Population Ecology 1

1. Population Ecology 1

2. Intraspecific Population Ecology: • Properties of Populations • A population is a group of organisms of the same species occupying a particular space at the same time. • Populations are unitary(individuals are easily defined, such as in most animals and many plants) or modular(the individual is difficult to define and may consist of many smaller units, e.g. genetically identical clumps of trees arising from root suckers, typical of many plant species, or colonial organisms).

3. Intraspecific Population Ecology: • Properties of Populations • Populations are both genetic and ecological units in which species’ members interact with one another. • In fact, a population’s characteristics are very significantly influenced by positive and negative interactions of its members among one another • and with members of other species

4. A metapopulation is a spatially separated, disjunct population • which is distributed in patches across a heterogeneous landscape • and interconnected by immigration

5. Metapopulations exist along a continuum from: • - the mainland-island population in which “island” populations are maintained by continuous immigration from large, extinction-resistant mainland populations • (e.g. the GMU “island” population is maintained by immigration from the large VA “mainland” population) • to -totally isolated populations that rarely receive immigrants and are doomed to eventual extinction. • “island” populations that go extinct could theoretically be re-colonized

6. Different types of metapopulations. Filled circles = occupied habitats; empty circles = vacant habitat patches; dotted lines = boundaries of local populations; arrows = dispersal. • Classic (Levins) • Mainland-island • Patchy population • Nonequilibrium metapopulation • Intermediate case combing features of a-d.

7. The gene pool is the sum of all genetic information carried by all individuals of an interbreeding population • – populations can be considered evolutionary units • (selective pressures act on and change the gene pool of a population)

8. Properties of populations include density, dispersion, age structure, sex ratios, and mortality and natality. • Density • Density-the number of organisms occupying a defined unit of space (e.g., number per acre). • Crude density is the number of individuals per unit area • Ecological density is measured in terms of area of available living space (actual useable habitat). BUT - ecological density is rarely estimated because it is difficult to determine what portion of a habitat represents living space (especially if organisms require different habitats during the year or during developmental stages)

9. Patterns of Dispersion • Density tells us nothing about how evenly individuals within the population are distributed over space and time. • Populations occur in one of three patterns of spatial dispersion • these are strongly influenced by the pattern of the landscape and interactions among members of the population:

10. Patterns of Dispersion • Random - the population is considered random if the position of each individual is independent of the others • or if the occupation of each spot is equally likely • (e.g. clams on mud flats) • Random distributions are rare, for they can occur only where: • - the environment is uniform, • - resources are equally available throughout the year, and • - interaction among members of the population produces no patterns of attraction or avoidance.

11. Uniform or regular - the population is considered to have a uniform or regular distribution if the spacing of individuals is more even than would occur by chance. • Regular patterns of distribution result from intraspecific competition among members of a population e.g. territoriality

12. Clumped, clustered, contagious, or aggregated - this pattern of dispersion is the most common and occurs as a result of responses by organisms to: • habitat differences, • daily and seasonal weather and environmental changes, • reproductive patterns, and social behavior. • Reasons for clumped distributions include: • poor dispersal capability of the young, • patchy distribution of resources, or • higher social organization.

13. Patterns of distribution

14. These dispersal patterns often have a temporal aspect related to daily changes in light and dark (circadian movements) or triggered by changes in: • humidity, • temperature, • seasons, • lunar cycles, and • tidal cycles.

15. Natal dispersal is the movement of the young away from the parents to occupy new habitat. • Breeding dispersal is the movement of adults from poor to better reproductive sites.

16. Murray’s rule of dispersal - move to the first uncontested site you find and no further. Animals will generally either disperse in a straight line from their natal area or make exploratory forays into surrounding areas before leaving the natal site. They should settle in the first empty site.

17. Observed and expected dispersal distances in (a) the deer mouse and (b) the white-crowned sparrow. The bar graphs indicate the observed dispersal distances. The reddish line indicates the dispersal distance expected if the search were along a radius away (straight line) from the natal site. The yellow line in (b) is the expected pattern if the search were to the nearest empty site (some prior exploration)

18. Migration - a two-way movement in response to an evolutionary or environmental adaptation or pressure. Repeated return trips may be daily or seasonal. Some migrations involve only one return trip, such as in Pacific salmon species.

