CCEA GCSE Biology - Denmour Boyd, James Napier 2017

Unit 1
Ecological relationships and energy flow

Specification points

This chapter covers specification points 1.7.1 to 1.7.17. It covers ecological terms, biotic and abiotic factors, sampling with a quadrat, adaptations and competition, the role of the Sun as an energy source, food chains and webs, energy flow, pyramids of number and biomass, the carbon cycle, decomposition and global warming, the nitrogen cycle, minerals and eutrophication and human activity on Earth.

In Double Award Science, specification points 1.7.1 to 1.7.13 are covered, which include ecological terms, biotic and abiotic factors, sampling with a quadrat, adaptations and competition, the role of the Sun as an energy source, food chains and webs, the carbon cycle, decomposition, the nitrogen cycle, minerals and eutrophication.

Biological terms

Ecology is the study of the relationships between organisms and their surroundings. Biologists often use more precise terms when describing these relationships. Although ecology can involve studying single organisms and their surroundings, more often it involves populations of organisms. A population is the number of organisms of the same species living in the same area. The area where a population lives is its habitat.

Where several populations of different species are found living in habitats close together, the populations are collectively a community and biodiversity is a measure of the number of different species living in the area.

Figure 7.1 shows a tree trunk habitat with a population of orange lichens growing on it. Since there is at least one other species of lichen (light grey and feathery) growing in the same habitat, it is also a community.

The ’surroundings’ can also be described in more detail as the environment. This is all the factors which act on an organism or population. They are often further divided into the non-living or abiotic factors such as temperature, humidity, light intensity and living or biotic factors such as the effect of a predator on its prey or where two species compete for the same food. The term ecosystem groups all these ideas together, being an area where a community of organisms live and are affected by a range of environmental factors.

Tip

Sometimes abiotic and biotic factors can interact, for example the shade in a woodland (abiotic factor) is created by the trees (biotic factor).

Fieldwork

During your studies, you will be able to investigate a habitat or ecosystem such as a woodland, shoreline or sand dune system, sampling the numbers of different types of organisms, taking measurements of environmental factors and using these to explain the distributions you find.

Sampling

Realistically, when investigating the number of plants and animals in a habitat it is not possible to cover the whole area (usually because of the time required). Only a small fraction, referred to as a sample of the area can be investigated — several subsections within the habitat are sampled to give an overall picture. In most cases, a square frame called a quadrat is used as the subsection. Some quadrats have 1 m sides giving a quadrat area of 1 m²; others have 0.5 m sides giving an area of 0.25 m².

The most important thing about sampling is that the method used should produce reliable results. For this to happen the sample needs to be as large as possible — the number selected will depend on the size of the habitat but for a reasonably large area it is usually necessary to use 20 or more quadrats. The sample should also be representative of the habitat area. In other words, it should be positioned randomly over the whole area and not just in one corner. This can be done by dividing the habitat into a grid and using random numbers to generate coordinates to find the location of each quadrat.

Tip

Random numbers can be created simply. Use the last four digits of phone numbers from a randomly chosen page in a phone book. The first two digits are for the x-axis and the last two the y-axis.

Collecting data

When investigating plant distribution, percentage cover is often used. This is the percentage of the quadrat covered by a particular type of plant. Estimating the exact percentage is a difficult skill so it is normal to round up to the nearest 10%. (An exception is if there are any plants with a percentage cover between one and five — this is recorded as one and not ten.) Therefore, when sampling for percentage cover using a quadrat, the possible values obtained are 0, 1, 10, 20 and so on up to 100%. Figure 7.2 shows the estimated percentage cover of three plant species in a quadrat.

Tip

In some cases, where some of the plants lie over each other it is possible to record percentage values which give a total greater than 100%.

When the data for all the quadrats is collected, the values for each species can be averaged to give an overall estimate of the percentage cover.

In other cases, where individual plants can be easily identified, percentage cover is not necessary and they can be counted. This data can also be averaged but the area of the quadrat can be used to estimate the average number of plants per square metre, which in turn, if multiplied by the total area of the quadrat, can give an estimate of the total number of that plant species present in the habitat.

Example

Table 7.1 shows the number of buttercup plants recorded in a field measuring 30 m × 20 m.

