Ecology, ecosystems, biotic & abiotic factors, organism interactions,
interdependence, parasitic & mutual relationships, interdependence, environmental changes,
effects on populations
IGCSE AQA GCSE Biology Edexcel
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Biology
Doc Brown's
school biology revision notes: GCSE biology, IGCSE biology, O level
biology, ~US grades 8, 9 and 10 school science courses or equivalent for ~14-16 year old
students of biology
This page will help you answer questions
such as ... What is an ecosystem? What are biotic factors? What are abiotic factors? Why do populations of species rise and
fall? How do environmental changes affect
communities?
Sub-index for this
page on ecosystems
(a)
Introduction: Ecology and ecosystems: technical terms and definitions
explained
(b)
Competition for resources
(c)
Effects of environmental changes on communities -
introduction and abiotic factors
(d)
Methods of surveying-monitoring pollution - measuring
abiotic factors
(e)
Changes in communities - biotic
factors and populations
(f)
Examples of how a population might change in
size when a biotic/abiotic factor changes
(g)
More examples of interactions -
interdependence - mutualism and parasitism
(h)
Learning objectives for this page
See also
Ecological
surveying - using quadrats and transects
and
Food chains
& webs, trophic
levels, pyramids of biomass & numbers, transfer efficiency
Biodiversity,
land management, waste management, maintaining ecosystems - conservation
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(a)
Introduction to ecology, ecosystems, definitions and
terms used in ecology
Ecology is the
study of how all organisms (plant and animals) survive in their physical
environment, how they relate to other organisms and what makes an
organism successful or unsuccessful as the case may be.
An
ecosystem is the interaction of a
community of living things (biotic) and non-living (abiotic)
factors affecting their environment.
Therefore an ecosystem includes all the
populations of all the living organisms (plant or animal) in a
specified area and such a description must also include all the non-living conditions/factors
such as temperature, soil quality-nutrients, water sources (or lack of them).
Typical ecosystems include:
Natural ecosystems like ancient
rainforest-jungle, tundra, swamps, oceans, lakes and even the humble
pond.
Artificial ecosystems like planted
highly managed forests, fish farms, horticulture in a greenhouse.
Within an ecosystem there is continuous competition
between organisms for food and other resources and coping with conditions such as availability
of water, food or the climate of the environment.
You can consider that ecosystems have different levels of
organisation - important terms listed below.
A
habitat means a specified area of the ecosystem involved
e.g. a wood, a pond, a sea shore etc. where the organism lives to
give fertile offspring.
Biotic factors are the 'living' factors of
the environment e.g. availability of food, availability of water, a new pathogen/predator and
competition for food between organisms - all of these affect the
distribution of organisms.
Abiotic conditions means the
'non-living' factors like temperature, water/soil pH, presence and
concentration of soil
nutrients, moisture level, light intensity and other climate conditions - again,
all of these affect the distribution of organisms.
Species is defined as a group of similar organisms that can
reproduce with each other to produce fertile offspring.
The abundance or population size of
an organism is how many individuals are present in a given area.
The distribution of an organism is where
you find it in its habitat e.g. part or the whole of a river, stream,
field, heathland etc.
Levels of organisation
- definitions in increasing size
1. Individual - a single organism of any species in
its population in the
ecosystem habitat.
e.g. a squirrel living in a wood.
2. Population - the total number of one particular
organism species
in a specified ecosystem habitat.
The number of squirrels that live in the
wood.
3. Community - all the populations of
organisms of different
species living in a specified ecosystem habitat.
The squirrels live along side other
animals like rabbits, foxes, insects, plants.
4. Ecosystem - the community of organisms and the conditions
under which they live - biotic and abiotic.
This community live in the specified
environment, the individuals and species interact with each
other (biotic factors) and their lives are affected by e.g. soil
type in the woodland (pH, nutrient content), ambient seasonal
temperatures, water resources (rain, pond) and light intensity.
A
stable community is where the biotic
and abiotic factors in balance and the population sizes remain fairly
constant.
Examples include ancient oak woodland in
England or tropical rainforests in South America.
Interdependence
Interdependence is the term to describe the
fact that ALL organisms depend on other organisms for resources such
as food or shelter in order to survive and reproduce.
As a consequence, any change in the population
of one species, can have knock on effect on another species in the
same community.
This can be best understood by studying food
webs. See
section (d) Food webs on Biomass page
Biodiversity and ecosystems
Since very ecosystem has a range of different
plants and animals, the term
biodiversity expresses
'width' of diversity of species.