19. Age Structure • Age structure (age distribution) - the ratio of one age class to another. • When deaths = births and the population is closed (no immigration or emigration), the population has reached a constant size, known as a stationary age distribution. • A stable age distribution (the ratio of each age group remains the same) is dependent on the existence of constant age-specific birth and death rates (an unlikely occurrence).

20. Age structure distributions sometimes are used as one tool in wildlife management for predicting future changes in a population. However, its use in this way is dependent on mortality and natality (birth) schedules remaining the same over extended periods of time (also not a likely occurrence).

21. Age structure determines in part population reproductive rates, death rates, vigor, survival, and other demographic attributes. Categories may be used rather than ages, such as life-history stages : • - prereproductive • - reproductive and • - postreproductive in birds and mammals • or eggs, pupae, larvae, and instars in insects • or size classes and diameters in trees -because reproduction is better predicted by size than by age in plants.

22. A relatively stable population of most mammals and birds has a ratio of young to adults of approximately 2:1 A normally increasing population should have an increasing proportion of young While a decreasing population will have a decreasing proportion of young.

23. Theoretical age pyramids, especially applicable to mammals and • birds. • and (b) growing populations with large number of young; • (c) aging population with high proportion of older age classes; and • (d) population with declining ratio of young to adults

24. Sex Ratio • Sex ratio - the proportion of males to females, which tends toward 1:1 in most sexually reproducing organisms. The primary sex ratio (the ratio at conception) also tends to be 1:1, but is difficult to determine. • The secondary sex ratio (the ratio at birth) among mammals is often weighted toward males but as age class increases there is a ratio shift towards females The opposite is true among birds

25. Mortality and Natality • Birth rate (natality) - crude birth rate is the number of births expressed in terms of population size (e.g., number/1000). • Specific birth rate a more accurate measure, is expressed relative to a specific criterion, such as age. • Age-specific schedule of births is the number of offspring produced per unit time by females in different age classes.

26. Related statistics are: physiological natality (the maximum possible number of births under ideal conditions, or the biological limit per individual); realized natality (the amount of successful reproduction that actually occurs over a period of time); fecundity (the number of offspring per individual by age class); and reproductive value (the average contribution to the next generation that members of a given age group make between their current age and death).

27. Death rate (mortality)- the number of deaths during a given time divided by the average population during that time. • The crude death rate is the number of deaths per 1000 in a given period of time.

28. Other related statistics are: • the probability of dying (the number that died during a given time interval divided by the number alive at the beginning of the period); • probability of surviving (number of survivors divided by the number alive at the beginning of the time period or 1 – the probability of dying); and • life expectancy (the average number of years to be lived in the future by members of a given age in the population).

29. Immigration rate - a dispersal movement of individuals into another habitat without return to the place of origin. • Emigration rate - Movement one-way out of one habitat. Emigrants from one area become immigrants to another. • Growth rate - the difference between gains (births and immigration) and losses (death and emigration) in the population for a given unit of time.

30. A life table consists of a series of columns, each of which describes an aspect of mortality statistics for members of a population according to age. Figures are presented in terms of a standard number of individuals (by convention, the initial number of individuals is set at 1000) all born at the same time, called a cohort Columns include: the unit of age or age level; the number of individuals in a cohort that survive to that particular age level; the number of a cohort that die in an age interval (from one age level listed to the next); and the probability of dying (age-specific mortalityrate), Columns for survival rate (1 – probability of dying) and life expectancy may also be added.

31. The life table X = Age or age level Nx = Number surviving to age X (number of individuals in the cohort) Ix = Survivorship dx = Number dying during the interval qx = the probability of dying or the age specific mortality rate (number of individuals that died during the age interval / the number alive at the beginning of the interval) ex = life expectancy (Number of time units left for all individuals to live from age x onward [Tx] for the specific age class x / survivors for that age [lx]

32. Three basic types of survivorship curves. The vertical axis may be scaled arithmetically or logarithmically. If logarithmic, the slope of the lines will show the following rates of change: Type I-curve for populations in which the survival of juveniles is high and mortality is concentrated among old individuals. Type II-curve for populations in which the rate of mortality is fairly constant for all age levels. Type III-curve for a population in which mortality is concentrated among juveniles.

33. Mortality curves Mortality curve = a plot of mortality rate (qx) from the life table vs. age Mortality curve for the red deer population on the Isle of Rum (1957) Note the break in the J-shaped curve caused by a sharp rise in mortality for deer between the sixth and tenth years.

