Category: Section 4: Ecology and the Environment

Human Influences on Greenhouse Effect: A* understanding for IGCSE Biology 4.12 4.13 4.14

I established in a previous post that the Greenhouse Effect is a good thing for life on the planet.  So what is the problem?  Well the simple idea is that human activities and the massive growth in human populations seen over the past two hundred years have changed the composition of the atmosphere.  The concentration of greenhouse gases has risen and this enhanced greenhouse effect is causing climate change.

The principal gases in the atmosphere responsible for the greenhouse effect are carbon dioxide, methane and water vapour.  Have a look at these two tables taken from the following website from the Center for Climate and Energy solutions:

The first image shows the main gases in the atmosphere that contribute to the Greenhouse Effect.


Anthropogenic means “caused by mankind” and so you can see what humans are doing to generate an enhanced greenhouse effect.  The GWP figure stands for Global Warming Potential and gives a relative value for how each gas might contribute to climate change.  One molecule of CFC-12 is as powerful as a greenhouse agent as 10,900 molecules of carbon dioxide.


This second table shown above demonstrates how the composition of the atmosphere has changed from pre-indutrial to modern times.   I am going to focus on the two greenhouse gases at the top of the list:  carbon dioxide and methane.

Carbon Dioxide concentrations in the atmosphere

Scientists at the Maunua Loa Observatory in Hawaii have been measuring atmospheric carbon dioxide concentrations since the 1950s.  Here is a graph of their results.


What do you notice about this graph?

  1. There is a gradual upward trend such that the average concentration has risen steadily over the 50 year period.
  2. Within each year, there is an annual peak and an annual trough in the carbon dioxide concentration.  The “peak” corresponds to northern hemisphere winters when there is less photosynthesis by plants and more fossil fuels are burned.  The “trough” is northern hemispere summer when photosynthesis rates are high and so carbon dioxide is removed from the atmosphere

If you want data going back further into the past, you need to look at ice core data.  Tiny volumes of the atmospheric gases are trapped within ice as it forms in Antartica and by drilling out a core and analysing the gases it contains, one can determine the concentration of the atmosphere when the ice was formed.  The deeper parts of the core formed longer ago so a journey through an ice core is like travelling back in time…..


What human activities might be responsible for these changes in carbon dioxide?

  1. Deforestation (see my post on this topic)
  2. Combustion of Fossil Fuels (coal, oil, gas)
  3. Cement production

 Methane concentrations in the atmosphere


Methane is also a potent greenhouse gas. It is produced as waste product of the bacterial reactions that happen inside the rumen and intestines of cattle.  (The rumen is the large first chamber of their stomach in which bacteria digest cellulose in the cow’s food)  I was once told that each cow produces 65 litres of methane a day but I have never measured it myself…….  Seeing as the world population of cattle is estimated at 1.4 billion, that is a lot of methane each day being released into the atmosphere.

Methane is also produced by bacteria that break down our domestic waste in land fill sites and by anaerobic bacteria that live in paddy fields in which rice is grown.    More humans means more cattle, more rice and more landfill and all of these are responsible for the rise in methane concentrations seen in the atmosphere in recent times.


Greenhouse Effect: A* understanding for IGCSE Biology 4.12 4.13 4.14

The Greenhouse Effect is the name given to the way in which the earth’s atmosphere acts to warm up the planet.  The earth and the moon gain almost exactly the same incident radiation from the sun and yet average temperature on the earth is stable at around 14 degrees Celsius.  On the moon the temperature fluctuates wildly from 1oo degrees Celsius during the day to minus 153 degrees Celsius at night.   Life would be impossible in such extreme and variable conditions and so the greenhouse effect is definitely a “good thing” for life on our planet.

How does the greenhouse effect work?

Well the main idea here is that there are certain gases in the atmosphere that can trap the infrared radiation that the earth emits and prevent it escaping the atmosphere.  These greenhouse gases are warmed as they absorb the infrared and so the atmosphere heats up.


Remember that because the sun is so hot, it emits radiation at a much higher frequency.  This is mostly in the “visible light” part of the spectrum together with some ultraviolet.  The gases in the atmosphere cannot trap visible light (air is transparent as you have probably noticed) and so most of the solar radiation passes through the atmosphere and hits the earth.


