Form 3:
Ecology kcse questions

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1. State how excessive use of pesticides may affect soil fertility

- May kill soil micro-organism that decompose humus to release mineral salts

- Soil structure interfered with encouraging soil erosion.

2. The graph below represents a population growth of a certain herbivore in a grassland ecosystem over a period of time.

Suggest three factors that could have caused the population change between C and D(3 marks)

- Drought/food shortage /overgrazing

- Fire

- Emigration

3. A biologist carried out a study to investigate the growth of a certain species of herbivorous bony fish and the factors influencing plant and animal life in four lakes A, B, C and D. The lakes were located in the same geographical area.

Two of the lakes A and B were found to contain hard water due to presence of high content of calcium slats. The mean body length of 2 year old fish, amount of plant life and invertebrates biomass in each lake were determines. The data was as shown in

(a) Describe the procedure that may have been used to determine the mean body length of the fish ( 6 marks)

-The fish were caught, their age determined and the 2 year olds were retained and their length measured and recorded.

- This was done repeatedly until a large number were measure; calculation was done by dividing the total length of all fish by the total number of fish.

(b) What are the likely reasons for the difference in the mean body length of the fish living in lakes A and D? ( 4 marks)

Lake A has hard water with more calcium while Lake D has soft water with no calcium. Calcium is necessary for bone formation. Fish in Lake A grow faster and greater bone length than fish in Lake D. Lake C has more food which fish eat than Lake D.

(c) Suggest one reason for the absence of snails in lakes C and D? ( 1 mark)

Lakes C and D have little or no calcium which is necessary for the formation of the shell in snails.

(d) (i) Name any six abiotic (physical) factors that are likely to influence the plant and animal life in lake A. ( 3 marks)

Light temperature, carbon dioxide concentration, oxygen concentration, PH and salinity

(ii) Explain how each of the factors named in (i) may influence the plant and animals life in Lake A. ( 6 marks)

Light-Affects the rate of photosynthesis.

Temperature- Affects enzyme activities hence photosynthesis.

CO2 concentration – Determines rate of photosynthesis.

O2 Concentration- affects rate of respiration.

Salinity – Osmoregulation in plants and animals.

PH – Affect enzyme activities.

4. During an ecological study of a lake a group of students recorded the following observations.

(i) Planktonic crustaceans feed on planktonic algae

(ii) Small fish feed on planktonic crustaceans worms and insect larvae

(iii) Worms feed on insect larvae

(iv) Bird species feeds on small fish planktonic crustaceans and worms

(v) Insect larvae feed on small fish

(a) From this record of observations construct a feed web (5 marks)

(b) From the food web you have constructed in (a) above isolate and write down a food chain that ends with

(i) Bird species as a secondary consumer (1 mark)

i) Planktonic algae → Planktonic crustacean → Birds

(ii) Large fish as tertiary consumer (1 mark)

ii) Planktonic algae → Planktonic crustacean → Small fish → Large fish

(c) The biomass of the producers in the lake was found to be greater than that of primary consumers. Give an explanation for this observation? (1 mark)

Producers must always have a higher biomass than consumers because they support the consumers which are at higher trophic levels

5. Give two reasons why primary productivity in an aquatic ecosystem decreases with depth(2 marks)

-Light intensity decreases with depth,less light limits photosynthesis

-Temperature decreases with depth,lowering rate of photosynthesis

6. An investigation was carried out between 1964 and 1973 to study the changes of fish population in a certain small lake. Four species of fish A, B, C and D were found to live in this lake. In 1965 a factory was built near the lake and was found to discharge hot water into the lake raising the average temperature from 250C to 300C. In 1967 sewage and industrial waste from a nearby town was diverted into the lake was stopped. The fish population during the period of investigation is shown in the table below.

(a) (i) In which year were the fish populations lowest?


(ii) State the factors that might have caused the lowest fish populations during the year you have stated in (a) (i) above ( 3 marks)

Hot water, sewage and industrial waste.

(iii) Explain how each factor you have stated in (a) (ii) above could have brought about the changes in fish populations (11 marks)

-High temperature reduces dissolved oxygen causing suffocation of fish.

- Sewage leads to eutrophication reducing oxygen concentration and reduces penetration of light. - Industrial waste

- toxic substance kills the organism.

