Transcript Population Biology

Population Biology
Population Growth
1.
2.
3.
4.
5.
6.
Review: What is a population?
What is population growth?
How is population growth determined?
When does it happen?
Why is it important?
How fast do populations grow…?
Biotic Potential
• The biotic potential of a population is its
potential to grow (in numbers) if all
offspring survive and reproduce at max
capacity
• Biotic potential is not reached in nature
– why?
• Offspring die
• Not all reproduce
• Limiting factors
Dynamics of Growth
• More births than deaths =
Growth
• More deaths than births =
Decrease
• Deaths = Births: No
population growth
Dynamics of Growth
• Exponential
Growth growth at a
constant
rate of
increase
per unit
time.
J Curve - Exponential Growth
• Nearly all
populations
will grow
exponentially
under ideal
conditions
• This type of
curve shows
indefinite
(unlimited)
growth
Exponential Growth
• As a population gets larger, it also grows
faster
• Example:
– Bacteria (per 24 hours): 2 – 4 – 8 – Million (25
days later)
• This type of growth occurs when:
1. Species moves into a new habitat
2. A new food supply is made available
Another Type of Growth: S Curve
• Most populations do not
grow exponentially for long
• Exponential growth stops
when a population
encounters limiting
factor(s) such as availability
of food and physical space
S Curve (Logistic growth)
• When limiting factors are
encountered, growth stops increasing,
and a “leveling-off” occurs – a
flattening out of the graph/curve
• This upper limit is called the carrying
capacity: the # of individuals the
environment can support
–Represented by the letter “K”
–Population stabilizes
Environmental Limits
To Population Growth
•
Limiting Factors: biotic or abiotic factors that
determine whether an organism can live in a
particular environment; controls growth of
population
1. Density Dependant: space (including
stress from overcrowding, parasitism
&disease, competition, food,
predation, herbivory
2. Density Independent (affect all
populations in similar ways):
temperature, storms, floods, droughts,
other disturbances
Organism Interactions Limit
Population Size
• Predation/herbivory
– Keeps prey and plant population down;
eventually predator population down
(fluctuates)
• Competition for resources
– Demand vs. supply
• Crowding and stress
– Organisms can exhibit aggression,
decreased resistance to disease,
decreased fertility
Summary
• Two opposing forces affect population
size: biotic potential and
environmental resistance
• A graph of typical population growth is an
S-curve, with K as the upper limit
• A number of environmental factors
(density de- and inde- pendant) impose
limits on population growth
• Predictions of future growth can be made
by graphing the age make-up of a
population in an age-structure graph