Which gases can act as greenhouse gases?

The two most prevalent gases in the atmosphere are nitrogen (N2) and oxygen (O2) and neither is able to trap infrared so cannot act as a greenhouse gas.  The principle greenhouse gases in the atmosphere are


  • carbon dioxide
  • water vapour
  • methane
  • ozone
  • nitrous oxide

So what’s the problem?

Well of course the problem is that human activities over the past century or so have altered the composition of the atmosphere so that the concentration of greenhouse gases has risen.  This has meant more heat is trapped and climate systems are altered in consequence.  This enhanced-greenhouse effect is the problem and I will look at this in the next post…..


Evidence for Climate Change: A* understanding for IGCSE Biology (part 2) 4.12 4.13 4.14

Before you can look at the science of climate change and how human activities are causing it, you first need to accept that we are currently undergoing a period in which our climate is changing at an unprecedented and rapid rate.

What would you expect to see in a warming world?


Well the first and most obvious point is that you would expect to see measurable changes in land and sea temperatures.

All three of these graphs represent in different ways the changes in temperature over the past century or so.  It is true of course that there have been times in the past when global temperatures were much warmer than they are now but the rate of change seen since the Industrial revolution is totally unique in the 4.6 billion year history of our planet.

The top ten hottest years on record have all happened since 1998.


There is excellent evidence from all over the planet of glaciers shrinking; polar ice caps are shrinking and it may be that the North Pole is free of summer sea ice for the first time in 100,000 years some time in the coming decades.  (This is controversial as it is a prediction based on computer models of future climate: different models make different predictions and with a system as chaotic and interwoven as global climate, it can be very difficult to predict)

As the oceans have warmed, this has impacted on extreme weather systems.  This graph shows the incidence of North Atlantic Tropical Storms over the past century or so.  It has continued to rise since 2007…..


When you look at all this evidence, it is hard to believe that our climate has been stable over the past century.  In fact very few people try to dispute the fact of climate change.  The dispute is whether human activities are causing this climate change and indeed whether it matters…..  I hope in later posts I can convince you the answers to these two questions are yes and yes.

Air pollution and climate change: A* understanding for iGCSE Biology (part 1) 4.12 4.13 4.14

Climate change remains one of the more controversial topics in the IGCSE Biology specification.  Just in the past few weeks, the USA (one of the finest nations on the planet) has elected as their President someone who has stated on record that he believes in some giant conspiracy theory about climate science centred around the Chinese….


The overwhelming majority of climate scientists do not support this interpretation of the facts.  They are able to provide evidence of rapid climate change over the past few decades and link this to human-induced changes in the composition of the atmosphere due to pollution.  When these facts are linked by a sensible scientific theory that proposes how and why certain gases might lead to an increased warming effect in the atmosphere (the so-called greenhouse effect) the evidence in support of human-induced climate change becomes compelling.  Keeping US manufacturing competitive is important of course, but not at the expense of the enormous environmental and financial costs of allowing our pollution of the atmosphere to continue unchecked.  I am not sure I will be able to convince President-Elect Trump (he probably doesn’t read my blog in any case) but perhaps I can show you the kind of understanding needed to generate A* answers in GCSE questions on this topic…?


I am going to organise this work into several sections and will post on each topic in the coming week….

  1. What is the evidence for climate change?
  2. What is the greenhouse effect?
  3. How are human activities altering the make up of the atmosphere?
  4. What are the predicted consequences of climate change in the coming years?

Air Pollution part 1: A* understanding for IGCSE Biology 4.11

The extended topic on “Human Influences on the environment” is one that is well worth revising thoroughly.  I haven’t done any analysis of past papers (life is too short) but my hunch is that questions on these topics have been over-represented in the past few years.   This first blog post on air pollution is just going to summarise the consequences of pollution of the atmosphere by one gas – sulphur dioxide.

