Factors That Determine
Population Growth
• Growth rate: change in a population’s size
over time.
• A population’s growth rate is determined by
births, deaths, immigration and emigration.
Why are there so many bacteria and so few
elephants?
1. Birth and Death Rates
• Mortality: rate of death
• Natality: rate of birth
 Survivorship Curves:
A graph that shows
how the likelihood of
death varies with age.
2. Age Structure
• Age structure of a population is
related to birth rates, death rates
and population growth.
• Young populations (pre and reproductive
age) = more births than deaths.
– Population increase
• Elderly population = more deaths
– Population decrease
• Sex Ratio also influences population size.
– Imbalance of males and females = changed
population size
Immigration and Emigration
• Immigration is the arrival of
individuals from outside a given
area.
• Emigration is the departure of
individuals from a given area.
• Migration: temporary movements into
and out of an area as a part of a
seasonal routine.
– Fish, mammals, insects and birds all
migrate
Calculating Population Growth
(birth rate + immigration rate) - (death rate + emigration rate) =
net change in population size
Population changes are often expressed as percentages, which we can
calculate using the following formula:
1.
GROWTH RATE x 100%
How Populations Grow
• Exponential Growth: when
a population increases by
a fixed percentage each
year.
• Logistic Growth: describes
how a population’s
increased growth is
slowed and stopped by
limiting factors.
Exponential Growth
• Faster and faster growth!
• Only occurs in nature when
populations have plenty of
food, water, space, and no
competition.
• Ex. Mold on bread
Logistic Growth
• Growth that slows and stops due
to limiting factors.
• Limiting factors:
Any environmental characteristic
that can slow population growth
and determine carrying capacity
• Water
• Space
• Food
• Predators
• Disease
What Limits Population Growth?
1. Carrying capacity
2. Limiting factors
3. Competition
Carrying Capacity
• Maximum population an ecosystem can
support.
• A population may grow past the carrying
capacity, but cannot stay at that size.
• Example: The state of Pennsylvania can only
support a certain number of white tailed deer.
There is a limited amount of food, water, and
shelter for the deer that live here. If the
population grows so much that deer do not
have enough of what they need, that
population has gone over the carrying capacity
and it will shrink.
Limiting Factors
• Whatever resource is lacking or missing
for a population
• Will ultimately cause a decline in the
population.
• Water
• Space
• Food
• Predators
• Disease
Competition
• Members within a population use
the same resources and therefore
will compete with each other.
• Part of natural selection: those
species that can access resources
and make best use of those
resources will survive.
• Examples:
Wolves: territory, food
Trees: sunlight
Biotic Potential
• Potential that each living thing has to produce offspring
in ideal conditions.
• Not every organism is created equally – some can
reproduce quickly and easily – others take quite a long
time!
Review Questions
1. What is the difference between exponential growth and
logistic growth? Which is more common over long
term in nature?
2. You are a population ecologist studying white-tailed
deer populations in your state. Populations have been
growing exponentially for some time, and food is
becoming a limiting factor. Many deer are dying of
starvation, and others are in bad health. What do you
recommend to state officials? Should people intervene
and try to limit deer populations through relocation or
hunting? Or should they do nothing and wait for the
population to regulate itself? Explain your reasoning.