Population Ecology: Population Growth and Regulation

1
Population Ecology Population Growth and
Regulation

• Starr/Taggarts
• Biology
• The Unity and Diversity of Life, 9e
• Chapter 46

2
Characteristics of Populations

• Demographics are characteristics of populations
• Populations can be characterized by their
• Population Size
• Population Density
• Population Distribution
• Age structure
• Reproductive base
• Reproductive and younger

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Age Structure Diagram
Green - Pre-reproductive years Dark Blue-
Reproductive years Light blue - Post-
reproductive years
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Age Structure Diagram U.S.
Zero growth births deaths and immigrate
emigrate
5
Regarding Population Density

• The same area may be home to many species but
  they may vary by density
• Crude density refers to the number of individuals
  in an area but not how they are dispersed
• Population density variations are largely due to
  ecological relationships and organism
  interactions in an area

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Patterns of Dispersion

• Members living in clumps is very common this is
  called clumped dispersion
• Reasons for clumping include
• Physical, chemical and biological conditions
  could be patchy
• Social grouping
• Many offspring are not
  highly mobile

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Patterns of Dispersion

• Nearly uniform dispersion
• Uniform dispersion is rare in nature
• If it does occur, it is due to fierce competition
  for resources
• Example
  creosote bushes

8
Patterns of Dispersion

• Random Dispersion
• Happens when environmental conditions are uniform
  and individuals are neither attracting or
  repelling one another
• This is the exception and not the rule
• Example Wolf spiders

9
Some Qualifiers

• The spatial pattern may differ based on the
  breadth of view
• Distribution can vary over time in response to
  environmental change

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Population Size and Exponential Growth

• How population size changes
• Size depends on births, immigration, deaths,
  emigration and migrations
• Zero population growth is a near balance between
  all the positive and negative factors
• There is a growth rate formula and it is G
  rN

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Defining variables

• G growth rate
• r births - deaths
  
• individuals individuals
• N number of individuals
• G rN
• As long as r is positive the population will rise!

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Ground Zero to Exponential Growth
Increase in population of field mice
J Shaped Growth Curve
13
Effects of Deaths on Populations
The effects of deaths on the rate of increases in
2 bacterial populations
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Regarding the Biotic Potential

• The biotic potential of a population is its
  maximum rate of increase under ideal conditions
• Biotic potentials vary from species to species
  based on
• What age reproduction begins
• How frequent reproduction is
• How many offspring are born at a time

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Limits on the Growth of Populations

• The actual rate of growth depends on the
  environmental conditions
• Limiting factors are collectively called
  environmental resistance and include
• Nutrient supply
• Predation
• Competition for space
• Pollution
• Metabolic wastes

16
Limiting Factors on Growth

• The sustainable supply of resources defines the
  carrying capacity for a population in an
  environment
• The capacity can vary over time
• Note the S shaped logistic growth

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Logistic Growth Equation
K carrying capacity!
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Carrying Capacity
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Density - Dependent Controls

• The main controls are
• Competition for resources
• Predation
• Parasitism
• Disease
• They exert their effects in proportion to the
  number of individuals present
• Density - dependent factors
• Events resulting in more deaths
• Fewer births

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Paramecium caudatum
Paramecium aurelia
Fig. 48.4, p. 856
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PREY POPULATION
PREDATOR POPULATION
Fig. 48.7, p. 858
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Fig. 48.8, p. 859
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Density Independent Factors

• Some events such as weather tend to increase the
  death rate without respect to the number of
  individuals present
• Lightning, floods, snowstorms affect both large
  and small populations!

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Life History Patterns

• Life Tables
• Demography is the study of the age-specific
  patterns of particular populations
• Life tables follow the fate of a group (cohort)
  and calculate survivorship schedules

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Patterns of Survivorship

• Type I
• High survivorship until late in life
• Elephants, bears, humans
• Type II
• Constant death rate at all ages
• Small mammals, lizards, songbirds
• Type III
• Death rate is highest early in life
• Sea stars, corals, marine invertebrates

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A note on Human Population
Growth

• Increase in growth rate
• Expansion of habitat
• Increased capacity in existing habitats
• Population has sidestepped limiting factors
• Technology
• Medicine

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Population Growth Rate in Different Regions
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Population Growth Rate in Different Regions
1996 - Orange 2025 - Blue