The square quadrat used had a side of 0.5 m.

The average number of buttercups in each quadrat = (3+2+4) ÷ 3 = 3 plants

The area of each quadrat = 0.5 m × 0.5 m = 0.25 m2

The average number of buttercups in a square metre = 3 × (1 ÷ 0.25) = 12 m²

The field area = 30 m × 20 m = 600 m2

The total number of buttercups = 600 × 12 = 7200

Belt transect sampling

Belt transect sampling is used in habitats where there is a gradual change from one side of the habitat to the other. Randomly placed quadrats would not identify the change in the habit. Figure 7.3 shows a rocky seashore where the types of plants and animals change between the low tide area at the bottom of the photograph and the high tide area at the top of the photograph.

In this habitat, a quadrat can be used to sample the animals as well as the plants because the animals do not move or do so very slowly. In a belt transect (Figure 7.4), quadrats are placed along a line from the bottom of the shore to the top of the shore. Quadrats can be placed continuously or at intervals along the line depending on the distance involved.

Test yourself

1 What is a population of organisms?

2 Name the apparatus usually used to take a sample of plants.

3 How is percentage cover used when sampling plants?

4 When is a belt transect method of sampling used?

Show you can

Describe what is needed to make a sample reliable.

Prescribed practical

Biology Practical 1.6 Double Award Science B4: Using quadrats to investigate the abundance of plants and animals in a habitat

Estimate the population of weed species in a lawn

Procedure

1 Place two 20 m measuring tapes at right angles to each other to mark out the area of grassland to be sampled.

2 Use random numbers to select five sets of coordinates to sample within the area of grassland.

3 Place a 1 m² quadrat at each coordinate and count the number of dandelion, plantain and daisy plants present.

4 Record the results in a table similar to table 7.2

5 Calculate the average number of each type of plant type per square metre and estimate the total population of each type in the grassland area sampled.

Sample results and questions

1 Explain why a sampling method is used instead of counting all the plants in the area.

2 Explain why random numbers are used to select the position of each quadrat.

3 Table 7.3 shows sample data for this investigation

Use the sample data to calculate the average density of each plant type and the total number of plants in the grassland area. Table 7.4 summarises the results.

4 Explain how more accurate results could be obtained in this investigation.

Investigate the abundance of species between high and low tide on a rocky shore

Procedure

(SAFETY - Follow instructions given by teacher. Be aware of the tidal movements during the activity and exercise care as the rocks may be slippery.)

1 Extend a measuring tape as a transect, from the high tide mark down to the low tide mark.

2 Place a quadrat (0.5 m × 0.5 m) beside the tape at the 0—0.5 m measurement. Record the position of the quadrat.

3 Identify and list the organisms present in the quadrat.

4 Repeat steps 2 and 3 with quadrats placed at regular intervals down the transect tape until the full length of the transect has been sampled.

5 Record the results in a table similar to Table 7.5.

6 Draw a bar graph of the number of species at each position along the transect.

7 Describe any trends in the results.

Sample results and questions

1 Table 7.6 shows sample data for this investigation.

2 Draw a bar graph of the number of species at each position along the transect.

3 Figure 7.5 shows a bar graph of the sample results. Describe any trend in the results.

Tip

This method can also be used to investigate any site where one habitat merges into another, for example from a pathway to grassland, conifer to deciduous woodland or stream to field.

The factors acting on organisms

To explain the distribution and abundance of a species we need an understanding of the different factors which affect each species. These factors have been divided into two groups.

Investigating abiotic factors

Most ecological investigations involve the analysis of some of the abiotic (non-living) factors that could affect the distribution of plants and animals. Examples of the abiotic factors that can be investigated include:

• Wind — wind speed can be analysed using anemometers. Wind speed can be very important, affecting the numbers and distribution of plants and animals in exposed habitats such as sand dune systems.

• Water — soil moisture levels can be calculated by taking soil samples and weighing them to find their mass. The soil samples are then dried in an oven until completely dry and reweighed. The difference in mass as a percentage of the original mass gives a value for the percentage soil moisture. Soil moisture levels can be an important factor in the distribution of many plants and animals.

