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# Classwork Exercise and Series (Biology- SS1): Population Studies Of An Eco-System

Introduction

Population is defined as the total number of organisms of the same species living together in a given area at a particular time. In an ecosystem, the community is made up of many populations of different species.

In population studies of a habitat, the following are usually studied:

• Types of organisms: This involves the listing of the various types of populations that are found in the particular habitat. It helps to determine the relationships that exist between the various organisms (plants and animals) in a habitat.
• Dominance: Dominance refers to those species that exert a greater influence or a major controlling effect on the other members of the community. The relative importance of a species in the community is expressed by dominance.

Dominance could be expressed in terms of:

o   Their number

o   Occupation of largest portion of space

o   Possession of the highest biomass and

o   The largest contribution to the energy flow in the habit. For a species to be dominant in a habitat, it should possess some of these attributes over other species.

Population Characteristics

These characteristics include:

• Population size: This refers to the total number of the species of the same kind in a given area or habitat. A large population stands a better chance of surviving dangerous and unfavourable conditions such as fire, diseases etc while a small population can easily be wiped out.
• Population density: Population density is defined as the number of individual organisms per unit area or volume of the habitat.

Mathematically, Population density = total population or population size / Area of habitat

Population density can be used to estimate the total number of individuals of a population or population size. Mathematically, population size = population density x area of habitat.

• Population frequency: This refers to the number of times an organism occurs within a given area a habitat.
• Percentage cover: This refers to the area or space covered or occupied by a given species in its habitat and it is expressed in percentage.
• Population growth rate: This refers to the net result of the influence of natality (birth rate) and mortality (death rate) of organisms in a giver habitat.

Factors Affecting Population

Factors which may affect the population of organisms in a given habitat include:

• Natality (birth rate): This refers to the rate of given birth to new organisms. This generally (be it plants or animals) leads to increase in population.
• Mortality (death rate): This refers to the rates at which organisms die in a habitat. Mortality generally leads to a decrease in population.
• Immigration (dispersal): This is the movement of organisms from different habitats into a new habitat. This tends to increase the population of this new area.
• Emigration: This is the movement of organisms out of a habitat due to either scarcity of food or unfavourable conditions or for breeding purposes. This, however, reduces the population of a habitat.
• Availability of food: The availability of food in a given habitat tends to increase the population of that habitat through rapid rate of reproduction and other organisms coming into feed.
• Seasonal Climatic Changes: Unfavourable climatic changes may result in the decrease of population since most organisms may die or migrate out of the habitat while the reverse is the case when the conditions are favourable.
• Breeding periods: Most organisms move out of a habitat during the breeding period or season, thereby reducing the population decreases during the breeding season in fish, toad, termite etc.
• Natural Disasters: Natural disasters like fire, drought, floods, earthquakes etc may lead to a decrease in population as many organisms may die or move out of that habitat to a new area.

Method of Population Study

1. Population Studies by Sampling Method

Population studies can easily be carried out in a habitat especially in terrestrial habitat by sampling method, making use of an instrument called the quadrat. A quadrat is made of a square or rectangular wire, plastic, wooden or metal frame with predetermined area. For example, the area of a quadrat may be 1.5m2.

How to Conduct Population Studies

The population size or total population and population density of a particular species in a terrestrial habitat can be estimated by the following procedure:

1. Choose and locate the sample plot
2. Then identify the species in the plot
3. Measure the area with a measuring tape to know the area of the habitat
4. Throw or toss the quadrat randomly at intervals for up to 10 or above times
5. After each throw or toss, the number of species within the area of quadrat is recorded
6. The density of species is calculated by dividing the average number of times the species occurs within the quadrat by the area of the quadrat.

The calculation can be worked out this way:

Frequency of species = X

Number of toss = Y/10

Average number of species per quadrat = X = Z

Example: Estimate the population density and population size of specific grasses using the quadrat.

Solution:

Total area of habitat = 1.5m2

Frequency of the organisms (grasses), that is, the total number of times an organisms occurs in all the quadrats thrown in the habitat. Assume 400 times.

Number of tosses = 50

Average number of organisms per quadrat tossed = 400/50 = 8

Density of organisms = 8 / 1.6 = 5 grasses per m2

Population size = Density x Area of habitat = 5/m2 x 16m2 = 80

The population size of grasses in the habitat = 80 grass plants.

Estimation of Population using the transect Method: The tape should stretch with marking at intervals. The plants within the various intervals are recorded. This procedure is repeated a number of times until an accurate estimate of the number and types of plants in the habitat are obtained.

Capture mark recapture method

Censuring wild animal population is fraught with many difficulties of sampling errors and statistical bias. Occasionally, direct counts are possible, as in aerial birds or sea bird colonies, but for most animals this is impossible and various kinds of sampling methods must be employed. Most animals are not readily visible because of their behavior and habitat, or because they exist in such abundance or scarcity that they cannot be readily counted. It therefore becomes necessary to estimate numbers through programmes of capture-mark recapture.

The ratio of marked to unmarked animals in subsequent trapping runs provides a population estimate known as the Lincoln index. This is mathematically expresses as follows:

P / M1  = T2 / M2

Where:

P = Unknown population

M1 = Total number of individuals marked in the first capture period

T2 = Total number of individuals captured in the second period

M2 = Total number of those in the second capture period, which were previously marked.

However, the validity of this method involves the assumption that the:

Marked animals mix randomly in the population

Probability of recapture is the same as for each individual

iii.     The system must be a closed one, i.e., there is no immigration or emigration, death or birth of the animal under investigation between the periods of sampling.

Ecological Factors

Ecological factors are those factors in the environment which can influence living organisms or cause changes in any habitat, be it aquatic or terrestrial habitat.

Ecological factors are grouped into two categories Biotic and Abiotic factors.

Biotic Factors

The biotic factors are made up of the effects of other plants and animals on a given organisms.

Examples of biotic factor are:

•     Competitions:Competition involves the interactions among two organisms of the same species or different species in which one neither outgrows the other nor survives while the other cannot grow nor survive.
•     Parasitism: This is a close association between two organisms in which one called the parasite lives in or on, and feeds at the expense of the other organisms called the host. The parasitic benefits from the association while the host usually suffers harm or even die.
•      Commensalism: Commensalism neither is an association between two organisms living together in which only one (the commensal) benefitted nor is harmed.
•     Predation: Predation is a type of association between two organisms in which the predator kills the other called the prey and feeds directly on it.
•     Pathogens: These are microorganisms which can cause diseases in plants and animals leading to their reduction through death.
•     Mortality: Mortality is the death rate of organisms (plants or animals) in an environment. Mortality generally reduces the population of organisms in any habitat.

Abiotic factors are: wind, temperature,soil, water, sunlight etc.

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