(b) (i) What is the difference in the rate of population recovery of species A and

B? (1 mark)

(ii) Suggest two biological factors that could have led to this difference

(2 marks)

- Reproductive rate

- Competition

- Predation

- Sex ratio

(c) (i) State a method that might have been used to estimate the fish population

in the lake ( 1 mark)

Capture/recapture method

(ii) State one disadvantage of the method you have stated in (c) (i) above

( 1 mark)

Marks may disappear

During marking fish may be killed

Predation/death may interfere

7. Industrial wastes may contain metallic pollutants. State how such pollutants may indirectly reach and accumulate in the human body if the wastes were dumped into rivers.

The pollutants may be absorbed by aquatic plants which in turn may be eaten by fishes. The pollutants therefore get into man through the food chain.

8. State three measures that can betaken to control infection of man by protozoan parasites ( 3 marks)

- Improving sanitation to prevent infection by parasite.

- Insecticide to kill vector like mosquitoes or tsetse fly

- Avoid indiscriminate sexual intercourse to prevent spread of parasites. 

9. The chart below shows a feeding relationship in a certain ecosystem

(a) Construct two food chains ending with a tertiary consumer in each case (2 marks)

Green plants → Grasshoppers → Lizards → Snakes

Green plants → Grasshoppers → Lizards → Cats

Green plants → Mice → Snakes → Hawk

Green plants → Mice → Snakes → Cats

(b) Which organisms has the largest variety of predators in the food web? (1 mark)


(c) Name secondary consumers in the food web ( 2 marks)

Lizard, cats, hawk, snakes.

(d) Suggest three ways in which the ecosystem would be affected if there was prolonged drought ( 3 marks)

Most plants will die

Some organisms will starve and die

Some organisms may migrate.

10. To estimate the population size of crabs in a certain lagoon, traps were laid at random. 400 crabs were caught, marked and released back into the lagoon. Four days later, traps were laid again and 374 crabs were caught. Out of the 374 crabs, 80 were found to have been marked. 

(a) Calculate the population size of the crabs in the lagoon using the formula below ( 2 marks)

(b) State two assumptions that were made during the investigation ( 2 marks)

There was even distribution of crabs; No movement in and out of lagoon/on migration. There was random distribution of crabs after first capture

(c) What is the name given to this method of estimating the population size? ( 1 mark)

Capture, mark, release then recapture/capture/recapture method.

11. The figure below represents a feeding relationship in an ecosystem

(a) Write down the food chains in which the guinea fowls are secondary consumers ( 1 mark)

Grass → Grasshopper → Guinea fowls

Grass → Termite → Guinea fowls

(b) What would be the short term effects on the eco- system of lions invaded the area? ( 3 marks)

Lions would complete with leopards

Gazelle number would reduce

Grass would be increased

(c) Name the organisms through which energy from the sun enters the food web. ( 1 mark)


12. The diagram below represents a simplified nitrogen cycle

(a) Name the organisms that causes processes E and J ( 2 marks)

E -Denitrifying bacteria

J - Nitrifying bacteria

(b) Name the processes represented by F and H (2 marks)

F- Nitrogen fixation


(c) Name the group of organisms represented by c (i)


13. (a) Distinguish between a community and a population (2 marks)

Community – it is the total number of plants and animals living together in an area.

Population- total number of organisms of a given species occupying an area at a certain trophic level

(b) Describe how a population of grasshopper in a given area can be estimated (5 marks)

- Use of net to capture the grasshopper

- Which are then counted and marked. They are then released.

- Total number of grasshoppers is determined by multiplying the grasshoppers captured second time by those captured and marked first time.

- The sum is divided by number of grasshoppers marked in second capture.

14. Explain how the various activities of man have caused pollution of air ( 20 marks)

- During manufacture of sulphuric acid and nitric acids oxide of sulphur and nitrogen are released into air causing acid rain.

- Motor vehicle exhaust fumes release carbon monoxide a respiratory poison.

- Combustion of fuels and coal increase concentration of carbon dioxide creating greenhouse effect.

- Aerosols containing CFC in herbicides and perfumes deplete the ozone layer.

- Smoke from factories mix with fog forming smog which reduces visibility.

- Exhaust fumes from vehicles contains lead from leaded petrol that poisons the body.

- Deforestation exposes top soil to air currents encouraging sheet erosion

- Leaded petrol that poisons the body.

