and oil.

Human activity has disrupted the balance of the global carbon cycle (carbon

budget) through increased combustion, land use changes and deforestation.

Nitrogen cycle

All living organisms use nitrogen to make molecules such as protein and DNA.

Nitrogen is the most abundant gas in the atmosphere but atmospheric nitrogen is

unavailable to plants and animals, though some specialized micro-organisms can fix

atmospheric nitrogen. The nitrogen cycle can be thought of in three basic stages.

Nitrogen fixation: atmospheric nitrogen is made available to plants through the

fixation of atmospheric nitrogen as ammonia by nitrogen fixing bacteria either free

living in the soil (Azotobacter) or symbiotically in root nodules (Rhizobium).


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Plant and Animal Biomasscontaining nitrogen

Excretion

AmmoniaNH3

Dead Organic Matter DOM

Decomposition

Nitrifyingbacteria

Nitrifyingbacteria

NitriteNO2

NitrateNO3

Nitrates taken up by plants

Animals consume plants

Nitrogen Cycle

NitrogenFixing

bacteria

Nitrogen gas in the atmosphereN2

Denitrifyingbacteria

xationduring lightning

storms



lightening also causes the oxidation of nitrogen gas

to nitrate which is washed into the soil.

Denitrification: denitrifying bacteria (Pseudomonas

denitrificans)in waterlogged and anearobic

conditions reverse this process converting ammonia,

nitrate and nitrite to nitrogen gas which escapes to

the atmosphere.

Nitrification: some nitrifying bacteria are able to

convert ammonium to nitrites (Nitrosomonas)

while other convert the nitrites to nitrates

(Nitrobacter) which is then available to be

absorbed by roots.

Humans have intervened in the nitrogen cycle by

applying large amounts of nitrogen fertilisers to

the land, either as organic sources (manure and

green crops) or as inorganic chemical fertilisers. overuse of fertilisers can lead to

serious pollution problems particularly to water supplies. (Eutrophication)

The Hydrological cycle

The hydrological or water cycle

involves the transfer of water

between atmosphere rivers lakes and

oceans and the lithosphere. To move

between these different sections

water molecules often need to be

transformed from one phase to

another. E.g. Liquid water evaporates

from surfaces allowing transfer to the

atmosphere as gas where it

condenses out in clouds to fall as

liquid precipitation again.

It is easy to think about Lakes, Oceans and Ground water as stores (sinks), but the

largest store of fresh water is held within snow and ice. The polar ice caps and

mountain glaciers in particular are an enormous sinks of water temporarily removed


29

AmmoniaNH3

Nitrifyingbacteria

Nitrifyingbacteria

NitriteNO2

NitrateNO3

Nitrogen Cycle

NitrogenFixing

bacteria

Nitrogen gas in the atmosphereN2

Denitrifyingbacteria

xationduring lightning

storms

Plant and Animal Biomasscontaining nitrogen

Excretion

AmmoniaNH3

Dead Organic Matter DOM

Decomposition

Nitrifyingbacteria

Nitrifyingbacteria

NitriteNO2

NitrateNO3

Nitrates taken up by plants

Animals consume plants

Nitrogen Cycle



from the cycle ( though this could be for thousands of years) and unavailable for

use by organisms.

As well as being essential for life it is the water cycle that drives the worlds weather

systems.

2.5. 6: Primary and Secondary Productivity

Organisms that use inorganic sources of

energy, and particularly plants are the

base unit of stored energy in any

ecosystem. Light energy is converted

to chemical energy by photosynthesis

within the cells of plants

Because all the energy fixed by plants is

converted to sugars it is in theory

possible to calculate a plant’s energy

uptake by measuring the amount of

sugar produced. This is Gross Primary Production (GPP), because it occurs in the

primary producers, an abstract that is difficult to measure. More useful is the

measure of Net Primary Production (NPP).

An ecosystems NPP is the rate at which plants accumulate dry mass, usually

measured in kg,m-2,yr-1, or as the energy value gained per unit time kJ,m-2,yr-1.

This store of energy is potential food for consumers within the ecosystem.

NPP represents the difference between

the rate at which plants photosynthesize

(GPP) and the rate, which they respire

(R). This is because the glucose produced

in photosynthesis has two main fates.

