Population structures

Population structures

1. POPULATION GROWTH Dr. Anukriti Nigam Fergusson College (Auto.) Pune

2. What is a population? A group of organism of the same species living in the same habitat at the same time where they can freely interbreed Or It is a group of individuals of a particular species occupiying a particular area at a particular time. Or The population that occupies a very small area, is smaller in size, such a population is called local population . A group of such a closely related local population is called meta-population.

3. What is a Demography? It is the statistical study of populations. It is used to predict how the size of a population will change

4. Population Dynamics Three Key Features of Populations Size Density Dispersion

5. Describing a population 1.Size and Density Crude density-density per unit total space. Specific density/ecological/economic -density per unit habitat space. Graph

6. Important Indexes Used: 1. Crude density: It is the number (or biomass) per unit of total space. 2. Ecological density: Ecological density is the number (or biomass) per unit of habitat space (area or volume available that can be colonised by the population) 3. Relative abundance: It is used to denote changing (increasing or decreasing) population and is time relative. For example, the number of birds seen on a tree per hour. 4. Frequency of occurrence: Frequency of occurrence is the per­centage of sample plots occupied by a species. 5. Importance value: The importance value of each species are formed by combi­ning density, dominance and frequency during the descriptive studies of vegetation.

7. Thus local density provides us with: ( 1) Information about the interaction of a population with its environment (2) Changes in density reflect changing local conditions. The factors that regulate population size can be classified as extrinsic and intrinsic. The populations own response to density is said to be intrinsic, while the interaction with the rest of the community is said to be the extrinsic factor . Intrinsic factors include intraspecific competition, immigration, emigration and physiological and behavioural changes affecting reproduction and survival. Extrinsic factors are interspecific competi­tion, predation, parasitism and disease.

8. Methods for Estimating Population Densities: Mark-recapture method . This method involves capturing of a fraction of the population and marking with tags, paint, radio collars etc. and releasing them back into the population enough time is allowed for the marked individuals to recover and mingle with the rest of the population. A second sample is taken, after a certain time period, from the mingled population.

9. The ratio of the marked to unmarked is noted and the esti­mate of the population size can be calculated by the following equation: X = nM/N where x =the number of marked individu­als recaptured, N = the total number or size of the second sample, = M = the number of individuals marked initially (first sampling), and N is the total size of the population. If the above equation is rearranged, then we get N = nM/x

10. Thus, the estimate of the total population density is called Lincoln index. The validity of the above equation is based upon the factors listed: 1. The marking technique has no negative effect. 2. The marked individuals were released at the same site of capture and were allowed to mix with the population, based on their natural behaviour. 3.Marking technique does not affect the probability of being recaptured. 4. The markings should be clear and firmly fixed so that they are not lost or over­looked. 5. There should not be any significant natality or mortality during the time interval under study. 6. No significant immigration or emigration of marked or unmarked individuals during the time interval under study.

11. Problem solving Example: Supposing 100 deers were radio collared and was allowed to mix with the population. After a certain time period 38 deers came to a particular site for licking salts. Of these 38 deers, the numbers of radio collared animals were 8. Thus, the density of deer population is that area is estimated to be ??????????

12. Solution :- Supposing 100 deers (M) were radio collared and was allowed to mix with the population. After a certain time period 38 deers (n) came to a particular site for licking salts. Of these 38 deers, the numbers of radio collared animals were 8(x). Thus, the density of deer population is that area is estimated to be N = 38(100)/8 = 475 However, for bigger carnivore animals the estimate of population is generally done by the pug mark method eg tigers. The other methods generally used are minimum known alive (MKA) total counts, quadrate or transect sampling, removal sampling, plotless methods etc.

13. Concept of Growth rate Growth Rate –ΔN/ Δ t Where-ΔN= is the change in number of organisms. Δ t = per time

14. Sources, Sinks and Meta-populations: When there is abundant resources in habitats, more offsprings are produced by individuals than required to replace themselves. In such cases, the surplus offspring may disperse to other areas. Such populations are said to be source populations .

