Interdependence - Key SQA Definitions:


An ecological Niche is the 'role' that a species plays within the wider biological community. It describes how an organism interacts with its environment and is made up of the following:- 

The fact-file below shows the niche of a Great Crested Newt, the largest Newt found in Scotland:-

Food Chains

A food chain shows how plants and animals get their energy. At each link in a food chain 90% of the energy is lost, so the organism next in the chain only gains 10%. This is why there are relatively few top predators (such as lions) compared to their prey (such as wildebeest). 

Energy can be lost from a food web in different ways:-

An example of a food chain is shown below, the arrows showing the direction of energy flow:-

Producers & Photosynthesis

A food chain always starts with a Producer. A producer is an organism that makes its own food. Most producers are green plants, algae or bacteria which make their food by Photosynthesis using energy from sunlight. 

In Photosynthesis, water and carbon dioxide are converted into glucose (a type of sugar) and oxygen. This oxygen is either used in respiration or released from the plant as a waste gas. 

The diagram below shows the word equation for Photosynthesis:-

There is a small number of producers that do not use Photosynthesis to produce their food, but use other chemical processes instead. An example of this can be found around Hydrothermal Vents on the ocean floor. 

Hydrothermal Vents form when seawater seeps down through the sea bed in a region of volcanic activity. As the water passes down through the rock, minerals become dissolved in the water. The water is then heated by underground magma and rises back up to the ocean floor. The hydrothermal vent forms where this mineral-rich water escapes.

The Producers around a hydrothermal vent are a type of bacteria. Instead of Photosynthesis, these bacteria use the chemicals released into the water by the hydrothermal vents to make their food, a process known as 'Chemosynthesis'. 

Consumers & Respiration

Organisms that cannot produce their own food are known as 'Consumers'. Consumers must eat other organisms to gain their energy. Consumers can be split into five groups, based on what they consume (eat):-


An animal that only eats plant material


An animal that only eats other animal material


An animal that eats both plant and animal material


An animal that feeds on dead organic matter


Bacteria and fungi that feed on dead organic matter at microscopic level

Both Producers and Consumers release the energy from their food through a process known as 'Respiration', occurring in all living cells. 

In Respiration, glucose and oxygen are converted into carbon dioxide and water. 

The diagram below shows the word equation for Respiration:-

Food Webs

The food chains described above show a simple explanation of the way energy moves from organism to organism in an area, but in reality it is more complex than this, as organisms usually consume more than one food source. When two or more food chains are linked together, they form a 'Food Web'. 

Terrestrial (Land-Based) Food Web

Aquatic (Water-Based) Food Web

Factors Affecting Food Webs

Food Webs show the complex interdependence between all of the organisms within an ecosystem, with a change in any part of the web affecting the whole ecosystem.

Due to this, it is very important that any changes we try to make to an ecosystem are very carefully managed. When a species is introduced to new area, either by accident or intentionally, the species has the potential to disrupt the entire food web. 

Factors affecting Food Webs include:-


In an ecosystem, the numbers of predators and prey will usually be found in a balanced state. An increase in the number of either species would result in a increase in the other, counteracting the change. This is known as a negative feedback loop. 

A classic example of this is the Fox/Rabbit population model. If the rabbit population in an ecosystem increases, then there will be more food available for foxes, so the fox population would increase. But an increased fox population would eat more rabbits, reducing the rabbit population. Less rabbits means less food for foxes, so the fox population would also decrease, and the cycle begins again.

If the numbers of either predator or prey changed dramatically over a short period of time, for example due to a disease,  then the ecosystem could be thrown out of balance and wide variations in populations could be seen. There is even the risk of a population failing to survive, with consequences for the entire ecosystem. 

The embedded website below runs a simulation of the Fox/Rabbit population model. Starting variables (number of fox to rabbits etc.) can be changed and the effects of these can be modelled:-


Diseases in a single species have the potential to effect the entire ecosystem. If a disease causes enough of a population drop in a particular species, this will cause changes throughout the whole food web. 

If the population drop occurs in a prey animal, then all predators of that animal will have less food, putting pressure on them and increasing competition. If the population drop occurs in a predator, then there is less pressure on their prey and the prey population can expand massively.  

An example of this effect can be seen in amphibians globally. Since the early 1980s, the chytrid fungus has contributed to the decline of more than 500 species of frogs, toads and salamanders, or nearly 7% of all amphibian species. In some species of amphibian, such as the European fire salamander, the infection with this fungus is nearly always fatal. 

Andean Toad with Chytrid infection

Since the Chytrid Fungus was first found to be infecting amphibians in Australia in 1993, the fungus has now spread globally, transferred by natural means through contact with spore in water, but the spread has been accelerated due to human activities, such as trade in amphibians for food or as pets or through infected amphibians accidently 'hitchhiking' with other trade goods. 