34. Animal natality Natality = the production of new individuals in a population Fecundity = rate of production of young by a female Fecundity curve for the red deer

35. Intraspecific Population Ecology: • Population Growth • Information from a life table provides the basis for constructing a population projection table, an age distribution table, and r, which represents the intrinsic rate of natural population increase. • Three indices serve as indicators of the direction of population growth:

36. R0 the net reproductive rate (the number of female offspring left during a lifetime by a newborn female), is obtained by • multiplying the age-specific survivorship • by age-specific fecundity and • summing them for the entire lifetime. • It provides a first indication of population growth from the point of view of whether or not an individual (usually a female) is replacing itself through reproduction during its life.

37. the finite multiplication rate, is found by dividing the total number of individuals in one year (x+1) by the total number of individuals in the previous year (x). It represents a ratio of population size between successive time periods. A ratio of 1 shows no population growth (stable population), while values greater than 1 are indicative of growing populations and less than 1 of declining populations. It can be used to project, but not predict population growth.

38. rthe per capita or intrinsic rate of increase, is the per capita rate of growth of a population that has reached a stable age distribution and is free from competition and other growth restraints It is the difference between the instantaneous birth rate and instantaneous death rate; in a closed population with no immigration or emigration, it is the intrinsic rate of increase. When r > 0, a population is growing, and if r < 0 the population is declining.

39. Population growth models • Exponential growth model - If a population were suddenly presented with an unlimited environment, it would tend to expand exponentially: • Nt = N0ert, where Nt is the number of individuals at time t, N0 is the number of individuals at time 0, e is the base of natural logarithms (2.71828…), r is the per capita rate of increase, and t is the unit of time. • The exponential growth model is applicable only to initial growth after colonization of an unexploited habitat so it is a transient phenomenon. • Exponential growth is not biologically realistic or sustainable.

40. Trajectories of exponential population growth calculated from a starting point of 100 individuals for several values of r

41. Logistic growth model - The environment is not constant and environmental resources are limited. • As population densityincreases, competition for available resources among its members increases (and disease etc). • The effects of increasing competition on the population begin to slow the rate of growth until • growth rate becomes zero and the population is at a theoretical equilibrium level with its environment.

42. The logistic growth model also uses r, but it includes the carrying capacity(K) of the environment • (the level at which available resources can sustain individuals in a population at a survival level) • which ultimately limits population size • The equation supposes that the source of supply of resources can support an equilibrium density of K individuals of the species. • The logistic growth equation involves several assumptions…

43. Age distribution is stable • No immigration or emigration takes place (closed population) • Increasing density depresses the rate of growth instantaneously without any time lags in reproductive responses • The relationship between population size and the rate of growth is linear • When plotted logarithmically, the rate of growth declines directly as population increases • A predetermined level for K is assumed Because these assumptions and the model are simplistic, the logistic growth equation should be used to view how populations might grow under favorable conditions and as a yardstick against which to measure actual population growth

44. The amount of resources present can vary – due to season, weather etc. The carrying capacity (K) is held down by the most limiting resource So K can vary considerably over time If the resource increases K can increase But there are other factors (disease), predation etc) that can prevent a population for reaching K These factors are often density dependent (e.g. more animals increases disease)

45. Logistic growth Exponential growth rate of the St. Paul reindeer herd. When the herd outstripped its resources, the population crashed.

46. When the population density of the deer was low, resources were abundant, death rate was low and the birth rate was high. As density increases, birth rate decreases death rate (due to predation, disease, competition) increases When birth rate and death rate are equal the population is “stable” and K is reached. If density increases further, death rate increases and birth rate decreases.

47. Density Dependent Responses to Growth Density-dependent per capita birth rates and death rates change as a function of density. The population reaches stability at N=K.

48. To make the logistic equation more realistic, you would need to factor in a reaction time lag a lag between environmental change and corresponding change in the rate of population growth. Another factor is a reproductive time lag, a lag between environmental change and change in the length of gestation or its equivalent.

49. Time lags result in population fluctuations. Populations may fluctuate widely without any reference to equilibrium size (K) especially if influenced by some external, or extrinsic force, such as weather Or, a population may fluctuate about the equilibrium level, K, rising and falling between some upper and lower limits.

50. Population Fluctuation and Cycles Fluctuation in a wintering population of black-capped chickadees