Population Dynamics

1. Population Dynamics

2. Characteristics of Populations • Population ecologyis the study of populations in relation to the environment, including environmental influences on population density and distribution, age structure, and population size. • A population is a group of individuals of a single species that live in the same general area. • Members of a population rely on the same resources, are influenced by similar environmental factors, and have a high likelihood of interacting with and breeding with one another.

3. Population Dynamics: change in size, density dispersion and age distribution in response to environmental conditions. • Population Size: the number of individuals in a population at a given time.

4. Ways of Examining Population 1) density of a population is measured as the number of individuals per unit area or volume. Ex: # of trees per acre # of deer per square mile # of Daphnia per liter of water

5. Difficulties in Measuring Density - counting all of the individuals – nearly impossible - population counts are estimates based on sampling techniques Ex: For Plants and Sessile Organisms: In a forest count the # of oak trees in many different areas Average the number and multiply by the area of the habitat

6. - only accurate if the sample sizes are the sameand the area being surveyed is homogenous which may not be the case due to possible variations in habitat that may make it inhospitable and different social interactions

7. For Animals: Mark and Recapture - a certain area is used and a sample of the population is caught and marked – tags, electronic devices, paint – and released - later the same areais used and more organisms are caught and examined for markings - based on the number of organisms that have marks and don’t have marks the actual population is calculated

8. - The mark-recapture method assumes that each marked individual has the same probability of being trapped as each unmarked individual. - This may not be a safe assumption, as trapped individuals may be more or less likely to be trapped a second time.

9. - Also affected by the changes in the population - birth (including all forms of reproduction) - immigration (the influx of new individuals from other areas). - death (mortality) - emigration (the movement of individuals out of a population). - Immigration and emigration may represent biologically significant exchanges between populations

10. 2) dispersion of a population is the pattern of spacing among individuals within the geographic boundaries. Dispersion is clumped when individuals aggregate in patches. Plants and fungi are often clumped where soil conditions favor germination and growth. Animals may clump in favorable microenvironments (such as isopods under a fallen log) or to facilitate mating interactions. Group living may increase the effectivenessof certain predators, such as a wolf pack.

11. Dispersion is uniform when individuals are evenly spaced. For example, some plants secrete chemicals that inhibit the germination and growth of nearby competitors. Animals often exhibit uniform dispersion as a result of territoriality, the defense of a bounded space against encroachment by others.

12. In random dispersion, the position of each individual is independent of the others, and spacing is unpredictable. Random dispersion occurs in the absence of strong attraction or repulsion among individuals in a population, or when key physical or chemical factors are relatively homogeneously distributed. For example, plants may grow where windblown seeds land. Random patterns are not commonin nature

13. Dispersion Patterns

14. Demography is the study of the vital statisticsof populations and how they change over time. Factors Influencing Demography: 1. Birth rates/Death Rates 2. Fecundity (fertility) of Organisms 3. Rate of Maturation to Reproductive Capacity 4. Life Span 5. Reproductive Strategies

15. Often represented by a life table- an age-specificsummary of the survival pattern of a population usually by studying a group of individuals (cohort) of the same age as they proceed through lifenoting when they die and calculating the survival ratio of the group

17. Tables can also be represented as a Survivorship Curve– Shows the overall pattern of surviving members of a population throughout the life of the group of individuals.

18. Types of Survivorship Curves • Type I curveis relatively flat at the start, reflecting a low deathrate in early and middle life, and drops steeply as death rates increase among older age groups. • Ex: humans, elephants • Indicates a high level of parental investmentin the raising of young. (Iteroparity)

19. Types of Survivorship Curves • Type II curveis intermediate, with constant mortalityover an organism’s life span. • Ex: rodents, insects, annual plants • equal chance of dying at all times of life

20. Types of Survivorship Curves • Type III curvedrops quickly at the start, reflecting very high death rates early in life, then flattens out as death rates decline for the few individuals that survive to a critical age. • Reflect the production of large numbers of offspring with little or no parental care. (Semelparity) • Ex: long lived plants, sea turtles, marine invertebrates, most fish.

21. Types of Survivorship Curves • Many species fall somewhere between these basic types of survivorship curves or show more complex curves. • Some invertebrates, such as crabs, show a “stair-stepped” curve, with increased mortality during molts.

22. POPULATION GROWTH Population Growth • Change in population size = Births - Deaths - for a certain time period Or • N/t = (B − D)where B is the number of birthsand D is the number of deaths. • N represents population size, and t represents time, then N is the change in population size and t is the time interval.