• pH — pH can be measured using soil test kits or probes. The pH of the soil is very important in the distribution of many plants. Some plants will only grow in relatively acidic soils, for example heathers, and some will only grow in relatively alkaline soils, for example some orchids, but most plants prefer soil pH to be around neutral.

• Light — can be measured using light meters. Light is particularly important in the distribution of plants. While all plants need light to photosynthesise, some need high light levels to thrive whereas others can survive in very low light levels.

• Temperature — temperature can be measured using a thermometer.

Biotic factors

Biotic factors are living features of the environment, or the ways in which the presence of one species interacts with another. One example is competition, where the organisms in a habitat each try to obtain enough of a resource they both need to reproduce and survive. There is usually not enough of the resource to allow both organisms to thrive so the reproduction and ultimately the survival of both organisms is affected.

Competition between animals is usually for food, water, territory and mates. The relationship between red and grey squirrels demonstrates how competition can affect populations. Although they both can eat similar types of tree seeds, the red squirrel is not able to digest seeds such as acorns as well as the grey squirrel. This means that in mixed woodland the grey squirrels have an advantage, being able to get more energy from their food, so have a better chance of surviving in this habitat. As a result of grey squirrels out-competing red squirrels in this way, very few red squirrels can survive in mixed woodland

However, grey squirrels are much larger and so need more energy to survive. This becomes a disadvantage in habitats like conifer forests where the seeds are small and so it is more difficult for the grey squirrels to eat enough seeds for the energy they require. In conifer forests red squirrels are therefore at an advantage and are often the largest population.

Another biotic factor affecting the relationship between grey and red squirrels is the disease squirrel pox, which is carried by the grey squirrel with little effect but is lethal if transferred to red squirrels.

Competition in plants is usually for light, carbon dioxide, water, minerals and space. The case study below is an example of how the plants of a sand dune system compete for these resources.

The following case study gives an example of abiotic and biotic data in a habitat.

Case study — plant distribution in a sand dune system

The following data was collected from a survey in a sand dune system. As the sand dune system extended for over 1 km the data could not be collected from a continuous belt transect but was collected from three ’interrupted’ belt transects. The first belt transect was from the start of the first dune nearest the shore and extended inland. The third belt transect was at the very end of the dune system, just before the typical dune system was replaced by woodland. The second transect was halfway between the other two. Data was collected from 20 quadrats in each transect.

Figure 7.8 shows some of the abiotic information gathered and the distribution of three plants typical of sand dunes. The appropriate abiotic data was collected for each quadrat sampled in each transect.

Marram grass is the grass typical of sand dunes and has an important role in binding the sand within the dunes together and enabling dunes to become stabilised. Unlike most other plants, it can grow in very unstable conditions such as those found near the shore. Heather, a small shrub, grows where the soil becomes more stable and moist. The larger woody shrub, gorse, will thrive when the soil becomes even more stable and contains more nutrients and moisture.

The graphs show that marram grass is most common in Transect 1, with little coverage in Transect 2 and 3. Heather is the most common plant found in Transect 2. Gorse is the most common species found in Transect 3 but is uncommon or not found in Transects 1 and 2. You should be able to interpret this data and give possible explanations for the plant distributions based on the information given.

Possible explanation

The marram grass is able to grow near the shore and is important in the formation and stabilisation of dunes (you have been given this information). It is also logical to conclude that it requires high light levels to grow and possibly grows best in slightly alkaline pH (from the abiotic data). However, it appears to be much less successful further inland where the soil becomes more stable, more moist and more acidic. It appears that the changing conditions further inland favour other plants such as heather and gorse and these outcompete the marram grass. The low light level in Transect 3 (created by shade from gorse shrubs) probably prevented the marram grass from growing.

The shrub, heather, gains a foothold in the more stable Transect 2 but cannot compete against the larger woody shrubs in Transect 3. Like the marram grass, the heather is probably unable to survive in the shade of the larger gorse.

It is important to note that plants and animals can influence the abiotic data — it is not only the other way around. In this example, the ground light levels in each transect are entirely controlled by the shade produced by plants growing there.

Validity and reliability

During this course, you have come across the term reliability on a number of occasions. If data is reliable, someone else could repeat the investigation and get similar results. Reliability can be increased in experiments by doing repeats or taking many measurements.