Biodiversity is extremely important for the
'health' of any ecosystem and phrases like 'high
biodiversity' or 'low biodiversity' sum up the good or bad
state of an ecosystem.
The living organisms in an ecosystem can be
described as producers, consumers and decomposers.
Producers initiate a food chain
e.g. plants producing food from photosynthesis.
Consumers are eating from a
previous level in the food chain, up which the biomass is
transferred.
Decomposers are live organisms that
break down other dead organisms, recycling materials in the
ecosystem.
An ecosystem with a high biodiversity has many
advantages over a low biodiversity e.g.
a wider variety of food resources,
reducing the dependency of a species on limited choices,
for our own needs, we are provided with
food supplies, medicines derived from natural products, oxygen
in the atmosphere and a water supply.
Biodiversity is reduced by removing too much
of a species from an ecosystem e.g. overfishing, overhunting,
deforestation, draining swamps or bogland.
Unless conservation-replacement measures
are not taken, the population of affected species are not
sustainable - possibly to the point of extinction.
A high level of biodiversity in a small
(e.g. field) or huge (a forest, country) is very good, especially if
environmental conditions change.
Some natural ecosystems like
rainforests and tropical reefs have a high biodiversity,
providing a variety of food and shelter for many species all
year round.
Other natural ecosystems have a low
biodiversity e.g. arctic tundra, deserts or deep-sea volcanic
thermal vents, where a relatively few highly adapted species
of plants or animals can survive.
All natural ecosystems are self-supporting,
meaning all the resources an organism needs survive and reproduce
are all present in their habitats.
However, such systems do need the input of
energy, usually from sunlight and,
all animals need plants for oxygen and
food,
and plants need carbon dioxide,
pollination and seed dispersal from animals.
These connections are called
interdependence.
Distribution
Distribution is a measure of how high or low
the population of species is from one area compared to another in an
ecosystem i.e. how they are dispersed around an ecosystem.
Herbivores such as rabbits and sheep will tend
to congregate where there is good grazing land.
BUT, there are other consequences for the
distribution of plants e.g.
If the intensity of grazing is low, just a
few plants will dominate their habitat and out-compete the
others.
As the grazing increases, more plant
species can thrive because the dominant plant population is
controlled by the grazers and the weaker species can grow, but
they are often specially adapted plants that can resist the
intensive grazing of the herbivores.
The same argument applies to other wild
animals such as wild cattle living on the e.g. the African
savannah.
For more on these see:
Food chains, food webs, trophic
levels, pyramids of biomass, transfer efficiency, pyramids of numbers
Biodiversity and ecological
surveying - using quadrats and transects, methods of trapping animals
Biodiversity, land management, waste management,
maintaining ecosystems - conservation
TOP OF PAGE and
sub-index for ecosystems
(b)
Competition for resources
All organisms need a variety things from their
environment and other organisms to survive and reproduce (and
therefore evolve to be better adapted to their habitat).
Organisms can only survive if they have enough
resources for their needs - individual growth and collective
reproduction. Within the same ecosystem and its habitats ...
... animals will compete for food,
water, territory (e.g. for hunting or gathering food) and
mates to produce the next generation,
... and plants will compete for light
for photosynthesis, nutrient mineral ions in the soil,
space to grow, and water for transport-transpiration
and photosynthesis.
Within an ecosystem, animals and
plants that get more of the resources from their environment, are more
likely to be successful than those organisms that get less.
Successful organisms are more
likely to survive and pass on their 'successful genes' in
reproduction.
Animals who have with a wider
range of food sources are more likely to survive than those e.g.
might rely on one food source - if this was affected, such animals
are less likely to survive or move to another location.
Most animals can, and will move
to, wherever food is available.
Changes in the environment can cause the
distribution of organisms to change - meaning a change in where members
of a population live.
Plants, the primary food producers at the base of many
food
chains, need light, mineral ion nutrients, space to grow, water and
oxygen and carbon dioxide from the atmosphere.
See
biomass - food chains
Animals need food and oxygen, mates for reproduction, water and space to
live in (the 'territory' of their habitat).
The size of a population is limited by competition
for these resources as well as pre-predator relationships.
One organism can outcompete another give increases
and decreases in populations, sometimes in complex cycles.
Types of competition
Interspecific competition is
competition between different species e.g. red and grey
squirrels.
Generally speaking animals try to avoid
competing with others, more chance of all surviving, but
competing for the same resources will affect the size and
distribution of their populations.
Intraspecific competition is
competition between members of the same species e.g. an
animal competing for its own territory and mates - a lot of effort
can be put in by a male to attract a female including displays of
colour/showing off and fighting competitors!