Sulphur Dioxide

99% of the sulphur dioxide in the air comes from human sources.  It is an acidic gas that acts as a pollutant in two ways.  Firstly direct on human lungs and airways where it is an irritant and can cause wheezing, tightness in the chest and lead to lung disease.  This can be a particular problem for people already suffering with asthma and other diseases of the lung.  Sulphur dioxide (along with various oxides of nitrogen) is also a major contributor to the environmental problem of acid rain.

Sulphur dioxide is produced when any fossil fuel containing sulphur is burned.  The biggest contributor of this pollutant gas (~70% of the total emissions) is the industrial combustion of coal and natural gas for electricity production.  ~20% of total emission comes from other industrial processes and the remainder from burning petrol and other domestic fossil fuels.


The major environmental problem with sulphur dioxide is acid rain.


Sulphur dioxide and various nitrogen oxides are released from coal-fired power stations when coal is burned.  These gases can react in the atmosphere under the influence of radiation from the sun and then dissolve in water to form sulphuric and nitric acids.  These then cause the rainfall to be excessively acidic and this often occurs long distances from where the pollutants were released.

NB All rainfall is slightly acidic due to carbon dioxide in the atmosphere forming carbonic acid.  Acid rain is therefore defined as rainfall (or other precipitation such as snow or hail) with a pH of less than pH5.6.


What are the consequences of acid rain?

As shown in the picture above, there are several biological impacts of acid rain.

Acid rain can cause coniferous trees (e.g. pine trees) to lose their needles (leaves).  This will kill the tree as it cannot photosynthesise and so has led to deforestation in certain parts of the world.

Acid rain leaches minerals (e.g calcium and potassium )from the soil more effectively than normal rain.  This leaves the soil very low in certain essential minerals and so makes it harder for plants to grow.

The minerals leached can then themselves become pollutants in fresh water.  Aluminium ions in fresh water can cause fish to overproduce mucus in their gills.  This will kill adult fish as they cannot get enough oxygen into their blood.  Fish eggs will not survive in acidic water and many small invertebrates are also killed directly by the acidity.  So freshwater ecosystems can collapse.

Deforestation: A* understanding for IGCSE 4.17


Deforestation means the cutting down of mature forest and woodland for non-forestry purposes.  No new trees are planted and so the total area of forest decreases.  Humans have cut down forests for many reasons and have built their economies on exploiting natural resources.  Forests today are cleared to provide wood for logging, to provide land for building homes, for subsistence farmers as well as for commercial growing of crops and cattle farming.


Deforestation happens all the world.  (It is worth noting that the only reason there are no European countries on the list below is because all our forests were cleared very effectively some time ago….)


What are the biological consequences of deforestation?

Wherever deforestation occurs, the biological consequences are the same:

  1. Atmospheric Gases
  2. Soil Erosion
  3. Disturbance to the Water Cycle
  4. Loss of Biodiversity (inexplicably omitted by the people who wrote the specification

Growing trees have a net uptake of carbon dioxide and a net loss of oxygen due to photosynthesis.  Carbon dioxide is a gas that acts as a pollutant in our atmosphere because it is a greenhouse gas.  Carbon dioxide concentrations have been rising over the past century and this is leading to permanent and potentially damaging alterations to the earth’s climate system – a process called climate change.  Oxygen is the gas that almost all organisms require for their respiration.


In many tropical regions, forests protect the sometimes violent tropical storms from hitting the ground. When forest cover is removed, rainfall hits the soil much harder and this can lead to loss of topsoil in a process called soil erosion.  As the water runs through the soil, it will dissolve minerals as it goes, thus leaving the soil that is left denuded of essential minerals for plant growth.  This leaching of minerals makes it difficult to use the land cleared for agriculture and so more forest is cleared.

Deforestation also disrupts the water cycle.  Trees move large volumes of water a year from the soil into the atmosphere in a process called transpiration.  So when trees are lost, less water evaporates from the soil, more water is lost in run-off and so rainfall can be reduced.