The validity of information provided is an indication of whether you are actually able to draw conclusions from the information. The concept of a ’fair test’ applies as much to ecological experiments as it does to laboratory-based ones. For example, you will only be able to say definitely that a particular abiotic factor is responsible for a change in vegetation if other abiotic factors are controlled.

Test yourself

5 What is an abiotic factor?

6 Describe how an oven and a balance can be used to find the water content of soils.

7 Explain why the pH of the soil is important to plants.

8 What is competition?

Show you can

Explain the difference between validity and reliability.

Transfer of energy and nutrients

Examples of ecosystems include grasslands, woodlands and lakes. If ecosystems can remain stable for long periods of time, there must be some way in which energy continually enters the system to replace the energy that is lost through respiration and the many activities which use energy that occur. Where does this energy come from?

The energy comes from the Sun and is trapped by green plants in the process of photosynthesis. Plants that can photosynthesise are known as producers as they produce their own food and they in turn provide food and energy for other organisms. The herbivores (plant-eating animals) that feed on plants are known as primary consumers and the carnivores (animals that eat other animals) that feed on the primary consumers are known as secondary consumers. Animals that feed on secondary consumers are tertiary consumers and so on.

The sequence of producers trapping the Sun’s energy and this energy then passing into other organisms as they feed is known as energy flow.

The different stages in the feeding sequence can also be referred to as trophic levels. Producers occur in trophic level 1 and primary consumers are trophic level 2.

Food chains and food webs

Figure 7.9 shows a sequence or chain of living things through which energy passes. It is an example of a food chain. Food chains show the feeding relationships which transfer substances, including carbon and nitrogen, as well as energy between several organisms (represented by the arrows). Examples of some other food chains are shown in Figure 7.10.

These examples show that in all food chains the first organism is the producer and it provides food and energy for primary consumers. Of course, food chains are very simplistic in that they do not show the complex interactions that usually exist. In reality very few animals have only one food source.

A food web shows how a number of food chains are interlinked and gives a much more realistic picture. Figure 7.11 shows how the food chains above are built up into a food web. The food web shown is only part of the story as there will still be many more links and organisms involved than those listed.

Test yourself

9 What are trophic levels?

10 What is the source of energy in a food chain?

11 Name one primary consumer in the grassland food web shown in Figure 7.11.

12 What do the arrows in a food chain represent?

Show you can

Explain fully why producers are important to the animals in a food chain.

Pyramids of number and biomass

For a food web to be sustainable, there must be enough food for all the organisms involved. There will usually be more producers than there are primary consumers and more primary consumers than secondary consumers. The number of organisms at each stage of a food chain can be represented in a pyramid of numbers. Figure 7.12 shows a typical pyramid of numbers. The term ’pyramid’ is used as the shape will usually resemble a pyramid.

Example

How to draw a pyramid of numbers

Table 7.7 shows data for a pyramid of numbers.

1 Draw the axes and decide on a suitable scale for the x-axis.

2 Draw a bar using the producer data.

3 Draw and label a bar for the primary consumer, secondary consumer and tertiary consumer.

Pyramid of biomass

Pyramids of numbers can sometimes be misleading as they do not always take into account the size of the organisms involved. The pyramids of numbers in Figure 7.16 highlight this problem. In b), the producer is a single tree. A tree is a relatively large organism on which many primary consumers can feed, therefore the pyramid of numbers gives an inverted shape. You can see this for yourself if you examine the leaves of a tree carefully. You will find many small insects on, and occasionally inside, the leaves of one tree.

When looking at energy flow through a food chain, it is sometimes more accurate to use a pyramid of biomass. These diagrams represent the mass of living tissue in the organisms concerned. Figure 7.17 shows that, if we use a pyramid of biomass for the woodland pyramid in Figure 7.16b, it is no longer inverted and now has a typical pyramid shape.

As we have learned, a pyramid of numbers has the advantage that it is simply a count of the number of organisms at each trophic level. The problem is that a 100 m tall tree has the same value as a 5 mm long insect. Both are equal to one and sometimes this means a pyramid of numbers is not an accurate representation of the relationships in the community.

