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Ww, , 4, , , , , BIOLOGY, , predator or the effect of a pesticide application, we always evaluate them, in terms of any change in the population size. 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). Population size, technically, called population density (designated as N), need not necessarily be, measured in numbers only. Although total number is generally the most, appropriate measure of population density, it is in some cases either, meaningless or difficult to determine. 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. In such cases, the per, cent cover or biomass is a more meaningful measure of the population, size. Total number is again not an easily adoptable measure if the, population is huge and counting is impossible or very time-consuming., 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. For instance, the number, of fish caught per trap is good enough-“measure of its total population, density in the lake. We are mostly-obliged to estimate population sizes, indirectly, without actually counting them or seeing them. The tiger census, in our national parks and tiger,reserves is often based on pug marks and, fecal pellets., , size of a population for any species is not a static parameter. It keeps, changing with_time, depending on various factors including food, availability, predation pressure and adverse weather. 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. 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., () 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., , 2020-21
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ORGANISMS AND POPULATIONS, , \W, , (L, , , , , , Immigration, , O, , |, , , +, , ane ——> Population — >, , Density, (N), , , , , , , , , , , , , , , , , , Emigration, , (©), , , , , , , , , , , , , , Figure 3138, So, if N is the population density at time t, then.its density at time t +1 is, Nu =N,+ (B+) (D+ E)], , You can see from the above equation (Fig-13.5) 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). Under normal ¢onditions, births and deaths are, the most important factors influencing population density, the other, two factors assuming importance*only under special conditions. 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., 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, , 2020-21
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/, J loc, , 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) x N, , Let (b-d) =r, then, , dN/dt =rN, 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., , 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. In, 1981, the r value for human population in India was 0.0205. Find, out what the current r value is. 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 13.6) and results in a J-shaped curve, when we plot N in relation to time.If you are familiar with basic, calculus, youyean derive the integral form of the, exponential‘growth equation as, , NAN, e*, where’, N, = Population density after time t, N, = Population density at time zero, r = intrinsic rate of natural increase, e = the base of natural logarithms (2.71828), Any species growing exponentially under unlimited, resource conditions can reach enormous population., \ densities in a short time. Darwin showed how even, Figure 13.6 Population growth curve a slow growing animal like elephant could reach, a when responses are not enormous numbers in the absence of checks. The, limiting the growth, plot is fojlowing is an anecdote popularly narrated to, exponential, imitt demonstrate dramatically how fast a huge, b when responses are limiting, the growth, plot is logistic, population could build up when growing, K is carrying capacity exponentially., , , , Population density (N)—>, , Time ()—> 1+, , The king and the minister sat for a chess game. The king, confident, , 230 of winning the game, was ready to accept any bet proposed by the, , minister. The minister humbly said that if he won, he wanted only, , ——— Tr 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. The king accepted the seemingly silly bet and started, , 2020-21
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\W, , ORGANISMS AND POPULATIONS (, , , , the game, but unluckily for him, the minister won. The king felt that fulfilling, the minister’s bet was so easy. 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. 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), , (ii) Logistic growth: No population of any species in nature has at its, disposal unlimited resources to permit exponential growth. This, leads to competition between individuals for limited resources., Eventually, the ‘fittest’ individual will survive and reproduce. The, governments of many countries have also realised this fact and, introduced various restraints with a view to limit human population, growth. In nature, a given habitat has enough resources to support, a maximum possible number, beyond which no further growth is, possible. Let us call this limit as nature’s carrying capaeity (K) for, that species in that habitat., , A population growing in a habitat withlimited’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. Avplot of,N in relation to time (t), results in a sigmoid curve. This type of population growth is called, Verhulst-Pearl Logistic Growth (Figtre 13.6) and is described by, the following equation:, , K-N, dN/dt = rN (, Where N = Population density at time t, r= Intrinsic rate of natural increase, , K = Carrying capacity, , 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., , Gather from Government Census data the population figures, Jor India for the last 100 years, plot them and check which growth, pattern is evident., , , , 13.2.3 Life History Variation, , Populations evolve to maximise their reproductive fitness, also called, Darwinian fitness (high r value), in the habitat in which they live. Under, , 2020-21
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yy), , Ww, , 4, , 13.2.4 Population Interaction:, , BIOLOGY, , a particular set of selection pressures, organisms evolve towards the most, efficient reproductive strategy. Some organisms breed only once in their, lifetime (Pacific salmon fish, bamboo) while others breed many times, during their lifetime (most birds and mammals). Some produce a large, number of small-sized offspring (Oysters, pelagic fishes) while others, produce a small number of large-sized offspring (birds, mammals). 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. Evolution of life history traits in different species is currently an, important area of research being conducted by ecologists., , Can you think of any natural habitat on earth that is inhabited just by a, single species? There is no such habitat and such a situation is even, inconceivable. For any species, the minimal requirement is one more, species on which it can feed. Even a plant species, which makes its own, food, cannot survive alone; it needs soil microbes to break down the organic, matter in soil and return the inorganic nutrients for absorption. And then,, how will the plant manage pollination without an animal agent? It is, obvious that in nature,animalsplants and microbes do not and cannot, live in isolation butdinteract.in various ways to form a biological, community. Even ‘in minimal communities, many interactive linkages, exist, although all may not be readily apparent., , 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 detrimental and 0 for neutral, interaction; let us look at all the possible outcomes of interspecific, interactions (Table 13.1)., , Table 13.1 : Population Interactions, , , , | Species A Species B Name of Interaction, | 2 + Mutualism, , | ad S Competition, , | ae i Predation, , | + = Parasitism, , | + Oo Commensalism, , = 0 Amensalism, , , , 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, , 2020-21
