Ecosystems and sustainability:
What is an ecosystem?
ECOSYSTEM: the living organisms and non living factors in a habitat
COMMUNITY: all the interacting populations of different species living together in the same area at the same time
POPULATION: a group of individuals of the same species in an area at the same time that can interbreed
HABITAT: the area in which an organism lives
NICHE: the role of an organism within its environment and community
ABIOTIC FACTORS: the non- living factors in an ecosystem
BIOTIC FACTORS: the living factors in an ecosystem
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Any group of living organisms and non-living things occurring together, and the interrelationships between
them, can be thought of as an ecosystem.
Ecosystems can be on a large scale such as the African grassland or on a smaller scale like a garden pond.
The role that each species plays in an ecosystem is called its niche.
Because each organism interacts with both living and non-living things, it is almost impossible to define its
niche entirely.
Description of its niche could include things like how and what it feeds on, what it excretes, how it
reproduces, etc.
It is impossible for two species to occupy exactly the same niche in the same ecosystem.
Depending on their niche, the living organisms in an ecosystem can affect each other; such biotic factors
include food supply, predation and disease.
Abiotic factors:
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Abiotic factors describe the effects of the non -living components of an ecosystem
Climatic factors: light, temperature, water availability
Edaphic factors: factors associated with the soil such as pH and organic content
Topographic factors: such as the angle of a slope
Ecosystems are dynamic:
In most ecosystems, population sizes fluctuate, either very slightly or very noticeably. This is because the community
of living things in an ecosystem interact with each other and with their physical environment. Any small changes in
one can affect the other. For example:
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If a predator’s population size goes up, the population size of the prey will go down because they are being
eaten more quickly.
The nitrogen levels in soil can affect the population size of plants growing there.
Nitrogen fixing plants would grow successfully in nitrogen deficient soli, but they would affect their
environment by increasing the soil nitrogen levels. This change would then help other plants to grow there
as well.
Energy and ecosystems:
PRODUCER: an autotrophic (produces own source of nutrition) organism that starts the food chain
CONSUMER: heterotrophic (use other organisms for nutrition) organism
DECOMPOSER: A saprophytic organism that feeds off decomposing organic material
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Matter is constantly recycled within an ecosystem, - nutrient cycles, such as the nitrogen cycle and carbon
cycle, are good examples.
Energy is not recycled – it flows through an ecosystem.
All living organisms need energy
Via respiration they release energy from organic molecules such as glucose, in their food.
This energy originally came from sunlight.
At the start of nearly all food chains is a plant, which captures light energy through photosynthesis, and
converts it to chemical energy stored in molecules like glucose.
Because plants and other photosynthetic organisms, such as algae and some bacteria, supply chemical
energy to other organisms, they are called producers.
Other organisms, like animals and fungi, are called consumers.
Primary consumers are herbivores, who feed on plants, are who are eaten by carnivorous secondary
consumers.
These in turn are eaten by carnivorous tertiary consumers.
Some living things called decomposers (bacteria, fungi and some animals) feed on waste material or dead
organisms.
Understanding energy transfer:
TROPHIC LEVEL: the level at which organisms feed in a food chain is called a trophic level.
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A food chain shows how energy is transferred from one living organism to another.
The level at which an organisms feeds is called its tropic level.
Within an ecosystem, living organisms are usually members of more than one food chain, and often feeds at
different trophic levels in different chains
Drawing food chains together as a food web show how energy flows through the whole ecosystem.
Efficiency of energy transfer:
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At each trophic level, some energy is lost from a food chain, and is therefore unavailable to the organism at
the next trophic level.
At each trophic level, living organisms need energy to carry out life processes
Respiration releases energy from organic molecules like glucose.
Some of this energy is eventually converted into heat.
Energy remains stored in dead organisms and waste material, which is then only available to decomposers,
such as fungi and bacteria.
This waste material includes parts of animals and plants that cannot be digested by consumers.
Because of this, there is less energy available to sustain living tissue at higher levels of the food chain, and so
less living tissue can be kept alive.
When the organisms in a food chain are about the same size, this means there will be fewer consumers at
the higher levels.
Ecologists draw pyramids of numbers to represent this idea.
The area of each bar in the pyramid is proportional to the number of individuals.
Pyramids can be drawn for individual food chains, or for an ecosystem as a whole.
Pyramids of number show population size but don’t show how much energy is in the ecosystem
Pyramids of biomass:
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Counting the number of organisms does not always produce an accurate picture about how much living
tissue exists at each level.
