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Overview: Counting Sheep Soay sheep were introduced to Hirta Island in 1932 Opportunity to study changes in population on isolated island with abundant.

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Presentation on theme: "Overview: Counting Sheep Soay sheep were introduced to Hirta Island in 1932 Opportunity to study changes in population on isolated island with abundant."— Presentation transcript:

1 Overview: Counting Sheep Soay sheep were introduced to Hirta Island in 1932 Opportunity to study changes in population on isolated island with abundant food and no predators Population ecology: study of populations in relation to environment; influences on density, distribution, age structure, population size © 2011 Pearson Education, Inc.

2 Population Ecology Chapter 53

3 Population density, dispersion, and demographics Population: group of individuals of one species, living in an area Density: number of individuals per unit area or vol. – Immigration and emigration – Impractical to count all individuals – Sampling techniques, as mark-recapture method © 2011 Pearson Education, Inc.

4 Figure 53.3 BirthsDeaths ImmigrationEmigration Births and immigration add individuals to a population. Deaths and emigration remove individuals from a population.

5 Patterns of Dispersion Dispersion: pattern of spacing among individuals Environmental and social factors influence spacing – Clumped: resource availability, behavior – Uniform: social interactions such as territoriality, defense of a space against other individuals – Random: absence of strong attractions or repulsions © 2011 Pearson Education, Inc.

6 Figure 53.4 (a) Clumped (b) Uniform (c) Random

7 Demographics Demography: study of vital statistics of a population Death rates and birth rates Life table: age-specific summary of survival pattern; follows fate of a cohort (individual) © 2011 Pearson Education, Inc.

8 Example of a Life Table

9 Figure 53.5 Males Females 1, Age (years) Number of survivors (log scale) Survivorship Curve for Belding’s ground squirrels

10 Survivorship Curves Type I – low death rates early, middle; death rates increase among old age – Large mammals, – Produce few offspring but give good care Type III – high death rate for young; few survive early dieoff – Large numbers of offspring, little or no care of young – Plants, fish, marine invertebrates Type II – constant death rate over lifespan – Rodents, invertebrates, some lizards Many species don’t fall into any of these categories © 2011 Pearson Education, Inc.

11 Figure ,000 III II I Percentage of maximum life span Number of survivors (log scale) Survivorship Curves are of three types

12 Reproductive Rates For species with sexual reproduction, demographers often concentrate on females Reproductive table or fertility schedule = age-specific summary of reproductive rates © 2011 Pearson Education, Inc.

13 Exponential model of population growth Useful to study population growth in an idealized situation Per capita rate of increase Often, migration is ignored © 2011 Pearson Education, Inc.

14 b = annual per capita birth rate m = per capita death rate N = population size B  bN D  mN Expected number of births/deaths per year

15 r = per capita rate of increase © 2011 Pearson Education, Inc. r  b  mr  b  m Zero population growth (ZPG) when r  0

16 Exponential Population Growth Population increase under ideal conditions Rate of increase at maximum – r max Exponential population growth is Exponential growth results in J-shaped curve © 2011 Pearson Education, Inc. dN dt  r max N

17 Number of generations Population size (N) ,000 1,500 1, dN dt = 1.0N = 0.5N Figure 53.7 Population growth predicted by the exponential model.

18 Figure 53.8 Year Elephant population 8,000 6,000 4,000 2, Exponential growth in the African elephant population

19 Logistic model Describes how a population grows more slowly as it reaches its Carrying Capacity Exponential growth cannot be sustained More realistic model incorporates carrying capacity (K); produces sigmoid (S-shaped) curve Carrying capacity varies with abundance of resources Per capita rate of increase declines as carrying capacity reached (as N approaches K) © 2011 Pearson Education, Inc.

20 Table 53.3

21 Number of generations Population growth begins slowing here. Exponential growth Logistic growth Population size (N) ,000 1,500 1, K = 1,500 dN dt = 1.0N dN dt = 1.0N 1,500 – N 1,500 () Figure 53.9 Population growth predicted by the logistic model.

22 Figure Time (days) (a) A Paramecium population in the lab (b) A Daphnia population in the lab Number of Paramecium/mL Number of Daphnia/50 mL 1, In a constant environment lacking competitors and predators, how well do these populations fit the logistic growth model?

23 Logistic Model and Real Populations Some populations overshoot K Some populations fluctuate greatly; difficult to define K Logistic model assumes instant adjustment to growth Logistic model can be used to estimate possible growth Conservation biologists use model to estimate critical size below which populations may become extinct © 2011 Pearson Education, Inc.

24 Life history traits are products of natural selection Life history comprises traits that affect schedule of reproduction and survival – Age at which reproduction begins – How often organism reproduces – How many offspring are produced during each reproductive cycle Life history traits – evolutionary outcomes reflected in development, physiology, behavior © 2011 Pearson Education, Inc.

25 Evolution and Life History Diversity Species that exhibit semelparity, or big-bang reproduction, reproduce once and die – Highly variable environments favor semelparity – Century plant or Agave Other species show iteroparity (repeated reproduction) – Dependable environments favor iteroparity – Some Lizards Organisms have finite resources – may lead to trade-offs – Trade-off between survival & paternal care in kestrels © 2011 Pearson Education, Inc.

