- Ecology is the study of organisms and their environment.
- All organisms show interdependence on one another.
- Organisms are affected by their environment, and they in turn affect the environment.
- Green plants manufacture food by photosynthesis which other organisms obtain directly or indirectly.
- Growth of plants is mainly affected by environmental factors such as soil and climatic factors.
- On the other hand, organisms modify the environment through various activities.
- This interrelationship comprises the study of ecology.
- The study of ecology is important in several fields of study such as agriculture and environmental studies.
Concepts and Terms Used in Ecology
a) Habitat:
- This is the place or "home" that an organism lives or is found,
- e.g., forest or grassland.
b) Niche:
- A niche is the functional unit in the habitat.
- It includes not only the specific place in which an organism lives but also how the organism functions.
- To avoid or reduce competition, organisms are separated or segregated by their niches.
- for example, different species of birds make their nest on one tree, some at tips of terminal branches, and others feed on leaves, some on flowers and yet others on fruits of the same tree, i.e., food niche.
- Yet others feed on same food, e.g., worms in the same place but at different times - time niche.
c) Population:
- The term population refers to the total number of individuals of a species living in a given area at a particular time.
- Density is the number of individuals of a population found in a unit area, i.e.,
- This is the distribution of individuals in the available space.
- Dispersion may be uniform as in maize plants in a plantation;
- random as in cactus plants in the savannah ecosystem or clumped together as in human population in cities.
e) Community:
- This is the term used to describe all the organisms living together in an area.
- During the development of an ecosystem, the species composition of a community changes progressively through stages.
- Finally a steady state is reached and this is described as the climax community.
- This development of an ecosystem is termed succession.
- Each stage in development of an ecosystem is a sere.
- Succession is primary when it starts with bare ground, and secondary when it starts in a previously inhabited area e.g. after clearing a forest.
f) The Ecosystem:
- The community and the abiotic or non-living environment together make up an ecosystem or ecological system.
- In this system energy flow is clearly defined from producers to consumers and nutrient cycling takes place in paths that links all the organisms and the non-living environment.
g) Biomass:
- This is the mass of all the organisms in a given area.
- Ideally, it is the dry mass that should be compared.
h) Carrying capacity:
- This is the maximum sustainable density in a given area e.g. the number of herbivores a given area can support without overgrazing.
Factors in an Ecosystem
Abiotic factors (environmental factors)1. Temperature
- Is the hotness or coldness of an area or habitat.
- It directly affects the distribution and productivity (yield) of populations and communities.
- Most organisms are found in areas where temperature is moderate.
- However, certain plants and animals have adaptations that enable them to live in areas where temperatures are in the extremes such as the hot deserts and the cold polar regions.
- Temperatures not only influence distribution of organisms but also determine the activities of animals.
- High temperature usually accelerates the rates of photosynthesis, transpiration, evaporation and the decomposition and recycling of organic matter in the ecosystem.
2. Light
- Light is required by green plants for photosynthesis.
- Light intensity, duration and quality affect organisms in one way or another.
3. Atmospheric Pressure
- The force per unit area of atmospheric air that is exerted on organisms at different altitudes.
- Growth of plants and activity of animals is affected by atmospheric pressure.
- e.g., rate of transpiration in plants and breathing in animals.
4. Salinity
- This is the salt content of soil or water.
- Animals and plants living in saline conditions have special adaptations.
5. Humidity
- This describes the amount of moisture (water vapour) in the air.
- Humidity affects the rate of transpiration in plants and evaporation in animals.
6. pH
- Is the measure of acidity or alkalinity of soil solution or water.
- pH is very important to organisms living in water and soil.
- Most prefer a neutral pH.
7. Wind:
- Is moving air currents and it influences the dispersion of certain plants by effecting the dispersal of spores, seeds and fruits.
- Air currents also modify the temperature and humidity of the surroundings.
8. Topography:
- These are surface features of a place.
- The topographical factors considered include altitudes, gradient (slope), depressions and hills.
- All these characteristics affect the distribution of organisms in an area.
- e.g., the leeward and windward sides of a hill.
