Biology A2 Revision cards.

Part of Unit 4, Topic 5, Revision cards. 


What is an Ecosystem?

An ecosystem is a life supporting environment which includes all of the living organisms which interact together, the nutrients that cycle through the system and the physical and chemical environment in which the organisms are living. 
An ecosystem consists of a network of habitats and the communities of organisms that are associated with them. 

A habitat is the place where an organism lives. Many organisms only live in a small part of a habitat, this is known as a microhabitat.

A population  is a group of organisms of the same species, living and breeding together in a habitat.

A community is all the populations of the different species of organisms living in a habitat at any one time. 

The ecological niche can be defined as 'the way an organism exploits (uses) its environment'. It can be broken down into more specific elements (i.e. food niche).


1 of 31

Biotic Factors

Biotic factors are the living elements of a habitat which affect the ability of a group of organisms to live there. 

  • Competition; for resources like food can be inter-specific or intra-specific.
  • Grazing, predation and paratism are all relationships between two organisms where one benefits at the others expense.
  • Mutalism is a relationship in which both partners benefit. 

Biotic factors are density dependent. The effects are related to the size of the population relative to the area available. 

2 of 31

Abiotic Factors

Abiotic factors are non-living or chemical factors; they including the following;

  • Solar energy input; is affected by latitude, season, cloud cover and chargers in the Earth's orbit. Light is vital to plants (e.g. photosynthesis and germination) as well affecting the behavior in animals. 
  • Climate; includes rainfall, wind exposure and temperature. 
  • Topography; includes altitude, slope, aspect and drainage.
  • Oxygen avaliability;  particularly important in aquatic systems. 
  • Edaphic factors; connected with the soil, includes soil pH and mineral avaliability -> itself affected by geology. The underlying geology of an area can have a significant effect on plant distribution. Edaphic factors also include soil texture. 
  • Pollution; can be air, water or land.
  • Catastrophes; are infrequent events that disturb conditions considerably. 

Anthropogenic factors; are those arising from human activity. They be either Abiotic or Biotic.  

3 of 31

Biomes - The major ecosystems

The biosphere could be considered the largest ecosystem on Earth. 

It is so large it is divided into smaller parts, distinguished by their similar climates and plant communities. 

These major ecosystems or biomes are generally subdivided into smaller ecosystems for ease of study. 

The major biomes of the Earth have developed over thousands or millions of years, from bare rock to the ecosystems present today. 

This has been bought about by the process of succession. 

Succession is the process by which communities of animals and plants colonize an area and then over time are replaced by other, usually more varied, communities. 

4 of 31

Primary Succession

  • Occurs from the startin point of an empty inorganic surface, such as bare rock.
  • The first plants are opportunists or pioner species, such as lichens, algae or mosses.
  • These organisms can penetrate the rocks surface, helping to break it into grains and trapping organic material that will break down to form humus.
  • The inorganic rock grains and organic humus are the start of the formation of soil.
  • Once their is soil, other species such as grasses and ferns can establish root systems.
  • The action of their roots and the humus they form when they die and decay adds to the soil.
  • As the soil layer develops, more water and nutrients are retained and become avaliable for plant roots, so that less hardy species can develop and survive.
  • Gradually larger plants can be supported and the diversity of species increases.
  • As plant biodiversity increases, so the diversity of animals that can also be supported increases.
  • Eventually a cliimax community is reached, where the biodiversity and range of species are generally constant.
  • A climax community is self-sustating and usually the most productive group of organisms which that environment can support.

Primary succession must have been of prime importance in the formation of the biosphere, but it is only found in a few places today.

5 of 31

Secondary Succession

  • Secondary succession is the evolution of an ecosystem from existing soil that is clear of vegetation.
  • It occurs when rivers change their course, after fires and floods, and after human disturbance.
  • The sequence is similar to that of primary succession but because the soil is already formed and contains seeds, roots and soil organisms, the number of plants and animals present right from the beginning is much higher.
  • The time it takes to go from an are of bare eart to a climax community varies largely.
  • It depends on many different factors, including temperature, rainfall leels and underlying fertility of the soil.
  • However... a succession of different types of plants and animals will always be seen.
  • The climax community will depend on not only climatic factors bus also on the plants and animals that are either within or able to colonise the area. This means the secondary succesion climax community may differ from the primary climax community.
  • Observing succesion is not always easy. Sand dunes can help overcome this problem because they can show complete records of the stages of succession. :)
6 of 31

The affect of Abiotic Factors

Abiotic factors can vary a great deal within a habitat to produce microclimates. These provide different niches and so determine the distribution and abundunce of different populations.


