| Topic Notes |
Additional Support Materials i.e. animations, quizzes, pictures, worksheets |
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Ecology
Definitions (doc)
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Relationships and the Environment |
Energy
and the Env't M.C Qu's |
Human Impact on the Environment |
Ecology
is the study of inter-relationships between organisms and their environment. Its
aim it to explain why organisms live where they do. To do this ecologists study ecosystems,
areas that can vary in size from a pond to the whole planet.
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Ecosystem |
|
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Habitat |
The
physical or abiotic part of an ecosystem, i.e. a defined area
with specific characteristics where the organisms live, e.g. oak forest,
deep sea, sand dune, rocky shore, moorland, hedgerow, garden pond, etc. |
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Community |
The
living or biotic part of an ecosystem, i.e. all the organisms of
all the different species living in one habitat. |
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Biotic |
Any
living or biological factor. |
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Abiotic |
Any
non-living or physical factor. |
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Population |
The
members of the same species living in one habitat. |
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Species |
A
group of organisms that can successfully interbreed |
To study the dynamics of a population,
or how the distribution of the members of a population is influenced by a biotic
or an abiotic factor, it is necessary to estimate the population size.
In other words, it will be necessary to count the number of individuals
in a population. Such counting is
usually carried out by taking samples
One
of the most fundamental problems faced by community and population ecologists is
that of measuring population sizes and distributions. The data is important for
comparing differences between communities and species.
It is necessary for impact assessments (measuring effects of disturbance)
and restoration ecology (restoring ecological systems).
It is also used to set harvest limits on
commercial and game species (e.g. fish, deer, etc.).
In
most cases it is either difficult or simply not possible to census all of the
individuals in the target area. The
only way around this problem is to estimate population size using some form of sampling technique. There
are numerous types of sampling techniques.
Some are designed for specific types of organisms (e.g. plants vs. mobile
animals). As well there are
numerous ways of arriving at estimates from each sampling technique.
All of these procedures have advantages and disadvantages.
In general, the accuracy of an estimate depends on 1) the number of
samples taken, 2) the method of collecting the samples, 3) the proportion of the
total population sampled.
Sampling
is viewed by statistical ecologists as a science in its own right.
In most cases, the object is to collect as many randomly selected samples
as possible (so as to increase the proportion of the total population sampled).
The accuracy of an estimate increases with the number of samples taken.
This is because the number of individuals found in any given sample will
vary from the number found in other samples.
By collecting numerous samples, the effect of these variations can be
averaged out. The
purpose for collecting the samples randomly is to avoid biasing the data.
Data can become biased when individuals of some species are
sampled more frequently, or less frequently, than expected at random.
Such biases can cause the population size to be either over estimated or
under estimated, and can lead to erroneous estimates of population size.
Population
size generally refers to the number of individuals present in the population,
and is self-explanatory. Density
refers to the number of individuals in a given area. For ecologists density is usually a more useful measure.
This is because density is standardized per unit area, and therefore, can
be correlated with environmental factors or used to compare different
populations.
The spatial distribution of a population is a much more complicated matter. Basically, there are three possible types of spatial distributions (dispersions) (see diagrams below). In a random dispersion, the locations of all individuals are independent of each other. In a uniform dispersion, the occurrence of one individual reduces the likelihood of finding another individual nearby. In this case the individuals tend to be spread out as far from each other as possible. In a clumped dispersion, the occurrence of one individual increases the likelihood of finding another individual nearby. In this case, individuals tend to form groups (or clumps).
Ecologists
are often interested in the spatial distribution of populations because
it provides information about the social behaviour and/or ecological
requirements of the species. For
example, some plants occur in clumped distributions because they propagate by
rhizomes (underground shoots) or because seed dispersal is limited.
Clumped distributions in plants may also occur because of slight
variations in soil chemistry or moisture content. Many animals exhibit rather
uniform distributions because they are territorial (especially birds), expelling
all intruders from their territories. Random
distributions are also common, but their precise cause is more difficult to
explain.
