POPULATION ESTIMATION METHODS
What is 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.
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Different sampling methods are thus used.A sample acts as a representative of the whole population.
- 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 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.
- 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.
- 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.
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 following assumptions are made:
- 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
Density is calculated by dividing the number of organisms by the size of the area studied.
Frequency is the number of times that a species occurs in the area being studied.
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.
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. Adaptations of Plants to Various Habitats Organisms have developed structural features that enable them to live successfully in their particular habitats. Plants found beneath the canopies of trees are adapted to low light intensities by having broad leaves.
These are plants that grow in dry habitats, i.e., in deserts and semi-deserts. They have adaptations to reduce the rate of transpiration in order to save on water consumption. Others have water storage structures.
- Reduction of leaf surface area by having needle-like leaves, rolling up of leaves and shedding of leaves during drought to reduce water loss or transpiration.
- Thick cuticle; epidermis consisting of several layers of cells;
- leaves covered with wax or resin to reduce evaporation.
- Sunken stomata, creating spaces with humid still air to reduce water holes.
- Few, small stomata, on lower epidermis to reduce water loss.
- Stomata open at night (reversed stomatal rhythm) to reduce water loss .
- Deep and extensive root systems for absorption of water.
- Development of flattened shoots and succulent tissue for water storage e.g. Opuntia.
These are the ordinary land plants which grow in well-watered habitats. They have no special adaptations. Stomata are found on both upper and lower leaf surfaces for efficient gaseous exchange and transpiration. However, those found in constantly wet places e.g. tropical rain forests, have features that increase transpiration.
These plants are called hygrophytes. The leaves are broad to increase surface areas for transpiration and thin to ensure short distance for carbon (IV) oxide to reach photosynthetic cells and for light penetration. The stomata are raised above the epidermis to increase the rate of transpiration.
They have grandular hairs or byhathodes that expel water into the saturated atmosphere. This phenomenon is called guttation.