The amount of light in a habitat - the solar energy input - has a direct effect on the numbers of organisms found there. Plants are dependant on light for photosynthesis. Any plant populations which are going to thrive in habitats with low light levels must be able to cope with this factor.

Animals are affected by light levels indirectly as a result of the distribution of food plants. Seasonal light changes can also affect reproductive pattersn and without cues from changing light levles, many aspects of animal behaviour would be lost.

7 of 31

The affect of Abiotic Factors


For any particular organism there is a range of temperatures within which it can grow and successfully reproduce. It is the extremes of temperature which detemine where an organism can live, not the average.

Temperature particulary effects the rate of enzyme controlled reactions in plants and ectothermic animals (whse body temp. is mainly determined by heat exchange).

Many animals have evolved behaviours and physiological features which enable them to cope.

Organisms without these adapations do not survive.

8 of 31

The affect of Abiotic Factors

Wind and water currents;

Wind has a direct effect on organisms in a habitat.

Wind increases water and heat loss from the body and so adds to the environmental stress an organism has to cope with.

Fewer species can survive in areas with strong prevailing winds.

Occasional gales and hurricanes can destroy populations. 

In water currents, organisms have to flow with the current, be strong swimmers, or be able too hang on tight and resist the force of the water. 

Currents are most damaging to populations when the strength increases suddenly, such as flooding.

9 of 31

The affect of Abiotic Factors

Water avaliability

In a terrestrial environment the avaliability of water is affected by several factors including the amount of precipiation, the rate of evaporation and the edaphic factors such as rate of loss by drainage. 

A limited supply of water can cause severe problems, unless organisms are adapted to cope.

An increase in the avaliability of water can lead to a huge change in habitat and massive increase in population. 

10 of 31

The affect of Abiotic Factors

Oxygen Avaliabilty;

Oxygen supply can be in short supply in both water and soil.

When water is cold or fast flowing sufficient oxygen dissolves in it to support life.

If the temperature rises, or it becomes still and stagnant then oxygen content will drop, making it a mcuh more difficult habitat and affecting the populations within it.

Soil is usually a well aerated habitat, however in waterlogged soil the air spaces are filled and plant roots may be deprived of oxygen. 

11 of 31

The affect of Abiotic Factors

Edaphic Factors;

Soil structures and mineral content. 

The structure of the soil on which organisms live and grow can affect the various populations associated with it.

Sand has a loose, shifting structure that allows very little to grow on it.

  • Marram grass can survive with massive roots and rhizome network.
  • It reproduces successful and binds sand together, which makes it more suited for colonization by other species. 

Soils that contain a high proportion of sand are light, easily worked and easily warmed, however are easily drained. Water passes through rapidly carrying with it essential minerals.

This leaching of minerals reduces population density.

The ideal soil is loam, which has particles of a wide range of sizes. It is heavier than sand and less prone to leaching than sandy soils, yet easier to warm and work than clay. 

12 of 31

The affect of Biotic Factors


A mathematical model that describes the relationships between predator and prey predicts that populations will oscillate in a repeating cycle.

As prey population increases there is more food for predators.

After an interval the predator population grows too.

Predators will increase to the point where they are eating more prey than are replaced by reproduction.

Number of prey will fall.

This will reduce the food supply of the predators as there wont be as many offspring, numbers will fall, allowing the abundance of prey to increase again.

13 of 31

The affect of Biotic Factors

Finding a mate;

Reproduction is a powerful driving force and the likelihood of finding a mate, or acheiving pollination will help determine the organisms which are found in any habitat. 

Avaliability of mates has a big effect on the abundance of any type of animal in an area. 


Many species of animals show clear territorial behavior.

The type and size of territory will help to determine which species live in a particular community.

14 of 31

The affect of Biotic Factors

Paratism and Disease

These can have a devastating effect on individuals. 

Diseased animals will be weakened and do not reproduce successfully. Sick predators will no be able to hunt well, where as diseased prey animals are more likely to be caught.

Some diseases are infectious and can spread without direct contact.

Parasites affect their host usually by feeding off the living body of their host and so weakening it.

Occasionally they can wipe out whole populations.

Parasites and infectious disease spread more rapidly where there a high population density, as individuals will be in greater proximity of each other.

In a community with greater biodiversity, although affect on individuals will be great, the effect on the whole communities will be smaller. 

15 of 31

Energy Transfer in Ecosystems;

Energy within an organism can be used to produce more body tissue, that is to increase biomass.Since the source of energy in most ecosystems is the sun, the rate at which producers convert the suns energy into organic material will determine the flow of energy within those ecosystems.

Only a small % of the solar energy input from the sun is actually transferred into plant material. Figures vary, but it is between 1 and 3%.