Unfortunately,
it is often difficult to visually assess the precise spatial distribution of a
population. Furthermore, it is
often useful to obtain some number (quantitative measure) that describes spatial
distribution in order to compare different populations.
For this reason, there are a variety of statistical procedures that are
used to describe spatial distributions.
Communities
are assemblages of many species living in a common environment.
Interactions between species can have profound influences of their
distributions and abundances. Comprehensive
understanding of how species interact can contribute to understanding how the
community is organized. One way to
look at species interactions is to evaluate the level of association between
them. Two species are said to be positively associated if
they are found together more often than expected by chance.
Positive associations can be expected if the species share similar
microhabitat needs or if the association provides some benefit to one (commensualisms)
or both (mutualism) of the species involved.
Two species are negatively associated if they are found together
less frequently than expected by chance. Such
a situation can arise if the species have very different microhabitat
requirements, or if one species, in some way, inhibits the other.
For example, some plants practice allelopathy, the production and release
of chemicals that inhibit the growth of other plant species.
Allelopathy results in a negative association between the allelopathic
species and those species whose growth is inhibited.
The
quadrat method is used primarily in studies of plant populations, or where
animals are immobile. The principal
assumptions of this technique are that the quadrats are chosen randomly, the
organisms do not move from one quadrat to another during the census period, and
that the samples taken are representative of the population as a whole.
It is often conducted by dividing the census area into a grid.
Each square within the grid is known as a quadrat and represents the
sample unit. Quadrats are chosen at random by using a random number generator or
a random number table to select coordinates.
The number of individuals of the target species is then counted in each
of the chosen quadrats.
Ecologists
use units to measure organisms within the quadrats.
Frequency (f) is an indication of the presence of an organism in a
quadrat area. This gives no
measure of numbers, however the usual unit is that of density – the
numbers of the organisms per unit area. Sometimes
percentage cover is used, an indication of how much the quadrat area is
occupied.
Transects
are used to describe the distribution of species in a straight line across a
habitat. Transects are particularly
useful for identifying and describing where there is a change in habitat.
A simple line transect records all of the species which actually touch
the rope or tape stretched across the habitat.
A belt transect records all the species present between two lines, and an
interrupted belt transect records all those species present in a number of
quadrats places at fixed points along a line stretched across the habitat.
This method of sampling is most useful when dealing with an animal population that moves around. Ecologists must always ensure minimum disturbance of the organism if results are to be truly representative and that the population will behave as normal. In this method individual organisms are captured, unharmed, using a quantitative technique. They are counted and then discretely marked or tagged in some way, and then released back into the environment. After leaving time for dispersal, the population is then recaptured, and another count is made. This gives the number of marked animals and the number unmarked. This can allow ecologists to estimate of the entire population in a given habitat.
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The
following equation is used to
estimate the population: S = total number of
individuals in the total population |
S = S1
X S2
S = 8 X 10 population = 40 individuals |
Diversity
depends on the number of species (species richness of a community) in an
ecosystem and the abundance of each species – the number of individuals of
each species. The populations of an
ecosystem can support demands on abiotic and biotic factors.
The growth of populations depends on limiting factors:
Abiotic factors
physiological adaptations of organisms only allow them to live in a certain range of pH, light, temp etc – it is part of what defines their niche.
Biotic factors (interactions between organisms)
Intraspecific competition occurs between individuals of the same species eg for a patch of soil to grow on, or a nesting site or food.
Interspecific competition occurs between different species needing the same resource – at the same trophic level.
Plant species compete for light, herbivore species compete for plants or carnivore species compete for prey
Predation – a predator is a limiting factor on growth on the population of its prey and the prey is a limiting factor on the predator population
An index of diversity is used as a measure of the range and numbers of species in an area. It usually takes into account the number of species present and the number of individuals of each species. It can be calculated by the following formulae:
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d
= |
N(N-1) |
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Sn(n-1) |
Where:
N = total number of organisms of all species in the area
d
= index of diversity
n = total number of organisms
of each species in the area
e.g.
