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Our living world is fascinatingly diverse and amazingly complex
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We can try to understand its complexity by investigating processes at various levels of biological organisation–macromolecules, cells, tissues, organs, individual organisms, population, communities, ecosystems and biomes
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At any level of biological organisation we can ask two types of questions – for example, when we hear the bulbul singing early morning in the garden, we may ask – ‘How does the bird sing?’ Or, ‘Why does the bird sing ?’
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The ‘how-type’ questions seek the mechanism behind the process while the ‘whytype’ questions seek the significance of the process
- For the first question in our example, the answer might be in terms of the operation of the voice box and the vibrating bone in the bird, whereas for the second question the answer may lie in the bird’s need to communicate with its mate during breeding season
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When you observe nature around you with a scientific frame of mind you will certainly come up with many interesting questions of both types - Why are night-blooming flowers generally white? How does the bee know which flower has nectar?
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Why does cactus have so many thorns?
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How does the chick spures recognise her own mother ?, and so on
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You have already learnt in previous classes that Ecology is a subject which studies the interactions among organisms and between the organism and its physical (abiotic) environment
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Ecology is basically concerned with four levels of biological organisation – organisms, populations, communities and biomes
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In this chapter we explore ecology at population levels
Population Attributes
- In nature, we rarely find isolated, single individuals of any species; majority of them live in groups in a well defined geographical area, share or compete for similar resources, potentially interbreed and thus constitute a population
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Although 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
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All the cormorants in a wetland, rats in an abandoned dwelling, teakwood trees in a forest tract, bacteria in a culture plate and lotus plants in a pond, are some examples of a population
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In earlier chapters you have learnt that although an individual organism is the one that has to cope with a changed environment, it is at the population level that natural selection operates to evolve the desired traits
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Population ecology is, therefore, an important area because it links ecology to population genetics and evolution
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A population has certain attributes whereas, an individual organism does not
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An individual may have births and deaths, but a population has birth rates and death rates
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In a population these rates refer to per capita births and deaths
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The rates, hence, expressed are change in numbers (increase or decrease) with respect to members of the population
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Here is an example
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If in a pond there were 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
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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 that period is 4/40 = 0.1 individuals per fruitfly per week
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Another attribute characteristic of a population is sex ratio
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An individual is either a male or a female but a population has a sex ratio (e.g., 60 per cent of the population are females and 40 per cent males)
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A population at any given time is composed of individuals of different ages
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If the age distribution (per cent individuals of a given age or age group) is plotted for the population, the resulting structure is called an age pyramid (Figure 11.1)
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For human population, the age pyramids generally show age distribution of males and females in a diagram
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The shape of the pyramids reflects the growth status of the population -
(a) whether it is growing,
(b) stable or
(c) declining
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The size of the population tells us a lot about its status in the habitat
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Whatever ecological processes we wish to investigate in a population, be it the outcome of competition with another species, the impact of a predator or the effect of a pesticide application, we always evaluate them in terms of any change in the population size
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The size, in nature, could be as low as <10 (Siberian cranes at Bharatpur wetlands in any year) or go into millions (Chlamydomonas in a pond)
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Population size, technically called population density (designated as N), need not necessarily be measured in numbers only
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Although total number is generally the most appropriate measure of population density, it is in some cases either meaningless or difficult to determine
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In an area, if there are 200 carrot grass (Parthenium hysterophorus) plants but only a single huge banyan tree with a large canopy, stating that the population density of banyan is low relative to that of carrot grass amounts to underestimating the enormous role of the Banyan in that community
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In such cases, the per cent cover or biomass is a more meaningful measure of the population size
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Total number is again not an easily adoptable measure if the population is huge and counting is impossible or very time-consuming
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If you have a dense laboratory culture of bacteria in a petri dish what is the best measure to report its density? Sometimes, for certain ecological investigations, there is no need to know the absolute population densities; relative densities serve the purpose equally well
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For instance, the number of fish caught per trap is good enough measure of its total population density in the lake
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We are mostly obliged to estimate population sizes indirectly, without actually counting them or seeing them
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The tiger census in our national parks and tiger reserves is often based on pug marks and fecal pellets
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The size of a population for any species is not a static parameter
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It keeps changing with time, depending on various factors including food availability, predation pressure and adverse weather
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In fact, it is these changes in population density that give us some idea of what is happening to the population whether it is flourishing or declining
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Whatever might be the ultimate reasons, 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
(i) Natality refers to the number of births during a given period in the population that are added to the initial density
(ii) Mortality is the number of deaths in the population during a given period
(iii) Immigration is the number of individuals of the same species that have come into the habitat from elsewhere during the time period under consideration