Organisms may compete with other species or with members of their own
species for the same resources in the same habitat.
Different predators might compete for the same prey.
e.g. in the oceans sharks and dolphins compete for the same
shoals of smaller fish.
Animals might compete for the same plant food.
e.g. red and grey squirrels compete for
the same food like nuts in the same woodland habitat. Grey
squirrels compete more strongly-efficiently for food resources,
depleting them to the point where red squirrel population drop
to zero
unless managed in protected areas.
'Winners and losers'
If competing species are not perfectly
matched, eventually one will become more successful than the other.
The less successful species may:
Be unable to do anything and become
extinct.
Remain in its original habitat and adopt a
different survival strategy.
More to another area and find a new
habitat and suitable resources.
In general in any ecosystem "the survival
of the fittest' determines which the population is stable.
In many respects humans are very successful
organisms, we are intelligent and adaptable, exploiting and competing
with many plants and animals across every continent.
TOP OF PAGE and
sub-index for ecosystems
(c)
Effects
of environmental
changes on communities - introduction and abiotic factors
Environmental conditions are changing all the time and are caused by
many factors and may be short-lived or long-lasting.
There are non-living (abiotic) factors and living (biotic)
factors.
Abiotic factors are physical conditions that
affect the distribution of organisms.
Environmental changes affect the distribution and
behaviour of organisms in their particular habitats.
These changes affect communities in several different ways e.g.
populations of species may rise or fall due to availability of
food,
or the distribution of a species might change - may involve
migration from one
location to another,
and the better adapted organisms are more
likely to survive the impact of environmental changes.
See also
environmental changes and effects on biodiversity
Origin of environmental changes:
(i)
Seasonal changes
e.g. change in temperature
from winter to summer, rainfall patterns etc.
Lower winter temperatures and scarcer food
resources can cause animals to migrate or hibernate.
Hot dry seasons cause draught and death
from dehydration or lack of food.
Rainy seasons cause flooding, drowning
animals.
High temperatures reduce the concentration
of oxygen in water, affecting aquatic life.
(The general rule for gases is they
become more soluble the lower the temperature.)
(ii) Impacts of human activity
Agriculture.
factories, building housing estates, mining, motorways etc.
Even within the same
area e.g. differences between neighbouring industrial, 'suburban' and rural
parts - you will observe the success and failure of organisms in
their respective ecosystems.
I've dealt with global warming separately
in the 3rd section below.
(iii) Global warming
'Climate change' is becoming one of
the biggest factors in he future of the environmental and also comes under the
heading of 'human activity' since the current evidence suggests it
is being accelerated by burning fossil fuels giving higher
levels of carbon dioxide in the atmosphere.
Seas and land are warming places, e.g.
causing species of animals to move further north in the northern
atmosphere.
The increase in atmospheric CO2
levels leads to more of the gas to dissolve in water - seas,
oceans etc. making the water more acidic (lowering the pH).
This, combined with an increase in
water temperature, is having major impact on the stability
of fragile ecosystems like coral reefs, which become
imbalanced and degraded e.g. coral dies, populations of
other coral residing organisms are reduced.
The warmer more acidic water causes
the shells of organisms like molluscs to dissolve and become
thinner.
With warmer temperatures expanding
water, plus rising sea levels due to melting ice, low lying
land is being flooded leading to loss of habitat and
migration of species and reduction of populations.
Increased rainfall OR drought are other
prediction of climate change science.
This can lead to increased flooding of
habitats, which can also be washed away,
OR lack of water, lack of plant
growth, both contributing to the death of animals.
(In Africa the Sahara Desert is
increasing in area all the time, no rain, no vegetation and
accompanying animals.)
(iv)
Geographic changes
In terms of thousands/millions of years,
land bridges between continents rise and fall, permitting or
inhibiting movement of animals.
On a smaller scale, after the last
ice-age, sea levels rose as the ice melted and Great Britain
becomes separated from continental Europe - this cuts off
populations of plants and animals who may evolve to form
sub-species.
Global warming is raising the temperature
of cold mountainous regions sufficiently to cause the melting of
glaciers. The warmer 'upland' climate means 'lowland' plants and
animals can also compete for the 'upland' resources, so that
'lowland' species can be found at higher altitudes than before.
See also
environmental changes and effects on biodiversity
Examples of abiotic factors (non-living factors)
(Overlaps with the initial discussion of with
causes environmental changes above)
A change in environmental conditions
usually means an increase or decrease in one of the abiotic factors
described.
A change in the environment can affect an
individual organism, both promoting or retarding its wellbeing.