A better approach is to draw a pyramid of biomass, where the area is representative to the dry mass of all
the organisms at that tropic level
To do this properly, an ecologist would collect all the organisms and put them in an over at 80 degrees until
all the water in them has been evaporated.
Doing this is rather destructive to the ecosystem being studied so ecologists often just measure the wet
mass of an organism and calculate the dry mass on the basis of previously published data.
Pyramids of energy:
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Pyramids of biomass still present problems, such as different species may release different amounts of
energy per unit mass.
Because of this, ecologists sometimes prefer to construct pyramids of energy.
This involves burning the organisms in a calorimeter and calculating how much heat energy is released per
gram – this is calculated from the temperature rise of a known mass of water.
Given that this too is destructive and time- consuming, ecologists revert to using pyramids of biomass
instead.
Only give an idea of energy in an ecosystem at one time
Efficiency of energy transfer may be incorrect due to population fluctuations
Productivity:
PRODUCTIVITY: the rate at which energy passes through each trophic level in a food chain is a measure of the
productivity.
Ecologists often look at the rate at which energy passes through each trophic level, drawing a pyramid of energy
flow. This rate of energy flow is called productivity.
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Productivity gives an idea of how much energy is available to the organisms at a particular trophic level, per
unit area, in a given amount of time
Measured in mega-joules of energy per square metre per year.
At the base of the food chain, the productivity of the producer trophic level is called the primary
productivity
Gross primary productivity is the rate at which plants convert light energy into chemical energy
Some of this energy is used in respiration so the remaining energy which can be passed onto the primary
consumer is called the net primary productivity
Manipulating energy transfer:
Primary productivity:
PRIMARY PRODUCTIVITY: the total amount of energy fixed by photosynthesis. It is the net flux of carbon from the
atmosphere to plants, per unit time. It is the rate and may be measured in terms of energy per unit time.
NET PRIMARY PRODUCTIVTY: the rate at which carbohydrates accumulates in the tissue of plants of an ecosystem
and is measured in dry organic mass.
NPP = PP – R
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Amount of energy available to heterotrophs in the ecosystem.
It is a fundamental ecological variable and is an important factor in determining the amount of biomass that
a particular ecosystem can support.
Less than 1% of the sunlight energy reaching Earth is used for photosynthesis.
The energy captured by leaves for photosynthesis is called the primary productivity.
Some of this will be used by the plant and lost as respiratory heat (R).
The difference between primary productivity and R is the Net Primary Productivity (NPP).
NPP is the rate of production of new biomass available for consumption by heterotrophs.
Improving primary productivity:
By manipulating environmental factors, humans can increase NPP – making energy conversion more efficient,
reducing energy loss and increasing crop yields.
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Light levels can limit the rate of photosynthesis, and hence NPP
Some crops are planted early to provide a longer growing season to harvest more light. Others are grown
under light banks.
Lack of water is important in many countries
As well as irrigating crops, drought resistant strains have been bred
E.g. drought resistant barley in North Africa, wheat in Australia and sugar beet in the UK.
Lack of available nutrients can slow the rate of photosynthesis and growth.
Crop rotation can help. Including a nitrogen fixing crop, like peas or beans, in that cycle replenishes levels of
nitrates in the soil.
Many field crops have been bred to be responsive to high levels of fertiliser.
Pests, such as insects, caterpillars or nematodes, eat plant crop plants
They remove biomass and stored energy from the food chain, and lower the yield.
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Spraying with pesticides can help to reduce this loss.
Some plants have also been bred to be pest resistant, or have been genetically modified with a bacterial
gene.
Fungal diseases of crop plants can reduce NPP.
Fungi causes root rot, damaged xylem vessels, damage foliage through wilting, blight or spotting, damage to
the phloem tubes, or damage to the flowers and fruit.
Farmers spray their crops with fungicides.
Many crops have been bred to be resistant to fungal infections. Potatoes have been genetically modified to
be resistant to potato blight.
Competition from weeds for light, water and nutrients can reduce a crops NPP.
Farmers use herbicides to kill weeds.
Improving secondary productivity:
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Energy transfer from producers to consumers is inefficient, as it transfers energy from the primary
consumers to the secondary consumers and beyond.
Primary consumers do not make full use of plants biomass – some plants die, consumers don’t eat every
part of the plant, and they do not digest everything that they eat. Egesting a lot of it in their faeces.
Even when food is digested and absorbed, much of the stored energy is used to keep the animal alive, with
only a small amount being stored when it grows.
It is this small amount that is available to the next consumers in the food chain
However, it is possible for humans to manipulate energy transfer from produce to consumer.