The diagram below shows a before and after explanation of how the water cycle is disrupted. (Evapotranspiration is a term for the total water evaporated from a piece of land, combining evaporation directly from the ground and transpiration lost from plants)


There is a final problem with deforestation although the examiners have omitted it from the specification for some inexplicable reason.  Forests provide a habitat for a wide variety of animal and plant species.  So when forests are lost, species become extinct.  This loss of biodiversity is a final terrible consequence of deforestation.  80% of known species live in tropical rainforest so the fact that in the last 50 years, over half of this area has been cleared is a major concern.  The rate of loss of rainforest is around 140,000 square kilometres a year although in some parts of the world, the rate of loss is slowing.

Deforestation is a complex issue and a GCSE revision blog like this is not the place to go into the interesting political and cultural details. I would direct you to the WWF site for more information and indeed some ideas as to what we can do to help.


Use of Quadrats: A* understanding for IGCSE Biology 4.2 4.3

This is going to be a dull post so I shall get my apologies in early…. Please do not read this if you are hoping to be inspired by the beauty of science nor if you want to learn about cutting edge technologies.  If you are struggling with insomnia or have a strange interest in square-shaped pieces of metal, this post might be suitable for you.

A quadrat is just a small square used in environmental biology to estimate populations of plants or to sample within an ecosystem.


This quadrat is 50cm by 50cm and divided into 25 smaller squares, just to make it slightly less dull….

There are two uses of quadrats.  The first is to make estimates of population, the second to investigate the distribution of organisms in an ecosystem.  Your school probably has playing fields and on the playing fields there will be plants such as dandelions.  But how many dandelions live on your playing field?  Well if you were feeling desperate, you might distract yourself by attempting to find this out.  You could cover every square inch of the field and keep an accurate tally count of the dandelion plants you find.  But this would take a long time and even plant ecologists have better things to do with their time.   So a quadrat can be used to randomly sample the field to save you the bother of counting every single plant.

How to use quadrats in random sampling?

Each quadrat has an area of 0.25m2 (50cm by 50cm).  So let’s imagine you randomly place your quadrat 10 times on the playing field and count the dandelion plants in it each time.   You get the following results:


This gives an average of 1.5 dandelions per quadrat.  If this is a representative sample of the total population, you can now estimate the total population of dandelion plants in the field.  You will need to know the total area of the field – let’s pretend it is 200m2.

So each quadrat contains an average of 1.5 dandelions.  How many quadrats in total represent the whole playing field?  You would need 800 quadrats to cover the whole field, so our estimate for the total population of dandelions is 1.5 x 800 = 1200 dandelions.

How do you get random samples?

The only way to sample randomly is to use truly random numbers to represent coordinates in the field.  Shutting your eyes and spinning round until you are disorientated before hurling the quadrat or perhaps alternatively dropping the quadrat from a great height both sound like random procedures but they are not….

Random numbers can be obtained from tables or from a website such as


Sampling with a quadrat can only be done if the organisms you are trying to count do not move around, so basically it works for plants and barnacles.

Right I am feeling sleepy just writing about quadrats so am off for a lie down.

Water cycle: the simplest topic in iGCSE Biology 4.17 4.8

I am wary of writing a post about the water cycle as I so rarely teach it.  It seems too much like common sense to me to require any elaboration in class, but perhaps writing this post will sooth my guilty conscience for Y10 and Y11 students?


The processes that happen in the water cycle are almost all nothing to do with Biology.  Water evaporates from lakes, streams and the sea.  Evaporation is when thermal energy from the sun changes water from a liquid to the vapour state.  The warmer the day, the more evaporation will occur.  The biological component here is that water evaporates from the above ground parts of a plant.  This process is called transpiration and mostly happens through the stomata (tiny pores in the lower epidermis of the leaves).  Geographers like to combine “transpiration” with the “evaporation” of water direct from the soil to come up with the exciting term “evapotranspiration”.  Water vapour condenses in the atmosphere to form clouds and then water falls as a liquid as rain/snow/hail which can be combined together as precipitation.

That’s the water cycle for you:  couldn’t be much simpler really, could it?

Just to finish, check your A* understanding of transpiration by answering these questions – if you are feeling really digital, why not add the answers as a comment at the foot of this post?

1) When are stomata open in the leaf and when do they close?

2) What four environmental factors can speed up rates of transpiration?

3) What is the name of the experimental set up that can be used to measure transpiration rates?  (Does it actually measure transpiration rate or does it really measure something else entirely?)