Chytrid fungus - 2014

Chytrid fungus - 2019

As the Chytrid infection can kill entire populations of amphibians, the effect it can have on an ecosystem is huge. One scientific study into the effects on the wider ecosystem in Panama showed changes in the abundance and diversity of snakes due to the loss of their main food source, as well as major changes in the diversity of invertebrates and plant life due to the lack of predation/consumption by the amphibians. 


As ecosystems develop, they form balances of all of the organisms within them. When a new species is introduced to an ecosystem, however, it can have devastating consequences for the ecosystem. This introduction can be through natural means, for example through migration of organisms in search of food, or through human influences, either accidental or deliberate.

Introduced species can spread rapidly and outcompete or prey on native species, as the usual checks that would keep them in balance are not present in the new ecosystem, for example no natural predators of the introduced species present. If a species outcompetes the native organisms in this way, it is known as a 'invasive species'. 

An example of an invasive species affecting Scotland is the Harlequin Ladybird. This is a invasive species of Ladybird that was first found in the UK in 2004. Since then it has spread rapidly, and can now be found throughout the whole UK. The diagram below shows the speed at which the Harlequin Ladybird spread across England and Wales in less than 10 years:-

The Harlequin Ladybird is larger than the native species and outcompetes them for food, as well as preying directly on them. The species is thought to be responsible for the drop in populations of at least seven native ladybirds, including the two-spot ladybird, which in 2012 was showing a drop of ~44%. 

Native Ladybird

Head nearly entirely black

Invasive Harlequin Ladybird

Head showing characteristic 'W' pattern

Human Impacts

Humans have throughout history had huge impacts on the natural world, either intentionally or by accident.

When Europeans first arrived in large number in the Americas in the 15th and 16th century, they brought within them lots of plants, animals and other organisms that where not present in the native ecosystems. This had devastating effects on the inhabitants of the Americas, with diseases carried by the Europeans killing millions.

Prior to the colonisation by Europeans, the Americas had a population of around 60 million indigenous people, but over the next two centuries this fell by ~90%, mostly due to diseases such as Smallpox. 

Countryside Ranger

You would do practical work to look after the countryside and conserve wildlife and habitats. You’d repair paths, fences and signs to assist people who visit the countryside. When you speak with the public and answer questions about the wildlife and landscapes you’d be able to convey your enthusiaism about the natural environment. 

As you would be using tools ands and equipment you would need an awareness of health and safety issues.

You could specialise in habitat management, fieldwork or education, or focus on managing certain types of habitat such as waterways, coasts or moorlands.

What Countryside Rangers do

Why become a Ranger? 

A Career as a Countryside Ranger

Salary : £18,000 to £28,000 per year

Working Hours : You would usually work around 37 hours a week, which may include evenings and weekends. Weekend work could increase significantly during the main visitor season. Some jobs are part-time or seasonal.

Typical Entry Requirements : You would usually require relevant qualifications such as countryside management or environmental studies, at Higher National Certificate (SCQF level 7), Higher National Diploma (SCQF level 8) or degree level (SCQF level 9/10).

Skills Required:

Carbon Cycle

The carbon cycle shows how carbon moves from the atmosphere, through animals and plants, then back into the atmosphere again.

All cells contain carbon compounds such as proteins, fats and carbohydrates. Carbon is passed from the atmosphere, as carbon dioxide, to living things. It is passed from one organism to the next in complex molecules, and returned to the atmosphere as carbon dioxide again.

In the atmosphere, carbon is attached to some oxygen in a gas called Carbon Dioxide.

Plants use Carbon Dioxide and the energy from sunlight to make their own food and grow. This is called Photosynthesis.

The Carbon becomes part of the plant. Plants that die and are buried may turn into fossil fuels made of carbon like Coal over millions of years.

When humans burn fossil fuels most of the carbon quickly enters the atmosphere as the gas Carbon Dioxide.

The carbon cycle shows where the carbon which you use comes from, and also where the carbon that you produce goes.

Carbon Cycle : Fossil Fuels

Many of the fuels we use in everyday life are obtained from fuels called fossil fuels. These fuels are mostly hydrocarbons – compounds that contain the elements carbon and hydrogen only but contain some impurities which can lead to pollution when we burn them.

Fossil fuels like coal, crude oil and natural gas have been formed over millions of years from dead plant and animal remains which have been buried under many layers sediment.

Carbon Cycle : Combustion

When a substance burns, it reacts with oxygen. This is known as combustion.

All combustion reactions are exothermic because they release energy, e.g. heat energy is given out when methane is burned in a Bunsen burner.

In order for combustion to happen, three things are needed:-

If any of the three are removed, the combustion will stop. This is how firefighters put out fires, by either:-

Fire Extinguishers

Fire extinguishers can be very useful in an emergency, but they must be used correctly or they can make the situation worse!

The safest option if you discover a fire is to leave the area and call 999 immediately.