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crested newt |
8 |
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stickleback |
20 |
|||||||||||||
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Leech |
15 |
|||||||||||||
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Great pond snail |
20 |
|||||||||||||
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Dragon fly larva |
2 |
|||||||||||||
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10 |
||||||||||||||
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Water boatman |
6 |
|||||||||||||
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Caddisfly larva |
30 |
|||||||||||||
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N
= |
111 |
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crested newt |
45 |
d = 2.6 Comparing both indices, 6.05
is an indicator of greater diversity.
The higher number indicates greater diversity |
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stickleback |
4 |
|
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Leech |
18 |
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Great pond snail |
10 |
In
extreme environments the diversity of organisms is usually low (has
a low index number). This may
result in an unstable ecosystem in which populations are usually dominated by abiotic
factors. The abiotic factor(s)
are extreme and few species have adaptations allowing them to survive. Therefore food webs are relatively simple, with few food
chains, or connections between them – because few producers survive.
This can produce an unstable ecosystem because a change in the population
of one species can cause big changes in populations of other species.
In
less hostile environments the diversity of organisms is usually high
(high index number). This may
result in a stable ecosystem in which populations are usually dominated by biotic
factors, and abiotic factors are not extreme. Many species have adaptations that allow them to survive,
including many plants/producers. Therefore
food webs are complex, with many inter-connected food chains.
This results in a stable ecosystem because if the population of one
species changes, there are alternative food sources for populations of other
species.
Population
Ecology is concerned with the question: why is a population the size it is? This
means understanding the various factors that affect the population.
When
a species is introduced into a new environment its population grows in a
characteristic way. This growth curve is often seen experimentally, for
example bees in a hive, sheep in Tasmania, bacteria in culture. The curve is
called a logistic or sigmoid growth curve.

The
growth curve has three phases, with different factors being responsible for the
shape of each phase. The actual factors depend on the ecosystem, and this can be
illustrated by considering two contrasting examples: yeast in a flask
(reproducing asexually), and rabbits in a field (reproducing sexually).
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Yeast
in a flask |
Rabbits
in a field |
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1.
Lag phase |
Little
growth while yeast starts transcribing genes and synthesising
appropriate enzymes for new conditions. |
Little
growth due to small population. Individuals may rarely meet, so few
matings. Long gestation so few births. |
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2.
Rapid Growth Phase |
Rapid
exponential growth. No limiting factors since relatively low density. |
Rapid
growth, though not exponential. Few limiting factors since relatively
low density. |
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3.
Stable Phase |
Slow
growth due to accumulation of toxic waste products (e.g. ethanol) or
lack of sugar. |
Slow
growth due to intraspecific competition for food/territory, predation,
etc. |
At
the end of phase 3 the population is stable. This population is called the carrying
capacity of the environment (K), and is the maximum population supported by
a particular ecosystem.
Many
different factors interact to determine population size, and it can be very
difficult to determine which factors are the most important. Factors can be
split into two broad group: abiotic factors and biotic factors. We’ll look at
7 different factors.
1.
Abiotic Factors
The
population is obviously affected by the abiotic environment such as:
temperature; water/humidity; pH; light/shade; soil (edaphic factors); mineral
supply; current (wind/water); topography (altitude, slope, aspect); catastrophes
(floods/fire/frost); pollution. Successful species are generally well adapted to
their abiotic environment.
In
harsh environments (very cold, very hot, very dry, very acid, etc.) only a few
species will have successfully adapted to the conditions so they will not have
much competition from other species, but in mild environments lots of different
species could live there, so there will be competition. In other words in harsh
environments abiotic factors govern who survives, while in mild environments
biotic factors (such as competition) govern who survives.
2.
Seasons
Many
abiotic factors vary with the seasons, and this can cause a periodic oscillation
in the population size.