4) In what ways would you think of transpiration as a “necessary evil”?


How is energy lost between one trophic level and the next? 4.4 4.5 4.6 4.7

Image The diagram above shows how energy moves up the food chain through feeding.  Remember that if you are asked what the arrows represent in a food chain, there is only one possible correct response.  “Arrows in a food chain show the flow of energy from one trophic level to the next”

The big idea here is that not all the energy in one trophic level can ever pass to the next.  The specification suggests that only 10% of the energy is transferred from one level to the next (but in fact the percentage varies between 0.1% to around 15%)

ImagekCal is a unit of energy and the pyramid shows that only 10% of the energy in one level is found in the next.

So there is a big question here – where does all the other 90% of the energy in one level end up?

There are a whole load of different ways energy is lost.  Consider the transfer of energy between mice and owls.

  • The mice use up energy in the process of respiration.  The glucose molecules that mice oxidise to provide the energy to move around are not available to an owl if the mouse is eaten.
  • Not all mice are eaten by owls or other predators. Many die of disease, starvation and exposure and a few might even live long enough to die peacefully in their sleep.  All these “dead mice” will have energy in their bodies that cannot pass up a food chain but instead passes to decomposers.
  • Even the mice that are eaten by owls are not eaten in their entirety.  The owl might only eat the energy-rich parts of a mouse and regurgitate out the bones and fur.  So some  energy is lost as not all the mouse is eaten and digested by the owl.
  • There will be parts of the mouse that even when swallowed and digested are not accessible.  Owl faeces will contain some molecules from the mice eaten that contain energy.   This energy is perhaps found in molecules that the owl digestive system cannot digest.  The energy present in the owl faeces is lost to the food chain and like the example above will pass to decomposers.

This energy adds up to around 90% of the energy in any trophic level.  Ultimately though where does it all go?  All the energy in all the organisms in an ecosystem has the same fate:  it ends up as heat that is dissipated into the system.  Energy can only enter an ecosystem in one way (as sunlight trapped in the process of photosynthesis in producers) and in the end, it all ends up leaving the system as heat energy.  This heat energy is a waste product of respiration.

4.9 Carbon Cycle – A* understanding for iGCSE


The Carbon cycle should really be much simpler to understand than the Nitrogen cycle I posted about yesterday.  This is because the processes involved in moving carbon atoms from one compartment to the next in an ecosystem are more straightforward.  There are four processes mentioned in the specification and you need to make sure you understand each.

  • Photosynthesis:  only happens in producers, takes CO2 from the air to produce complex molecules (carbohydrates/proteins/fats) that can be passed up food chain.
  • Respiration:  happens in all organisms (producers, consumers, decomposers) and turns carbohydrates into carbon dioxide
  • Combustion: fossil fuels and plants can be burnt for fuel releasing carbon dioxide into the atmosphere
  • Decomposition:  two types of decomposition – in aerobic conditions decomposer organisms (bacteria/fungi) convert complex molecules in faeces/dead organisms into carbon dioxide:  in anaerobic conditions, dead organisms can be turned into fossil fuels.

If you want to draw a carbon cycle from scratch to check you understand it, follow the procedure below.

1) Draw the following boxes showing where carbon atoms are found in an ecosystem – CO2 in air, carbon compounds in plants, carbon compounds in animals, fossil fuels and limestone, detritus in soil

2) Draw arrows linking the boxes with the following labels:  photosynthesis, respiration, feeding, combustion, death and decay, death and no decay

That’s about as complicated as it gets.

Warning:  Do not under any circumstances draw an arrow from the detritus in the soil directly to plants.  Plants do not absorb any carbon containing molecules from the soil into their roots.  Honestly, please believe me they don’t however much you want them to….  It would make the cycle more straightforward but they don’t – sorry.  The only carbon-containing molecule plants absorb from their environment is CO2 and that as you all know is absorbed from the air in leaves in the process of photosynthesis.

Zondle quiz on carbon cycle to follow in due course:  keep working hard!  (I am jealous of those of you in Portugal at the moment although weather has been fine today in Northants!)