- Loud noise from factories, aeroplanes and Jua Kali workshops can lead to poor hearing ability. - Radio active emissions can lead to mutations.

15. Explain how birds of prey are adapted to obtaining their food ( 2 marks)

Curved sharp hooked strong beaks for killing or tearing flesh from bones.

Curved strong sharp claws for holding prey

16. (a) Name the crop infested by phytophthora infestants and the disease it


Crop- potato/tomato

Disease- potato blight/tomatoes rot

(b) State four control measures against the diseases ( 4 marks)

- Use of fungicides

- Uprooting and burning infected plants

- Crop rotation

- Use biological control

- Use disease resistant varieties

17. Explain why the carrying capacity for wild animals is higher than for cattle in a given piece of land ( 2 marks)

Cattle are mainly grazers while most wild animals are browsers

18. (a) What is meant by

(i) Autecology ( 1 mark)

Study of a single species within a community or ecosystem.

(ii) Synecology ( 1 mark)

Study of different species of organisms in a natural community in an ecosystem.

(b) The number and distribution of stomata on three different leaves are shown in the table below

Suggest the possible habitat of the plants from which the leaves were obtained.

( 3 marks)

Leaf Habitat

A-Aquatic/Fresh water

B- Forest

C- Arid- Semi Arid

(c) State the modification found in the stomata of leaf C

- Sunken stomata,

- Reversed rhythm

- Small stomatal pores

19. After an ecological study of feeding relationships students constructed the food web below

(a) Name the process through which energy from the sun is incorporated into the food web ( 1 mark)


(b) State the mode of feeding of the birds in the food web (1 mark)


(c) Name two ecosystem in which the organisms in the food web live( 2 marks)

Aquatic (pond) and terrestrial (forest)

(d) From the information in the food web construct a food chain with the large bird as a quarter – nary consumer ( 1 mark)

Algae → Zooplankton → Small fish → Bird J → Large bird

(e) What would happen to the organisms in the food web if bird N migrated?

- Number of snails would increase

- Green plants would decrease.

- Bird M would increase

(f) Not all energy from one trophic level is available to the next level. Explain  ( 3 marks)

In a lower trophic level energy is lost through respiration, excretion and death of some organisms.

(g) (i) Two organisms, which display a role in the ecosystems, are not included in the food web. Name them. ( 1 mark)

- Vultures

- Decomposers

(ii) State the role played by the organisms named g (i) above. (1 mark)

Vultures- control population of the large birds.

Decomposers- cause decay of dead organisms recycling the nutrients.

(h) (i) State three human activities that would affect the ecosystems ( 3 marks)

- Deforestation

- Bird hunting

- Over fishing

(ii) How would the activities stated in h (i) above affect the ecosystems?  ( 3 marks)

-Removal of the trees destroys the habitat for birds, they therefore migrate.

- Bird hunting kills the birds reducing their number and increasing small fish, mussels and snails. - Over fishing reduces the number of small fish increasing zooplankton and reducing the algae

20. How is aerenchyma tissue adapted to its functions? ( 2 marks)

- Cells have large air spaces between them to enhance buoyancy. - Cells are air filled reducing their density.

21. Explain how abiotic factors affect plants ( 20 marks)

Light- high light intensity increase the rate of photosynthesis

Temperature- low temperatures lower metabolic activities while moderate temperature increase metabolic activities. High temperature increases transpiration.

Wind- Strong air current increase rate of transpiration and deforms the plants according to direction of the wind.

Atmosphere pressure-High pressures decrease the rate of transpiration and also reduces rate of photosynthesis.

Ph value-some plants thrive well in acid soils while others thrive better in alkaline soils.

Radioactive radiations- Cause mutations of the offspring Oxides of sulphur and nitrogen-cause acid rain that corrodes plant leaves.

22. What is the importance of the following in an ecosystem? ( 3 marks)

(a) Decomposers

Cause breakdown of organic matter enhancing recycling of nutrients

(b) Predation

The organism feeds on whole or part of another organism and therefore control their population.

23. Chart below represents a simplified nitrogen cycle

What is represented by X, Y and Z? ( 3 marks)

X- Denitrification

Y- Animals

Z- Nitrification

24 (a) Distinguish between pyramid of numbers and pyramid of Biomass

( 2 marks)

a) Pyramid of number Way numbers of individuals occurring at each trophic levels of a food chain may be diagrammatically represented.

b)Pyramid of biomass The way the total amount of living matter occurring at each trophic level of food chain may be diagrammatically represented.