On a larger scale, such changes can also
affect the size of a population of a species in a community AND
therefore affect the size of the populations of other species of
organisms if they depend on it higher up a food chain.
A change in an abiotic factor can not only
change population sizes, but also their distribution - climate
change, especially rise in temperature, is leading to significant
geographical movement of species of birds, fish and other marine
organisms.
Various abiotic factors are discussed below
with suitable examples of plants or animals.
1. Light intensity
Generally speaking the abiotic factor of
light intensity only affects plants.
Lack of light inhibits plant growth - rate
of
photosynthesis slowed
down.
All 'green' plants need good access to
light for photosynthesis, but in growing they produce areas of shade
from sunlight (and sometimes more moist conditions too).
Under the shade of trees, the grass
population might be reduced, but replaced by fungi and mosses which can
cope much better with lower intensity sunlight.
2. Water access - moisture level
- usually a weather factor
Most plants and animals need access to a continuous
source of water - without access to water for a long period plant growth
will be stunted and eventually most plants will wither and die
and animals will die of dehydration.
Water is needed for all biochemical
process of living organisms - water is both a reactant and a
solvent.
Some plants are adapted to survive in relatively
dry conditions, but most cannot and their populations decline if the
climate changes.
Desert plants are adapted to
retain water.
However, with the opposite situation, most
plants cannot survive in very boggy waterlogged ground, but a
few species are adapted to marshy conditions.
The availability of water also greatly
affects the distribution of animals too.
The wet and dry season sequence e.g.
Africa, causes great migrations of wild animal herds seeking
food and water.
The geographical migration routes
follow the seasonal changes - often to the north and
returning to the south following the pattern of rainfall -
the rain promotes plant growth and pools of water to drink
from.
Drought conditions will affect almost all
species of plants and animals e.g. scenes you see in parts of
Africa where every food chain is affected.
Plants are rare in deserts, but after
rain their distribution of changes dramatically - after
heavy rain the flowers grow in abundance and produce seeds
while adequate water is available.
3. pH and chemical composition of the soil
The relative acidity
of land and water mainly depends on the local geology.
For more on pH
e.g. the pH of the soil.
Distribution of plants and animals can
depend on changes in soil composition due to underlying
differences in geology e.g. limestone (slightly alkaline) and
sandstone (slightly acidic).
Some plants are adapted to live in quite
acid conditions e.g. heather grow best on acidic soils and other
plant species have adapted to moorland peat conditions.
Other plants are
adapted to live in the mildly alkaline soils in limestone country.
If you swap the locations for these groups of plants they will not
flourish to the same extent as when grown in their native habitat.
In the garden and on the farm you treat
soil that is too acidic (too low a pH) with lime to increase the
pH and the soil fertility.
4. Mineral content of soil
(can depend
on local geology and pH of soil)
Apart from water, oxygen and carbon
dioxide (not regarded as minerals), plants also require
important nutrients from the soil e.g. absorbing ions that
supply the plant with elements such as nitrogen (nitrate),
phosphorus (phosphate), potassium, calcium, iron, magnesium and
sulfur (sulfate).
Deficiency of any important nutrient in
soil can inhibit the growth of healthy plants and the
population will decrease - in turn this can affect-decrease the
populations of species that feed on the plants.
Deficiencies in minerals can be remedied
with fertilisers containing nitrogen, phosphorus,
potassium, iron, calcium etc., but don't apply to much!
5. Temperature (usually a climate abiotic
factor)
Many populations are only stable if the
ambient environmental temperature lies within a certain range - a
climate-weather factor.
Populations of fish and other aquatic life are quite sensitive to
the temperature of the water.
Global warming is having an effect on
populations and the distribution of many species of plants and
animals - both aquatic and land based e.g. species are moving
further north in Europe.
There is evidence of populations of
certain marine species moving to more northerly waters as sea
temperatures rise around the UK coast e.g. cod and haddock which
prefer colder water are moving northwards towards the Icelandic
fishing grounds.
Similarly there is evidence
of both bird and insects species (e.g. certain butterflies) extending their territory further
north on mainland Britain as our climate is warming up.
Species of birds usually associated with
Mediterranean countries are now regularly appearing in much greater
numbers in northern European
countries than previous decades.
Warm water discharged from power
stations can cause a local warming effect and species
preferring a milder climate can flourish in the river, and
sometimes species not native to surrounding ecosystem!
6. Wind intensity and direction
(another climate-weather abiotic factor)
Plants in particular can be affected by
weather conditions such as the effect of wind.
Some plants can cope with harsh weather
conditions, but others will only survive in a sheltered habitat.