Some provides for growth, maintenance

and reproduction with energy being lost

as heat during processes such as

respiration. The remainder is deposited in

and around cells represents the

stored dry mass (NPP=GPP-R).


30



The accumulation of dry mass is more usually termed biomass, and provides a

useful measure of the production and use of resources.

Primary production is the foundation of all metabolic processes in an ecosystem,

and the distribution of production has a key part in determining the structure of an

ecosystem. Biological communities include more than just plants, they also

include herbivores, carnivores and detritivores.

Production also occurs in

animals as

Secondary Production.

Importantly though animals do

not use all the biomass they

consume. Some passes

through to become feces. Gross

production in animals equals

the amount of biomass or

energy assimilated or biomass

eaten less feces.

As with plants some of the energy assimilated by animals is used to drive cellular

processes via respiration the remainder is available to be laid down as new

biomass. This is Net Secondary Production. Net secondary productivity (NSP )

= food eaten - faeces - respiration energy so NSP = GSP- R (just like plants)

Total energytaken in

(food eaten)

Energy to drive cellularprocesses

(Respiration)

NewBiomass

Waste(faeces)

NSP = GSP - R(Food eaten - Energy in faeces) - Respiration


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Total energytaken in

(food eaten)

Usable energy(Assimilation)

Waste(faeces)

Gross Secondary Production = Energy assimilatedFood eaten - faeces



2.6. 2: Population GrowthDecember 15th, 2009

Over time the numbers within a population change. If we were to collect a few

bacterial cells, place them in a suitable supply of nutrients and then under a

microscope cont the number of cells every hour we would find that there would be

many more bacteria at the end of a 24 hour period than at the start.

It is possible to model this growth as a mathematical equation:

Population growth = change in

population number / change in time

or dN/dt

Where dN = change in numbers and dt

= change in time

This equation can also be written using

the symbol delta to represent time:

Exponential Growth

Thinking about the bacteria above, if I started out with one bacteria (bacteria

reproduce asexually so a population can start with one)and if the bacteria

reproduced one after 5 minutes and then died every, after 30 minutes I would have

64 bacteria - the population size would double every 5 minutes. This means that

the each time the population changes it increases the amount of population change

next time. More simply the rate of population growth increases as the population

grows or exponential growth. This can be expressed as a rate equation.


32



This produces a J shaped curve. As long as

their is a plentiful supply of the resources

that the organism needs. in the case of

bacteria: sugars, moisture and warm the

population would keep growing indefinitely.

Obviously in nature this does not happen.

Darwin in Origin of Species (p117 -119)

recognizes that this would be an

absurd proposition

“There is no exception to the rule that every

organic being increases at so high a rate,

that if not destroyed, the earth would soon be covered by the progeny of a single

pair. Even slow-breeding man has doubled in twenty-five years, and at this rate, in

a few thousand years, there would literally not be standing room for his progeny.

Linnaeus has calculated that if an annual plant produced only two seeds - and there

is no plant so unproductive as this - and their seedlings next year produced two,

and so on, then in twenty years there would be a million plants. The elephant is

reckoned to be the slowest breeder of all known animals, and I have taken some

pains to estimate its probable minimum rate of natural increase: it will be under the

mark to assume that it breeds when thirty years old, and goes on breeding till

ninety years old, bringing forth three pairs of young in this interval; if this be so, at

the end of the fifth century there would be alive fifteen million elephants,

descended from the first pair.”

So what stops the planet being knee deep in elephant dung?

Limited resources

Al resources in an ecosystem are limited. there

is only so much food, only so much space, only

so many mates even. The results of these

ecological limits or ECOLOGICAL RESISTANCE is

that no population can keep growing forever.

There is a ceiling limit that each ecosystem sets.

This limit set by the resources of the ecosystem

is the CARRYING CAPACITY, confusingly given

the symbol K in ecology.


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http://sciencebitz.com/wp-content/uploads/2009/05/rn.png

http://sciencebitz.com/wp-content/uploads/2009/05/rn.png



2.6. 3: Population Regulation

The number of organisms in any population continually

change. They never remain constant for ever. Many

factors can affect population size, but their combined

effect is going to be seen in an alteration in one of four

conditions: birth rate, death rate, gains from immigration

and losses from emigration.

