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School biology notes: Ecosystems biotic & abiotic factors, interactions, interdependence

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Ecology, ecosystems, biotic & abiotic factors, organism interactions, interdependence, parasitic & mutual relationships, interdependence, environmental changes, effects on populations

IGCSE AQA GCSE Biology Edexcel GCSE Biology OCR Gateway Science Biology OCR 21st Century Science 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 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 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.

TOP OF PAGE and 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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(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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(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.
    • Know and understand that the mass of living material (the biomass) at each stage in a food chain is less than it was at the previous stage.

      • Appreciate that the biomass at each stage can be drawn to scale and shown as a pyramid of biomass.

      • Up the food chain: producer ==> primary consumer ==> secondary consumer ==> tertiary consumer etc.

        • The producer is usually a photosynthesising plant or algae.

      • In a biomass pyramid, each horizontal bar (drawn to scale) is proportional to the mass of the living material at that producing level and feeding levels (trophic levels).

      • How to construct a biomass pyramid: To draw to scale, you can keep the vertical height the same for each level and make the horizontal length of the bar proportional to the biomass of that level in the pyramid.

        • Obviously, the bigger the bar, the greater the biomass at the producer/feeding-trophic level.

      • Up the food chain and 'up the pyramid' the biomass gets less because of loss of organic material, waste energy and even the energy from respiration, required to sustain life, eventually becomes waste energy too eg heat energy to the surroundings. More in section (c).

      • Know and understand that the amounts of material and energy contained in the biomass of organisms is reduced at each successive stage in a food chain because:

      • (i) some materials and energy are always lost in the organismsí waste materials by eg excretion (urine, droppings), fallen leaves from trees etc.

      • (ii) respiration supplies all the energy needs for living processes, including movement and much of this energy is eventually transferred to the surroundings, particularly with warm blooded mammals where much energy is spent in maintaining their raised body temperature.

        • the overall simplistic equation for respiration is the opposite of photosynthesis

        • glucose + oxygen ==> water + carbon dioxide (+ energy)

        • This energy is needed for all life processes, energy to do things like movement of any organism, heat to keep mammals warm,

        • The fact of the matter is, that up a food chain/biomass pyramid, only a small percentage of the mass is passed on eg

          • plants producers (100%) ==> primary consumers (caterpillars, 40%) ==> secondary consumers (small birds 5%) ==> bird of prey (owl, 0.5%)

          • This means in this particular food chain, that of all the mass /energy you start with, only 0.5% (1/200th) eventually ends up as the owl.

          • In the food chain: plants ==> rabbits ==> foxes, all these fields of plants of large areas of grass support a relatively smaller population of rabbits, which in turn support a very small number of foxes - you only get a relatively small numbers of a top predator!

          • This is the reason why you rarely get food chains of more than five stages (feeding/trophic levels) because there is so little mass/energy left in the end.

          • Once the energy is lost, it can't be used by the animal in the next stage of the food chain i.e. the next 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.
  • Know and understand that changes in the environment affect the distribution of living organisms.

    • Exam question examples might include, but not limited to, the changing distribution of some bird species and the disappearance of pollinating insects, including bees.

  • Know and understand that animals and plants are subjected to environmental changes.

    • Realise that such changes may be caused by living or non-living factors

      living: Change in competitor (a new or rise/fall in native ones), spread of an infectious disease from parasites and pathogens, levels of prey available to hunt,

      • One species population might be affected by a 'living' factor. If it is the prey for some other animal, then in turn the predator is affected, so population changes are frequent in the animal world and can rise or fall significantly with the availability of food.

      • The decline in the bee population in many countries is attributed to them carrying pathogens/parasites and their food supply contaminated with pesticides - but nobody is quite sure, what is sure, is that bees immune system can't cope.

      • The spread of Dutch elm disease, and other diseases, are devastating tree populations.

    • non-living: Change in the average temperature or rainfall,

      • The average temperature in some northern European countries has risen, so populations of some bird species from southern areas eg the Mediterranean countries, are beginning to increase in northern Europe.

      • Acid rain, from the industrial revolution onwards, has affected forests and ecosystems in lake by decreasing the pH of water.

      • The English Channel separating England and France has become slightly warmer (only by 0.5oC in 100 years), so species of animals from warmer waters are moving north-east into warmer water ie the geographical distribution of marine life is changing.

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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