Option G.3 – Impact of Humans on Ecosystems
G.3.1 – Calculate the Simpson diversity index for two local communities
During early succession, diversity is low. The populations are usually dominated by abiotic factors. In populations that are larger and more diverse, abiotic changes do not have such a dramatic effect. As dominant plants provide nutrients and determine the habitats that exist, they set the way of life for other inhabitants. They will also modify and reduce the effects of abiotic factors.
From this, we see that the diversity of species present in a habitat is an indicator of the stability of the community. We measure species richness with the Simpson Diversity index:
N = total number of organisms of all species found
n = number of individuals of each species
A high value of D would indicate an ancient, stable site, while a low value indicates recent colonisation, pollution or agricultural management.
G.3.2 – Analyse the biodiversity of the two local communities using the Simpson index
When the values have been calculated, they can be compared. The higher value would suggest that the site is healthier, with greater diversity of species.
G.3.3 – Discuss reasons for the conservation of biodiversity using rainforests as an example
Unfortunately, tropical rainforests are rapidly destroyed. We know this through satellite imaging, with the only real ones left being in South America, West Africa and the Far East.
It is estimated that the current rate of destruction is about one hectare every second. As a result, the extinction of many species is coming about more rapidly than ever.
Conservation involves applying the principles of ecology to manage the environment.
G.3.4 – List three examples of the introduction of alien species that have had significant impacts on ecosystems
Alien species are plants and animals that have been accidentally or deliberately transferred from their habitat to a new environment where the abiotic conditions are still suitable for them. Should they take over in an aggressive way, which is detrimental to food chains of their new habitat, then they will be described as an invasive species.
Rabbits
These were deliberately introduced to Australia from Europe. The myxoma virus from South America is a disease of rabbits, and was introduced as a biological control. The rabbit numbers have declined since this introduction.
Once in Australia, the rabbits spread very rapidly. There were no natural predators, so interspecific competition failed, so they very soon over-ran large parts of the continent. The rabbits caused a great deal of damage to grassland for cows and sheep.
Out of desperation, the myxoma virus was introduced from South America in 1950, which was found in rabbits. It caused a parasitic disease called myxomatosis. The native rabbits of South America were only mildly affected by it. Over a few years, the virulence of the virus changed, indicating that a small number of the population already had immunity. As the others died, they thrived on the weakened competition, and soon the bulk of the population were immune.
Japanese Knotweed
This was deliberately introduced to northern Europe as an ornamental plant for garden ponds and lakes in the early nineteenth century.
When Knotweed was introduced to Britain, the natural parasites and predators it had come to exist in balance with were not present. Natural herbivores did not browse it, so it was able to escape into natural waterways. Eventually, it grew to the point that it crowded out native species, in dense, submerged thickets. It also blocked waterways and public access to stream banks.
In Japan, it lived amongst its natural population of predators. However, in Britain, the chemicals in its leaves and roots discouraged predation from leaf browsers or root parasites, which was not the case in Japan.
American Grey Squirrel
This was accidentally introduced to Britain in the nineteenth century.
The indigenous red squirrel of Britain is under serious threat of extinction from the American grey squirrel’s invasive pattern of spread. The grey squirrel consumes a wide range of growing nuts, more than the red squirrel, causing faster breeding. The grey squirrel now competes for hazel nuts and pine cones, causing the diet of the red squirrel to be very limited.
G.3.5 – Discuss the impacts of alien species on ecosystems
G.3.6 – Outline one example of biological control of invasive species
Once in Australia, the rabbits spread very rapidly. There were no natural predators, so interspecific competition failed, so they very soon over-ran large parts of the continent. The rabbits caused a great deal of damage to grassland for cows and sheep.
Out of desperation, the myxoma virus was introduced from South America in 1950, which was found in rabbits. It caused a parasitic disease called myxomatosis and acted as a biological control. The native rabbits of South America were only mildly affected by it. Over a few years, the virulence of the virus changed, indicating that a small number of the population already had immunity. As the other died, they thrived on the weakened competition, and soon the bulk of the population were immune. Any that didn’t develop immunity were soon taken out.
G.3.7 – Define biomagnification
Biomagnification is a process in which chemical substances become more concentrated at each trophic level.
It is the bioaccumulation of a substance up the food chain by transfer of residues if the substance in smaller organisms. If the accumulation of the substance comes only through contact with water, then it is known instead as bioconcentration.