Birds can be blown off course and not
reach their breeding grounds causing a decline in population.
In fact any sort of bad weather can takes
it toll on migrating flocks of birds e.g. becoming exhausted and
dying.
7. Carbon dioxide level
Plants need carbon dioxide for
photosynthesis, although
only around 0.04% of air, it would be unusual to fall below this
level and affect plant growth and for animals its a waste gas
from respiration.
On the other hand, oceans are becoming
slightly more concentrated with carbon dioxide from fossil fuel
burning. This has two effects.
(i) This will increase the rate of photosynthesis of marine
organisms in the surface water.
(ii) The pH falls slightly, making the
water more acidic - this is having detrimental effects on
coral reefs.
8. Oxygen level
- usually a pollution effect - see below in next section
Since 21% of air is oxygen, all plants and
animals not living in water are highly unlikely to suffer from
lack of oxygen.
However, the oxygen concentration in water
(fresh or sea) is crucial for aquatic animals.
e.g. if water is polluted with fertiliser
runoff from farmland (nitrates and phosphate), you get excess
growth of weeds and algae. They decay and use up all the oxygen
in the water, killing most aquatic life (fish, molluscs etc.) -
a phenomena called eutrophication.
9. Pollution
This a relatively 'modern' abiotic factor
is entirely due to human activity!
The level and nature of pollution can
affect the populations of both plants and animals and their
distribution.
Unfortunately chemicals like pesticides
build up in food chains and at stage they get more
concentrated as you move from one trophic level to another.
This is an example of
bioaccumulation.
A 'classic' tragic case is small
mammals taking in pesticides in their plant food and then
birds of prey catching and eating the small contaminated
mammals. The poisoning effects on the birds included reduced
fertility, decline in successful hatchings and death.
Excessive use of artificial fertilisers
results in too high a level of nutrients in ponds, streams,
lakes or rivers.
This causes eutrophication -
excessive growth of algae that blocks out sunlight to plants
below the surface. The plants decay as microorganisms feed
on them and using any oxygen around to respire. When most of
the oxygen is used up, most respiring aquatic life dies e.g.
fish, insects and crustaceans.
Lichen are used as a pollution indicator
species - they are NOT tolerant of high pollution levels -
particularly acidic gases like sulfur dioxide and oxidising
gases like ozone and nitrogen dioxide.
Lichen are unable to survive if the level
of sulfur dioxide is too high (from burning fossil fuels).
If the local rocks and walls are covered
in lichen, its usually a good sign of relatively low pollution
levels - but you will find far species and higher numbers of
lichens in rural locations compared to urban locations - less
traffic, less pollution from acidic gases like nitrogen oxides
and sulfur dioxide.
Statistical surveys show that the
number of lichen species steadily increases as you move out
from a town centre out into the countryside well away from
any busy main roads/
You should have noticed that most of the
abiotic factors discussed above seem only to apply to plants,
BUT NOTE, plants are usually the primary food producer at the
bottom of a food chain.
If plants are affected by an
adverse change in environmental conditions, so are all the
animals in the succeeding food chain - and that may
include us too!
See also
environmental changes and effects on biodiversity
TOP OF PAGE and
sub-index for ecosystems
(d)
Methods of
surveying-monitoring pollution - measuring abiotic
factors
(This section is repeated in the
biodiversity notes
and see also
methods of ecological surveying)
Abiotic conditions means the
'non-living' factors like temperature, water/soil pH, soil
nutrients, moisture level, light intensity, climate conditions - again,
all of these affect the distribution of organisms.
Measurement of abiotic factors may
help to explain differences in the populations and distributions of
organisms.
As described above, you can survey
and compare one location with another, looking particularly for
indicator species of plants or animals.
You might survey the length of a
stream or a stretch of a river, sampling-surveying at regular
intervals, looking at what species are present or not present -
relatively quick to do.
This might help to trace the
source of pollution in a polluted stream or river.
However, surveys based on
'present/not-present' do not tell you how polluted the specific
environment is.
To get quantitative data
you would need to count the numbers of each indicator species
present in a measured area or volume of water - this set of
numerical data takes longer and more costly to do, but gives a
better estimate of pollution levels.
But, even doing species counts does
still not give you very accurate data on levels of pollution, but using
modern analytical instrumental methods. These non-living
indicators and allow rapid and regular checks to be carried out to monitor pollution levels and see
how they change with time and
location.
You can analyse air or water
samples for traces of polluting chemicals even if their
concentrations are only in ppm with electronic sampling devices.
You can monitor ozone, carbon
monoxide, nitrogen oxides and sulfur dioxide in the polluted air
of towns and cities with an air quality meter.