15. The reverse occurs in case of poor habitats . Here, few offsprings are produced locally , to replace loss due to mortality. Thus, to maintain the population , individuals immigrate from other habitats. These populations are said to be sink populations

16. There are populations that exist as a set of subpopulations referred to as meta-populations by Richard Levins (1970). These meta-populations are more or less isolated but there exist some exchange of individuals (and genes) by way of dispersal . These concepts of sources, sinks and meta-populations are important as they serve as a framework for studying many of our threatened and endangered species.

17. 2.Dispersion -Dispersion is the spatial pattern of individuals in a population relative to one another . In nature, due to various biotic interactions and influence of abiotic factors, the following four basic population distributions can be observed:- Regular Random Clumped Regular clumped

18. Regular dispersion - Here the individuals are more or less spaced at equal distance from one another. This is rare in nature but in common is cropland. Examples: monoculture crops, orchard or pine plantation, desert shrubs etc (b) Random dispersion: Here the position of one individual is unrelated to the positions of its neighbours. This is also relatively rare in nature. Examples: Lone parasites or preda tors show a random distribution as they are often engaged in random searching behaviour for their host or prey.

19. (c) Clumped dispersion: Most populations exhibit this dispersion ,individuals aggregated into patches interspersed with no or few individuals. Such aggregations may result from social aggregations, such as family groups or may be due to certain patches of the environment being more favourable for the population concerned. Examples: Salamanders prefer to live in clumps under logs. Birds travel in large flocks. Trees form clumps of individuals through vegetative reproduction. D) In regular clumped distribution: I ndividuals are clumped and are spaced out evenly from other similar clumps. Examples: Herds of animals or vegeta­tive clones in plants show either random or are clumped in a regular pattern.

20. Population Dispersion

21. To determine the type of spacing and the degree of clumping, several methods have been suggested of which two are men­tioned: 1. To compare the actual frequency of occurrence of different sized groups obtained in a series of samples. If the occurrence of small sized and large sized groups is more frequent and the occurrence of mid-sized groups less frequent than expected, then the distribution is clumped. The reverse is seen in uniform distribution. 2. The distance between individuals are measured and the square root of the distance is plotted against frequency. The shape of the resulting polygon indicates the pattern of distribution. A symmetrical polygon (bell-shaped) indicates random distribution, a slanted polygon to the right indicates a uniform distribution, and one slanted to the left indicates a clumped distri­bution.

22. The Allee principle: Aggregation will subsequently increase competition . This often is counter-balanced by the increased survival of the group due to its ability to defend itself, or to find resource, or to modify microhabitat conditions. Thus, both under-crowding (lack of aggregation) and over-crowding may be limiting. This view was put forward by W. C. Allee, a Quaker and V. E. Shelford, and was termed as the Allee effect or Allee principle of aggregation. Allee effect stresses that any optimal function (faster body growth, increased reproduction, or longer life) takes place at an intermediate rather than at minimal density. For instance, at low density, a drop in reproductive rate takes place as some females may go unmated because they were not found by males or because of an unbalanced sex ratio.

23. 3 . Age structure- In most types of populations, individuals are of different age. The proportion of individuals in each age group is called age structure of that population . The ratio of the various age groups in a population determines the current reproductive status of the popu­lation, thus anticipating its future. From an ecological view point there are three major ecological ages in any population. These are, pre-reproductive, reproductive and post reproductive. The relative duration of these age groups in proportion to the life span varies greatly with different organisms.

24. Age pyramids Young population (Triangular)-It indicates a high percentage of young individuals. In rapidly growing young populations birth rate is high and population growth may be exponential as in yeasty house fly, Paramecium, etc Stable population (Bell)- It indicates a stationary population having an equal number of young and middle aged individuals. Declining population (Urn)- It indicates a low percentage of young individuals and shows a declining population

25. Population Growth: The size of a population for any species is not a static parameter, it keeps changing with time. It depends on the following factors: (i) Food availability (ii) Predation pressure (iii) Weather