A young animal invests a larger proportion of its energy into growth than an adult does. Harvesting animals
just before adulthood minimises loss of energy from the food chain.
In the past some farm animals have been treated with steroids to make them grow even more quickly,
increasing the population of energy allocated to growth. However, this practise has been outlawed in the EU
for many years.
Selective breeding has been used to produce breeds with faster growth rates, increased egg production and
increased milk production
Animal may be treated with antibiotics to avoid unnecessary loss of energy to pathogens or parasites.
Mammals and birds waste lots of energy walking around to find food, and keeping their body temperature
stable.
Approaches such as zero grazing for pig and cattle farming maximise energy allocated to muscle meat
production by stopping the animals from moving about by supplying food to them and by keeping
environmental temperature constant.
Although we say that transfer of energy from producers to consumers is inefficient and that grain could be
used to feed humans directly as opposed to feeding cattle or pigs first, there are some areas where grain
cannot be grown but animals can exist.
For example, sheep can live on mountainside
These areas are largely infertile and cannot be used to grow grain, but humans can eat the lamb produce.
Many people have serious concerns about modern farming practices and animal welfare. Deciding where
the balance lies between welfare and efficient food production is a contentious topic that is constantly kept
under review and should include informed public debate.
Succession:
Changing ecosystems:
SUCCESSION: is a directional change in a community of organisms over a period of time
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A community consists of all the plants and animals that occupy a particular area
Any change in a community of organisms can cause a change in their habitat.
Any change in a habitat can also cause a change in the makeup of the community.
The community is a constantly changing and dynamic unit, which passes through a number of stages from its
origin to its climax
These ideas can help explain why gradual directional changes happen in a community over time.
Such a process of directional change is called succession.
There are two types of succession:
1) Primary succession: the starting point is bare ground (e.g. rock)
2) Secondary succession: Here a community is damaged and the soil is left. Plants then colonies (e.g. a
woodland had been felled)
The END POINT of both types of succession is the climax community
How Does Succession Happen: The Island of Surtsey
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Created by a volcanic eruption in the 1960s, Iceland
Developments of such a community from the bare ground are called primary succession
Algae and lichens began to live on the bare rock
Pioneer community.
Erosion of the rock and a build up of dead and rotting organism, produces enough soil for larger plants like
mosses and ferns to grown.
These replace or succeed the algae and lichens.
Larger plants succeeded the moss and ferns and a final, stable community is reached:
Climax community
In the UK, climax communities are often woodland communities.
Succession does not always start from bare ground. Secondary succession takes place on a previously colonised, but
disturbed or damaged, habitat.
Typical colonisers are/have:
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Large quantities of wind-dispersed seeds/spores
Rapid germination
Photosynthetic
Fix nitrogen
Tolerate extreme conditions
Succession on sand dunes:
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Sand dunes are interesting because they display all the stages of succession in the same place at the same
time.
Because the sea deposits sand to the beach, the sand nearest to the sea is deposited more recently than the
sand further away.
Eventually, a dune’s community may develop into grassland, and then into woodland.
Nearest the sea, only a few species can colonise the sand
Sand constantly moves
High salt concentration
Loose, unstable sands
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Pioneer species Caikle maritime (sea rocket) and Salsola kali (prickly sandwort) colonise the sand above the
high water mark
A mini dune builds up as plants die and decay and windblown sand accumulates around the base of these
plants
The mini dune is stabilised as it is colonised by species with underground stems such as Agrophyron
junceiforme (sea couch grass)
The dune becomes more stable and has more nutrients and Ammophilia arenaria (Marram grass) grows
Marram grass catches sand in shoots and this accumulates into a larger dunes
Leguminous plants like Triflolium arvense (hare’s foot clover) begin to grow which fix nitrogen in the dune
allowing even more species to grow
LOOK AT AS SAMPLING NOTES.
Decomposers and Recycling:
Decomposing organic material:
Bacteria and fungi involved in decomposition feed in a different way from animals. They feed saprotrophically so
they are described as saprotrophs.
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Saprotrophs secrete enzymes onto dead and waste material.
These enzymes digest the material into small molecules, which are then absorbed into the organism’s body.
Having been absorbed, the molecules are stored or respired to release energy.
If bacteria and fungi did not break down dead organisms, energy and valuable nutrients would remain
trapped within the dead organisms.
By digesting dead and waste material, microbes get a supply of energy to stay alive, and the trapped
nutrients are recycled
Microorganisms have a particularly important role in the cycling of carbon and nitrogen within ecosystems.