You can analyse for traces of
toxic organic chemicals or heavy metal compounds in water.
Electronic pH meters tell
how acidic or alkaline a water system is - choosing different
locations along a stretch of stream or river.
You can use a simple visual
indicator strip to measure the pH of soil.
It uses a sort of universal
indicator where match the colour strip turns in a soli/water
mixture and match the colour it turns to a pH chart.
You can also dip the
electronic pH probe into a soil/water mixture - a more accurate
measurement.
Another electronic probe
instrument can directly measure the oxygen concentration in
water - just dip in and press the button! - again, choosing
different locations along a stretch of stream or river to broaden
the survey.
You can use an electronic
thermometer probe to measure the temperature of land or water -
again, choosing different locations along a transect to broaden the
survey.
You can employ a light meter
sensor to measure light intensity in different locations e.g.
comparing open and shaded areas of a habitat.
You can use a soil moisture
meter to measure the relative water content of soil.
You can compare the results of
measuring abiotic factors with the distribution of selected
organisms to look for similarities or differences.
TOP OF PAGE and
sub-index for ecosystems
(e)
Examples of
biotic factors (living factors) - changes
in community populations
Any changes in a biotic factor can also change
population size up or down and also affect geographical distribution
of species.
Competition between species for resources
In a given environment, all organisms are
competing with each other for the same resources they need to
survive and reproduce.
Plants need mineral ion nutrients
from the soil, light and carbon dioxide for photosynthesis,
space to grow and water and nutrients from the soil.
Weeds compete with crop plants for
light, nutrients and water.
Unfortunately, this leads to the use
of weedkillers to reduce the crop's competitors and allow
the crop to have a greater access to the resources.
Animals need food (plant or
animal), mates for offspring, water and territory - the space
for
their habit.
One species may out compete for food or
territory leading to the rise of one species population and the
decline of another to the point where breeding is low that the
population is unsustainable.
e.g. In most areas grey squirrels have
outcompeted native red squirrels in the UK so their population
falls and the population of grey squirrels rises.
In some cases, a species outcompeted might
move to another area where there is less competition.
The availability of food is one of the most
important biotic factors that supports a stable population of a
species.
The more abundant the food supply, the more organisms can
survive and reproduce to maintain or increase their population.
The
food ranges from plants for animals, animal prey for animal predators
higher up the food chain.
The
abiotic factor of the weather can
considerably affect food availability.
Milder winter weather allows populations
of birds to keep at higher levels - less dying form low
temperatures.
Poor weather might mean less food is
available for the birds to survive and reproduce, increasing
competition and decreasing
population numbers.
Predators and prey
- predation
Predation:
Wild animals like lions, cheetahs and hyenas on the African
savanna are competing for the same prey (food resources e.g.
gazelle, zebra), same grass land territory and drinking water.
The relative populations of predators and
their prey will affect each other.
If a predator eats all their
chosen prey, it will die out for lack of food or move to another
habitat.
In stable communities the
numbers of predators and preys rises and falls but in a balanced
way.
If a prey is abundant at one
point in time, then the population of a predator can increase
because there is more food available to eat.
However, in doing so,
the population of the prey will decline soon after.
This leads to
complex cycles in the rise and fall of the populations of the
species involved.
A classic example is the relationship between rabbit (the prey)
and fox (predator) populations in a particular community.
The population of a species (e.g. foxes) is often limited by
the amount of food available (e.g. rabbits).
If the population of prey increases e.g. lots of rich green
grass for the rabbits, their population can increase - the
crests in the upper graph line.
This means more food for the foxes!, so their population
increases too - upper crests on the lower graph line. There is a
time lag to allow for reproduction!
BUT, the extra foxes eat more rabbits and so the number of
rabbits decreases - troughs in the upper graph line.
The decline in rabbits once again limits the food for the
foxes, so their population begins to decrease - troughs on the
lower graph line.
This rise and fall in prey-predator populations is very
typical and produces these 'wave-like' graphs and is a classic
case of interdependence.
The two interdependent populations
forming a predator-prey cycle will always be 'out
of phase' with each other because there will be a time lag
(dotted line) as one change in population gradually affects the
other population - one population cannot respond immediately to
the change in the other.
New predators
If a new predator arrives in an ecosystem,
it can displace another species that cannot compete with the
arrival.
There might be similar species
competing for the same food resource and if the 'invasion'
is rapid the native population of the original organism
might not be able to adapt in time before its population
declines.
Competition for food is a biotic
factor - see prey and predators in the previous section.
New pathogens
If a new pathogen enters an ecosystem,
organisms may be very susceptible to attack.
