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CHAPTER 52
Population Ecology
Lecture Outline
• The Characteristics of Populations
• Density and spacing
• Population Growth
• Exponential growth model
• Logistic growth model
• Demography
• Human Population Growth

Characteristics of Populations
• Population: group of individuals of a single species that simultaneously occupy the same general area

• Population ecology: scientific study of population changes
- in size,
- age distribution,
- and the ecological causes of these changes

• Biggest influences on populations are:
- interactions among individuals within the
population = intraspecific

- interactions with individuals from other populations of
other species = interspecific

- interactions of individuals with their environment
-> all these are under the influence of natural selection processes

Density and spacing
• Populations have size and geographical boundaries:
• Population size is the absolute number of individuals present in a geographical area.
• The density of a population is measured as the number of individuals per unit area.
• The dispersion of a population is the pattern of spacing among individuals within the geographic boundaries.

Figure 52.2 Patterns of dispersion within a population’s geographic range

Changes in population size
dN/dt = r N

A population can decline, stay stable or increase:
-> negative r indicates a declining population
-> r = 0 indicates a stable population
= zero population growth (ZPG)
-> positive r indicates an increasing population
= exponential population growth under ideal conditions

Exponential model of population growth
-> describes growth of an idealized population in an unlimited environment
-> continuing increase

Logistic model of population growth
Typically, unlimited resources are rare.
-> population growth is therefore regulated by carrying capacity, which is the maximum stable population size a particular environment can support

Carrying capacity depends on:
-
-

as a population’s size approaches the carrying capacity of its environment, its growth slows, stabilizing at the carrying capacity
->

How well does the logistic model fit the growth of real populations?
-> the growth of laboratory populations of some animals fits the S-shaped curves fairly well
Examples:

The logistic model does not apply to all populations.
Example:
-> it is an idealized model which provides a basis from which we can compare real populations

Summary population growth models
• Exponential & logistic models are idealized models of population growth
-> very few populations grow exponentially (paramecium example)
-> not all populations reach and stay at carrying capacity (sparrow example)

• But, these two models capture the two key concepts in understanding how population size is regulated.

Demography
=
• Population growth:
• by birth (B)
• by immigration
= addition of individuals to a population from outside
• Population decline:
• by death (D)
• by emigration
= departure of individuals from a population

• Causes of mortality:
• predation
• food limitation
• habitat loss
• extremes of weather (heat, cold, drought …)

• Life stages can be affected to a different extent:
• increased death rates of immature stages (e.g. marine larvae)
• increased death rates of old individuals
• increased mortality during molting of invertebrates
(e.g. ‘soft-shell’ crabs)

• life tables & reproductive tables
= age-specific summary of the survival pattern and reproductive rates of a population
e.g. cohort of ground squirrels (simplified from Fig. 52.1 & 52.2)

• survivorship curves = describe patterns of survival and death in a population

Human population growth
• slow increase until about 1650
• decrease around 1650: plague took numerous lives
• since then nearly exponential increase:

• Exponential increase of human populations: WHY?
-> humans can increase their own carrying capacity:
- by increasing food production (agriculture)
- by reducing mortality (improved sanitation,
healthcare, nutrition)

-> human population cannot continue to increase indefinitely
-> projected population of Earth for 2050 ranges from 7.3 to 10.7 billion people
-> overpopulated??

Human population dynamics
• Regional variation in population growth:
-> two major patterns:
1. Zero or negative population growth in developed
regions (e.g. Canada, Europe)
- few young individuals of reproductive age
- low mortality rates
2. Underdeveloped regions with high population
growth
- many individuals of reproductive age
- high mortality rates & shorter life span

Fig. 52.22: Age-structure pyramids for the human
population of Kenya, the United States, and Italy
Growth rates: 2.1 % 0.6 % 0 %
(per year) fast slow zero

Key terms 52
Population
Density
Dispersion
clumped
uniform
random
Life table / reproductive table
Demography
Survivorship curve
Zero population growth (ZPG)
Exponential population growth
Carrying capacity
Logistic population growth