An advantage of a pyramid of biomass is that it takes these size differences into account. However, the disadvantage is that biomass data is difficult to obtain. Biomass is actually the dry mass of the organism (heated until all the water is removed). Consequently, some of the organisms from each trophic level must be killed to obtain the data.

Energy loss and trophic levels

All the food chains in Figure 7.10 are relatively short, containing no more than four organisms. This is because energy is lost at each stage of energy transfer. Even the absorption of light by plants is not particularly efficient — energy is lost as light is reflected or passes through leaves and misses the chloroplasts, or for many other reasons. However, this loss of energy is not significant as there is no shortage of light energy coming from the Sun.

The transfer of energy between plants and animals and between animals at different trophic levels is usually 10—20%. This means that for every 100 g of plant material available, only between 10 g and 20 g is built up as tissue in the herbivore’s body. The same applies when carnivores eat herbivores. This loss of energy is due to three main reasons:

• Not all the available food is eaten. Most carnivores do not eat the skeleton or fur of their prey, for example.

• Not all the food is digested; some passes through the animal and is egested as faeces.

• A lot of energy is lost as heat in respiration. Respiration provides energy for movement, growth and reproduction. Heat is produced as a by-product of respiration. The heat is lost and cannot be passed on to the next trophic level.

Example

You will be expected to calculate the energy at different trophic levels and the percentage efficiency of energy transfer between trophic levels.

Figure 7.19 shows the energy flow through a food chain.

To calculate the energy in the primary consumer, subtract the total losses (respiration + wastes) from the energy coming from the producer.

To calculate the percentage efficiency of the energy transferred from the secondary consumer to the tertiary consumer:

Figure 7.19 shows why food chains are relatively short. There is a limited amount of the Sun’s energy trapped by the producer. Energy is lost at each trophic level reducing the overall amount available. Eventually so much energy has been lost through the food chain that there is not enough to support another trophic level. In effect, the shorter the food chain, the more energy efficient it is.

Show you can

Explain why food chains have usually no more than three or four trophic levels.

Nutrient cycles

We have already noticed that energy flows through food chains and webs as part of the feeding process. Figure 7.18 shows that energy must continually enter the system from sunlight as it is lost from all living organisms during the process of respiration. This is why we use the term energy flow.

If we look at the flow of nutrients (like carbon and nitrogen) in more detail, we see that it differs from the flow of energy in important ways. In a stable ecosystem, the overall gain or loss of nutrients from the system will be small and, unlike energy, the nutrients can be recycled as part of a nutrient cycle.

Test yourself

13 What is a pyramid of numbers?

14 Explain why a pyramid of numbers sometimes has a narrow base.

15 What is biomass?

16 How is energy lost as it passes through a food chain?

Nutrient cycling involves the process of decay and decomposition. For recycling to take place, dead organisms must first be broken down during the decay process. Many organisms such as earthworms, woodlice, and various types of insects are involved in the initial stages of breaking down dead organisms into small pieces. Fungi and bacteria are the decomposers that break down the organic compounds into their simplest components, which plants can absorb and use again.

These decomposing bacteria and fungi have a special way of breaking down dead organic matter, and so are described as saprophytic. They secrete enzymes into the soil or dead organism. The enzymes break down the organic material and it is then absorbed by the bacteria or fungi. When digestion happens like this, outside cells, it is described as extracellular digestion. Humus is the organic content of the soil formed from decomposing plant and animal material. Decomposition takes place more quickly when conditions are optimum.

These include:

• a warm temperature

• adequate moisture

• a large surface area in the decomposing organism.

Figure 7.20 shows one method that can be used to investigate these optimum conditions.

The carbon cycle

The carbon cycle is an example of a very important nutrient cycle. Carbon is an essential element in every living organism. For example, proteins, carbohydrates and fats all contain carbon. The carbon cycle involves the exchange of carbon between living organisms, but also includes transfer between these organisms and carbon dioxide in the atmosphere. The main processes in the carbon cycle are:

• Photosynthesis — carbon dioxide is taken in by plants and built up into sugar, starch and other organic compounds.

• Feeding — animals eat the plants (or other animals) and the carbon is built up into other organic compounds that can be transferred further along the food chain.