Gross primary productivity (GPP) in plants is the rate at which energy is incorporated into the plants. Plants use up to 25% of this accumulated energy for their own metabolic needs, mostly respiration.The rest of the energy is stored in their body tissues. This stored energy is known as the net primary productivity (NPP)

NPP = GPP - plant respiration

The NPP will depend on all the abiotic and biotic factors that affect plant growth within the ecosystem.If you combine the NPP of each type of ecosystem with the area of the Earths surface it covers you can see how much each type contributes to the overall NPP of the Earth.

The NPP is available to the rest of the ecosystem. 

16 of 31

Transfer of energy and feeding relationships

Some energy is fixed within organic molecules by autotrophs (aka producers). 

Organisms that obtain energy as 'ready-made' organic matter by ingesting material from other organisms are known as heterotrophs. 

Heterotrophs include all animals, all fungi and most bacteria.

Heterotrophs cannot make their own food; instead they must consume it. They are all consumers and depend on producers for their food. 

  • Primary consumers, also called herbivores are Heterotrophs that eat plant material. 
  • Secondary consumers, also called carnivores, feed on primary consumerrs
  • Tertiary consumers eat other consumers.
17 of 31

Transfer of energy and feeding relationships

Animals that eat and kill other animals are known as predators and carnivores.

Animals that eat plants and other animals are known as omnivores.

Energy is transferred;

  • producers --> primary consumers --> secondary consumers --> tertiary consumers.
  • Such feeding relationships are known as a food chain.

The position a species occupies in a food called is called its trophic level.

In reality an ecosystem has a complex food web in which each organism eats or is eaten by several other organisms.

18 of 31

Transfer of energy and feeding relationships


are primary consumers that feed on dead organic material called detritus (E.g. woodlice, earthworms and fresh water shrimps).


are specific of bacteria and fungi that feed on the dead remains of organisms and animal faeces. They are heterotrophs and they secrete enzymes and digest their food before absorption takes place.

Decomposers and detritivores play an important role in the recycling of organic matter from dead remains and waste.

19 of 31

Disappearing energy

Producers to primary consumers: Transfer of energy from producers to primary consumers is not very efficient. Only about 2-10% goes to new herbivore biomass. What happens to the rest?

  • Not all avaliable food is eaten: this may be due to limitations of the animals feeding methods. Some parts of the plants, such as the roots, twigs and parts protected by spines or thorns will not get eaten by herbivores.
  • Some energy is lost in faeces and urine: one of the main components of plant material is cellulose. Non mammals have enzymes to help break it down so much of it remains intact as it passes through the guy and comes out in the faeces.
  • Much of the energy absorbed by the consumers is used in respiration: for movement and chemica reactions in the body and is lost to the environment through heat.
20 of 31

Disappearing energy

Primary to secondary consumers:

The transfer of energy from primary to secondary is often more efficient. Often over 10% of the energy in herbivores ends up in carnivores.

This is because most of a herbivore may be eaten by a carnivore and the protein rich diet is easily digested so there is a less lost in faeces.

The fate of energy within a trophic level:

The energy entering a trophic level must equal the amount used or lost by a trophic level.

Energy entering a trophic level = Energy lost in respiration, faeces,                                                           urine and energy in new biomass.

21 of 31

The carbon cycle

The inefficient transfer of energy between organisms is not a problem because there is a constant supply of fresh enregy from the sun. However this is not the same for things such as water and carbon.

Complex cycles have evolved which ensures that the chemical consistuents of life are continually recycled.

Carbon is fundamental to the formation of complex organic molecules.

There is a massive pool of carbon in the CO2 present in the atmosphere and dissolved in water.

This CO2 is absorbed and the carbon incorporated into complex compounds in plants during photosynthesis. The carbon then passes to animals through food chains.

22 of 31

The carbon cycle

The CO2 is continually returned to atmosphere through respiration.
There are massive abiotic and biotic carbon sinks in nature. These are reservoirs where carbon is removed from the atmosphere and 'locked up' in organic or inorganic compounds.

Biotic; removed by photosynthesis and stored in the bodies of living organisms.
Soil also contains masses of organic, carbon-rich material in the form of humus.

Abiotic; rocks such as limestone and chalk, and fossil fuels such as oil, coal and natural gas hold vast amounts of carbon.

The oceans contain around 50 x more dissolved inorganic carbon than is present in the atmosphere. The CO2 is in continual exchange at the air-water surface.

Left to itself the carbon cycle is self-regulating, the amount of carbon released in respirationg and other natural processess and absorbed by photosynthesis remains in balance so that the atmospheric CO2 levels ramin relatively stale.