This is only seen in species with a
short life cycle compared to the seasons, such as insects. Species with long
life cycles (longer than a year) do not change with the seasons like this.
3. Food
Supply
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A
population obviously depends on the population of its food supply: if
there is plenty of food the population increases and vice versa. For
example red deer introduced to an Alaskan island at first showed a
population increase, but this large population grazed the vegetation too
quickly for the slow growth to recover, so the food supply dwindled and
the deer population crashed. |
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4.
Interspecific Competition
Interspecific
competition is competition
for resources (such as food, space, water, light, etc.) between members of different
species, and in general one species will out-compete another one. This can
be demonstrated by growing two different species of the protozoan Paramecium in flasks in a lab. They both grow well in lab flasks
when grown separately, but when grown together P.aurelia out-competes P.caudatum
for food, so the population of P.caudatum
falls due to interspecific competition:
5.
Intraspecific
Competition
Intraspecific
competition is competition
for resources between members of the same species. This is more
significant than interspecific competition, since member of the same species
have the same niche and so compete for exactly the same resources.
Intraspecific
competition tends to have a stabilising influence on population size. If the
population gets too big, intraspecific population increases, so the population
falls again. If the population gets too small, intraspecific population
decreases, so the population increases again:
Intraspecific
competition is also the driving force behind natural selection, since the
individuals with the “best” genes are more likely to win the competition and
pass on their genes. Some species use aggressive behaviour to minimise real
competition. Ritual fights, displays, threat postures are used to allow some
individuals (the “best”) to reproduce and exclude others (the
“weakest”). This avoids real fights or shortages, and results in an optimum
size for a population.
6.
Predation
The
populations of predators and their prey depend on each other, so they tend to
show cyclical changes. This has been famously measured for populations of lynx
(predator) and hare (prey) in Canada, and can also be demonstrated in a lab
experiment using two species of mite: Eotetranchus
(a herbivore) and Typhlodromus (a
predator). If the population of the prey increases, the predator will have more
food, so its population will start to increase. This means that more prey will
be eaten, so its population will decrease, so causing a cycle in both
populations:
7.
Parasitism and Disease
Parasites
and their hosts have a close symbiotic relationship, so their populations also
oscillate. This is demonstrated by winter moth caterpillars (the host species)
and wasp larvae (parasites on the caterpillars). If the population of parasite
increases, they kill their hosts, so their population decreases. This means
there are fewer hosts for the parasite, so their population decreases. This
allows the host population to recover, so the parasite population also recovers:
A similar pattern is seen for
pathogens and their hosts.
A
population’s niche refers to its role in its ecosystem. This usually
means its feeding role in the food chain, so a particular population’s niche
could be a producer, a predator, a parasite, a leaf-eater, etc. A more detailed
description of a niche should really include many different aspects such as its
food, its habitat, its reproduction method etc, so gerbils are desert
seed-eating mammals; seaweed is an inter-tidal autotroph; fungi are asexual
soil-living saprophytes. Identifying the different niches in an ecosystem helps
us to understand the interactions between populations. Members of the same
population always have the same niche, and will be well-adapted to that niche,
e.g. nectar feeding birds have long thin beaks.
Species
with narrow niches are called specialists (e.g. anteater). Many different
specialists can coexist in the same habitat because they are not competing, so
this can lead to high diversity, for example warblers in a coniferous forest
feed on insects found at different heights. Specialists rely on a constant
supply of their food, so are generally found in abundant, stable habitats such
as the tropics.
Species
with broad niches are called generalists (e.g. common crow). Generalists
in the same habitat will compete, so there can only be a few, so this can lead
to low diversity. Generalists can cope with a changing food supply (such as
seasonal changes) since they can switch from one food to another or even one
habitat to another (for example by migrating).
The
niche concept was investigated in some classic experiments in the 1930s by Gause.
He used flasks of different species of the protozoan Paramecium,
which eats bacteria.
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Experiment.
