Population Ecology

1
Population Ecology

• Chapter 52

2
Population

• population a group of individuals of a single
  species living in the same general area
• members of a population
• rely on the same resources
• are influenced by similar environmental factors
• have a high likelihood of interacting with and
  breeding with one another
• can evolve through natural selection
• can be defined in terms of density dispersion

3
Density

• density the number of individuals per unit area
  or volume
• methods used to estimate density
• count of individuals in sample plots, then
  estimate the population size in the entire area
• estimate density from some index of population
  size (ie of nests)
• markrecapture method
• density is the result of a dynamic interplay
  between processes that add individuals to a
  population (natality immigration) and those
  that remove individuals from it (mortality
  emigration)

4
Dispersion

• dispersion the pattern of spacing among
  individuals within the boundaries of the
  population
• influenced by environment social interactions
• 3 dispersion patterns
• clumped (most common)
• uniform
• random

5
Demography

• demography the study of the vital statistics of
  populations and how they change over time
• life tables agespecific summaries of the
  survival pattern of a population
• follow the fate of a cohort (a group of
  individuals of the same age) from birth until all
  are dead
• construction involves
• determining the number of individuals that die in
  each age group
• calculating the proportion of the cohort
  surviving from one age to the next

6
Life Table
7
Survivorship Curve

• a graphic way of representing the data in a life
  table is a survivorship curve a plot of the
  proportion or numbers in a cohort still alive at
  each age
• usually constructed by beginning with a cohort of
  1,000 individuals from a population multiplying
  the proportion alive at the start of each year by
  1,000
• example

8
Idealized Survivorship Curve

• Type I reflects low death rates during early
  and middle life, then drops steeply as death
  rates increase among older age groups
• associated with organisms that produce few
  offspring but provide them with good care
• Type II reflects constant death rate over an
  organism's life span
• Type III reflects very high death rates for the
  young, but then flattens out as death rates
  decline for those few individuals that have
  survived to a certain critical age
• associated with organisms that produce very large
  numbers of offspring but provide little or no care

9
Reproductive Rate

• another key factor used to determine population
  trends is reproductive rate
• concentrates on females in a population because
  only females produce offspring
• simplest way to describe the reproductive pattern
  of a population is to ask how reproductive output
  varies with the ages of females
• reproductive table (fertility schedule) an
  agespecific summary of the reproductive rates in
  a population
• construction involves measuring the reproductive
  output of a cohort from birth until death
• reproductive tables vary greatly, depending on
  the species

10
Life History

• traits that affect an organism's schedule of
  reproduction and survival (from birth through
  reproduction to death) make up its life history
• three basic variables
• when reproduction begins (the age at first
  reproduction or age at maturity)
• how often the organism reproduces
• how many offspring are produced during each
  reproductive episode

11
Life History Diversity

• semelparity a life history in which adults have
  but a single reproductive opportunity to produce
  large numbers of offspring (aka big-bang
  reproduction)
• favored where the survival rate of offspring is
  low (highly variable or unpredictable
  environments)
• iteroparity a life history in which adults
  produce large numbers of offspring over many
  years (aka repeated reproduction)
• favored in more dependable environments where
  competition for resources may be intense

12
Trade-Offs

• trade-offs exist between
• reproduction survival
• number size of offspring
• plants and animals whose young are subject to
  high mortality rates often produce large numbers
  of relatively small offspring
• animals that suffer high predation rates also
  tend to produce large numbers of offspring

13
Change in Population Size

• the change in population size during a particular
  time interval can be most simply expressed as
  births deaths
• this simple equation can be converted into one
  that reflects the average number of births and
  deaths per individual during the specified time
  interval by incorporating per capita birth
  death rates
• per capita birth rate (b) the of offspring
  produced per unit time by an average member of
  the population
• per capita death rate (m) the expected number
  of deaths per unit time in a population
• population ecologists are most interested in the
  per capita rate of increase (r) b m