Biotic factors:
- These are the living components in an ecosystem,
- competition
- predation
- symbiosis
- parasitism
- human activities
- The relationships between organisms in a given ecosystem is primarily a feeding one.
- Organisms in a particular habitat have different feeding levels referred to as trophic levels.
- There are two main trophic levels:
1. Producers:
- These organisms that occupy the first trophic level.
- They manufacture their own food hence are autotrophic.
2. Consumers:
- These are the organisms that feed on organic substances manufactured by green plants.
They occupy different trophic levels as follows:
Primary consumers:
- These are herbivores and feed on green plants.
Secondary consumers:
- These are carnivores and feed on flesh.
- First order carnivores feed on herbivores while second order carnivores feed on other carnivores, i.e., tertiary consumers.
Omnivores:
- These are animals that feed on both plant and animal material.
- They can be primary, secondary or tertiary consumers.
Competition:
- This describes the situation where two or more organisms in the same habitat require or depend on the same resources.
- Organisms in an ecosystem compete for resources like food, space, light, water and mineral nutrients.
- Competition takes place when the environmental resource is not adequate for all.
Intraspecific competition. - This is competition between organisms of the same species.
- For example, maize plants in a field compete for water and nutrients among themselves.
Interspecific competition.- This refers to competition between organisms of different species, e.g., different species of predators can compete for water and prey among themselves.
3. Predation
- It is a relationship whereby one animal (the predator) feeds on another (the prey).
Saprophytism
- Saprophytism is the mode of nutrition common in certain species of fungi and bacteria.
- Such organisms feed on dead organic material and release nutrients through the process of decomposition or decay.
- Saprophytes produce enzymes, which digest the substrates externally.
- The simpler substances are then absorbed.
- Saprophytes help in reducing the accumulation of dead bodies of plants and animals.
- Harmful saprophytes cause rapid decay of foods such as fruits, vegetables, milk and meat.
- Others damage buildings by causing wood rot.
- Some fungi produce poisonous substances called aflatoxins.
- These substances are associated with cereal crops which are stored under warm, moist conditions.
- If the infected grain is eaten, it may cause serious illness, and death.
Parasitism
- This is an association between members of different species.
- The parasite lives on or in the body of another organism, the host.
- The parasite derives benefits such as food and shelter from the host but the heist suffers harm as a result.
Symbiosis
- This is an association in which organisms of different species derive mutual benefit from one another.
- Some symbiotic associations are loose and the two partners gain very little from each other.
- Other symbiotic associations are more intimate and the organisms show a high degree of interdependence.
Nitrogen cycle
- Is the interdependence of organisms on one another and the physical environment as nitrogen is traced from and back into the atmosphere.- Although nitrogen is abundant in the atmosphere, most organisms are not able to utilise it directly.
- Some bacteria are capable of converting atmospheric nitrogen into forms which can be used by other living organisms.
- These bacteria are referred to as nitrogen fIxing bacteria.
- Symbiotic nitrogen fixing bacteria live in the root nodules of leguminous plants such as beans and peas.
- Non-symbiotic nitrogen fixing bacteria live in the soil.
- Nitrifying' bacteria convert ammonia into nitrites and nitrates.
- Denitrifying bacteria convert nitrates into atmospheric nitrogen.
Energy Flow in an Ecosystem
- Most of the energy used in an ecosystem is derived from the sun.
- Solar energy is trapped by photosynthetic plants.
- It flows through different trophic levels.
- At each level energy is lost as heat to space and also through respiration.
- Besides animals lose energy through excretion and defecation.
- The amount of energy passed on as food from one trophic level to another decreases progressively.
- The energy in the organisms is recycled back to plants through the various nutrient or material cycles.
Food Chains
- A food chain is a linear relationship between producers and consumers.- It represents the transfer of food energy from green plants through repeated stages of eating and being eaten.
Types of Food Chain
- Grazing food chain - starts with green plants.
- Detritus food chain - starts with dead organic material (debris or detritus).
Detritivores:
- Detritivores feed on organic wastes and dead matter derived from the grazing food chain.
- Many different types of organisms feed on detritus.
- They include fungi, protozoa, insects, mites annelids and nematodes.
