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# Organisms and population

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## Population :-

A population may be defined as a group of organisms of the same species occupying a certain area or space at a given time .

The term interbreeding implies sexual reproduction , a group of individuals resulting from even asexual reproduction is also generally considered a population for the purpose of ecological studies .

## population Attributes :-

A population has certain attributes that and individual organism does not . An individual may have births and deaths , but a population has birth rates and death rates .

• In a population these rates refer to per capita births and deaths respectively . The rates hence , expressed is change in numbers ( increase or decrease ) with respect to members of the population .

Examples – If in a pond there are 20 Lotus plants last year and through reproduction 8 new plants are added , taking the current population to 28 , we calculate the birth rate as 8 / 20 = 0.4 offspring per lotus per year .

If 4 individuals in a laboratory population of 40 fruitflies died during a specified time interval , say a week , the death rate in the population during the period is 4 / 40 = 0.1 individuals per fruitflies per week .

• Another attribute characteristic of a population is sex ratio . An individual is either a male or a female but a population has a sex ratio ( example – 60% of the population are females and 40% males ) .
• A population at any given time is composed of individuals of different ages . If the age distribution ( percent individuals of given age or age group ) is plotted for the population , the resulting structure is called an age pyramid . For human population , that age pyramids generally show age distribution of males and females in a combined diagram . The shape of the pyramids reflects the growth status of the population – ( a ) Expanding . ( b ) Stable . ( c ) Declining .
• The size of the population tells us a lot about its status in the habitat , population size more technically called population density ( designated as N ) , need not necessarily be measured in number only . Although total number is generally the most appropriate measure of population density . It is in some cases either meaningless or difficult to determine .

Population density ( N ) = Total number of organisms ( n ) / Area ( S )

## Population growth :-

The size of a population for any species is not a static parameter . It keeps changing in time , depending on various factors including food availability , Predation pressure and adverse weather .

• The density of a population in a given habitat during a given period . fluctuates due to changes in four basic processes , two of which + natality and immigration ) contribute to an increase in population density and two ( mortality and emigration ) to a decrease .

### 1 . Natality ( B ) :-

It refers to the number of births during a given period in the population that are added to the initial density .

### 2 . Mortality ( D ) :-

It is the number of deaths in the population during a given period .

### 3 . Immigration ( I ) :-

It is the number of individuals of the same species that have come into the habitat from elsewhere during the time period under consideration .

### 4 . Emigration ( E ) :-

It is the number of individuals of the population who left the habitat and gone elsewhere during the time period under consideration .

If N is the population density at time t ,then its density at time t + I is

N’ = N + { ( B + I ) – ( D + E ) }

From the above equation that population density will increase if the number of births + number of immigration ( B + I ) is more than the number of deaths + the number of emigrants ( D + E ) , otherwise it will decrease .

Under normal conditions , births and deaths are the most important factors influencing population density , the other two factors assuming importance only under special conditions , if a new habitat is just being colonised , immigration may contribute more significantly to population growth than birth rates .

## Exponential growth :-

Resource ( food and space ) availability is obviously essential for the unimpeded growth of a population .

Ideally , when resources in the habitat are unlimited , each species has the ability to realise fully its innate potential to grow in number , as Darwin observed while developing his theory of natural selection . then the population grows in an exponential or geometric fashion .

• If in a population of size N , the birth rates ( not total number but per capita births ) are represented as b and therefore it raids organ per capita death rates ( again , per capita death rates ) as d , then the increase or decrease in N during a unit time period t ( dN / dt ) will be –

dN / dt = ( b – d ) × N

Let ( b – d ) = r , then

dN / dt = rN

The r in this equation is called the intrinsic rate of natural increase and is very important parameter chosen for assessing impacts of any biotic or abiotic factor on population growth .

The above equation describes the exponential or geometric growth pattern of a population and results in a J shaped curve when we plot N in relation to time .

## Logistic growth :-

No population of any species in nature has its disposal unlimited resources to permit exponential growth .This leads to competition between individuals for limited resources .

A population growing in a habitat with limited resources show initially a large phase . Followed by phases of acceleration and deceleration and finally an asymptote ,

when the population density reaches the carrying capacity . A lot of N in relation to time ‘t’ results in a sigmoid curve . This type of population growth is called Verhulst-Pear Logistic Growth and is described by the following equation –

dN / dt = rN ( K – N ) / K

Where N = population density at time t , r = Intrinsic rate of natural increase and K = Carrying capacity .