Each fire extinguisher is designed to work with a specific type of fire:-

Combustion - Hydrocarbons

Crude oil is a mixture of hydrocarbons. Energy is released during burning/oxidation and respiration. The most common form of oxidation is the direct reaction of a fuel with oxygen through combustion. Combustion is the reaction of burning a fuel in oxygen. Burning natural gas means reacting with the Oxygen in the air. There will be products from the reaction, these products can be collected and identified.

Combustion can take two forms:


Detritivores are organisms that feed on dead organic material (Detritus). 

Examples of detritus are:- 

Examples of Detritivores (both terrestrial and aquatic) include:-


Dung Beetle


Fiddler Crab

Detritivores consume the detritus and the waste they produce allows other plants an animals access to nutrients that would otherwise not be available to them. For example, the waste expelled by Earthworms can contain 5 times more nitrogen, 7 times more phosphorus, and 1000 times more beneficial bacteria than the original soil, in a form which can be used by other organisms such as plants. 


Decomposers are bacteria and fungi, such as Bacillus subtilis (found in the soil that decomposes dead plant matter) or Penicillium (the mould that decomposes bread). 

Decomposers do not 'eat' dead organisms by ingesting them, instead they release enzymes onto the dead matter digesting it outside of their bodies, before consuming the resulting nutrients. This process is known as 'decomposition'

They form a vital role in the carbon (and nitrogen) cycle.  When organisms die and decompose, producers such as plants absorb the broken down nutrients through their roots, to be used in growth.

Decomposers : The Rumen and Bacteria

Bacteria that is found within the digestive system of animals that ruminate (cows, sheep, alpacas etc.) assist the animal through decomposition. Rumination in these animals forms a vital part of the digestive process, allowing nutrients that would not be available to be accessed. 

In rumination, plant matter is chewed, mixing it with saliva (containing digestive enzymes) and then swallowed. The plant matter enters the first part of the stomach known as the Rumen. In the Rumen it is mixed with more digestive enzymes, but more importantly with a large range of bacteria present naturally in the Rumen. The plant matter (now known as the 'cud') is then returned to the mouth and chewed again, breaking it down further before it is swallowed again and passes into the rest of the digestive system. 

The bacteria in the Rumen break down (decompose) cellulose, part of the structure of plant cells that would not normally be digestible by the animal, releasing nutrients in a form that the animal (as well as the bacteria) can absorb. This is known as a 'symbiotic relationship'.

Without this, the ruminating animal would not be able to gain enough energy from their food to survive. It is part of this decomposition process within the Rumen that produces the waste gas methane, a gas which contributes to climate change. 

Decomposers : Mycelial Networks (Fungi)

When thinking of Fungi, most people with think of Mushrooms or Toadstools. These, however, are only one small part of a much larger structure, the Mycelial Network. Mushrooms or Toadstools are the 'fruiting bodies' of the fungus (equivalent to flowers in plants) whose role is to spread the 'seeds' of the fungus, known as Spores. It is estimated that on average the mushrooms only make up about 5% of the fungus by mass, the rest being Mycelium.

The fugus gains its nutrients through decomposition by the mycelium of the fungus, an underground filament network (equivalent to the roots of a plant) that can spread over huge areas. For example, the Armillaria Ostoyae fungus' mycelium covers an area of over 4 square miles. 

Mushroom 'Fruiting Bodies' with Mycelium below the soil

Fungal Mycelium under a log

The mycelium break down and consume leaves, needles, and other forest litter by secreting digestive enzymes into the soil, then absorb the resulting nutrients. These nutrients can also be used by other organisms, again giving the organisms access to nutrients they would not otherwise have. 

Nitrogen Cycle

The Nitrogen cycle (just like the carbon cycle for carbon) shows how nitrogen moves from the atmosphere, through animals and plants, then back into the atmosphere again.

Plants require nitrogen to grow, specifically in order to produce chlorophyll which is vital for photosynthesis to take place. Even through the atmosphere is ~78% nitrogen, plants cannot use this, it can only be absorbed through the roots in the form of nitrates in the soil. These nitrate compounds are produced only by nitrogen-fixing bacteria that live in the soil. These bacteria convert nitrogen in the air into nitrates (a waste product to the bacteria) as part of their respiration.

These bacteria can live freely in the soil or can live in parts of the a plants root system, for example root nodules on a pea plant:-

Some bacteria don't use nitrogen in the air to produce nitrates, they can use another chemical present, ammonia. This ammonia can come from other decomposers breaking down animal waste, such as manure, or can be applied to fields as artificial fertiliser. These Nitrifying bacteria convert the ammonia into nitrates, again as part of their respiration process. 

If the soil becomes waterlogged, the conditions are not well suited for Nitrifying bacteria to live. In these conditions, a different type of bacteria, known as Denitrifying bacteria can thrive. These bacteria decompose the nitrates in the soil and release nitrogen gas into the air as a waste product. 

The graph below shows how the levels of nitrifying and denitrifying bacteria change with water levels in the soil:-

This is one of the reasons that plants struggle to grow in waterlogged soils, as these Denitrifying bacteria reduce the amount of nitrates available to the plants.