Decay:
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Energy and biomass is lost from a food chain due to
Death
Waste
This organic material can be broken down by decomposers
There are three types of decomposer:
Fungi
Bacteria
Detritus feeders
Saprotrophs:
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Bacteria and fungi involved with decomposition are saprophytic
Enzymes are secreted onto organic material
The enzymes digest the organic material
The molecules are used or stored
Recycling nitrogen within an ecosystem:
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Living things need nitrogen to make proteins and nucleic acids.
Below shows how nitrogen atoms are cycled between the biotic and abiotic components of an ecosystem.
Bacteria are involved in ammonification, nitrogen fixation, nitrification and denitrification.
The nitrogen cycle:
4 biological processes are involved in the cycling of nitrogen through the environment
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4.
Decomposition/ammonification (purification of proteins)
Nitrogen fixation
Nitrification
Denitrification
Three processes are responsible for most of the nitrogen fixation in the environment:
1. Biological fixation by certain microbes
2. Atmospheric fixation by lightning
3. Industrial fixation-Haber process
Nitrogen fixation:
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Very un-reactive
Impossible for plants to use it directly
Instead they need a supply of ‘fixed’ nitrogen such as ammonium ions (NH4+) or nitrate ions (NO3-).
Nitrogen fixation can occur when lightning strikes occur or through the Haber cycle. However, these
processed only account for 10% of nitrogen fixation.
Nitrogen Fixing Bacteria account for the rest. Many of these live freely in the soil and fix nitrogen gas, which
is in the air within the soil, using it to manufacture amino acids.
Nitrogen fixing bacteria such as Rhizobium, also live inside the root nodules of plants such as peas, beans
and clover, which are all members of the bean family.
They have a mutual relationship with the plant: the bacteria provide the plant with fixed nitrogen and
receive carbon compounds, such as glucose, in return.
Proteins, such as leghaemoglobin, in the nodules absorb oxygen and keep the conditions anaerobic.
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Under these conditions the bacteria use an enzyme, nitrogen reductase, to reduce nitrogen gas to
ammonium ions that can be used by the host plants.
Nitrogen fixing bacteria:
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Free-living bacteria such as Azotobacter
Mutualistic bacteria Rhizobium in root nodules of legumes
Rhizobium contains an enzyme called nitrogen reductase that converts nitrogen to ammonia ions
The ammonia is used to make amino acids which the legume can use to make proteins in return for
supplying the bacteria with carbohydrates
A protein found in the nodules called leghaemoglobin absorbs oxygen to maintain aerobic conditions
Nitrification:
Nitrification happens when chemoautotrophic bacteria in the soil absorb ammonium ions.
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Ammonium ions are released by bacteria involved in putrefaction of proteins found in dead or waste
organic matter, by decay
Rather than getting their energy from sunlight (like photoautotrophic bacteria, algae and plants),
chemoautotrophic bacteria obtain energy by oxidising ammonium ions to nitrites (Nitrosomonas Bacteria),
or by oxidising nitrites to nitrates (Nitrobacter bacteria).
Because this oxidisation requires oxygen, these reactions only happen in well aerated soils.
Nitrates can be absorbed from the soil by plants and used to make nucleotide bases (for nucleic acids) and
amino acids (for proteins)
Bacteria of the genus Nitrosomonas oxidise NH3 to nitrites (NO2-)
Bacteria of the genus Nitrobacter oxidise the nitrites to nitrates (NO3-)
These two groups of autotrophic bacteria are called nitrifying bacteria
This process can only occur in soil with good oxygen supply
Denitrification:
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Other bacteria convert nitrates back to nitrogen gas
When the bacteria involved are growing under anaerobic conditions, such as waterlogged soils, they use
nitrates as an alternative to oxygen for respiration
Bacteria such as Pseudomonas are the agents
N2 and N2O are produced
What affects population size?
