Population Size

 The overall health of a population can often be
monitored by tracking how its size changes.
 Population size – the number of individual
organisms present in a given population at a given
time
 It may increase, decrease, undergo cyclic change, or
remain the same over time
Determining Population Size

 The most direct way to determine population size is
to directly count…however, this is rarely possible!
 Sampling is the most common method of
determining population size
 Ecologists count the number of individuals in a
smaller, sample area
 That number is then extrapolated to estimate the entire
population
 EX – if there are 100 oak trees in 1 km2 – it can be
assumed there are about 1000 oak trees in 10 km2
Determining Population
Size, con’t

 Sometimes it is not possible to count even a small
sample of the organisms of the population
 Rare organisms, or those that prefer to remain hidden
can be particularly difficult to count
 In these cases, scientists often use signs of the
organisms – tracks, droppings or other signs can be
used to estimate population size
Population Density

 Population density – the number of individuals
within a population per unit area
 Ex – 1500 toads/4 km2 or 300 toads/km2
 Essentially, population density is a measure of how
crowded the population is
 In general, larger organisms have lower population
densities – as they require larger habitats to meet
their needs
Different Densities

 High population density can make it easier for
organisms to group together and find mates – but it
can also make for limited resources
 Overcrowded populations may also be more
vulnerable to predators and disease
 Populations must find the right balance
Population Dispersion

 Population dispersion– how organisms are arranged
within their population area
 Also called population distribution
 Population dispersion can be random, uniform, or
clumped
Random Dispersion

 There is no discernable
pattern to how individual
organisms arrange
themselves
 This occurs when the
resources needed are
readily available in all
parts of the area
 Another factor is the lack
of a strong influence from
other organisms
Uniform Dispersion

 Individual organisms
are evenly distributed
throughout the area
 May occur because of
competition for
territory or space
Clumped Dispersion

 Individual organisms
arrange themselves
according to the
availability of needed
resources
 This is the most
common pattern in
nature
Factors That Determine
Population Growth

 Populations increase in size when more organisms
enter than leave
 Entering a population occurs primarily through
immigration and birth
 Populations decrease in size when more organisms
leave than enter
 Leaving a population occurs primarily through
emigration and death
Birth and Death Rates

 The rate at which individuals is born is called
natality
 The rate at which individuals die is called mortality
 Both natality and mortality are generally expressed
per 1000 individuals over a given time
 Ex: 25 deaths/1000 in the year 2013
 If all other factors are equal, if birth rate is higher
than death rate, the population size will increase
 If death rate is higher than birth rate, the population
size will decrease
Survivorship

 Survivorship refers to the
likelihood that an
individual will survive to
a certain age
 Individuals of different
ages have different
probabilities of dying
 A survivorship curve is
used to show how the
likelihood of death varies
with age within a
population
Migration

 Sometimes organisms move around – staying in
certain locations for specific times/seasons
 This seasonal movement into and out of areas is
called migration
 Depending on the season can either increase or
decrease population size
Immigration & Emigration

 Immigration – arrival
of organisms to an area
 Increases the
population size
 Emigration –
organisms leaving an
area
 Decreases the
population size
Calculating Population
Growth

 A population’s overall growth rate can therefore be
expressed as:
 (Individuals added) – (Individuals subtracted)
Calculating Population
Growth

 A sample population has an annual birth rate of
18/1000; their death rate is 10/1000. Annual
immigration is 5/1000 and the emigration rate is
7/1000.
 What is the overall growth rate of this population? Is
this population growing?
How Populations Grow

 Populations increase in size according to 2 patterns
 Exponential Growth
 Logistical Growth
 Growth rates tend to change according to the
resources available to the organisms in the
population
Exponential Growth

 When populations increase
by a fixed percentage each
year – they are said to be
undergoing exponential
growth
 Normally only occurs in
nature when the starting
population is small and
environmental conditions
are ideal
 Can not continue
indefinitely!
Logistic Growth

 In reality – exponential growth rarely lasts long
 Limiting factors come into play and limit the
population growth
 Water, space, food, predators, disease
 Carrying capacity – the largest population size a
given environment can sustainably support
 The regulation of an initial exponential growth curve
by limiting factors is called logistic growth
Logistic Growth

Population Growth in
Nature

 The logistic curve is a simplified model – and real
populations can behave differently
 Some populations fluctuate or cycle
 Others rise quickly, only to overshoot and then crash
 Carrying capacity is not fixed – as environments
change, so do their capacities to support populations
Population Growth in
Nature

Limiting Factors

 Limiting factors regulate population growth in a
variety of ways
 Some limiting factors are more effective in dense
populations
 Density-dependent factors
 Some limiting factors effect all populations the same
 Density-independent factors
Density-Dependent
Factors

 Density-dependent factors include competition,
predation, and disease
 Each of these factors has a greater effect on a dense
population
 A less-dense population would be better able to adjust
to these factors and prevent a change in growth
Density-Independent
Factors

 Density-independent factors are generally
catastrophic events that change an environment
dramatically
 Fires, floods, landslides, volcanic eruptions, etc.
 These events effect all populations equally – rapid
reduction in population size
Reproductive Potential

 Reproductive potential indicates the maximum
ability of an organism to produce offspring in ideal
conditions
 This is a characteristic of the organism, NOT the
environment
 Many factors can affect reproductive potential
 Gestation time – how long it takes for an embryo to
develop and be born
 Gestation number – how many babies are born at a time
 Generation time – span from an organism’s birth to
when it can have its own offspring
Reproductive Potential

 Reproductive potential can be affected by
environmental change and can determine how well a
population will rebound from a decline
 Reproductive potential is measured under ideal
conditions – so less than ideal situations will result in
lower actual numbers
 A population with a high reproductive potential will
rebound more quickly from an environmentallytriggered decline