Chapter 15
Section 1
How Populations Grow
Objectives
Distinguish among the 3 patterns of
dispersion in a population
Contrast exponential growth and
logistic growth
Differentiate R-strategists from Kstrategists
Population
Consists of all the individuals of a
species that live together in one place at
one time
– Populations tend to grow: multiple offspring
– Limits resources limit population
Ex. Population of walleye in Maple Lake
Population
Demography
Statistical study of all populations
Study the composition of a population
and try to predict how the size will
change
Ex. Growth of the population of Canada
in the next 10 years
Population Size
Number of individuals in a population
Can affect the populations ability to
survive
Ex. 40 moose in Glacier National Park
Population Density
Number of individuals that live in a
given area
If individuals of a population are few and
spaced widely apart, they may seldom
encounter one another, making
reproduction rare
Ex. If 20 walleye live by the dock of my
cottage
Population Density
Dispersion
The way the individuals of the population are
arranged in space
3 patterns:
– Random Distribution: location of each individual is
self determined or determined by chance
– Even Distribution: located at regular intervals
– Clumped Distribution: individuals are bunched
together in clusters
Dispersion
Population Model
Hypothetical population that attempts to
exhibit the key characteristics of a real
population
By making a change and observing the
outcomes, demographers can predict
what might occur in a real population
Growth Rate
Population grows when more individuals are
born than die
A simple population model describes the rate
of population growth as the difference
between the birth and death rates
Ex. Find the rate of population growth where
there are 360 births and 250 deaths in a year
– 360 - 250 = 110 (population is increasing by 110
individuals/year)
Exponential Growth Curve
Curve in which the
rate of population
growth stays the
same, as a result the
population size
increases steadily
Population size vs.
time, J shaped
Exponential Growth Curve
N = size of the current population
r = rate of growth
K = carrying capacity, population size
that an environment can sustain
Density Dependent Factors
Resources are density dependent factors
The rate at which they become depleted
depends upon the population density of the
population that uses them
Ex. Population of 200 seagulls on lake Erie
vs. Population of 50 seagulls on lake Erie
– Which will use up resources more quickly?
Logistic Model
Population model that takes into
account the declining resources
available to a population
Exponential growth is limited by a
density dependent factor
Assumes birth and death rates vary
Logistic Model
Logistic Model
When population is below carrying capacity,
growth rate is rapid
As population approaches carrying capacity
death rates rise, birth rates slow
– Result: rate of growth slows
Population stops growing when birth and
death rates are equal
If population exceeds carrying capacity,
deaths will increase and outnumber births
until population falls to carrying capacity
Population Growth Models
Simple model (part one): calculating the
population growth rate
– r (rate of growth) = birthrate - death rate
The rate of population growth equals
the rate of births minus the rate of
deaths
Population Growth Models
Simple model (part 2): exponential growth
curve
– Delta N (change in population) = rN
Once r has been determined for a population
(part 1) the number of individuals that will be
added to a population as it grows is equal to
the rate of growth multiplied by the number of
individuals in the current population (N)
Population Growth Models
More realistic model: logistic model
– Delta N = rN (K-N)/K
Population size calculations often need
to be adjusted by the number of
members of the population at carrying
capacity (K)
Density Independent Factors
Environmental conditions
Weather and climate
Ex. Mosquito populations
increase in the summer
while the weather is
warm, but decrease in the
winter
R-strategist
Strategy means pattern of living
R-strategist: grow exponentially when
environmental conditions allow them to
reproduce
Results in temporarily large populations
When environmental conditions worsen
population size drops quickly
Usually have short life span, reproduce early,
and have many offspring, offspring are small
and mature rapidly on their own
R-strategist & K-strategist
K-Stategists
K-strategists: population
density is usually near carrying
capacity
Characterized by long life
span, few young, slow
maturing process,
reproduction late in life,
provide care of young, live in
stable environments
R vs. K Strategist
Ex. R-strategist
– Insects like mosquitoes
Ex. K-strategist
– Whales, tigers
Review
1. Identify the pattern of dispersion of fans
attending a basketball game as random,
even, or clumped. Explain your answer.
2. Differentiate a logistic growth pattern from
an exponential growth pattern.
3. Describe why an R-strategist might be
better suited for an unpredictable
environment than a K-strategist.
Answers
1. The pattern of dispersion of fans at a
basketball game could be described as
clumped. This could be because usually
friends and family sit together, making a
clumped pattern.
2. A logistic growth pattern is a population
model that takes into account the declining
resources available to a population.
Whereas a exponential growth pattern is a
curve in which the rate of population growth
stays the same, as a result the population
size increases steadily.
Answers
3. An R- strategist may be better suited for an
unpredictable environment because they
are able to reproduce exponentially when
environmental conditions are favorable.
They would be able to build up a large
population and be able to handle an
unpredictable environment when their
population size would drop.