1:
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Conclusion:
These two species of Paramecium share
the same niche, so they compete. P.
aurelia is faster-growing, so it out-competes P.
caudatum. |
|
Experiment. 2:
|
Conclusion:
These two species of Paramecium have
slightly different niches, so they don't compete and can coexist. |
It
is important to understand the distribution in experiment 2. P.
caudatum lives in the upper part of the flask because only it is adapted to
that niche and it has no competition. In the lower part of the flask both
species could survive, but only P.
bursaria is found because it out-competes P.
caudatum. If P. caudatum was
faster-growing it would be found throughout the flask.
The
niche concept is summarised in the competitive exclusion principle: Two
species cannot coexist in the same habitat if they have the same niche.
Ecosystems
are not fixed, but constantly change with time. This change is called succession.
Imagine a lifeless area of bare rock. What will happen to it as time passes?
1.
Very few species can live on bare rock since it stores little water and
has few available nutrients. The first colonisers are usually lichens,
which have a mutualistic relationship between an alga and a fungus. The alga
photosynthesises and makes organic compounds, while the fungus absorbs water and
minerals and clings to the rock. Lichens are such good colonisers that almost
all “bare rock” is actually covered in a thin layer of lichen. Mosses
can grow on top of the lichens. Between them, these colonisers start to erode
the rock and so form a thin soil. Colonisers are slow growing and tolerant of
extreme conditions.
2.
Pioneer species
such as grasses and ferns grow in the thin soil and their roots
accelerate soil formation. They have a larger photosynthetic area, so they grow
faster, so they make more detritus, so they form better soil, which holds more
water.
3.
Herbaceous Plants
such as dandelion, goosegrass (“weeds”) have small wind-dispersed seeds and
rapid growth, so they become established before larger plants.
4.
Larger plants (shrubs) such as bramble, gorse, hawthorn, broom and
rhododendron can now grow in the good soil. These grow faster and so out-compete
the slower-growing pioneers.
5.
Trees
grow slowly, but eventually shade and out-compete the shrubs, which are replaced
by shade-tolerant forest-floor species. A complex food web is now established
with many trophic levels and interactions. This is called the climax
community.
These
stages are called seral stages, or seral communities, and the
whole succession is called a sere. Each organism modifies the
environment, so creating opportunities for other species. As the succession
proceeds the community becomes more diverse, with more complex food webs being
supported. The final seral stage is stable (assuming the environment doesn’t
change), so succession stops at the climax stage. In England the natural climax
community is oak or beech woodland (depending on the underlying rock), and in
the highlands of Scotland it is pine forests. In Roman times the country was
covered in oak and beech woodlands with herbivores such as deer, omnivores such
as bear and carnivores such as wolves and lynxes. It was said that a squirrel
could travel from coast to coast without touching ground.
Humans
interfere with succession, and have done so since Neolithic times, so in the UK
there are few examples of a natural climax left (except perhaps small areas of
the Caledonian pine forest in the Scottish Highlands). Common landscapes today
like farmland, grassland, moorland and gardens are all maintained at pre-climax
stages by constant human interventions, including ploughing, weeding,
herbicides, burning, crop planting and grazing animals. These are examples of an
artificial climax, or plagioclimax.
Primary
succession starts with
bare rock or sand, such as behind a retreating glacier, after a
volcanic eruption, following the silting of a shallow lake or
seashore, on a new sand dune, or on rock scree from erosion and weathering
of a mountain.
Secondary
succession starts with
soil, but no (or only a few) species, such as in a forest clearing,
following a forest fire, or when soil is deposited by a meandering river.
Before
studying ecosystems in any further detail, it is important to appreciate the
difference between energy and matter. Energy and matter are quite different
things and cannot be inter-converted.
All
living organisms need energy and matter from their environment. Matter is needed
to make new cells (growth) and to create now organisms (reproduction), while
energy is needed to drive all the chemical and physical processes of life, such
as biosynthesis, active transport and movement.