26 Figure Male Female RESULTS Reduced brood size Normal brood size Enlarged brood size Parents surviving the following winter (%) How does caring for offspring affect parental survival in kestrels?

27 Trade offs between reproduction and survival Some plants produce large number of small seeds, ensuring that some will grow & eventually reproduce Other plants produce moderate number of large seeds that provide large store of energy that will help seedlings become established – r-selection (density-independent) – selects for life history traits that maximize reproduction – K-selection (density-dependent) – selects for life history traits sensitive to population density © 2011 Pearson Education, Inc. f

28 Figure (a) Dandelion (b) Brazil nut tree (right) and seeds in pod (above)

29 Density-dependent factors Density-independent = birth and death rates do not change with population density Density- dependent = birth and death rates do change with population density © 2011 Pearson Education, Inc.

30 Density-dependent factors Density-dependent birth and death rates Negative feedback between population density and birth/death rates Affected by competition for resources, territoriality, disease, predation, toxic wastes © 2011 Pearson Education, Inc.

31 When population density is low, b > m. As a result, the population grows until the density reaches Q. When population density is high, m > b, and the population shrinks until the density reaches Q. Equilibrium density (Q) Density-independent death rate (m) Density-dependent birth rate (b) Population density Birth or death rate per capita Figure Determining equilibrium for population density.

32 Figure Population size % of young sheep producing lambs Decreased reproduction at high population densities.

33 Population Dynamics Focuses on complex interactions between biotic and abiotic factors that cause variation in population size Long-term studies have challenged hypothesis that populations of large mammals are relatively stable Weather and predators can affect population size – Moose population on Isle Royale © 2011 Pearson Education, Inc.

34 WolvesMoose Year Number of wolves Number of moose ,500 2,000 1,500 1, Figure Fluctuations in moose and wolf populations on Isle Royale, 1959–2008.

35 Population Cycles Some populations undergo regular boom-and- bust – Lynx populations follow 10-year boom-and-bust cycle of hare populations – Three hypotheses have been proposed © 2011 Pearson Education, Inc.

36 Snowshoe hare Lynx Year Number of hares (thousands) Number of lynx (thousands) Figure 53.19

37 Hypothesis 1 Hare’s cycle follows cycle of winter food supply If hypothesis correct, then cycles should stop if food supply is increased Additional food was provided experimentally; whole population increased, but continued to cycle Rejected! © 2011 Pearson Education, Inc.

38 Hypothesis 2 Hare’s cycle driven by pressure from other predators In study by field ecologists, 90% of hares were killed by predators Second hypothesis supported! © 2011 Pearson Education, Inc.

39 Hypothesis 3 Hare’s cycle linked to sunspot cycles Sunspot activity affects light quality, which in turn affects quality of hares’ food Good correlation between sunspot activity & population size © 2011 Pearson Education, Inc.

40 Figure Topsoil Bacteria EXPERIMENT Dictyostelium amoebas Dictyostelium discoideum slug 200  m Immigration / Emigration – Dictyostelium amoebas can emigrate and forage better than individual amoebas How does food availability affect emigration and foraging in a cellular slime mold? ` Dictyostelium movement

41 Aland Islands ˚ EUROPE Occupied patch Unoccupied patch 5 km Figure Metapopulations – groups of populations linked by immigration and emigration Glanville fritillary

42 Human population Human population increased relatively slowly until about 1650, then began to grow exponentially Global population now ~7 billion people Though global population still growing, rate of growth began to slow during 1960s © 2011 Pearson Education, Inc.

43 Figure The Plague Human population (billions) 8000 BCE 4000 BCE 2000 CE 1000 BCE 2000 BCE 3000 BCE 1000 CE

44 Projected data 2009 Annual percent increase Year Figure Annual percent increase in the global human population (as of 2009).

45 Regional Patterns of Population Change To maintain stability, regional human population can exist in one of two configurations ZPG = High birth rate – High death rate or ZPG = Low birth rate – Low death rate Demographic transition = move from first state to second state – Associated with increase in quality of health care and improved access to education – Most of current population growth concentrated in developing countries © 2011 Pearson Education, Inc.

46 Age Structure Important demographic factor in present and future growth trends Relative number of individuals at each age Diagrams can predict growth trends & help future planning © 2011 Pearson Education, Inc.

47 Figure Percent of population Rapid growth Afghanistan Slow growth United States No growth Italy Male Female Age –84 75–79 70–74 65–69 60–64 55–59 50–54 45–49 40–44 35–39 30–34 25–29 20–24 15–19 10–14 5–9 0–4 Age –84 75–79 70–74 65–69 60–64 55–59 50–54 45–49 40–44 35–39 30–34 25–29 20–24 15–19 10–14 5–9 0– Age-structure pyramids for the human population of three countries (2009).

48 Indus- trialized countries Less indus- trialized countries Infant mortality (deaths per 1,000 births) Life expectancy (years) Figure Infant Mortality and Life Expectancy

49 Global Carrying Capacity Predicted population of 7.8  10.8 billion in 2050 Carrying capacity of Earth for humans is uncertain Average estimate is 10–15 billion Ecological footprint – aggregate land and water area needed to sustain people Countries vary greatly in footprint size and available ecological capacity © 2011 Pearson Education, Inc.

50 Figure Gigajoules > –300 50–150 10–50 < 10 Average per capita energy use

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