BIOLOGY 103
      Chapter 48:  POPULATION GROWTH

 

 

The most important concept 
Most populations grow exponentially unless some limiting factor changes the rate of birth or death or migration.

This chapter and Lab 9 emphasize two problems:
  1. The exponential explosion problem, when populations grow too fast.
  2. The extinction problem, just the opposite.

Both problems are nearly always caused by damage to a habitat or by moving a population out of its natural habitat.  So both the textbook and the lab emphasize habitat conservation as well as the basic facts of population growth.


Key Textbook Points
Wednesday
Sample Quiz 1 for Wednesday
lecture review

Friday
Sample Quiz 2 for Friday
lecture review

Other Science News

The Scientific Method in Ch. 48

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1. The exponential explosion problem, when populations grow too fast.
Nearly all populations are capable of exponential growth.  How fast the exponential growth will be depends on the energy available to the population and these factors:
  • survivorship (opposite of mortality rate)
  • fecundity (birth rate, especially age-specific rates)
  • age structure of the population (age pyramids)
  • immigration and emigration rates.

All of these factors above, in turn, are controlled by

  • interactions with other species (predators, prey,  pollinators, parasitic diseases, etc.)
  • chance (weather, fires, tidal waves, volcanoes, asteroids, the effects of chance on other populations, etc.)
  • the population's genetic composition, its adaptations for its specific niche; the relative frequency of alleles which contribute to migration and survival and reproduction under the specific conditions the population faces.
2. The extinction problem.  The latest research, published 1 November 2002,  
( http://www.sciencemag.org/cgi/content/short/298/5595/989 , also see http://news.bbc.co.uk/go/em/fr/-/1/hi/sci/tech/2385591.stm   , http://www.nature.com/nsu/021028/021028-11.html ) indicates that almost half of all plant species could be facing extinction.  Since animals are nearly always dependent on specific plants, directly or indirectly, for food and nesting sites, the outlook is horrible. In preparing for the exam (in 4 weeks, not counting the Thanksgiving vacation), keep notes on how your experiences in Labs 9, 10, and 8 (field trip) relate to these components of the extinction problem:
  • habitat destruction
  • habitat fragmentation
  • edge effects (note:  speciation seems to increase at habitat edges; but speciation is too slow to make up for the losses of other species when patches become too small)
  • gene pool bottlenecks (inbreeding problems)
  • Random accident effects
    • population size (demographic stochasticity)
    • loss of genetic variability (genetic stochasticity), becoming worse with smaller populations.
  • corridors

 

Key textbook points for Wednesday, pages 932-940

  • pp. 932 -935:  The growth of exponential (all  graphs on fig. 48.2) and density-dependent (blue curve on fig. 48.3) populations:  the graphs are the key to understanding this chapter.  
    • Population growth rates depend on the rates of birth, death, and net migration. In nature, these rates are sometimes controlled by limiting factors which are density-dependent, factors which change because the population size (its density) has changed. Density-dependent limiting factors include energy and material sources which decrease as density increases and other factors which may increase as population size increases: waste products, predators, and parasites.

    • Sometimes population growth is changed by random factors, accidents, storms, or other events which are not influenced by the population itself. These external forces are called density-independent factors.

    • As you study this chapter, note how the human population could be affected by each factor or topic.  To what extent is the human population an exception to the population limitations for plants and animals in nature?  What impact does our population growth have on plant and animal population growth?

    • The math.  Know what all these terms mean (biologically) and how they are used in  the graphs
      • J-curve ( fig. 48.2)
      • S-curve ( fig. 48.3))
      • lambda
      • r
      • e
      • per capita rates of birth, death, and net migration 
      • N (density)
      • t
      • N0
      • Nt
      • K or carrying capacity (from the original German "Kapazität.") (think of K as the critical point at which the first limiting resource runs out)
    • Be able to work the three problems on p. 934
    • CD activity 48.1 helps explain the math