## Population Interactions :-

Interspecific interactions arise from the interaction of populations of two different species . They could be beneficial , detrimental or neutral ( neither harm nor benefit ) to one of the species or both . Assigning a ( + ) sign for beneficial interaction , ( – ) sign for mental and (0 ) for neutral interaction .

Both the species benefit in mutualism and both lose in competition in their interactions with each other .

In both parasitism and predation only one species benefits ( parasite and Predator , respectively ) and the interaction is detrimental to the other species ( host and pray , respectively ) .

• The interaction where one species is benefited and the other is neither benefited nor harmed is called commensalism . In amensalism on the other hand one species is harmed whereas the other is unaffected . predation , parasitism and commensalism share of common characteristic – the interacting species live closely together .

### Predation :-

Predation only one species benefits ( parasite and Predator , respectively ) and the the interaction is detrimental to the other species ( host and prey respectively ) .

For energy transfer across trophic levels , Predator play Other important roles . they keep prey populations under control . But for Predators , Prey species could achieve high population densities and cause ecosystem instability .

when certain exotic species are introduced into a geographical area , they come invasive and start spreading fast because the invaded land does not have its natural predators .

If a Predator is too efficient and overexploits its sprey , then the prey might become extinct and following it . The Predator will also become extinct for lack of food .

### Competition :-

When Darwin spoke of the struggle for existence and survival of the fittest in nature , he was convinced that interspecific competition is a potent force in organic evolution .

It is generally believed that competition occurs when closely related species compete for the same resources that are limiting , totally unrelated species could also compete for the same resource .

For instance ,in some shallow South American lakes visiting flamingoes and resident fishes compete for their common food .,the zooplankton in the lake . Secondly , resources need not be limiting for competition to occur , in interference competition , the feeding efficiency of one species might be reduced due to the interfering and inhibitory presence of the other species , even if resources ( food and space ) are aboundant .

Therefore , competition is best defined as a process in which the fitness of one species ( measured in terms of its ‘ r ‘ the intrinsic rate of increase ) is significantly lower in the presence of another species .

• Gauss’s Competitive Exclusion Principle states that two closely related species competing for the same resources cannot co -exist indefinitely and the competitively inferior one will be eliminated eventually .

### Parasitism :-

Main parasites have evolved to be host-specific ( they can parasitise only a single species of host ) in such a way that both host and the parasite tend to co-evolve that is , if the host evolves special mechanisms for rejecting or resisting the parasite , the parasite has to evolve mechanism to counteract and neutralise them , in order to be successful with the same host species .

• The life cycle of parasites are often Complex , involving one or two intermediate hosts or vectors to facilitate parasitisation of its primary host .
• The human liver fluke ( a trematodes parasite ) depends on two intermediate hosts ( a snail and fish ) to complete its life cycle .
• The malaria parasite needs a vector ( mosquito ) to spread to other hosts . Majority of the parasites harm the host , they may reduce the survival , growth and reproduction of the host and reduce its population density .
• Parasites that feed on the external surface of the host organism are called ectoparasites . The most familiar examples of this group are the lice on humans and ticks on dogs .

Cuscuta , a parasitic plant that is commonly found growing on hedge plants , has lost its chlorophyll and leaves in the cource of evolution .

• In contrast , endoparasites are those that live inside the host body at different sites ( liver , kidney , lungs , red blood cells etc .) . The life cycle of endoparasites are more Complex because of their extreme specialisation .

### Commensalism :-

There is the interaction in which one species benefits and the other is neither harmed nor benefited . An Orchid growing as an epiphyte on a mango branch , and barnacles growing on the back of a whale benefit while neither the mango tree not the whale derives any apparent benefit .

### Mutualism :-

This interaction confers benefits on both the interacting species . Lichens represent an intimate mutualistic relationship between a fungus and photosynthesising algae or cyanobacteria .

Similarly , the Mycorrhizae are associations between fungi and the roots of higher plants . The fungi help the plant in the absorption of essential nutrients from the soil while the plant in turn provides the fungi with energy yielding carbohydrates .