An animals immune system might not be able
to cope leading to disease and death.
This happened to humans when explorers
went to places like South America, taking with them 'western
diseases' that led to the deaths of many native people.
The gene pool of an organism might not
produce enough mutations fast enough to evolve species that can
survive the new pathogen.
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sub-index for ecosystems
(f)
Some examples
of how a population might change in size when a
biotic or abiotic factor changes
A change in the environment can be due to either a
new biotic or abiotic factor.
Such a change can therefore affect the size of the
population of one or more organisms in a community.
This can have 'knock-on' effects because of the
nature of interdependence of organisms in a given habitat.
What the graph shows from the 'timing points'
a, b, c and d
From points
a to b the
population of the organism X
grows steadily -
perhaps a good supply of food and good weather.
At point
b some
biotic/abiotic factor changes with a
negative
effect. on the organism X, hence a decline in the population
At
c, the population of X begins to
recover and from
c to d
increases at a rate similar to before the negative impact of some
biotic/abiotic factor
Example 1. The emergence of a new predator
From
a to b,
no new predator, population of X growing steadily.
At
b
new predator enters the habit of the organism X.
From
b to c,
population of organism declines due to predator attack on organism
X.
In some cases the population of X may not
recover in this particular habitat and the graph
stops at c.
However, food for the predator is becoming
scarcer, so its population either declines or this particular
predator moves on to another 'hunting ground'.
This allows the population of the organism X
to start to grow steadily again from
c to d.
Example 2. The emergence of a new pathogen
From
a to b,
no new pathogen around in the habitat of X, so population of X grows
steadily.
At
b a
new pathogen enters the habit of the organism X.
From
b to c,
population of organism declines because organism X is susceptible to
attack from the new pathogen.
However, some of the X organisms may have
some genetic resistance to the pathogen, either already present
in the gene pool or gained through mutation - this effect
begins to show up at point
c.
The X organisms which are resistant to the new
pathogen begin to multiply and more frequently as they grow in
number, so the population of the organism X to starts to grow
steadily again from c to
d.
Example 3. A change in the weather
From
a to b,
the weather is warm and sunny with plenty of food around in habitat
of insect X, so population of insect X grows steadily.
At
b
the weather deteriorates, temperature falls, less sun and less food
available.
From
b to c,
population of insect X declines because its colder and less food
available.
At point
c, the weather
improves, temperature increases, more sun and more food.
Therefore the population of the insect X to
starts to grow steadily again from
c to d.
Example 4. The emergence of a new
competitor Y
From
a to b,
no new competitor for the same food source, population of
animal X grows steadily.
At
b a
new competitor for the same food, animal Y enters the habit
of animal X.
From
b to c,
population of animal X declines due to be outcompeted for the food
source by animal Y.
In some cases the population of X may not
recover in this particular habitat and the graph
stops at c.
This has happened with the introduction of
the grey squirrel into the UK in the 1870s.
The grey squirrel outcompetes the native
red squirrel for the same food source - mainly nuts.
Whole areas of the UK have no red
squirrels.
However, by trapping and shooting grey
squirrels, such management of selected woodland habitats has
allowed the red squirrel population to rise - sometimes drastic
measures are called for in conservation projects.
(Not sure whether you call the trapping
and shoot a biotic or abiotic factor!)
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sub-index for ecosystems
(g)
More examples of
interactions
between organisms - mutualism and parasitism
Types of interdependence -
parasitic and mutual relationships winners and loses!
An organism may depend partly/entirely on another species
to survive - interdependence.
Therefore, where an organism lives in its habitat, the size of
its population can be influenced by the distribution and abundance of
this other species.
Parasitism - Parasites live off a host (the other
organism).
From the host they extract what they need to survive
without giving anything in return.
This process can be harmful to
the host, one organism gains and the other loses e.g.
Fleas are a common parasite on humans,
cats and dogs amongst other animals. The bite through the skin to
feed on blood for nutrients and lay eggs after! There can be
allergic reactions to the presence of fleas.
Tapeworms happily live in the
intestines of many animals, including humans.
In their parasitic action they consume
large quantities of nutrients, depriving the host animals of some of
its digested food causing malnutrition.
Head lice thrive on human scalps.
In amongst your hair, head lice live on
your on your skin of your head and suck your blood for food and
making your scalp very itchy!
Mistletoe is a parasitic plant.
Mistletoe grows on the branches of trees
such as apple or hawthorn and obtains water and mineral ions
from the 'host' plant and gives nothing in return.