26. 4. Natality It is simply a broader term covering the production of new individuals by birth, hatching, by fission, etc. The natality rate may be expressed as the number of organisms born per female per unit time. In human population, the natality rate is equivalent to the birth-rate. Increases population size Each species will have its own maximum birth rate Maximum birth rates are seen when conditions are ideal This can lead to exponential growth MAXIMUM NATALITY/FECUNDITY ECOLOGICAL NATILITY/FERTILITY RATE

27. (a) Maximum natality: Also called as absolute or potential or physiological natality , it is the theoretical maximum production of new individuals under ideal conditions. It is a constant for a given population. This is also called fecundity rate. (b) Ecological natality: Also called realized natality or simply natality , it is the population increase under an actual, existing specific condition. Thus it takes into account all possible existing environmental conditions. This is also designated as fertility rate.

28. Natality is expressed as ∆N n /∆ t = Absolute Natality rate (B) ∆N n /N ∆ t = Specific natality rate (b) (i.e., natality rate per unit of population). Where N = initial number of organisms. n = new individuals in the population. t = time.

29. 5. Mortality Mortality reduces population growth It operates more when conditions are not ideal Overcrowding leading to competition, spread of infectious disease TYPES MINIMUM /SPECIFIC/POTENTIAL MORTALITY ECOLOGICAL/REALISED MORTALITY

30. (a) Minimum mortality: Also called specific or potential mortality, it represents the theo­retical minimum loss under ideal or non-limiting conditions. It is a constant for a population. (b) Ecological or realised mortality: It is the actual loss of individuals under a given environmental condition. Ecological mortality is not constant for a population and varies with population and environmental conditions, such as predation, disease and other ecological hazards.

31. Vital index and survivorship curves A birth-death ratio (100 x births/deaths) is called vital index. For a population, the surviving individuals are more significant for a population than the dead ones. The survival rates are generally expressed by survivorship curves. A survivorship curve is a graph of the survival rate of a group of organisms. In making a survivorship graph, a cohort is used; in this case, a group of individuals who were born at roughly the same time. This cohort is then graphed with the number of surviving individuals on the y-axis and the age of the cohort on the x-axis. Generally, a species will have one of three types of survivorship curves.

32. Survivorship Survivorship is the percentage of newborn individuals in a population that can be expected to survive to a given age. It is used as another way to predict population trends. To predict survivorship, demographers study a group of people born at the same time and notes when each member of the group dies .

33. Survivorship Wealthy developed countries such as Japan and Germany currently have a Type I survivorship curve because most people live to be very old. Type II populations have a similar death rate at all ages. Type III survivorship is the pattern in very poor human populations in which many children die. Both Type I and Type III may result in populations that remain the same size or grow slowly. The results of these studies are then plotted on a graph and might look like one of the types of survivorship graphs

34. Age, years
(x) Probability of 
surviving to age x 
(lx) No. of female offspring 
born to a mother of age x 
(mx) 1.000 0.000 1 0.845 0.045 2 0.824 0.391 3 0.795 0.472 4 0.755 0.484 5 0.699 0.546 6 0.626 0.543 7 0.532 0.502 8 0.418 0.468 9 0.289 0.459 10 0.162 0.433 11 0.060 0.421 6 . LIFE TABLES- Information on mortality , natality in different ages and sexes can be combined to form life table. Example-Consider a sheep population which is censused once a year immediately after breeding season

35. TYPES OF LIFE TABLES:- Cohort or age-specific or dynamic life tables: - data are collected by following a cohort throughout its life. This is rarely possible with natural populations of animals. Note: a cohort is a group of individuals all born during the same time interval. Static or time-specific life tables:- age-distribution data are collected from a cross-section of the population at one particular time or during a short segment of time, such as through mortality data. Composite :- data are gathered over a number of years and generations using cohort or time-specific techniques.

36. Factors that affect density 1. Immigration- movement of individuals into a population 2. Emigration- movement of individuals out of a population

37. The density of a population in a given habitat during a given period, fluctuates due to the four basic processes: (a) Natality refers to the number of births during a given period in the population that are added to initial density. (b) Mortality is the number of deaths in the population during a given period. (c) Immigration is the number of individuals of the same species that have come into the habitat from elsewhere during the time period under consideration. (d) Emigration is the number of individuals of population who left the habitat and moved elsewhere during a given period of time.

