Dynamics of Natural Populations
• Population growth curves
• Biotic potential versus environmental
resistance
• Density dependence and critical number
Population Equilibrium
A dynamic balance between births and deaths.
Births
Deaths
Population Growth Curves
Population Growth Curves
• Reproductive strategies:
Many offspring with
low parental care
J-shaped growth curve
Few offspring with
high parental care
S-shaped growth curve
Population Dynamics
• Environmental resistance: combination of
biotic and abiotic factors that may limit
population increase.
• Biotic potential: combination of biotic and
abiotic factors that enhance population
increase.
Density Dependence and Critical Numbers
• Factors of environmental resistance are
either:
– density-independent: effect does not vary
with population density; e.g., adverse weather
– density-dependent: effect varies with
population density; e.g., infectious disease
• Critical number: the lowest population
level for survival and recovery
Mechanisms of Population Equilibrium
• Predator-prey dynamics
• Competition
– Interspecific
– Intraspecific
• Introduced species
Predator-prey Balance: Wolves and
Moose
Lessons to Be Learned About Predator-Prey
Balance
• Absence of natural enemies allows an
herbivore population to exceed carrying
capacity which results in overgrazing of
the habitat.
• The herbivore population subsequently
crashes.
• The size of the herbivore population is
maintained so that overgrazing or other
overuse does not occur.
Plant-Herbivore Dynamics
Reindeer on St. Matthew Island
• No regulatory control
(predation) on
herbivores
• Went into exponential
growth pattern
• Overgrazed habitat
• Massive die-off of
herbivores
Mechanisms of Population Equilibrium:
Plant-Herbivore
• Compare the predator-prey with plant-herbivore
methods of controlling the size of the herbivore
population.
• How would the herbivore population growth
curve look if diseases or predators were used as
the control mechanism?
Keystone Species
• A single species that
maintains biotic
structure of the
ecosystem
• Pisaster ochraeceus a starfish that feeds
on mussels, keeping
them from blanketing
the rocks.
http://www.marine.gov/
Competition: Intraspecific
• Territoriality: defense of a resource
against individuals of the same species
– Examples of wolves and songbirds
– Results in priority access and use of
resources
• How do wolves and songbirds establish
territory?
Competition: Interspecific
• Grassland contains
plants with both
fibrous and tap roots
• Coexist by accessing
resources from
different soil levels
Introduced Species
• Rabbits in Australia and on Phillip Island
(next slide)
• Chestnut blight in United States
• Japanese beetles, fire ants, gypsy moths
in United States
• Water hyacinth, kudzu, spotted knapweed,
purple loosestrife (see Fig. 4-13 in text) in
United States
Introduced Species
• Why have introductions of nonnative and
exotic species resulted in a degradation of
ecosystems? (Think in terms of
environmental resistance and biotic
potential.)
• An example of the answer to this question
is given in the next slide.
Introduced Species: Rabbits in
Australia
• Introduced into Australia from England in
1859
• No natural enemies – rabbit population
exploded
• Overabundant herbivore population
devastated natural vegetation (see Fig. 411 in text).
• Using disease as control measure – why
will this procedure fail in the long-term?
Rabbits Overgrazing in Australia
Phillip Island 1978
Largely devoid of vegetation
Phillip Island 1988 After Rabbit Removal
Vulnerability of different organisms to
environmental changes.
Which Species Will Survive in a
Rapidly Changing Environment?
• Panda
Narrow distribution
Small population
Low genetic variation
Large size
Small # of offspring
Limited migration
• Mouse
Wide distribution
Large population
High genetic variation
Small size
Large # of offspring
High migration
Pressures on Ecosystems
• What is the carrying
capacity for the
human population on
Earth?
• How will the human
ecological footprint
impact on nature’s
goods and services?