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Chapter 52

Chapter 52. Population Ecology. Population Ecology. Population ecology is the study of the fluctuations in population size and composition and their ecological causes

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Chapter 52

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  1. Chapter 52 Population Ecology

  2. Population Ecology • Population ecology is the study of the fluctuations in population size and composition and their ecological causes • A population is a group of individuals of a single species that simultaneously occupy the same general area, use the same resources, and respond to similar environmental factors

  3. Characteristics of Populations • Density – the number of individuals per unit area; the mark-recapture method is an estimate of population density • N = (No. marked in 1st catch) X (total no. in 2nd catch) No. of recaptures (marked) in second catch • Dispersion – pattern of spacing among individuals • Can be clumped (associated with food and mating), uniform (associated with competition), or random (independent)

  4. Types of Dispersion

  5. Demography • Demography is the study of vital statistics that affect population size • Life table – an age-specific summary of the survival pattern of a population

  6. Demography • Survivorship curve – plot of number in cohort (a group of individuals of the same age) alive at each age • Type I – low mortality of young and mortality increases with age; few offspring produced with good parental care (humans, mammals) • Type II – constant death rate (some squirrels, hydra, and some lizard species) • Type III – high mortality of young; organisms that produce a large number of offspring with little or no parental care (fish and marine invertebrates)

  7. Demography • Survivorship curves

  8. Reproductive Table • A reproductive table is an age specific summary of reproductive rates in a population

  9. Life History • Life history – the traits that affect an organism’s schedule of reproduction and survival • Big-bang reproduction (semelparity)– a single reproductive episode (salmon and agaves) • Repeated reproduction (iteroparity) – adults produce large numbers of offspring over many years (oaks and lizards)

  10. Limited Resources • Trade offs between investments in reproduction and survival • Darwinian fitness is measured by how many offspring survive to reproduce offspring themselves • Finite resources mean trade offs for reproducing organisms • Reproduce at an early age • Produce many offspring each time they reproduce • Reproduce many times in a lifetime • It is best to have few, well-cared for offspring

  11. Limited Resources • It is best to have few, well-cared for offspring

  12. Population Growth • Δ pop. = births – deaths (no emigration or immigration) • Let N = population size and let t = time Δ NB = no. of births Δ t D = no. of deaths • Now, let births = (per capita rate) X (pop.) B = bN • We can now rewrite the equation as Δ N Δ t = B – D = bN – dN

  13. Population Growth • We can now rewrite the equation as Δ N Δ t • We then use r to identify the differences in the per capita birth rates and death rates: r = b – d • If r is positive the pop. is growing; if r is negative the pop. is declining; and if r is 0 there is ZPG • We can now rewrite the equation as Δ N or dN Δ t dt = bN – dN = rN = rN

  14. Exponential Population Growth • Exponential growth – idealized population in an unlimited environment (no limiting factors) dN dt • This type of pop. growth produces the J-shaped curve = rmaxN

  15. Exponential Population Growth • An example of exponential growth in nature

  16. Population Growth • Exponential growth assumes unlimited resources which is not very realistic • What are some limiting resources you can think of? • Ultimately, there is a limit to the number of individuals that can occupy a habitat • Carrying capacity (K ) is the maximum population size that a particular environment can support at a particular time • The logistic population growth model incorporates the effect of population density on the per capita rate of increase

  17. = rmaxN(K-N ) N Logistic Population Growth dN dt

  18. Logistic Population Growth

  19. Population Limiting Factors • Density dependent limiting factors (K – selection) – selection for life history traits that are sensitive to population density • d increases with pop. density; b decreases with population density • An example of negative feedback • Density independent limiting factors (r – selection) – selection for life history traits that maximize reproductive success in uncrowded environments • No effect on d or b

  20. Population Limiting Factors • What are some population-limiting factors? • What is the relationship between population growth and biotic and abiotic influences? Population decline of the northern pintail

  21. Population Limiting Factors • What are some population-limiting factors? • What is the relationship between population growth and biotic and abiotic influences?

  22. Human Population Growth • It took all of human time on Earth (3 million years) for the pop. to reach 1 billion in 1800 • It took 130 years to reach the second billion • The third billion was added 30 years later in 1960 • The fourth billion was added 15 years later in 1975 • In 1987, only 12 years later, the world population reached 5 billion • It reached 6 billion in 1999 • The human population is projected to reach 7.8 billion by the year 2025 • The world’s pop. increases by 214,000 people per day • It takes 12 years to add 1 billion people to our pop.

  23. Human Population Growth

  24. Human Population Growth • A regional human population can exist in one of two configurations to maintain population stability: • Zero population growth = high birth rates – high death rates • Zero population growth = low birth rates – low death rates • The movement from the first toward the second state is called the demographic transition • The world’s population growth is regional; it is near equilibrium (0.1%) in developed nations and 1.7% in less developed countries, where 80% of the world’s population lives

  25. Demographic Transition • It took Sweden 150 years for the transition, and Mexico only half the time

  26. Age Structure • The age structure of a population influences present and future growth • A large proportion of individuals of reproductive age or younger results in more rapid growth • Age structure also predicts future social conditions and needs

  27. Age Structure Growing at 2.1% per year Growing at 0.6% per year Zero growth

  28. Ecological Footprint • The concept of ecological footprint takes into account the multiple constraints involved in estimating human carrying capacity • Ecologically productive areas are defined as arable land, pasture, forest, ocean, built-up land, and fossil energy land • Taking into account ecologically productive areas and converting it into land area per person gives us an estimate of about 1.7 hectares per person.

  29. Ecological Footprint

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