Endangered Southern Sea Otter
Chapter 8:
Population Ecology
Laura Cunningham
3rd Hour
Objectives
1) What are the major characteristics of
populations?
Organisms disperse in their environment in three
general patterns of population distribution:
clumping, uniform dispersion, and random
dispersion. Populations increase through births and
immigration and decrease through deaths and
emigration. When a population exceeds its resource
supplies, many of its members will die unless they
can switch to new resources or move to an area with
more resources.
Objectives
2) How do populations respond to changes in
environmental conditions?
If a population overshoots its carrying capacity,
the group must switch to new resources or
move to an area with more resources. Species
can increase their carrying capacity by
developing adaptive traits through natural
selection that reduce environmental resistance
to their population growth. Weather, climate,
and other factors can increase or decrease
carrying capacity.
Objectives
3) How do species differ in their reproductive
patterns?
Asexual vs. sexual reproduction; some are
opportunists (r-selected species), some are
competitors (k-selected species). Reproductive
patterns may give a species a temporary
advantage, but the availability of a suitable
habitat for individuals of a population in a
particular area determines its ultimate population
size.
Population Dynamics and
Carrying Capacity
• Population Dynamics – how and why populations
change in their distribution, numbers, age structure, and
density in response to changes in environmental
conditions.
• Population Density – the number of individuals in a
population found in a particular area or volume.
• Age Structure – the proportions of individuals at various
ages in a population.
Limits on Population Growth
• Biotic Potential – maximum rate at which the population of a
given species can increase when there are no limits on its
rate of growth. The intrinsic rate of increase (r) is the rate at
which a population would grow if it had unlimited resources.
• Environmental Resistance – all factors that act to limit the
growth of a population.
Biotic Potential + Environmental Resistance =
Carrying Capacity
(The maximum population of a given species that a particular
habitat can sustain indefinitely without degrading the habitat.)
Exponential vs. Logistic
Population Growth
No Population can increase
in size indefinitely.
• Exponential Growth: starts
slowly but then accelerates
as the population increases
because the base size of
the population is increasing.
(J-Shaped Curve)
- Occurs when resources are
not limited and a population
can grow at its intrinsic rate of
increase, or biotic potential.
Exponential vs. Logistic
Population Growth
Exponential growth is converted
to logistic growth, in which the
growth rate decreases as the
population becomes larger and
faces environmental resistance.
• Logistic Growth:
involves rapid
exponential population
growth followed by a
steady decrease in
population growth with
time until the population
size levels off.
(S-Shaped Curve)
Reproductive Patterns
• Asexual Reproduction – offspring are exact genetic
copies (clones) of a single parent. Common in species
such as single-celled bacteria. Many plants and animals
such as coral reproduce this way.
• Sexual Reproduction – mixes the genetic material of
two individuals and produces offspring with combinations
of genetic traits from each parent. Used by 97% of
earth’s species.
Opportunists and Competitors
• Opportunists (R-Selected Species) - reproduce early
in their lifespan and produce large numbers of usually
small and short-lived offspring in a short period. They
have a high capacity for population increase.
Ex) Dandelions, Cockroaches, Bacteria, Rodents
- They reproduce and disperse rapidly when conditions
are favorable or when a disturbance opens up a new
habitat for invasion.
Opportunists and Competitors
• Competitors (K-Selected Species) – produce a few,
often fairly large offspring but invest a great deal of time
and energy to ensure that most of those offspring reach
reproductive age.
Ex) Elephants, Humans, Saguaro Cactus
- They do well in competitive conditions when their
population is near the carrying capacity. Their
populations typically follow a logistic growth curve.
Survivorship Curve
• Graph that shows the
percentages of the
members of a population
surviving at different ages.
• Three general curve types:
late loss, which live to an
old age (elephants,
humans); early loss,
where most members die
at a young age (annual
plants, bony fish species);
and constant loss, which
die at all ages (songbirds).
“We cannot command nature
except by obeying her.”
Sir Francis Bacon