• Respiration — when plants, animals and decomposers respire they return carbon compounds to the atmosphere as carbon dioxide (a form of excretion).

• Decomposition — carbon compounds in dead organisms and from egestion (for example in faeces) are broken down into simpler products. As the decomposers break them down, they respire and release carbon dioxide into the atmosphere.

• Fossilisation — when some plants and animals die, the environmental conditions are not favourable for the process of decomposition (for example the waterlogged, acid conditions of a bog). The dead organisms do not decay or do so very slowly and thus accumulate and are preserved in large quantities (the peat in a bog). Further changes can happen to this fossil material over millions of years producing fossil fuels (peat, lignite, coal, oil and gas).

• Combustion — when fossil fuels are burned the carbon is returned to the atmosphere as carbon dioxide.

Figure 7.21 shows how the different processes of the carbon cycle are linked.

Tip

Photosynthesis is the only process in the carbon cycle which removes carbon dioxide from the atmosphere.

Revisiting the carbon cycle — global warming

There is increasing evidence that the level of carbon dioxide in the atmosphere is rising. There is also evidence that humans are responsible. Figure 7.21 shows the carbon cycle.

Over the last 150 years or so, there have been two major changes to the way the cycle works.

• Increased combustion of fossil fuels has added more carbon dioxide to the atmosphere.

• Increased deforestation has removed many forests, meaning that less carbon dioxide can be taken out of the atmosphere by photosynthesis.

These changes mean that the amount of carbon dioxide has become unbalanced leading to an increase of carbon dioxide in the atmosphere.

The link between increased carbon dioxide concentrations and global warming

It is known that carbon dioxide and some other gases in the atmosphere form a ’greenhouse blanket’, trapping the heat from the Sun’s rays within the atmosphere. This is explained in more detail in Figure 7.22. It is thought that the increase in carbon dioxide is increasing the ’greenhouse effect’ and this is leading to rising temperatures, or global warming.

The evidence for global warming

It is only recently that many politicians and members of the public have accepted that it is the increase in carbon dioxide concentrations that causes global warming.

The effects of global warming

The warming of the atmosphere causes:

• climate change — more weather extremes such as droughts and severe storms

• polar ice caps to melt

• sea levels to rise and increased flooding

• more land to become desert

• loss of habitats.

What can be done?

• Plant more trees.

• Reduce deforestation.

• Burn fewer fossil fuels by using alternative fuels and/or becoming more energy efficient.

Many people believe that it is very important to act now before it is too late. Although it may not be possible to stop global warming, the process can be managed better and slowed down. For example, it would be better not to build houses on areas that are likely to flood, unless we make sure there are better flood defences.

There is still considerable controversy surrounding global warming — the evidence, causes and possible solutions.

Tip

Reducing the amount of fossil fuels burned is considered to be the most important strategy in the reduction of global warming and is the main focus of international agreements.

Test yourself

17 Name two types of organism that are decomposers.

18 Explain what is meant by the term fossilisation.

19 Give two reasons why the carbon cycle has become unbalanced.

20 Give two effects of global warming.

Show you can

Describe saprophytic digestion.

The nitrogen cycle

Most of the nitrogen in plants and animals is in the form of amino acids and protein, while most nitrogen in the environment is in the form of nitrogen gas and soluble nitrates. The ways that nitrogen moves between these different forms are shown in Figure 7.23 and can be split into three phases. These three phases are:

1 Nitrification

The build up of nitrogen into amino acids and protein in plants and animals and the eventual breakdown of these compounds into nitrates. Plants absorb nitrogen as nitrates and use them to make protein. As plants (and animals) are eaten, they are digested and then built up into other proteins in sequence. Eventually, the nitrogen is returned to the ground as urine or through the process of death and decay. During decay, bacteria and fungi break down the proteins to release ammonia. A second very important group of bacteria, nitrifying bacteria, convert the ammonia or ammonium compounds into nitrates (nitrification) and the cycle can continue.