     
  •  
  • Here is a good link for understanding how the graphs connect to populations growing:    www.bio.brandeis.edu/biomath/top.html.   (Go to the main menu and then click on exponential growth under "Dixon's Population....")
  • Some Case Histories
    • pp 935 - 6:  Gobie example.  It's important to understand (and to be able to explain examples of) what limits population growth when the population is density-dependent.   For a population to be truly density-dependent, something must keep it from maximizing its genetic potential to reproduce or survive or immigrate.   Also, that  something must be sensitive to the population's density so that the something has a stronger impact when the population gets larger.  Here are some somethings and maybe you will think of more before you are asked to do so on a test or something: (which of these might apply to gobies?)
      • a larger population will attract more parasites (diseases), which will increase the mortality rate
      • larger populations run out of nesting sites so that fewer individuals reproduce or so that more individuals emigrate to other habitats
      • larger populations produce higher concentrations of wastes which may poison some individuals or their mutualists or maybe attract more predators or diseases
      • when food becomes scarce, some individuals may not have enough energy left to produce eggs or some individuals may forage for food at more dangerous times and in riskier places so that survivorship.....
    • pp. 936-7.  Human example:  What could make human population growth density-dependent?  CD activity 48.2 helps (the activity seems sort of lame at the beginning, but it gets better)
    • pp. 937-8  Red grouse example:  What causes these population cycles?  Are they typical of density-dependent growth?
    • pp. 938-940.  Exon Valdez example:  What kinds of populations recover fastest from environmental disasters?  How are these ideas related to the factors involved in density-dependent growth?.
  • Checklist of terms:  population, evolutionary biology, ecology, birthrate, immigration, death rate, emigration, per capita, finite rate of increase (lambda), per-capita rate of increase (r), per-capita birthrate, per-capita death rate, exponential growth, density-independent growth, density-dependent growth, carrying capacity, fertility rates, population cycles, recovery, trauma  

SAMPLE QUIZ #1

  • http://wps.prenhall.com/esm_freeman_biosci_1/0,6452,501216-,00.html 

    • summary review #1, 2, 3, 4, 5, 6; figure review # 1

    • Content Review #1, 2, 3, 4; Conceptual #1, 3; Applying #1, 3, 4

  • Activity 48.2 Human Population Growth and Regulation prequiz and postquiz questions (4 total)

  • And these:

    1.  The size of a population is called its
      [a] age distribution.     [b]  cohort.    [c] density.     [d] reproductive value.

    2. The density of a population =     [a]  lambda      [b] K         [c] N            [d] r

    3. As a logistic population approaches K, ideally which of these should decrease?
      [a] birth, survival, or immigration rate       [b] K          [c] N           [d] r

    4. A graph of a population which has density-dependent growth is shaped like a(n)
      [a] flat line.         [b] J.            [c] S.

    5. A density-dependent factor theoretically decreases a population's fertility or survival
      [a] more when the population gets bigger.      
      [b] more when the population is small.
      [c] regardless of the size of the population.                                      Answers 

Key textbook points for Friday, pages 940-948

  • pp. 940-2.  The density, spatial distribution and age distribution of populations reflect their recent history.  Density, spatial distribution, and age distribution are all limited by the environment (including other species) as well as the genetic potential of the population.  

  • pp. 943-6 .  Similarly, each species has a particular genetic potential for the rates and probabilities for demographic events (birth rates or fecundity or fertility rates, death or mortality rates, and migration rates, survivorship curves, tables of age-specific life expectancies and fecundity), but the actual occurrence of demographic events is also limited by other species and by chance.  Life history traits and the Life tables for natural populations are genetically determined.   Each species evolves a specific set of life history traits.  Be able to list some examples of life history traits; then for each trait be able to suggest what type of selective force or "agent of selection" could affect the evolution of a particular trait.  For example, why don't caterpillars or fruit fly larvae have babies?   Why don't we Homo sapiens have a pupal stage (like having our young adolescents transforming themselves inside cocoons throughout the obnoxious and dangerous middle-school years)?   Why do horses usually produce only one colt, while fruit flies lay thousands of eggs?  Why do petunias and Pacific salmon die after they create babies?  Why don't oak trees die after they create acorns?     

    • Can you draw survivorship curves for these examples above?  Drawing and interpreting the curves is important; remembering which shape is called Type I or II or III is not important.-

    • Notice that density, spatial distribution, age distribution, birth rates or fecundity, death or mortality rates, and migration rates, survivorship curves, tables of age-specific life expectancies and fecundity are all emergent properties of populations; an individual cannot have birth rates or fecundity, death or mortality rates, and migration rates, survivorship curves, tables of age-specific life expectancies and fecundity, density, spatial distribution, and age distribution.  

    • Life history traits are properties of individuals as well as species.  An individual with an advantageous life history trait has higher Darwinian fitness; a individual with higher fitness leaves more offspring who also survive to reproduce.   

    • Note that life history traits have a major connection to the ideas in Chapter 22 "Genetic Variation and Selection."

  •  p. 946  PVAs:  Why does each endangered species need a different population viability analysis?

  •  

  • pp. 617-624.  Conservation  

    • What are we humans doing to affect our K?  What are we doing to the Ks for other gene pools?  