Changes in these four conditions determine later

population numbers. These conditions in-turn are

affected by the available resources in any ecosystems. As populations grow then

competition tends to come into play and resources start to become limited, this can

continue until a theoretical point is reached where the amount resources available

can not support the current population. The resource available define the maximum

number of species (or individuals within a species) that habitat can support

throughout their complete life cycle. This is the Carrying Capacity of that

ecosystem.

It is the environmental carrying capacity that limits how large a population can

grow to. I reality, often the number of individuals in a population is greater than the

carrying capacity. When this occurs competition between individuals for the limiting

resources takes place. These resources could be; food, mates, breeding sites,

water, soil nutrients or anything else. This is an example of Density dependent

competition (see 2.1.7: Population Interactions)

With Density dependent competition the larger the population the greater

the degree of competition for the limited resources. As the population grows so

fewer individuals will get the resources they require to survive.

This need not in itself result increased mortality but may also cause emigration of

individuals to areas where the resource is in greater supply or a lowering of

reproductive success. This can be illustrated by examining the declining Swallow

population in Europe. Since the 1970’s concern has been raised about an apparent

decline in the breeding populations of these birds across Europe. Two possible

explanations are linked to changes in farming practices. Extensive use of pesticides

on crop plants has reduced the number of insects that swallows can feed on.


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http://www.sciencebitz.com/wp-content/uploads/2007/09/growth_curve.jpg



However a another factor is the loss of old brick and stone farm buildings that

Swallows require for nest sites: this is an effect on their reproductive success, fewer

breeding sites means fewer young and so eventually fewer adults.

2.6. 5: Succession

ECOLOGICAL SUCCESSION

After the retreat of glaciers following the last ice age, new virgin land was exposed

with nothing living on it. It didn’t remain that way for long. Soon the land was

covered with mosses and lichen. Gradually organic material was added to the

simple mineral soils left behind by the glaciers and from the erosion of bare rock.

This created conditions that allowed, first grasses and small herbs to establish, and

eventually over time for northern Europe to be covered by woodland.

This directional change in community is termed succession.

PRIMARY SUCCESSION

Involves the colonization of newly

created land by organisms.

o River deltas

o Volcanic larva fields

o Sand dunes

o Glacial deposits

Simple mineral soils evolved from

erosion are, slowly invaded by

mosses and lichen. These and

other early plants are adapted to

survive periods of drought as

water drains quickly away from

the mineral soils. These contribute

organic matter to the soil as they

die and spread, this creates

conditions that allow larger


35



mosses to invade. These help add more

organic matter to the soil, which improves

its water holding capacity, and provide a

habitat for soil organisms that help speed

up the breakdown of organic matter and

release of nutrients.

Conditions then become favorable for ferns

and higher plants to establish on the

primitive soils and humus forms as more

organic matter is added. Eventually shrubs and trees invade, first from wind-

dispersed seeds and eventfully by animal dispersal. Eventually over time a stable

woodland community develops.

Succession progress in stages from;

Pioneer species that are adapted to develop in limiting environments to a stable

developed community. This final community is termed a CLIMAX COMMUNITY .

As the community develops, so biodiversity also increases.

The entire process from bare rock to climax is called a SERE and that progress

directionally through SERAL STAGES.

An example of primary succession can be seen in the development of the natural

broad-leafed forest that covered much of Northern Europe following the end of the

last Ice Age.

We know that following the retreat of ice around 10,000 years until around 7,500

years ago, a Boreal community formed. First of Juniper then birch and later pine. As


36



the climate warmed so the community changed from a dominance of birch to Oak

with abundant wych elm, alder and lime, marking a change to warm, moist Atlantic

period until about 5,000 years ago. Much of Northern Europe would still be covered

in this mixed broad leaf forest if Neolithic man had not started changes the plant

community around him as agriculture developed.

Ecological period Years ago Community

Pre-Boreal 10,000 - 9,500 Tundra with patches of willow, birch and

pine

Boreal 9,500 - 7,500 Hazel, Pine

Atlantic 7,500 - 5,000 Hazel, Oak, Elm

Lime , Ash, Alder

Sub-Boreal 5,000 - 2,500 Mixed Oak with many cleared areas

either being farmed or abandoned and

returned to woodland

If primary succession starts on dry land it is a XEROSERE

If it starts in water (a pond) it is a

HYDROSERE.

Pond and lakes get continuous inputs

of sediment from streams and rivers

that open into them. Some of this

sediment passes through but a lot

sinks to the pond bottom. As plant

communities develop they add dead

organic material to these sediments.

