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Ecology revision


Answering questions correctly makes all the difference. Here are some model questions and answers collected from previous papers and their mark schemes. The asterisks (*) indicate what a mark was given for on the mark scheme. Those without asterisks are answers that got full marks, or are copied from books.

Population biology


Describe the techniques that the students could use to obtain the data shown in the diagram

Use a quadrat to sample every metre along the transect. Lay the frame quadrat on the ground at one metre intervals, every time estimating the percentage cover, and recording the dominant species.

Describe how the mark-release-recapture method could be used to estimate the population of mice in the area being studied. What assumptions would you have made in using this method?

First, one could put traps in the area to catch mice, checking them every two hours. Mice caught within the first day should be marked and counted. After 6 hours all the mice should be released. Set the traps again the next day and the mice captured within 6 hours counted and released. The number in the area would therefore be the number:

[caught and marked in first sample] x [marked animals caught in second sample]

[all caught in second sample]

The assumptions made are that:

  1. The mice are marked in a way that will not affect their survival or movement
  2. The animals mix randomly within the population
  3. No immigration, emigration, death or birth occurs within the time of the sampling

Describe and explain the pattern shown by the population of rabbits shortly after their introduction to the island, and when myxomatosis was introduced

The population was initially very small. In the lag phase, the rabbits were adapting their behaviour to fit the environment. It then showed an increasingly fast increase. In the logarithmic phase, there is an unlimited supply of nutrients and no predators or disease. The population increases exponentially. As the population density increases, food sources are used up and disease spreads more quickly, so environmental resistance slows the growth of the population, and it establishes a carrying capacity during its stationary phase. When myxomatosis was introduced, it spread very quickly, and the population died out.

E.g. Niche

Explain what is meant by "different niche" and how occupying different niches as young and adults helps the survival of amphibians

Occupying a "different niche" means that they have different habitat/abiotic factors/environment*, and different feeding role/food source/biotic factors. This reduces competition.


Describe the process of succession

First the land is colonised/pioneers grow on it*, as seeds are blown onto the land and germinate, (or a root system exists in the case of secondary)*. The death of the colonisers results in modification of the habitat* e.g. by increasing the humus (DO NOT mention nutrient levels for secondary succession), making it more suitable for shrubs and small trees*. These further modify the habitat* and eventually a climax woodland community is achieved.

Explain how and why the number of different species of animals in the habitat might change as "set-aside" farmland reverts to forest.

Monoculture nature of crop* and the use of herbicides* initially means that few species of plant grow, so there is low plant diversity* and there is little "choice" of food*, leading to low animal diversity*. Increase in plant diversity* with succession leads to a greater variety of habitats*, more "cover"* and a greater variety of food, so the environment becomes less hostile* and able to support a more diverse* community. Mature forests have reduced ground flora* due to low light intensity*, so have a lower intensity of animals*.

Explain the relationship between abiotic factors and the diversity of organisms in alpine tundra.

Abiotic factors/named factors are unfavourable/extreme/harsh*. This limits the food sources/plants which limits the diversity*, as few species can survive.

Explain how diversity is related to the diversity of an ecosystem

High diversity confers greater stability on an ecosystem*. Low diversity renders an ecosystem succeptible to change*. For example if the diversity is high, then if one species is affected the others will be available as food*.

Describe the behavioural, structural and physiological adaptations that may help organisms to survive in this habitat (alpine tundra, mean temp 0C)

Depression of freezing point by having concentrated cell contents or synthesising antifreeze*prevents the cells' membranes from fracturing in cold. Xerophytic adaptations reduce transpiration*, for example reduced leaf size/leaf hairs/stomata reduced in number or positioned under leaf*to reduce rate of water loss. Perrenation* or hibernation enables the plants/animals to save their energy during the coldest periods and grow when conditions are most favourable*. Deep/shallow/extensive root system captures maximum water*. Dormancy of seeds* until more favourable conditions are present increases chances of survival. Heat loss by conduction/radiation* is reduced by having a thick winter layer of fur or a fat layer* and lowering of metabolic rate*. Reduction in size of protruding features* and larger volume increases surface area to volume ratio*.



Describe how nutrients running into the lake led to eutrophication and how eutrophication could have led to a reduction in the number of fish living in the lake

Large amount of nutrients results in a plant/plankton boom* followed by nutrient depletion in surface layers*. Plankton/plants block light from plants below*, which are then unable to photosynthesise*. The plants/plankton therefore die* and the animals which feed on them have no food and die too*. There is an increase in the number of decomposers present which feed on dead matter*. Their respiration* causes oxygen depletion*, and fish suffocate*, especially bottom-dwellers*. There is a high rate of respiration as the temperature remains high*, resulting in a build up of toxins* from dead matter.

Human pollution (1)

Explain the link between tourist pollution, planktonic algae and a decrease in corals

First, an increase in the concentration of nitrates/phosphates* from sewage/organic pollution/effluent (from increasing tourist industry)* stimulates the growth of seaweed/plants/plankton*. Increased number of planktonic algae increases the cloudiness of the water*, so less light is able to penetrate to algae in coral.

Human pollution (2)

Suggest sources of the following pollutants in the passage and explain how they may effect amphibians and their habitat

(i) Acid rain

Acid rain is formed from SO2/NOx* from burning of fossil fuels/cars*. This reduces the amphibians' resistance to disease*. It also causes leaching of nutrients/Al3+ from soil into water*, which affects food chains*. The decrease in water pH affects gills* and the enzymes* of the amphibians and their eggs/larvae*.

(ii) Pesticides

This is from run-off from farms*. It causes a reduction in the number of invertebrates*, which has a knock-on effect on the food chain* and is toxic/poisonous to amphibians.


You are basically guaranteed to get a 1-3 mark question based purely on one of these definitions, as well as needing them to correctly answer questions. Learn them.

Ecology is the study of living organisms in relation to their environment.

The environment is the sum total of external factors that directly or indirectly affects the life of an individual or community.

Competition is that which occurs when more than one organism require the same resource. The species that is better adapted to the environment will survive, while the other will decrease in number.

Intraspecific competition is between organisms of the same species

Interspecific competition is between organisms of different species

Factors affecting population size:

Density dependent factors affect a greater proportion of the population at higher population densities than at lower densities.

Density independent factors affect the same proportion of the population whatever the population density.

The ecosystem is a dynamic system involving the interaction of biotic and abiotic components.

Biotic factors are interactions between organisms affecting the survival of species (e.g. competition, predation)

Abiotic factors are the physical and chemical factors affecting individuals in a community (e.g. climatic: temperature, rainfall, wind; edaphic: soil pH, salinity)

The population is the number of individuals of the same species living within a definite area within a definite framework of time (whatever that means)

A species is a group of similar organisms that are able to breed together and produce fertile offspring.

A community consists of all the organisms that live in a particular habitat in a time interval.

The habitat of an organism is the specific locality where it lives.

The niche of an organism is the precise way in which it fits into its environment. If two species have the same ecological niche, either one or both adapt to occupy separate niches or one becomes extinct in that location (the competitive exclusion principle).

The carrying capacity of an environment is the maximum number of organisms (of a particular species) that the environment can support.

Environmental resistance is the sum of the biotic and abiotic factors which prevent the number of organisms from increasing much above the carrying capacity.


Copyright 2000 Simon the A-Level Man. Full reproduction permission is granted provided the URL is included.

If you are reproducing, please bear in mind that every third baby born on Earth is Chinese so maybe you should stop at two (Note: the author has nothing against Chinese)