    • Earlier we considered the importance of human activity in altering the limiting resources of habitats, and you might have even thought how that could make a difference in the species diversity questions we considered in Labs #3 and #9 and #11. Again, for this chapter you should expect questions where you can apply the most important concept to specific situations involving the growth of human populations and how larger human populations may affect the "K" or mortality or immigration rates of populations of other critters, like when more human beings leads to more highways and more fragmented forests or fewer predators or eutrophication of aquatic habitats or the greenhouse effect. The conservation info at the end of this chapter and the extinction vortex concept at the beginning of the chapter will be important in future chapters and (of course) on the exam.

  • REVIEW for entire chapter.  Make sure you understand how the graphs on page 610 connect to the other diagrams and concepts throughout chapter 48. 

  •  
  • Checklist of terms:  population, age structure, geographic structure, metapopulation, mark-recapture study, fragmented habitats, corridors, conservation, "pure research," "applied research," demography, life table, fecundity (mx), survivorship (lx), age-specific mortality, survivorship curves, population viability analysis (PVA)
  • Sample Quiz #2
    1. The age structure diagram of Mexico is much broader toward its base than near its middle. This diagram shape suggests that
      [a] Mexico has more children than adults of typical parent age.
      [b] The number of Mexicans of reproductive age will stay about the same for the next few decades.
      [c] The Mexican population growth rate (      D N/D t) is not different from the U.S. population growth rate.

    2. All the individuals in the same age category are called a(n)
      [a] age distribution.     [b] biomass.               [c] clutch or litter.   
      [d] cohort.                   [e] population.           [f] species.

    3. Which of these is NOT a life history trait?
      [a] age distribution.     [b] age at first reproduction.     [c] average clutch or litter size.
      [d] average number of litters per year.                
      [e] actually all of these ARE life history traits.

    4. The opposite of  "mortality" is
      [a] fecundity.     [b] fitness.   [c] survivorship.

    5. "Fitness" is a property of a(an)
      [a] age cohort.     [b] clutch.     [c] individual.   [d] population.          

    6. What is the global Homo sapiens population density today?

    click here for answers /\/\/\/\/\/\/\/\/\/\ back to top

     

     

     

Most populations grow exponentially unless some limiting factor changes the rate of birth or death or migration.

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The Scientific Method in Chapter 48

A model is a theory expressed in mathematical terms
or in a diagram.

The exponential population growth model explains how fast a population size will change when its critters are insensitive to crowding until it's too late. This theory works perfectly for many real populations, like rabbits, weeds, pondscum, breadmold, houseflies, and many germs. The other model presented in this chapter is the logistic growth equation, which models growth of populations when critters are perfectly density-dependent, fine-tuning their reproductive or migratory or mortality rates according to the current population size. Very few real populations come close to the logistic theoretical ideal.

In between these two very different different models are many other equations which do a good job of explaining the population growth of real populations. You will be spared these models until you take Biology 304; but if you can't wait until next year, check out the mother of all population ecology web pages: http://www.gypsymoth.ento.vt.edu/~sharov/popechome/welcome.html.

Even though real populations hardly ever grow just like the logistic equation predicts, ecologists find the model useful to contemplate the extremes of the wide range of possibilities for millions of different critter populations. 

Other scientific method issues may arise on upcoming events where you may be asked to analyze some science news reports or to figure out some differences in "how" and "why" questions. 

  "r and K types"
      another common application of the two models:

  • Here is a summary with some parts (marked with "?") left blank for you to fill in yourself:

   "r-selection" populations  "K-selection" populations
Population Growth Type Exponential Logistic (Density-dependent)
Shape of growth curve "J" "S" = sigmoid
Effect of density-dependent factors none until population exceeds carrying capacity and crashes         ?
Typical life history characteristics fertility rates:  high or low?         ?
        ? reproductive maturity: early or late?
juvenile mortality:  high or low?         ?
        ? litter size:  small or large?
offspring  size:  small or large?         ?
        ? life spans:  long or short?
high ability to immigrate, emigrate, recolonize or "rescue" from metapopulation         ?
little parental care               ?
Type of Environment Unpredictable               ?
Shape of age structure pyramid Much broader at base         ?
Potential for Extinction         ? more vulnerable
Impact on Biodiversity likely to increase or decrease biodiversity? increase or decrease?
other?   often good competitors
    may require mutualists
  more common in early successional stages  
  "fugitive lifestyle"  
     

It's important to be able to explain the differences in this table, too,

 

Most populations grow exponentially unless some limiting factor changes the rate of birth or death or migration.

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Answers to Sample quiz #1
1c. 2c. 3a.  4.c  5a.   

answers to sample quiz #2
1a. 2d. 3a. 4c. 5c. 6 It reached 6 billion in October, 1999 (remember Y6B?)

 

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