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Population Biology
Population: A group of organisms of the same species living
in the same area and interacting with one another.
How Study Populations?
Geographic Range
Density and Distribution
Growth rate
Age structure
Geographic Range
• The area inhabited by a
population
• Can vary greatly depending
on the species
 (bacteria on a rotting
pumpkin vs. cod in the western
Atlantic ocean from Greenland
to North Carolina
Density and Distribution
• Population density refers to the
number of individuals per unit area
• Distribution refers to how individuals
in a population are spaced out across
the range of the population:
 random, uniform or clumps.
Clumped
Random
Uniform
Exponential Growth – “J” curve
1.
Rapid rate of increase when resources are unlimited
and optimal growing conditions are present.
2.
The populations double with each generation, such
that the larger the population is, the faster it grows.
3.
Obviously this rate of growth cannot continue
indefinitely.
Number of Species over Time
Logistic Growth “S” Curve
1.
This type of growth grows rapidly at first resembling
exponential growth, but levels off toward later stages.
2.
Related to the fact that resources are limited, and as the
population grows larger, it is harder for each individual
to get what it needs, so growth slows.
3. Population may slow down either because of decrease in
birth rate or an increase in death rate (or both)
4. Populations tend to stabilize around a specific number,
although there may be slight fluctuations around that
amount.
Logistic Curve
Carrying Capacity –
the maximum number of
organisms of the same
species that can be
supported in a specific
are indefinitely.
Once the carrying capacity is reached, certain factors
work to keep population in check (including lack of
food, overcrowding, predations, accumulation of
waste)
If a natural population overshoots the carrying
capacity, three thing can happen:
1. It will die back to the original cc.
2. It will die back, but because of damage to the
environment, cc. is lower
3. It will become extinct.
Age Structure
Consider especially the pre-reproductive compared to
the post-reproductive years.
– Very young and very old and more susceptive to
disease
– If there are huge numbers of young adults, the
population will grow; if there are mostly elderly,
the population will decline.
Sex Ratios
• In a monogamous species, the ratio of males to
females should be about equal.
• In deer for example, that is not as important,
because one male often fertilizes many females.
Factors that Restrict the Size of a
Population
Limiting Factors of
Environmental Resistance
Density-Dependent Limiting Factors
1.
2.
3.
4.
The degree of influence depends on the size of the
population.
Examples: competition, predation, parasitism, stress caused
by crowding, disease
Seldom totally eliminate a population
Some populations are self-controlling in that they have
mechanisms to reduce competition – delayed maturation
caused by malnutrition, migration, cannibalism.
Density-Independent Limiting Factors
1.
2.
3.
The degree of influence is not related to population
density.
Examples: pollution, habitat destruction, natural
disasters, weather
Usually just reduce the population below the cc. but
they have the ability to totally eliminate a population
Relationships Within a
Population
Population Biology
Population: A group of organisms of the same species living
in the same area and interacting with one another.
Relationships Between
Populations of Different Species
Relationships Between Populations of
Different Species
A. Predator-Prey
B. Competition
C. Symbiosis
Relationships Between Populations of
Different Species: Predator- Prey
• Predation is any interaction between
two organisms in which one organism
(the predator) consumes all or part of
another organism (the prey).
What actually happens in nature is that a cycle develops where
at some time the prey may be abundant and the predators few.
Because of the abundance of prey, the predator population
grows and reduces the population of prey. This results in a
reduction of predators and consequent increase of prey and the
cycle continues.
-Predator
-Prey
Competition
• Competition in an interaction between two
organisms that are using the same limited
resource.
• Competition can be within the same species
(intraspecific) or between different species
(interspecific).
Competition
Intraspecific – between the same species
Interspecific – between different species
Symbiosis
Any relationship which two species live closely
together (and at least 1 benefits)
The three types are:
Mutualism
Commensalism
Parasitism
Mutualism
an interaction
between species in
which BOTH
benefit
(+,+)
Commensalism
an interaction
between species in
which one benefits,
and the other is not
helped or harmed
(+,0)
Parasitism
an interaction
between species in
which one benefits
and one is harmed
(+,-)