If too much mistletoe grows on the tree,
it can be killed due to lack of water e.g. in very dry weather
there might not be enough water for both plants.
Mutualism is a situation where both
organisms benefit from the relationship. Two winners! e.g.
Many plants are pollinated by insects,
because of the pollen grains carried on their legs.
The pollen brushes onto the insect's body
as it probes for the sugary liquid in the flower.
The insect (e.g. bee or wasp) then moves
onto anther flower and so on.
The pollination
allows the plants to sexually reproduce after the pollen has been
transferred.
In the process the insects
have access to food in the form of a sugary solution - nectar.
Both
organisms benefit from the pollination process.
Microorganisms in a cow's stomach survive and
flourish by helping to break down the hard to digest grass.
Both the
host cow and microorganisms benefit from this interaction, hence one
is mutually dependent on the other.
Cleaner fish eat dead skin and
parasites of the surface of larger fish.
The smaller fish get food and avoid being
eaten by larger fish.
The larger fish get their skin cleaned,
especially from parasitic organisms, so improving their health!
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sub-index for ecosystems
(h) Learning objectives for this page
- Know that
interdependence is the dynamic relationship between all living things.
-
It is important to understand
that all living things are interdependent on each other, especially through
the pathways of food chains, which are effectively energy chains too.
-
Apart from the obvious need for
food and energy to survive and reproduce, there are many other factors too
for particular organisms e.g. most flowering plants rely on insect
pollination,
- Be able to demonstrate an understanding of how some energy is transferred to less useful
forms at each trophic level and this limits the length of a food chain.
- Be able to show an understanding that the shape
of a pyramid of biomass is determined by energy transferred at each trophic
level.
- Be able to explain how the survival of some organisms may depend on the presence
of another species:
- a) parasitism - where one organism,
to survive, feeds off another that acts as the host - parasites 'take with
no give', live in or on the host which they may harm in the process!, including:
- (i) fleas - insects that live in the fur of
live animals and in the bedding of us humans. They feed by sucking the blood
of their host provides all their feeding needs and helps them to reproduce
rather too efficiently for our liking!
- (ii) head lice - insects that live on the
upper skin layer of the human scalp. Like fleas, they suck human blood for
all their feeding needs and make your head feel itchy!
- (iii) tapeworms - a parasite that can live
in a person's intestines (bowel) and they tend to be flat, segmented and
ribbon-like. Humans can catch them by touching contaminated faeces (stools)
and then placing their hands near their mouth, swallowing food or water
containing traces of contaminated faeces or eating raw contaminated pork,
beef or fish. Tapeworms are common in many animals and feed by attaching
themselves to the walls of an animal's intestine and absorb food through
their outer body covering. In extreme cases you can suffer from malnutrition
- all take and no give!
- (iv) mistletoe - is a parasitic plant that
attaches itself to trees and shrubs and grows by penetrating between the
branches and absorbs nutrients and water from the host plant. Like the
tapeworm producing malnutrition in animals, mistletoe can affect and reduce
the host plant's growth.
- b) mutualism - where two organisms
mutually benefit from a relationship - 'give and take' in a good
evolutionary Darwinian deal! - known as a mutualistic relationship!, including:
- (i) oxpeckers that clean other species -
these are birds that live on the backs of grazing animals (e.g. large
mammals like buffalo, oxen, rhinos etc.) and eat large quantities of ticks,
flies and maggots to feed themselves. In doing so they remove unwanted
parasites from the animal, hence they are classed as a 'cleaner species'.
- (ii)
cleaner fish - these small fish feed off dead skin and parasites on the skin of
larger fishes. In doing so they feed well, remove unwanted parasites from
the big host fish and don't get eaten by the host fish!
- (iii) nitrogen-fixing bacteria in legumes
- most plants cannot absorb and chemically process the nitrogen in air to
help synthesise amino acids to convert into proteins. However, leguminous
plants (e.g. beans, clover, peas etc.), have in their root nodules, bacteria
with the right enzymes to convert the nitrogen in air into nitrates, which
the plant needs and can use to make proteins. In return the bacteria get a
regular supply of water and sugar for energy, to everyone's mutual
satisfaction!
- (iv) chemosynthetic bacteria
in tube worms in deep-sea vents - these extremophiles mutually depend on each
other to survive. The bacteria get their necessary 'life chemicals' from the
tube worms and in reproducing themselves they become food for the tube worms
which act as the host.
See also
Carbon cycle,
nitrogen cycle, water cycle and decomposition
Food chains, food webs and biomass
Biodiversity, land management, waste management, maintaining ecosystems
- conservation
Ecological
surveying - using quadrats and transects
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