(iv) Emigration is the number of individuals of the population who left the habitat and gone elsewhere during the time period under consideration
- So, if N is the population density at time t, then its density at time t +1 is
Nt+1=Nt+[(B+I)–(D+E)]
- You can see from the above equation (Fig. - 11.2) that population density will increase if the number of births plus the number of immigrants (B + I) is more than the number of deaths plus the number of emigrants (D + E)
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Under normal conditions, births and deaths are the most important factors influencing population density, the other two factors assuming importance only under special conditions
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For instance, if a new habitat is just being colonised, immigration may contribute more significantly to population growth than birth rates
- Growth Models: Does the growth of a population with time show any specific and predictable pattern? We have been concerned about unbridled human population growth and problems created by it in our country and it is therefore natural for us to be curious if different animal populations in nature behave the same way or show some restraints on growth
- Perhaps we can learn a lesson or two from nature on how to control population growth
(i) Exponential growth: Resource (food and space) availability is obviously essential for the unimpeded growth of a population
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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
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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 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
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The r in this equation is called the ‘intrinsic rate of natural increase’ and is a very important parameter chosen for assessing impacts of any biotic or abiotic factor on population growth
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To give you some idea about the magnitude of r values, for the Norway rat the r is 0.015, and for the flour beetle it is 0.12
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In 1981, the r value for human population in India was 0.0205
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Find out what the current r value is
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For calculating it, you need to know the birth rates and death rates
- The above equation describes the exponential or geometric growth pattern of a population (Figure 11.3) and results in a J-shaped curve when we plot N in relation to time
- If you are familiar with basic calculus, you can derive the integral form of the exponential growth equation as
Nt=N0ert
where Nt = Population density after time t
N0 = Population density at time zero
r = intrinsic rate of natural increase
e = the base of natural logarithms (2.71828)
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Any species growing exponentially under unlimited resource conditions can reach enormous population densities in a short time
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Darwin showed how even a slow growing animal like elephant could reach enormous numbers in the absence of checks
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The following is an anecdote popularly narrated to demonstrate dramatically how fast a huge population could build up when growing exponentially
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The king and the minister sat for a chess game
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The king, confident of winning the game, was ready to accept any bet proposed by the minister
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The minister humbly said that if he won, he wanted only some wheat grains, the quantity of which is to be calculated by placing on the chess board one grain in Square 1, then two in Square 2, then four in Square 3, and eight in Square 4, and so on, doubling each time the previous quantity of wheat on the next square until all the 64 squares were filled
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The king accepted the seemingly silly bet and started the game, but unluckily for him, the minister won
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The king felt that fulfilling the minister’s bet was so easy
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He started with a single grain on the first square and proceeded to fill the other squares following minister’s suggested procedure, but by the time he covered half the chess board, the king realised to his dismay that all the wheat produced in his entire kingdom pooled together would still be inadequate to cover all the 64 squares
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Now think of a tiny Paramecium starting with just one individual and through binary fission, doubling in numbers every day, and imagine what a mindboggling population size it would reach in 64 days. (provided food and space remain unlimited)
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(ii) Logistic growth: No population of any species in nature has at its disposal unlimited resources to permit exponential growth
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This leads to competition between individuals for limited resources
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Eventually, the ‘fittest’ individual will survive and reproduce
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The governments of many countries have also realised this fact and introduced various restraints with a view to limit human population growth
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In nature, a given habitat has enough resources to support a maximum possible number, beyond which no further growth is possible
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Let us call this limit as nature’s carrying capacity (K) for that species in that habitat
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A population growing in a habitat with limited resources show initially a lag phase, followed by phases of acceleration and deceleration and finally an asymptote, when the population density reaches the carrying capacity
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A plot of N in relation to time (t) results in a sigmoid curve
- This type of population growth is called Verhulst-Pearl Logistic Growth (Figure 11.3) and is described by the following equation:
dtdN = rN (KK−N)
Where
N = Population density at time t
r = Intrinsic rate of natural increase
K = Carrying capacity
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Since resources for growth for most animal populations are finite and become limiting sooner or later, the logistic growth model is considered a more realistic one
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Gather from Government Census data the population figures for India for the last 100 years, plot them and check which growth pattern is evident
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Populations evolve to maximise their reproductive fitness, also called Darwinian fitness (high r value), in the habitat in which they live
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Under a particular set of selection pressures, organisms evolve towards the most efficient reproductive strategy
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Some organisms breed only once in their lifetime (Pacific salmon fish, bamboo) while others breed many times during their lifetime (most birds and mammals)
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Some produce a large number of small-sized offspring (Oysters, pelagic fishes) while others produce a small number of large-sized offspring (birds, mammals)
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So, which is desirable for maximising fitness? Ecologists suggest that life history traits of organisms have evolved in relation to the constraints imposed by the abiotic and biotic components of the habitat in which they live
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Evolution of life history traits in different species is currently an important area of research being conducted by ecologists
Organisms and Populations Introduction Our living world is fascinatingly diverse and amazingly complex We can try to understand its complexity by investigating processes at various levels of biological organisation–macromolecules, cells, tissues, organs, individual organisms, population, communities, ecosystems and biomes Introduction $\rarr$ Populations $\rarr$ Population Growth $\rarr$ Life History Variation $\rarr$ Population Interactions