23. Ignores immigration and emigration • The per capita birth rateis the number of offspring produced per unit time by an average member of the population. • If there are 34 births per year in a population of 1,000 individuals, the annual per capita birth rate is 34/1000, or 0.034 or 3.4%

24. ESTIMATING DOUBLING TIME: Rule of 70 • - once a growth rate has been determined the estimated time for the population to doublecan be determined by dividing 70 by the rate as a percentage. • Ex: 70/3.4% = 20.6

25. If we know the annual per capita birth rate (expressed as b), we can use the formula B = bNto calculate the expected number of births per year in a population of any size. • Similarly, the per capita death rate(symbolized by m for mortality) allows us to calculate the expected number of deaths per unit time for a population of any size. D = mN

26. Population ecologists are most interested in the differences between the per capita birth rate and the per capita death rate. • This difference is the per capita rate of increaseor rwhich equals b − m. r = b - m

27. The value of rindicates whether a population is growing (r > 0)or declining (r < 0). • If r = 0, then there is zero population growth (ZPG). • Births and deaths still occur, but they balance exactly.

28. Population Growth Models • 1) Exponential: population growth in an idealized, unlimited environment. • All populations have a tremendous capacity for growth. • A hypothetical population living in an ideal, unlimitedenvironment. • Under these conditions, we may assume the maximum growth ratefor the population (rmax), called the intrinsic rate of increase.

29. The size of a population that is growing exponentially increases at a constant rate, resulting in a J-shaped growth curvewhen the population size is plotted over time. • J-shaped curves are characteristic of populations that are introduced into a new or unfilled environment or whose numbers have been drastically reducedby a catastrophic event and are rebounding.

31. Exponential Growth • dN/dt = change in number over time • r = growth rate as a decimal • N = population size • EX: How many individuals will be added to a population of 500 if the growth rate is 25% per year? • EX: If 100 individuals are added to a population in a year, what is the rate if the original population was 400? • EX: If a population has a growth rate of 15%, what is the original size of the population if 150 new individuals are added to the population?

32. 2) The logistic modelof population growth incorporates the concept of carrying capacity. • Typically, resources are limited. • As population density increases, each individual has access to an increasingly smaller share of available resources. • Ultimately, there is a limit to the number of individuals that can occupy a habitat. • Ecologists define carrying capacity (K)as the maximum stable population size that a particular environment can support

33. FACTORS DETERMINING CARRYING CAPACITY Environmental Resistance– all the factors acting jointly to limit the growth of a population 1) Energy Limitation 2) Shelter and Breeding Sites 3) Predators 4) Soil Nutrients 5) Water

35. If individuals cannot obtain sufficient resources to reproduce, the per capita birth rate bwill decline. • If they cannot find and consume enough energy to maintain themselves, the per capita death rate m may increase. • the per capita rate of increase declines as carrying capacity is reached

36. Population growth is greatest when the population is approximately half of the carrying capacity. • At this population size, there are many reproducing individuals but still enough resources, and the per capita rate of increase remains relatively high.

37. The logistic model of population growth produces a sigmoid (S-shaped) growth curve when N is plotted over time. • Population growth rate slows dramatically as N approaches K. • In most natural populations, there is a lag timebefore the negative effects of increasing population are realized. • Populations may overshoot their carrying capacity before settling down to a relatively stable density.

40. Logistic Growth dN/dt = rmaxN ((K-N)/K) dN/dt = growth of the population (number of new individuals added) rmax = growth rate as a decimal K = carrying capacity population N = number of individuals in the population If N < K then r > 0, if N = K then r = 0, and if N > K then r < 0

41. Ex: If a population of 200 is at half the carrying capacity and growing at a rate of 20 individuals per year, what is the growth rate of the population?

42. If a population of 500 is at its carrying capacity, what is the growth rate?

43. A population exhibits logistic growth. If the carrying capacity is 200 for a given area. If the growth rate is 2% at the carrying capacity, what is the maximum growth rate? (Hint: the population at the maximum growth rate is half the population at carrying capacity or N at rmax = K/2)

44. How Does Population Density Affect Population Growth? • Density-independent population controls affect a population size regardless of its density. • Ex: floods, hurricanes, earthquakes, landslides, drought, fire, habitat destruction, pesticide spraying

45. Density-dependent population controlshave a greater effect on a population as the population size increases • Ex: competition for resources (Intraspecific or Interspecific), territory. Health, predation, parasitism, disease

46. Dense populations have lower birth rates, higher death rates. • Ex: In mice, overcrowding causes hormonal changes that inhibit sexual activity, lower sexual activity, reduced milk production. Stress from overcrowding reduces the number of offspring produced (spontaneous abortion). May also lead to cannibalism and killing of the young.

47. Population Change Curves (Population Cycles) • Stable – population fluctuates slightly above and below its carrying capacity • Irruptive • population occasionally explodes (irrupts) to a high peakand then crashes to a very low level. • caused by some factor that temporarily increases the carrying capacityEx: increase in rain = increased growth of seeds  increase in mice

48. Cyclic – boom-bust cycles; poorly understood and involve a number of factors.

49. Factors Influencing Population Growth Life Histories- the traits that affect an organism’s schedule of reproduction and survival Factors Affecting Life Histories 1) when reproduction begins 2) how often the organism reproduces 3) the number of offspring are produced during each reproductive episode.

50. Types of Life Histories: 1) Semelparity: big-bang reproduction- an individual produces a large numberof offspring and then dies. 2) Iteroparity: Repeatedreproduction - some organisms produce only a few offspring