The
many relationships between the members of a community in an ecosystem can be
described by food chains and webs. Each stage in a food chain is
called a trophic level, and the arrows represent the flow of energy and
matter through the food chain. Food chains always start with photosynthetic producers
(plants, algae, plankton and photosynthetic bacteria) because, uniquely,
producers are able to extract both energy and matter from the abiotic
environment (energy from the sun, and 98% of their matter from carbon dioxide in
the air, with the remaining 2% from water and minerals in soil). All other
living organisms get both their energy and matter by eating other organisms.
Although
this represents a “typical” food chain, with producers being eaten by animal
consumers, different organisms use a large range of feeding strategies (other
than consuming), leading to a range of different types of food chain. Some of
these strategies are defined below, together with other terms associated with
food chains.
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Producer |
|
|
Consumer |
An
animal that eats other organisms |
|
Herbivore |
A
consumer that eats plants (= primary consumer). |
|
Carnivore |
A
consumer that eats other animals (= secondary consumer). |
|
Top
carnivore |
A
consumer at the top of a food chain with no predators. |
|
Omnivore |
A
consumer that eats plants or animals. |
|
Vegetarian |
A
human that chooses not to eat animals (humans are omnivores) |
|
Autotroph |
An
organism that manufactures its own food (= producer) |
|
Heterotroph |
An organism that obtains its energy and mass from other organisms (=consumers
+ decomposers) |
|
Plankton |
Microscopic
marine organisms. |
|
Phytoplankton |
“Plant
plankton” i.e. microscopic marine producers. |
|
Zooplankton |
“Animal
plankton” i.e. microscopic marine consumers. |
|
Predator |
An
animal that hunts and kills animals for food. |
|
Prey |
An
animal that is hunted and killed for food. |
|
Scavenger |
An
animal that eats dead animals, but doesn't kill them |
|
Detritus |
Dead
and waste matter that is not eaten by consumers |
|
Carrion |
Alternative word for detritus |
|
Decomposer |
An
organism that consumes detritus (= detrivores + saprophytes) |
|
Detrivore |
An
animal that eats detritus. |
|
Saprophyte |
A
microbe (bacterium or fungus) that lives on detritus. |
|
Symbiosis |
Organisms
living together in a close relationship (= parasitism, mutualism,
pathogen). |
|
Mutualism |
Two
organisms living together for mutual benefit. |
|
Commensalism |
Relationship
in which only one organism benefits |
|
Parasite |
An
organism that feeds on a larger living host organism, harming it |
|
Pathogen |
A
microbe that causes a disease. |
So
food chains need not end with a consumer, and need not even start with a
producer, e.g.:
In general as you go up a food chain
the size of the individuals increases and the number of individuals decreases.
These sorts of observations can be displayed in ecological pyramids, which are
used to quantify food chains. There are three kinds:
These show the numbers of organisms at
each trophic level in a food chain. The width of the bars represent the numbers
using a linear or logarithmic scale, or the bars may be purely qualitative. The
numbers should be normalised for a given area for a terrestrial habitat (usually
m²), or volume for a marine habitat (m³). Pyramids of numbers are most often
triangular (or pyramid) shaped, but can be almost any shape. In the pyramids
below, A shows a typical pyramid of numbers for carnivores; B shows the effect
of a single large producer such as a tree; and C shows a typical parasite food
chain.
These
convey more information, since they consider the total mass of living organisms
(i.e. the biomass) at each trophic level. The biomass should be dry mass
(since water stores no energy) and is measured in kg m-2. The biomass
may be found by drying and weighing the organisms at each trophic level, or by
counting them and multiplying by an average individual mass. Pyramids of biomass
are always pyramid shaped, since if a trophic level gains all its mass
from the level below, then it cannot have more mass than that level (you cannot
weigh more than you eat). The "missing" mass, which is not eaten by
consumers, becomes detritus and is decomposed.
Food chains represent flows of matter and ene