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In some circumstances, a species’ population size may stay fairly stable
Population size can also fluctuate quite suddenly
The size of a population depends upon the balance between mortality and rate of reproduction
Population growth:
A=Lag phase
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May only be a few individuals, still acclimatising to their habitat
Rate of reproduction is low
Find shelter
Find food
Gestate
B= Exponential Phase
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Population increases
Resources are plentiful
Slutty Pigeons
Reproduce
Birth rates greater than death
C= Stationary phase
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Population reached carrying capacity
Habitat cannot support a larger population
Birth rates are equal to death rates
k-strategist
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Parental care
R-strategist:
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Bacteria populations
Colonising community in succession
Pioneer species
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Population exceeds carrying capacity
Over utilised resources in the habitat
Insufficient resources to support the population
Death rates exceed birth rates
Population decreases
Limiting factors:
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Limiting factors prevent the habitat from supporting a larger population than the carrying capacity
Abiotic factors:
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Water
Light
Oxygen concentration
Territory (nesting sites/shelter)
Biotic factors:
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Parasites
Predators
Food availability
Competition
Predator and prey:
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The population of prey increases
There is greater food available for predators leading to an increase in predator population
Prey population will decrease due to increased predation
Predator population will decrease as competition for food increases
The population of prey increases and population numbers cycle
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E.g. Lynx and snowshoe hares
Competition:
COMPETITION: the shared demand by two or more organisms for limited environmental resources
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As competition increases the rate of reproduction decreases as resources become more limited
Death rate increases as fewer organisms have enough resources to survive
There are 2 types of competition:
Intraspecific
Interspecific
Intraspecific:
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Competition occurs between members of the same species
Individuals will compete for factors such as food, mates and territory
As factors, such as food, become limiting the best adapted individuals will survive and reproduce
Slows down population growth and population enters stationary phase
Intraspecific competition maintains a stable population size:
Population size decreases
Competition decreases
Population size increases
Competition increases
Population size decreases
Interspecific competition:
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Interspecific competition occurs between members of different species:
Affects population size and distribution of species
If competitors occupy very similar niches Interspecific competition can lead to extinction of one competitor
In other competitions one species may just have a much larger population than its competition
Competitive exclusion theory
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E.g., 2 species of Paramecium
Grown separately, and together
Together there was competition for food with P.aurelia obtaining more effectively than P.caudatum
P.caudatum died out
Overlap in 2 species’ niches would result in more intense competition
If species have exactly the same niche, one would be out-competed by the other and would die out or
become extinct in that habitat
Allelopathy and competition in plants:
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Allelopathy refers to the chemical inhibition of one species by another.
The "inhibitory" chemical is released into the environment where it affects the development and growth of
neighboring plants.
Allelopathic chemicals can be present in any part of the plant. They can be found in leaves, flowers, roots,
fruits, or stems. They can also be found in the surrounding soil.
Target species are affected by these toxins in many different ways. The toxic chemicals may inhibit
shoot/root growth, they may inhibit nutrient uptake, or they may attack a naturally occurring symbiotic
relationship thereby destroying the plant's usable source of a nutrient.
Prevent other plants from using the available resources and thus influence the evolution and distribution of
other species.
Black Walnut:
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Black Walnut (Juglans nigra)
Uses chemical Juglone (5 hydroxy-1,4 napthoquinone) and is a respiration inhibitor.
Solanaceous plants, such as tomato, pepper, and eggplant, are especially susceptible to Juglone.
These plants, when exposed to the allelotoxin, exhibit symptoms such as wilting, chlorosis (foliar yellowing),
and eventually death.
Gives a young walnut tree better access to light by preventing other plants from crowding it out
Sorghum:
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Cereal plant
The major constituent of sorghum that causes allelopathic activity is sorgolene
Very potent allelotoxin that disrupts mitochondrial functions and inhibits photosynthesis.
It is being researched extensively as a weed suppressant.
ABIOTIC FACTORS: the non- living factors in an ecosystem
BIOTIC FACTORS: the living factors in an ecosystem
COMMUNITY: all the interacting populations of different species living together in the same area at the same time
COMPETITION: the shared demand by two or more organisms for limited environmental resources
CONSUMER: heterotrophic (use other organisms for nutrition) organism
DECOMPOSER: A saprophytic organism that feeds off decomposing organic material
ECOSYSTEM: the living organisms and non living factors in a habitat
HABITAT: the area in which an organism lives
NET PRIMARY PRODUCTIVTY: the rate at which carbohydrates accumulates in the tissue of plants of an ecosystem
and is measured in dry organic mass.
NICHE: the role of an organism within its environment and community
NPP = PP – R
POPULATION: a group of individuals of the same species in an area at the same time that can interbreed
PRIMARY PRODUCTIVITY: the total amount of energy fixed by photosynthesis. It is the net flux of carbon from the
atmosphere to plants, per unit time. It is the rate and may be measured in terms of energy per unit time.
PRODUCER: an autotrophic (produces own source of nutrition) organism that starts the food chain
PRODUCTIVITY: the rate at which energy passes through each trophic level in a food chain is a measure of the
productivity.
SUCCESSION: is a directional change in a community of organisms over a period of time
TROPHIC LEVEL: the level at which organisms feed in a food chain is called a trophic level.