2 Nitrogen fixing bacteria

Nitrogen fixing bacteria are a special group of bacteria that can convert nitrogen gas into nitrates. These bacteria can be found in the soil or frequently in small swellings (root nodules) in the roots of a particular group of plants called legumes. Legumes include peas, beans and clover. The relationship between the legumes and the bacteria is complex, but an important one in which both partners benefit. The bacteria gain carbohydrates from the legumes and the legumes in turn provide a ready source of nitrates for the benefit of the plants. The process of converting nitrogen from the atmosphere into nitrates is called nitrogen fixation and can also be carried out by other bacteria in the soil.

3 Denitrification bacteria

Denitrifying bacteria convert nitrates into atmospheric nitrogen. This is a wasteful and undesirable process. Denitrifying bacteria are anaerobic and are most commonly found in waterlogged soils. Their effect in well drained soils is much reduced. The process of converting nitrates into nitrogen gas is called denitrification.

The processes of nitrification and nitrogen fixation can be accelerated by higher temperatures and aerated soil.

Root hair cells

Root hair cells are specialised cells in the root that are adapted by having a large surface area (due to their finger-like shape) for the uptake of nitrates, other minerals and water.

Figure 7.24 shows that there are more nitrate ions inside the root hair cell than outside. The nitrate ions are taken into the root hair cell by the process of active transport (uptake). This process requires oxygen for aerobic respiration to produce the energy needed to move the nitrates against the concentration gradient.

Replacing lost nitrogen — the use of fertilisers

Agricultural land needs to be fertilised on a regular basis. When plants are harvested and animals are slaughtered the nutrients they took from the soil are lost. To replace these lost nutrients growers add either natural fertiliser (manure, slurry and compost) or artificial fertiliser to their crops. Fertilisers mainly contain nitrates which the plants absorb and convert into amino acids and proteins for growth.

Tip

Slurry is a liquid manure; a mixture of cattle faeces, urine and water.

Fertilisers also add calcium (needed for plant cell walls) and magnesium (needed to make chlorophyll).

Water pollution

Water pollution can have a particularly harmful effect on our rivers, lakes and seas as they are relatively fragile environments and are easily damaged. Every so often we hear about substantial fish kills in local rivers or lakes. This may be due to harmful chemicals being accidently released from a factory, but many fish kills are the result of sewage or fertiliser runoff draining from farmland that borders the river or lake. The sewage (or slurry) and fertiliser runoff adds to the nitrate concentration in waterways. But why does the increased nitrate concentration kill the fish?

The high nitrate concentration causes aquatic plants in the water to grow much faster by providing the nitrates needed for growth. The extra nitrates have a particular effect on algae (microscopic organisms which live in water). The algae grow so quickly that the water surface becomes green in colour which is described as an algal bloom.

This may block the light from plants lower in the water causing them to die. The excessive growth of the surface plants uses up the nitrates and so they also die. These dead plants are then decomposed by aerobic bacteria which use up the oxygen dissolved in the water and the fish and other animals die due to lack of oxygen. This process is called eutrophication (see Figure 7.25).

This type of pollution can be reduced by increasing the environmental awareness of farmers to encourage better control of fertiliser use and more secure storage of farmyard manure and slurry.

Human activity and biodiversity

We have seen that human activity can have potentially damaging effects on biodiversity (deforestation, burning fossil fuels and using excess fertilisers), but other activities try to reverse or slow such changes.

Figure 7.26 shows satellite images of the Eastern part of the Rondonia state in Brazil taken 1990 and 2000. The light coloured areas in the dark green forest show where the trees have been cut down and the increase that has taken place over ten years. If this level of deforestation continues, the rainforests and the resources they contain will be destroyed in a few decades.

Some countries have developed a controlled way of harvesting this valuable timber through developing sustainable woodlands. The types of strategies used in these woodlands are:

• Only a small number of ’large’ trees are harvested at a time.

• Saplings are planted to replace the trees harvested, a process called reforestation.

• Harvesting of that same area does not happen again until the medium trees have grown to become ’large’ (25—30 years).

Another way in which humans are attempting to bring about positive changes, or at least reduce the negative ones, is to agree international strategies. This started in 1997 with the Kyoto Protocol and continued in Paris 2015, when 195 countries of the world agreed a legally binding global climate change deal in an attempt to reduce global warming.

Practice questions

1 Figure 7.27 shows a decayed leaf found in soil.