14
Zero Population Growth (ZPG)

• ZPG occurs when the per capita birth and death
  rates are equal (r 0)
• NOTE
• when r gt 0, a population is growing
• when r lt 0, a population is in decline

15
Exponential Growth

• exponential population growth rapid increase in
  population size under ideal conditions including
• access to abundant food space
• no competition or predators
• free to reproduce at physiological capacity
• under ideal conditions, the per capita rate of
  increase may assume the maximum rate for the
  species intrinsic rate of increase (rmax)
• population size increases at a constant rate,
  resulting in a Jshaped growth curve

16
Carrying Capacity (K)

• K the maximum population size that a particular
  environment can support
• varies over space and time with the abundance of
  limiting resources

17
Logistic Growth

• as population growth nears K, the per capita rate
  of increase declines
• either the birth rate (b) decreases, the death
  rate (m) increases, or both

• the logistic model of population growth produces
  a sigmoid (Sshaped) growth curve
• assumes that populations adjust instantaneously
  to growth and approach carrying capacity
  smoothly, which is not usually the case in nature

18
Logistic Growth Life Histories

• Kselection (densitydependent selection)
  selection for life history traits that are
  sensitive to population density
• tends to maximize population size operates in
  populations living at a density near the carrying
  capacity
• rselection (densityindependent selection)
  selection for life history traits that maximize
  reproductive success in uncrowded environments
  (low densities)
• tends to maximize r (the rate of increase)
  occurs in environments in which population
  densities fluctuate below carrying capacity or
  individuals are likely to face little competition
• characteristics of most species place them
  somewhere in between the extremes represented by
  r and K selection

19
What causes a population to stop growing?

• density-dependent population regulation
• death rate that ? as population density ?
• birth rate that ? as population density ?
• density-dependent growth factors include

• Factors that lower birth rate

• Factors that raise death rate

• competition for resources
• territoriality

• disease
• predation
• pollution (toxic waste)

20
Why do some populations fluctuate dramatically
over time while others remain more stable?

• factors that cause variation in population size
• interaction between biotic abiotic factors
• immigration emigration between linked
  populations (metapopulation)

21
Population Cycles

• some populations show regular fluctuations in
  density (boom-and-bust cycles)
• example snowshoe hare lynx

22
Human Population

• increased relatively slowly until about 1650, at
  which time the population was 500 million
  people
• doubled to 1 billion within the next two
  centuries
• doubled again to 2 billion between 1850 and 1930
• doubled again by 1975 to more than 4 billion
• today it numbers over 6 billion people and is
  increasing by about 73 million each year
• it takes only 4 years for world population growth
  to add the equivalent of another United States
• population ecologists predict a population of
  7.38.4 billion people by the year 2025

23
Exponential Growth

• though the global population is still growing,
  the rate of growth began to slow during the 1960s
• this reduction in growth rate shows that the
  human population has departed from true
  exponential growth
• declines in growth rate are due to
• diseases (ex AIDS)
• voluntary population control

24
Age Structure

• age structure the relative of individuals of
  each age
• helps determine present future growth trends
• commonly represented in pyramids
• a small base projects continuing population
  decrease b/c the population has few individuals
  younger than reproductive age
• a large base projects continuing population
  growth b/c the population has many young
  individuals who will grow up and may sustain the
  explosive growth with their own reproduction

25
Global Carrying Capacity

• difficult to estimate
• one method of determining human carrying capacity
  is to calculate the ecological footprints of
  various countries
• ecological footprint the aggregate land and
  water area appropriated by each nation to produce
  all the resources it consumes and to absorb all
  the waste it generates
• this method shows that some countries are already
  over K while others are still below it
• exactly what the world's human carrying capacity
  is under what circumstances we will approach it
  are topics of great concern and debate
• unlike other organisms, humans have the capacity
  to decide whether ZPG will be attained through
  social changes based on human choices or through
  increased mortality due to resource limitation,
  plagues, war, and environmental degradation