Over time these sediments build up allowing rooted plants to invade the pond

margins as the pond slowly fills in. This eventually leads to the establishment of

climax communities around the pond margins and in smaller ponds the eventual

disappearance of the pond. In regions where rainfall is high, the xerosere climax

community mat not establish after a hydrosere. The wet conditions creat the

development of raised bogs as the climax following hydrosere succession.


37



SECONDARY SUCCESSION

Where an already established community is suddenly

destroyed, such as following fire or flood or even human

activity (ploughing) an abridged version of succession

occurs.

This secondary succession occurs on soils that are

already developed and ready to accept seeds carried in

by the wind. Also there are often dormant seeds left in

the soil from the previous community. This shortens the

number of seral stages the community goes through .


38



Good examples of secondary succession have been studied in abandoned form land

in North Carolina in the United States. The farmland had become infertile through

not enough nutrients being returned after crops had been taken and through wind

erosion. As the land became unproductive and uneconomical to farm, farmers

simply abandoned the land. This left patches of former farmland of various ages.

Years after

abandonment

Dominant species Predominant Forest Vegetation

0 Crabgrass Developing grassland community

1 Crabgrass,

Horseweeds, Pigweed

Community starts to be dominated by

Horseweed

2 Aster, Crabgrass,

Ragweed

Grass scrub is developing

3-18 Broomsedge, Pine

later

Developed grass scrub community with

later invading pine

18-30 Pine Immature pine forest

30-70 Pine, Young Hickories

and Oaks later

Mature pine wood with understorey of

young hardwoods later

70-100 Pine, Hickories, Oaks Transitonal stage between Pine and

Hardwood forest

100+ Oak, Hickory Mature mixed hardwood forest


39



2.6. 6: Succession, Productivity and Diversity.

Productivity

During succession Gross Primary Productivity

tends to increase through the pioneer and early

wooded stages and then decreases as climax

community reaches maturity. This increase in

productivity is linked to growth and biomass.

Early seral stages are usually marked by rapid

growth and biomass biomass accumulation -

grasses, herbs and small shrubs. Gross Primary

Productivity is low but Net Primary Productivity

tends to be be a large proportion of GPP as with little biomass in the early seral

stages respiration is low. As the community develops towards woodland and

biomass increases so does productivity. But NPP as a percentage of GPP can fall as

respiration rates increase with more biomass.

Studies have shown that standing crop (biomass) in succession to deciduous

woodland reaches a peak within the first few centuries. Following the establishment

of mature climax forest biomass tends to fall as trees age growths slows and an

extended canopy crowds out ground cover. Also Older trees become less

photosynthetically efficient and more NPP is allocated to none photosynthetic

structural biomass such as root systems.

Biomass Accumulation and Successional Stage:

Early Stage Middle Stage Late Stage

Low GPP but High

percetage NPP

Little increase in

biomass

Gross Productivity high

increased

photosynthesis

Increases in biomass as

plant forms become

bigger

Tree reach their maximum size

Ratio of NPP to R is roughly equal


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Biodiversity

Early stages of succession tend to be marked

by few species within the community. As the

community passes through subsequent seral

stages so the number of species found

increases. Very few pioneer species are ever

totally replaced as succession continues. The

result is increasing diversity - more species.

This increase tends to continue until a balance

is reached between possibilities for new

species to establish, existing species to

expand their range and local extinction.

Evidence following the eruption of the Mount St Helens volcano in 1980 has

provided ecologists with a natural laboratory to study succession. In the first 10

years after the eruption species diversity increased dramatically but after 20 years

very little additional increase in the diversity occurred1

Disturbance

Early ideas about succession suggested that the Climax community of any area was

almost self perpetuating. This is unrealistic as communities are affected by periods

of disturbance to greater or lesser extent. Even in large forests trees eventually

age, die and fall over leaving a gap. Other communities are affected by flood, fire,

land slides earthquakes, hurricanes etc. All of these have an effect of making gaps

available that can be colonised by pioneer species within the surrounding

community. This adds to both the productivity and diversity of the community.

1. Carey, Susan, John Harte & R. del Moral. 2006. Effect of community assembly and primary succession on

the species-area relationship in disturbed systems. Ecography 29:866-872. [↩]


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Environmental Systems and Society Guide Part 1