38. Out of these four, natality and immigration contribute an increase in population density while mortality and emigration contribute to the decrease in population density. So, if N is the population density at time t, then its density at time t +1 is N t+1 = N t + [(B + I) – (D + E)] Where, N = Population density t = Time, B = Birth rate, I = Immigration, D = Death rate, E = Emigration From the above equations, we can see that population density will increase if, (B + I) is more than (D + E).

39. Immigration Emigration Natality Mortality Population + + - - Factors That Affect Future Population Growth

40. KEY FEATURES OF POPULATIONS, con’t Population size is limited by: density-dependent factors Disease Competition Predators Parasites Food Crowding The greater the population, the greater effect these factors have. Ex. Black plague in the Middle Ages – more deaths in cities density-independent factors Volcanic eruptions Temperature Storms Floods Drought Chemical pesticides Major habitat disruption (as in the New Orleans flooding) Most are abiotic factor s

41. Other factors that affect population growth Limiting factor- any biotic or abiotic factor that restricts the existence of organisms in a specific environment. EX.- Amount of water Amount of food Temperature

42. Phases of population growth Phase 1: Log or exponential phase Unlimited population growth The intrinsic rate of increase ( r ) Abundant food, no disease, no predators etc Phase 2: Decline or transitional phase Limiting factors slowing population growth

43. Phase 3 Plateau or stationary phase No growth The limiting factors balance the population’s capacity to increase The population reaches the Carrying Capacity ( K ) of the environment Added limiting factors will lower K Removing a limiting factor will raise K

44. Factors affecting the carrying capacity Food supply Infectious disease/parasites Competition Predation Nesting sites

45. Many organisms present Few organisms present Few organisms present None None Limiting Factor- Zone of Tolerance

46. Carrying Capacity- the maximum population size that can be supported by the available resources There can only be as many organisms as the environmental resources can support Other factors that affect population growth

47. Carrying Capacity Carrying Capacity (k) Nu m b e r Time J-shaped curve (exponential growth) S-shaped curve (logistic growth)

48. PREDICTING POPULATION GROWTH Model: A hypothetical population that has key characteristics of the real population being studied. Used by demographers to predict how a population will grow.

49. PREDICTING POPULATION GROWTH, con’t Nearly all populations will tend to grow exponentially as long as there are resources available. Two of the most basic factors that affect the rate of population growth are the birth rate, and the death rate. r(rate of growth)=birth rate – death rate

50. Growth Models Studying about the behaviour and pattern of different animals can help us to learn a lesson on how to control the human population growth. There are following two models of population growth: Exponential Growth: Logistic Growth:

51. Availability of resources (food and space) is essential for the growth of population. The unlimited availability results in population exponential. The increase or decrease in population density (N) at a unit time period (t) is calculated as (dN/dt) Let dN/dt = (b – d)  N Let (b-d) = r, then, dN/dt = rN Where, N is population size, b is birth per capita d is death per capita, t is time period r is intrinsic rate of natural increase. r, is an important parameter that assess the effects of biotic and abiotic factors on population growth. It is different for different organisms.

52. Figure 35.3A Exponential Growth Curve

53. Logistic Growth: Practically, no population of any species in nature has unlimited resources at its disposal. This leads to competition among the individuals and the survival of the fittest. Therefore, a given habitat has enough resources to support a maximum possible number, beyond which no further growth is possible. This is called the carrying capacity (K) for that species in that habitat.

54. When N is plotted in relation to time t, the logistic growth show sigmoid curve and is also called Verhulst-Pearl Logistic Growth and is calculated as dN/dt = rN (K – N/K) Where, N is population density at time t K is carrying capacity r is intrinsic rate of natural increase. This model is more realistic in nature because no population growth can sustain exponential growth indefinitely as there will be completion for the basic needs. Human population growth curve will become S-shaped, if efforts are being made throughout the world to reduce the rate of population growth and make it stationary.

55. Thanks.