Examples of Food Chains
- Green plants → aphids → lady-bird beetle
- Green plants → antelope → lion
- Algae → Tilapia → kingfisher
- Plant debris → bacteria → eprotozoa → mosquito larva
- Phytoplankron → eZooplankton → Tilapia → Nile perch → Human
- In a natural community, several food chains are interlinked to form a food web.
- Several herbivores may feed on one plant.
- Similarly, a given herbivore may feed on different plants and may in turn be eaten by different carnivores.
Decomposers
- These are mainly bacteria and fungi.
- These organisms feed on dead organic matter thereby causing decomposition and decay and releasing nutrients for plants.
- They form a link between the biotic and the abiotic components.
Pyramid of Numbers
- Refers to the number of organisms in each trophic level presented in a graphic form and a pyramid shape is obtained.
- The length of each bar is drawn proportional to the number of organisms represented at that level.
- This is because a herbivore feeds on many green plants.
- One carnivore also feeds on many herbivores.
- In a forest the shape of the pyramid is not perfect.
- This is because very many small animals such as insects, rodents and birds feed on one tree.
Pyramid of Biomass
- This is the mass of the producers and consumers at each trophic level drawn graphically.
Population Estimation Methods
- It is important to find or estimate the sizes of the different populations in a habitat.
- Direct counting or head count which involves the counting of every individual, is not always applicable for all organisms.
- e.g., it is impossible to count directly the numbers of grasshoppers in an area.
- Different sampling methods are thus used.
- A sample acts as a representative of the whole population.
Sampling Methods
Quadrat Method- A Quadrat is a square, made of woos metal/hard plastic.
- It can also be established on the ground using pegs, rope/permanent coloured ink, using metre rule or measuring tape.
- The size is usually one square metre (1M2), in grassland.
- In wooded or forest habitat it is usually larger, and can reach upto 20 m2 depending on particular species under investigation.
- The number of each species found within the quadrat is counted and recorded.
- Total number of organisms is then calculated by, finding the average quadrats and multiplying it with the total area of the whole habitat.
- The number of quadrats and their positions is determined by the type of vegetation studied.
- In a grassland, the quadrat frame can be thrown at random.
- In other habitats of forest, random numbers that determine the locus at which to establish a quadrat are used.
Line Transect
- A line transect is a string or rope that is stretched along across the area in which all the plants that are touched are counted.
- It is tied on to a pole or tent peg.
- It is particularly useful where there is change of populations traversing through grassland, to woodland to forest land.
- This method can also be used in studying the changes in growth patterns in plants over a period of time.
Belt Transect
- Two line transects are set parallel to each other to enclose a strip through the habitat to be studied.
- The width is determined by the type of habitat, i.e., grass or forest and by the nature of investigation.
- In grassland it can be 0.5 m or 1 m.
- Sometimes it can be 20 metres or more especially when counting large herbivores.
- The number of organisms within the belt is counted and recorded.
Capture-recapture method
- This is used for animals such as fish, rodents, arthropods and birds.
- The animals are caught, marked, counted and released.
- For example, grasshoppers can be caught with a net and marked using permanent ink.
- After sometime, the same area is sampled again, i.e., the grasshoppers are caught again.
- The total number caught during the second catch is recorded.
The number of marked ones is also recorded:
- Let the number caught and marked be a.
- The total number in the second catch be b.
- The number of marked ones in the second catch be c.
- The total number of grasshoppers in the area be T.
The total number T can be estimated using the following formula:
Total Number =
- No migration, i.e., no movement in and out of the study area.
- There is even distribution of the organisms in the study area.
- There is random distribution of the organisms after the first capture.
- No births or deaths during the activity.
- After the estimation, the results can be used to show anyone of the following population characteristics:
- Density is calculated by dividing the number of organisms by the size of the area studied.
Frequency:
- Frequency is the number of times that a species occurs in the area being studied.
Percentage Cover:
- This is the proportion of the area covered by a particular species.
- For example, a given plant species may cover the whole. of a given area.
- In this case the plant is said to have 100% cover.
Dominance: - This is the term used to describe a species that exerts the most effect on others.
- The dominance may be in terms of high frequency or high density.