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Ecological Definitions
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Exam Note
The Exam Boards report that ecology, after genetics, is the area
of the syllabus in which students attain the lowest marks.
The definitions that follow are deceptively simple, yet students
often fail to learn them precisely. A typical ecology question
might ask for the definition of one of these words applied to an
example of a particular ecosystem.
Biosphere
The part of the Earth in which organisms live –
the atmosphere, seas, lakes, rivers, earth etc.
Biome
Areas on the earth with similar climate, soil, plants and animals
and which have similar ecosystems e.g.
tropical rainforest; deciduous woodland.
Ecosystem
A community of organisms interacting with one another, plus the
abiotic environment in which they live and also interact. e.g.
a pond; a woodland.
The abiotic environment is the non-living part of the ecosystem.
Community
All of the populations living in the same habitat.
This is the biotic, living part of the ecosystem.
Population
A group of organisms of the same species living within a given area.
Species
A group of organisms – with similar morphology, physiology &
behaviour – that interbreed to produce fertile offspring & which are
reproductively isolated – in place, time or behaviour – from other
species.
Habitat
A well-defined locality in which organisms live e.g. a fallen log;
a birch tree.
Niche
The role, activities and location of an organism within a habitat;
the sum total of all the relationships an organism has with its
environment.
Environment
A description of the biotic & abiotic conditions in which an
organism lives.
Autotroph
An organism that can synthesise complex organic molecules
from simple ones. There are two types of autotroph, depending
on how they obtain their energy:
i) Phototrophs: autotrophs that use light energy e.g. plants
ii) Chemotrophs: autotrophs that use inorganic chemical
energy e.g. nitrifying bacteria; sulphur bacteria
Heterotroph
An organism that feeds on complex, ready-made organic
matter (molecules made of carbon). It uses them for:
i)
Respiration: the organic molecules are oxidised to provide
energy for vital activities
ii)
Building materials: organic molecules e.g. amino acids are
used for the repair and growth of the body
iii) Vitamins: organic molecules that cannot be synthesised by
organisms and that are essential for normal cellular processes
Respiration
The process by which all organisms oxidise organic matter
to liberate energy. There are two types:
i)
Anaerobic respiration: which does not require oxygen,
is less efficient and which usually produces ethanol and
carbon dioxide, or lactic acid as end products
ii) Aerobic respiration: which requires oxygen, is much
more efficient than anaerobic and produces carbon
dioxide and water as end products
Saprotroph*
Organisms e.g. fungi and bacteria that break down dead organic matter.
*(Also called saprophyte or saprobiont)
Trophic Levels
....troph as in autotroph and heterotroph means feeding. Within ecosystems*
there may be several different trophic (feeding) levels:
Producers: these are the autotrophs that are at the base of food
chains in all ecosystems e.g. grass.
All the other organisms in an ecosystem are heterotrophs and consumers.
Primary consumer: these are the organisms, usually called herbivores e.g.
rabbits that directly consume the primary producers.
Secondary consumer: these are the organisms, usually called carnivores e.g.
foxes that consume the primary consumers.
Tertiary consumer: these organisms consume the secondary consumers. They
might be saprobionts such as fungi and bacteria.
Saprobiont: organism that breaks down dead organic matter (D.O.M).
*In real ecosystems a consumer may occupy more than one trophic level e.g.
badgers which consume plants, insects, earthworms etc.
Food Chains
Energy-containing organic molecules produced by
autotrophs are a source of food (materials and energy)
for heterorophic organisms. A typical sequence would
start with a plant being eaten by an animal herbivore
(primary consumer). The animal is in turn eaten by
another animal (secondary consumer) and so on. At
each stage energy and materials are passed on to the
next level. This sequence is called a food chain with
each stage being a trophic level. There are usually four
or five trophic levels and rarely more than six.
Tertiary consumer
Secondary consumer
Primary consumer
Producers
Fresh Water Food Webs
Insectivorous birds
Stickleback
Adult dragonfly
Carnivorous insect larvae
Carnivorous beetles
Dragonfly nymph
Leeches
Adult caddis fly
Daphnids/Copepods
Rotifer
Adult midge
Caddis fly larvae
Midge larvae
Oligochaetes
Adult mayfly
Ciliates
Bivalves
Water snails
Mayfly nymphs
Herbivorous beetles
Plankton
Filamentous algae
Aquatic flowering plants
The feeding relationships between organisms are complex and a species may feed on more than one
organism in the same food chain or at different trophic levels. Food chains interconnect to produce a food
web. All of the organisms in the food web are dependent on the sunlight energy gathered by the plants.
Gross Primary Productivity
Gross Primary Productivity: the total amount of organic matter made
by a producer in a given time period.
Imagine you owned a shop selling newspapers, sweets etc. At the end
of one week you find that your earnings amount to £1000.
Plants manufacture organic molecules such as sugars. In a given
time period the plant may manufacture 1000 units of sugar.
Question: If a shopkeeper has a £1000 in his till at the end of a week,
is this how much profit he has made?
Net Primary Productivity
Net Primary Productivity: the total amount of organic matter made by a
producer in a given time period, minus the amount lost in respiration in
the same time period.
The shopkeeper selling newspapers, sweets etc. has £1000 in his till at
the end of the week. However this is not all profit, for in the same time
period he will have had to spend some of his earnings on buying
papers, sweets etc. paying for staff, lighting etc.
In a similar way plants manufacture organic molecules - say 1000 units
of sugars - in a given time period. They will have to oxidise some of
these molecules in respiration. This will release energy that they need
for growth, active uptake, reproduction etc.
