Population ecology

1. Population ecology

2. Population Ecology Population ecology is the study of populations in relation to the environment. It includes environmental influences on population density and distribution, age structure, and variations in population size.

3. Characteristics of Population Population size Population density Dispersion patterns Demographics Survivorship curves Population growth

4. Population size “ In population genetics and population ecology , population size (usually denoted N) is the number of individual organisms in a population ” . Factors that Govern Population Size Crude Birth Rate (CBR) Crude Death Rate (CDR) Immigration Emigration

5. Natality The birthrate, which is the ratio of total live births to total population in a particular area over a specified period of time Mortality The death rate, which is also the ratio of the total number of deaths to the total population. Immigration The number of organisms moving into area occupied by the population is called immigration. Emigration The number of organisms moving out of the area occupied by the population is called emigration.

6. Factors that increase population size Natality is recruitment to a population through reproduction. Immigration from external populations e.g. Bird migration. Factor reducing population size Mortality which is the death rate from any source e.g. predation. Emigration, where individuals leave the population for another habitat.

7. Population Change = (births + immigration) – (deaths + emigration) Parameters that effect size or density of a population

8. Population Density “Population density is a measurement of the number of people in an area. It is an average number ” . It is usually shown as the number of people per square kilometer. Density = Population/ Area

9. need to estimate population density How to quantify nature – ecologist role Estimates are allows for comparisons between different populations in terms of space and time measure.

10. 2 attributes Mobility Based on movements of these organisms Size Small animals/plants are usually more abundant than large animals/plants

11. Measurements of density Density is measured by two methods. Total count method Sampling method

12. Total Count Method Direct counting of populations. Possible for few animals. Breeding colonies can be photographed then later counted. Examples Human pop census Trees in a given area

13. Sampling Methods Depends on the type of organism and its natural abundance and distribution. Two broad categories: Plot-based (quadrant) methods Capture-based methods

14. Quadrat Sampling Method Widely used in plant studies Total population = Average number per quadrat × Total area/Area of quadrat

15.   Mark-Recapture Method Used for very mobile or elusive species First used in the 1890s by C. G. Peterson to estimate fish abundance.

16. Population Dispersion The dispersion of a population is the pattern of spacing among individuals within the geographic boundaries. 3 types uniform clumped random

17. Clumped Dispersion Clumped is a pattern when individuals are aggregated in patches. Most frequent pattern of distribution in a population Reasons: Some area of habitat are more suitable than others Heterogeneous environment with resources concentrated in patches Tendency of offspring to remain with parents Mating or social behavior of the individuals

18. Uniform Dispersion Evenly spaced distributions, in which members of the population maintain a minimum distance from one another. In plants due to competition for water, sunlight, or available nutrients Example Creosote bushes in the Mojave desert In animals due to strong territoriality Example The desert lizard Uta sp

19. Random Dispersion It is a spacing pattern based on total unpredictability. Least common pattern of distribution Reasons Members of a species do not frequently interact with one another Not heavily influenced by the microenvironments within their habitat

20. Factors Affecting Distribution Density Independent Floods Hurricanes Unseasonable Weather Fire Clear Cutting Pesticide Spraying Density Dependant Competition for Resources Predation Parasitism Infectious Disease

21. Demography Demography is the study of the vital statistics of a population and how they change over time Two statistics that are of particular import are a population's age structure and a population's sex ratio.

22. Sex Ratio It is the ratio of males to females in a population. Primary sex ratio Secondary sex ratio Tertiary sex ratio The human sex ratio is of particular interest to anthropologists and demographers.

23. Age structure The age structure describes the number of individuals in each age class as a ratio of one class to another. Age classes can be specific categories, such as people in the same age range.

24. population pyramid Age structure diagram Graphical illustration that shows the distribution of various age groups & sex ratio in a population. Three age categories: Prereproductive (ages 0-14) Reproductive (ages 15-44) Postreproductive (ages 45 and up)

25. Life table A life table ( mortality table ) is a table which shows, for each age, what the probability is that a person of that age will die before his or her next birthday. From this starting point, a number of inferences can be derived. The probability of surviving any particular year of age Remaining life expectancy for people at different ages Separately for men and for women because of their substantially different mortality rates.

26. Survivorship curve Graph showing the number or proportion of individuals surviving at each age for a given species or group (e.g. males/females). Constructed for a given cohort (a group of individuals of roughly the same age) based on a life table. Three types Type I Type II Type III

27. Type I survivorship curves are characterized by high survival in early and middle life, followed a rapid decline in survivorship in later life. Example: Humans Type II curves are an intermediate between Type I and III, where roughly constant mortality rate is experienced regardless of age. Example: Some birds In Type III curves, the greatest mortality is experienced early on in life, with relatively low rates of death for those surviving this bottleneck. Example: Octopus

28. Population Growth Refers to how the number of individuals in a population increases (or decreases) with time. Controlled by the rate at which new individuals are added to the population -- the birth rate, and the rate at which individuals leave the population -- the death rate.

29. 2 types of pop growth Exponential population growth dN = r max N dt Logistic population growth dN = r max N (K-N) dt K Population Growth Mathematically Defined

30. N=K/2

31. Exponential Growth If a population has a constant birth rate through time and is never limited by food or disease, it has what is known as exponential growth. With exponential growth the birth rate alone controls how fast (or slow) the population grows.

32. Exponential Growth Continuous population growth in an unlimited environment can be modeled exponentially. dN / dt = r max N As population size (N) increases, rate of population increase ( dN / dt ) gets larger.

33. Exponential Growth For an exponentially growing population, size at any time can be calculated as: N t = N o e rt N t = number individuals at time t. N = initial number of individuals. e = base of natural logarithms. r ( r max ) = per capita rate of increase. t = number of time intervals.

34. Logistic Population Growth As resources are depleted, population growth rate slows and eventually stops: logistic population growth. Sigmoid (S-shaped) curve Carrying capacity (K) is the number of individuals of a population the environment can support. Finite amount of resources can only support a finite number of individuals .

35. Logistic Population Growth dN / dt = r max N (1-N/K) r max = Maximum per capita rate of increase under ideal conditions. When N nears K, the right side of the equation nears zero. As population size increases, logistic growth rate becomes a small fraction of growth rate . Highest when N=K/2

36. Logistic Growth Logistic growth curve Environmental limits result in logistic growth Carrying capacity New or changed environment No limits

37. Smooth Transition

38. Limits to Population Growth Environment limits population growth by altering birth and death rates. Density-dependent factors Disease, Parasites, Resource Competition Populations do not show continuous geometric increase When density increases other organisms reduces the fertility and longevity of the individuals in the population This reduces the rate of increase of the pop until eventually the pop ceases to grow The growth curve is defined as the sigmoid curve, S – shaped K = carrying capacity (upper asymptote or maximum value) – the maximum number of individuals that environment can support Density-independent factors Natural disasters Climat e

39. Life history strategies K and r selection (MacArthur and Wilson 1967) r-selected species r refers to the per capita rate of increase Selection favoring rapid growth Should be favored in new or disturbed environments Less competition K-selected species K refers to carrying capacity More prominent in species that are typically at their carrying capacity Favors more efficient use of resources Live with competition

40. r versus K Selection

41. Human Growth

42. The history of human population growth Figure 35.8A