Chapter 39, Populations 786-800
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The content of today's lecture will help you complete these
After studying this material you should be able to:
Define the term biological population and provide examples.
Draw a simple graph illustrating a population that is
growing at an
exponential rate of increase (J-shaped curve).
Describe how population growth rate is calculated.
Describe how and explain why each of the following factors
the growth of a population:
population growth rate
biotic or intrinsic factors
exponential population growth
logistic population growth
density dependent environmental resistance factors
density independent environmental resistance factors
Distinguish between density dependent and
independent environmental resistance factors that regulate population
Create a diagram, concept map, or text that represents the
relationships among the concepts;
resistance factors (density dependent and density independent), birth
rate, death rate, population growth rate, logistic population growth,
General Web Resources
What is a Population?
"A population is a group of organisms of the same species
geographic location at the same time." (Hoefnagels, text pg. 788)
The Glossary on pg. G-17 gives a slightly different wording of
same concept, "members of the same species occupying a region".
Understanding the demography of populations has many critical
Demography is the statistical study of populations, and
such statistics as population size, density, and distribution.
Factors that affect the
growth of Populations
What is population "growth"?
- What we might talk about as population size is actually
population density, the number of individuals per unit area (or
Population growth is based on four fundamental factors: birth
rate, death rate, immigration, and emigration.
Population growth rate = (birth rate + immigration) -
"Per capita rates" are calculated as the number of events
deaths, or growth) divided by the number of individuals in the
population over a specific time period.
Biotic or Intrinsic factors that affect
Population growth is affected by biotic or intrinsic
that are built into the genetic basis of each species.
Biotic or Intrinsic factors are specific to each
The age of reproductive maturity
The number of offspring produced per reproductive event
The number of reproductive events in an individual's
The three factors above are refered to together as fecundity,
or the number of offspring an individual produces in its lifetime.
Exponential (J-Shaped) Population Growth
This is the sort of population growth that occurs when
biotic or intrinsic factors affect a population.
Exponential growth assumes that environmental factors
water supply, space, shelter, disease organisms, predators, weather
conditions, and natural disasters do not affect the birth or
Examine the shape of the curve of population growth
over time. Graph: Hoefnagels text, Fig. 39.5, pg. 791
- Exponential Population Growth Simulation
As long as birth rate exceeds death rate (even
size will increase exponentially.
If death rate exceeds birth rate population size will
The human population is growing exponentially. Graph: Hoefnagels text Fig. 39.11, pg. 795
Movie of exponential bacterial growth. (16 Sec)
Population Regulation &
Environmental Resistance Factors
that Affect Birth and Death Rates
Obviously, populations cannot realistically grow
are environmental limits, called environmental resistance
factors, that affect the number of individuals that can survive and
reproduce in a given habitat.
"The power of population is so superior to the power of
the earth to produce subsistence for man, that premature death must in
some shape or other visit the human race. The vices of mankind are
active and able ministers of depopulation. They are the precursors in
the great army of destruction, and often finish the dreadful work
themselves. But should they fail in this war of extermination, sickly
seasons, epidemics, pestilence, and plague advance in terrific array,
and sweep off their thousands and tens of thousands. Should success be
still incomplete, gigantic inevitable famine stalks in the rear, and
with one mighty blow levels the population with the food of the world."
Thomas Malthus, 1798
Environmental resistance factors fall into two
categories: density dependent and density independent.
A population explosion and crash. Graph: Hoefnagels text, Fig. 39.7, pg. 793
Density dependent environmental resistance factors
(See Hoefnagels text, Pg 792)
Density dependent factors include the
needed by the individuals of a population. Competition for food, water,
shelter, etc., results as the population density increases. The
survival, health, and reproduction of individuals will be affected if
they cannot acquire the basic requirements of life.
Density dependent factors ALSO include
such as predators, infectious disease organisms, and parasites
that do not necessarily result in competition for needed resources, but
do affect the health, survival, and reproduction of individuals in the
population as population density increases. Individuals that are
diseased may have a reduced ability to reproduce. Dead individuals
Density dependent factors are referred to as Environmental
Resistance Factors that determine the Carrying Capacity of
the environment for a population.
Carrying Capacity - "The theoretical maximum
individuals that an environment can support for an indefinite time
period is its carrying capacity." (Horfnagels. pg. 792)
Logistic Population Growth
In the presence of density dependent
population growth is constrained at high population densities. This is
because the impact of density dependent factors depends on the
density of the population.
- Logistic Population Growth Simulation
At low population densities, density dependent
factors exert little
influence on population growth, which initially grows rapidly.
This is to say that individuals have an abundance of
their health is good. They have a high capacity to reproduce and are
less likely to die.
At high population densities, density dependent
factors exert an
increasing negative effect on population growth which slows and finally
stops at the carrying capacity.
The health of individuals is stressed because of
crowding, prevalent diseases, etc. Their reproductive capacity is
reduced and their liklihood of dying is greater.
Density dependent factors therefore produce an S-shaped
Graph: Hoefnagels text, Fig. 39.6, pg. 792
Population Reference Bureau 2009 World Population Data
Sheet (Downloadable PDF)
Activity - Graph:
Lewis et al., Life, Fig. 43.8, pg. 857
Density Independent environmental
resistance Factors (See Hoefnagels Pg 793)
Density Independent factors are Environmental
Factors that occur or have an effect on a population regardless of
the density of the population.
Density independent factors include weather phenomena
disasters that affect the population, but the chance of their
or level of severity is unrelated to the density of the population.
Density independent factors may affect the
that are required by the population (density dependent factors),
indirectly affecting the carrying capacity of the environment.
might destroy trees that are the required nesting site for a population
of birds. (from weatherunderground.com)