Sunlight
1x
Productivity
106
0.99 x 106
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Net
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Death and Excretion
Death
Death and Excretion
Respiration
Detritivores and Decomposers
Death and Excretion
Food Pyramids
The purpose of food pyramids is to understand the relationship between the
different trophic levels.
There are three types of pyramid:
i) Number
ii) Biomass
iii) Energy
Each has its own particular advantage and disadvantage – though it is energy
that is the most important.
Food Pyramids
Pyramids of Number
In a pyramid of number the population size of each trophic level is shown by
the width of each band.
Fox
Secondary consumer
Rabbit
Primary consumer
Producer
Advantages
1. Relatively simple to collect the data
Grass
1.
Disadvantages
Can be difficult to place a species in one
trophic level e.g. badgers are omnivorous
2.
The range of numbers is often so large
that it can be difficult to draw a pyramid
to scale
3.
Inverted pyramids are produced when
organisms vary greatly in size
Food Pyramids
Inverted Pyramids of Number
Ladybird
Secondary consumer
Aphid
Primary consumer
Producer
Oak
Pyramids of number can be inverted particularly if the species involved vary
greatly in size e.g. one oak tree can support thousands of aphids and many
predatory ladybirds. Since the producer must be the most important part of the
food chain this shows the limitations of this type of food pyramid.
Food Pyramids
Pyramids of Biomass
Ladybird
Secondary consumer
Aphid
Primary consumer
Producer
Oak
In a pyramid of biomass the width of each band is equal to the amount of
biomass in each trophic level.
Biomass = number of individuals x mass of each individual at each trophic level
at any one time
1.
2.
Advantages
Takes account of the different size of
organisms & gives a more realistic
picture of the relationships between the
trophic levels
Avoids the difficulty of scale
1.
Disadvantages
Only takes account of biomass i.e.
standing crop at the time of sampling, not
productivity over time & inverted
pyramids may be produced
Food Pyramids
Inverted Pyramids of Biomass
Zooplankton
Primary consumer
Producer
Phytoplankton
Pyramids of biomass can be inverted. The example above is for the English
Channel in spring. This is possible because the productivity of the
phytoplankton is far greater than the zooplankton. The phytoplankton are
constantly being eaten by the zooplankton but reproduce very rapidly & so
produce many more individuals over a period of time.
Food Pyramids
‘Standing biomass is not a measure of productivity’
Mown once
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hasthe
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by mowing.
the end of the summer and the second
which he mows every few weeks.
Mown at regular intervals
Food Pyramids
Pyramids of Energy
20%
Fox
Primary consumer
5-10%
Rabbit
Producer
0.5-1%
Grass
Secondary consumer
Pyramids of energy show the true ecological relationship between the different trophic levels.
Plants use about 1% of the sunlight they receive.
Primary consumers are about 5-10% efficient – digesting plants is difficult and costs a
considerable amount of energy.
Secondary consumers are about 20% efficient.
At each trophic level a large proportion of the energy is lost. This loss of energy limits the
number of trophic levels.
1.
Advantages
Takes account of productivity
2.
Allows comparisons to be made between
different populations and ecosystems
3.
Can include the input of (solar) energy
into the ecosystem
4.
No inverted pyramids are produced
1.
Disadvantages
It is very difficult to collect the data
Food Pyramids
A Comparison
Species
Approximate density/
no. m-2
Biomass/
g m-2
Energy/
kJ m-2 day-1
Soil Bacteria
1011
0.001
4.2
Deer
10-5
1.1
2.1
The table above compares numbers, biomass and energy in bacteria and deer
living in an English woodland.
The bacteria and the deer vary enormously in size. The density of the bacteria
is many millions of times greater than the deer. However, the figures show
that the biomass of the deer is twice that of the bacteria. The final column for
energy shows that the bacteria use twice that of the deer.
Zonation
Zonation: the gradual change in communities up (or down) an
environmental gradient.
Examples can be found from low to high water on a rocky shore and from valley
floor to mountain top in the Scottish mountains. As the abiotic environment
changes so too do the species.
Succession
Succession: the progressive change in composition of a community of organisms
towards a stable climax community.
Examples:
Primary succession occurs where there is no soil, organic matter
or seeds present e.g. after a lava flow has cooled or after a
retreating ice sheet exposes bare rock.
Secondary succession occurs when a pre-existing community is
disrupted e.g. by fire.
Deflected succession occurs when the usual progression of
communities is altered, usually by human interference; this may
result in a plagioclimax.
Pioneer Species
Pioneer Species: the first colonisers of a new habitat; the first individuals
present in a succession e.g. marram grass on sand dunes or lichen on bare rock.
The presence of these species changes the environment in some way and allows
other organisms to become established.
Example: Marram grass (Ammophila maritima) is a pioneer of sand dune
ecosystems. In the early stages, the dune is unstable, the sand constantly
moving and the dune has poor water and nutrient retention. The root system of
the grass stabilises the sand and as dead organic matter from the plant
accumulates, water retention increases. These changes allow other competing
species to establish themselves and in time the marram grass will be pushed
out.
Climax Community
Climax community: The more-or-less stable community of organisms that
arises at the end of a succession e.g. oak woodland.
The composition of this community may be determined by the climate – the
climatic climax and/or by the soil conditions – the edaphic climax.
Chalk
Downland
Lowland
Heath
Plagioclimax
Plagioclimax: The stable community of organisms produced at the end of a
deflected succession e.g. lowland heath and chalk downland ecosystems. If
the deflecting factor e.g. grazing pressure is removed then a secondary
succession will eventually establish the climax community.