23 of 31

The human influence?

Evidence is building that the enourmous increase in the production of CO2 by people since the indrustrial revolution coupled with the development of the internal combustion engine and its use in cars is not threatening the balance of the carbon cycle.

This could have major effects on climate, geology and the distribution of organisms.

24 of 31

Greenhouse Gases

Carbon dioxide is one of the greenhouse gaes, others include methane and water vapour. These gases have an important role in the atmosphere.

Greenhouse gases reduce heat loss from the surface of the Eart in a way similar a greenhouse functions. this is called the greenhouse effect.
When the radtion from the sun reaches the earth some is reflected back into space by the atmosphere and the Earths surface.

Some is absorbed by the atmosphere.

Infared radation that reaches the Earths surface is of fairly short wavelength, it is absorbed by the Earths surface and then radiated from the surface at a longer wavelength.
Some of this is absorbed by the and re-radiated back to Earth by GHG's and maintains the Earths temp in such a way life is possible.

25 of 31

The global warming debate

A lot of evidence from many studies now suggest a clear correlation between the increase in temperature and carbon dioxide levels.

However... one problem is that the correlation is so close that it is difficult to know whether increases in GHG's are causing the increase in temperature - or whether the rise in GHG's is the result of rising carbon dioxide levels.

To say that there is a causal relationship we need some mechanism that explains how one factor changes another.

Proving a causal link is almost impossible however it is believed that there is sufficient evidence from research into different aspects to state that their is a causal link.

However.. it will almost certainly turn out that global warming is multi factorial.  

26 of 31

Predicting the future

We can extrapolate the data on GHG's and use them to make predictions about what will happen to temperature in the future.

These extrapoliations can be used in other models to predict the long term effects of increased temperature on the environment.

The models are useful :) However.. there are many limitations;

  • It is impossible to tell the exact impact of carbon dioxide on global warming.
  • It is impossible to predict the impact of GW on particular aspects of the worlds climate.
  • Extrapolations from past data cannot take into account unknown factors in the future.  
27 of 31

Risk of flooding

Antarctic temperatures have increased faster than anywhere else on Earth.

Many scientists believe that the thinning of the ice is a clear indication that global warming is actually occurring.

Antarctic ice contains about 70% of the worlds fresh water.

As the ice melts the volume of water in the sea's and oceans rises which in turn causes an increase in sea levels.

As water gets warmer its volume also increases, which means an even bigger impact on sea levels.

An increase in sea levels could increase the risk of flooding. 

28 of 31

Climate change

Rising temperatures affect weather and rainfall patterns. 

It it impossible to link any one weather event to GW but statistical evidence suggests that there is an increase in extreme weather conditions which has been linked to GW.

Rainfall patterns are complex but have also been shown to be changing.

The effect on organisms;

Temperature has an effect on enzyme activity which in turn affects the whole organism. A ten degree increase will double the rate of an enzyme controlled reaction.

there is an optimum temperature for many enzyme controlled reactions, and if the temp increases beyond this point the enzyme starts to denature and the reaction rate falls.  

29 of 31

Climate change

As a result increasing temperature could have a different effect on processes including the rate of growth and reproduction.
If plants grow faster they will take up more carbon dioxide and therefore reduce atmosphere carbon dioxide levels.
In other places, temperature may exceed optimum temperature for some enzymes and organisms will die.
The majority of animals and plant species in the tropics have little tolerance to change, as conditions tend to vary very little in the tropics.
Experimental evidence suggests a change of a few degrees could have a devastating affect on many species.  
In higher latitude, seasonal cycles affect life cycles.
GW appears to be affecting the onset of seasonal cycles, affecting both life cycles and species distribution.
Warmer temps -> plants grow and flower earlier -> some insects become active earlier -> available plant food -> some birds can adapt :) -> other birds emerge earlier -> missing peak population of insects -> raise fewer chicks. 
For some animal species breeding earlier in the year -> more than one breeding cycle -> increased population.  
Changes in temperature could also effect the embryos of some reptiles.
Crocodiles for example, males will only develop if the temperature is 32-33 degrees, any higher or lower will produce the development of females. GW could be the end of this species and others similar, as only females will be produced.  

30 of 31

Changes in species distribution

A change in climate could affect the range of many organisms. 

Animals can move more easily than plants, they can often survive change more easily, (i.e. by extending their range).

Others may be able to colonize bigger areas.

If organisms involved in the spread of disease are affect, patterns of world health could change.

GW could be responsible for a major increase in insect borne diseases in Britan and Europe.  

31 of 31




Thank you!

Similar Biology resources:

See all Biology resources »