G.3.8 – Explain the cause and consequences of biomagnification, using a named example
The use of pesticides to control harmful organisms has greatly improved agriculture, but at the same time causes problems in the environment.
The chemical DDT (dichlorodiphenyltrichloroethane) was found to be an effective insecticide. It is a molecule with chlorine atoms attached to hydrocarbon rings. It acts as a nerve poison to insects, causing rapid death, even in low concentrations. It remains lethal for a long time, as it is stable when dispersed into the biosphere. It did not do any harm to vertebrates, so it was liberally used. DDT was especially effective against mosquitoes, which were a vector in the spread of malaria. Many insect predators of pest species were also killed by DDT.
Since DDT is fat soluble, and is selectively retained in fatty tissues of animals instead of being excreted by the kidneys, it becomes more concentrated at each stage of the food chain.
In non-vertebrates such as fish and birds, DDT concentrations can reach toxic levels. In top 
carnivores, it was concentrated with devastating consequences. While DDT is not a nerve poison for birds and mammals, it does inhibit the deposition of calcium in the eggshell of breeding birds. The eggs are thin-shelled and easily crack, causing a rapid decline in numbers of birds of prey.
Once these effects were realised, a ban was placed on DDT, and the quality of stability was renamed ‘persistence.’
Now, insecticides that are biodegradable are sought, and are more specific in their actions. Organophosphates were tried, but are suspected to be harmful in humans. They act as synapse blockers in insects. As a result, synthetic derivatives of pyrethrum have been developed, as they are much less toxic in mammals, and are biodegradable. However, they are lethal to fish, and must not be used near waterways.
G.3.9 – Outline the effects of ultraviolet (UV) radiation on living tissues and biological productivity
UV radiation is absorbed by organic bases (adenine, guanine, thymine, cytosine and uracil) of nucleic acids (DNA and RNA), which causes them to be modified.
UV radiation kills phytoplankton, which form a significant portion of net photosythesis in the biosphere. The whole oceanic food web is affected by their death.
Many terrestrial plants suffer retarded growth as their rate of photosynthesis slows, resulting from radioactive damage and mutation in plant leaves. It may also kill symbiotic bacteria which fix nitrogen in the root nodules of legumes In animals, increases in UV exposure increases rates of skin cancer, sunburn and eye cataracts.
G.3.10 – Outline the effect of chlorofluorocarbons (CFCs) on the ozone layer
Substances made by industry, like chlorofluorocarbons (CFCs), are threatening the ozone layer. They are very unreactive and stable, deliberately manufactured for use as propellants in aerosol cans and as 
coolants in refrigerators. The gases escape into the atmosphere from time to time, and are slowly carried into the stratosphere, taking up to five years. Once in the stratosphere, they are exposed to high levels of UV light, and a broken down. The highly reactive chlorine atoms are then released, and break down ozone in a cyclic reaction.
The result is that ozone molecules are broken down faster than they can be reformed. Large quantities of CFCs have been released, and despite current steps to replace them with safer chemicals, the time taken for them to reach our atmosphere allows for ozone depletion to continue.
The ozone hole, or thinning of the ozone layer, is a potential problem for all organisms exposed to sunlight on land. Care must be taken for people in Australia, New Zealand, South Africa and Chile that they do not become over-exposed to sunlight and develop skin cancer.
G.3.11 – State that ozone in the stratosphere absorbs UV radiation
Ozone is a molecule of three oxygen atoms, which occurs naturally in the Earth’s atmosphere in an ozone layer, found in the stratosphere.
Ozone is formed by the action of UV radiation in the upper atmosphere on O2
The highest concentration of ozone is at midpoint of the stratosphere, as there is little oxygen at the top where there is much UV radiation, and little radiation closer to the Earth’s surface where there is the highest concentration of oxygen.
As the ceaseless cycle continues, most of the incoming UV light is absorbed. The stratosphere is slightly warmed by the reactions, but the heat is lost to space.
Any UV radiation that reaches Earth is harmful to living things as it is absorbed by the organic bases of nucleic acids, and causes them to be modified. UV radiation may also damage proteins and lipids. Exposure gives rise to increase in cancer rates, glaucoma, cataracts and skin aging. In plant and phytoplankton, exposure may negatively affect productivity.
UV radiation does have beneficial qualities. It is used to treat jaundice in newborn infants, and is used in vitamin D synthesis. During water purification, UV radiation is used to kill microbes. As a result, it is extremely important that the high-level ozone layer is maintained.