(b) Give three reasons for loss of energy from one trophic level to another in a food chain. ( 3 marks)

-Loss through excretion e.g. egestion

- Heat

- Respiration

(c) Describe how the belt transect can be used in estimating the population of a shrub in a grassland ( 2 marks)

-Two parallel strings are laid down over a determined length and width within study area.

- No. of organisms in belt transect are counted.

- Area transect is worked out.

- Number of organisms per unit area is worked out.

25 (a) Distinguish between population and community ( 2 marks)

Population – It is all members of a given species in a particular habitat at a particular time. Community – All organisms belonging to different species that interact in the same habitat

(b) Name a method that could be used to estimate the population size of the following organisms

(i) Fish in a pond ( 1 mark)

Capture and recapture method.

(ii) Black jack in a garden ( 1 mark)

Line transect  

26.State two ways in which schistosoma species is adapted to parasitic mode of life

- Produce large number of eggs for increased survival.

- Produce enzymes to digest human skin when penetrating.

- Can withstand low oxygen concentration.

- Have hook-like structures to attach to the intestinal walls.

27.Describe causes and methods of controlling water pollution ( 20 marks)

The causes of water pollution include;

-Untreated sewage that causes water borne diseases. It can be controlled by treating sewage before being discharged.

-Detergents that cause eutrophication causing reduced oxygen concentration. It is controlled by banning phosphate based detergents. 

-Pesticides and herbicides that have heavy metals that they may gammulates along the food chain killing the higher animals. It is controlled by banning phosphate based detergents.

 - Inorganic fertilizers that have nitrates and sulphates that cause eutrophication. It is controlled by use of organic fertilizers.

28. (a) What is biological control of population growth? ( 2 marks)

It is use of natural predator to kill a prey e.g pest instead of use of pesticide

(b) Describe one example where biological control has been used successfully ( 2 marks)

The aphids are pest found in plants. The ladybirds can be used to control the aphids as they feed on them but not destroy the plants

(c) Explain why the number of predators in nay ecosystem is less than the number of their prey ( 1 mark)

Prey is the source of food for predators. If the number of prey is smaller than the predators they would be depleted.

29. Suggest reasons to account for the following observations.

(b) Antelopes are more commonly found in open grassland while giraffe while giraffes are commonly found in wooded areas. ( 2 marks)

Antelopes are grazers while giraffes are browsers. Antelopes have brown fur being camouflaged by the colour of grass while giraffes are camouflaged by the trees.

(b) In the savannah there is a wider variety of herbivores in wooded areas than in open grassland ( 1 mark)

Trees are camouflage against the herbivores preventing them from being spotted by predators. In open grassland herbivores are easily spotted.

(c) Removal of predators for an herbivore may in the long run lead to a decrease in its population

The population will first increase leading to competition of resource e.g. food or mates. This causes death of the weak herbivores or migrations to new habitats.

30. Explain why primary productivity decreases with depth in aquatic environments.

- As it gets deeper light penetration decreases reducing rate of photosynthesis hence less productivity.

- As it gets deeper carbon dioxide concentration decrease hence reducing rate of photosynthesis hence less productivity

( 2 marks)

31. The following is a simplified drawing of nitrogen cycle.

(a) Identify the compound named A ( 1 mark)

Plant protein

(b) Name the processes


Y-Nitrogen fixation

Z- Dentrification

(c) In what form is nitrogen found in plants and animals?


32. An investigation was carried out to study the type of food eaten by birds found in forest and savannah in a certain area. The table below compares the feeding habitats of the birds found in a closed forest area and an open dry savannah of the area.

(a) Work out the difference in the number of bird species the feed on:

(i) Fruits found in forest and savannah ( 2 marks)

25-10 = 15 birds

(ii) Seed found in forest and savannah ( 2 marks)

20-5 = 15 birds

(b) State two factors that may cause this difference in (a) above ( 2 marks)

i) The number of species in forest are more than in the number in the savannah hence higher change. Fruits more abundant in forest than in savannah Selectively reduces with forest birds because they are many and competition is stiffer than savannah

ii) Seeds more abundant in savannah than in forest-the