Download Populations - OnMyCalendar

Population Dynamics
UNIT II
Characteristics of Populations
 Three important characteristics of a population are:
 Its geographic distribution (range)


Population Density


Describes the area inhabited by a population – can vary in size
The number of individuals per unit area
Growth rate

The number of births minus the number of deaths
Patterns of Dispersion
 Environmental and social
factors influence the spacing
of individuals in a
population.
 3 Patterns:
Clumped
 Uniform
 random

Survivorship Curves
 A life table is an age-specific summary of the
survival pattern of a population
 Data in a life table can be represented graphically
by a survival curve.
 Curve usually based on a standardized population
of 1000 individuals and the X-axis scale is
logarithmic.
Type 1
 Type 2
 Type 3

Survivorship Curves
 Survivorship curves can be classified into
three general types
Type I, Type II, and Type III
Number of survivors (log
scale)

1,000
I
TYPE 1: high survivorship until
old age.
100
II
TYPE 2: constant proportion of
individuals die at each age.
10
III
1
0
100
50
Percentage of maximum life span
TYPE 3: experience high mortality
as larvae but decreased mortality
later in life.
Type I curve
 Type I curve typical of animals that produce few
young but care for them well


(e.g. humans, elephants)
Death rate low until late in life where rate increases sharply as
a result of old age (wear and tear, accumulation of cellular
damage, cancer).
Type II curve
 Type II curve has fairly steady death rate throughout
life (e.g. rodents).
 Death is usually a result of chance processes over
which the organism has little control (e.g. predation)
Type III curve
 Type III curve typical of species that produce large
numbers of young which receive little or no care
(e.g. Oyster).
 Survival of young is dependent on luck. Larvae
released into sea have only a small chance of
settling on a suitable substrate.

Once settled however, prospects of survival are much
better and a long life is possible.
Population Growth
 Three factors that can affect population size:
 Number of births
 Number of deaths
 Number of individuals that enter or leave the population
(immigration & emigration)
 A population will increase or decrease in size
depending on how many individuals are added to it
or removed from it
Exponential Population Growth (EPG)
 Describes population growth in an idealized,
unlimited environment.
 During EPG the rate of reproduction is at its
maximum.
Exponential Growth
Exponential Growth Trends
 If a population has abundant space and food, and is
protected from predators and disease, then organisms in
that population will multiply and the population size will
increase.
 The J shaped curve indicates that the population is
undergoing exponential growth. This occurs when the
individuals in a population reproduce at a constant rate.
 Under ideal conditions with unlimited resources, a
population will grow exponentially and reach BIOTIC
POTENTIAL.
Carrying Capacity
 Exponential growth cannot be sustained for long in
any population.
 A more realistic population model limits growth by
incorporating carrying capacity.

Carrying Capacity (K) is the maximum population size the
environment can support.
The Logistic Growth Model
 In the logistic population growth model the rate of increase
declines as carrying capacity is approached.
Logistic Growth Trends
 As resources become less available, the growth of a population slows
or stops. The general S shaped curve of this growth pattern is called
logistic growth.
 Logistic growth occurs when a population’s growth slows or stops
following a period of exponential growth.



Can slow when the birthrate decreases, the death rate increases, or when
both occur at the same rate
Can slow when the rate of immigration decreases, the rate of emigration
increases, or both
Can slow as the population encounters a limiting factor
 The point at which carrying capacity line intercepts the y axis tells
you the size of the population when the average growth rate is zero.
 That number represents the largest number of individuals that a
given environment can support (carrying capacity).
Phases of Logistic Growth Curve
3
2
1
Logistic model produces a sigmoid (S-shaped) population growth curve.
K vs. R Reproduction Strategies
 K-selection, or density-dependent selection
 Selects for life history traits that are sensitive to population density.
 Produce relative FEW offspring that have a GOOD chance of survival.
 r-selection, or density-independent selection
 Selects for life history traits that maximize reproduction.
 High reproductive rate is the chief determinant of life history.
 The concepts of K-selection and r-selection have been
criticized by ecologists as oversimplifications.

Most organisms exhibit intermediate traits or can adjust their behavior
to different conditions.
Limits to Growth
 Limiting Factors- any factor that causes population
growth to decrease
Limits to Growth
 In the context of populations, a limiting factor is a
factor that causes population growth to decrease





Competition
Predation
Parasitism
Drought & other climate extremes
Human disturbances
Density & Growth Regulation
 Density Dependent factors – include
disease, competition, parasites and food.
These have an increasing effect as the
population increases.
 Density Independent factors – affect all
populations regardless of their density
(numbers) Most are abiotic factors such as
temperature, storms, floods, droughts and
habitat destruction.
Population Regulation
 Populations are regulated by a complex interaction of
biotic and abiotic influences:

In density-independent populations birth rate and death
rate do not change with population density.


For example, in dune fescue grass environmental conditions kill a
similar proportion of individuals regardless of density.
In contrast in density-dependent populations birth rates fall
and death rates rise with population density.

Density-dependent population regulation much more common than
density- independent
Density-Dependent Factors
 A limiting factor that depends on population size is
called a density-dependent limiting factor

These factors become limiting only when the number of
organisms per unit area reaches a certain level
Competition
 Predation
 Parasitism
 Disease

Competition
 When populations become crowded, organisms
compete, or struggle, with one another for food,
water, space, sunlight, and other essential elements
of life
 The more individuals that live in an area, the
sooner they will use up an available resource


Interspecific competition occurs between members of two
different species.
Intraspecific competition occurs between members of the
same species.
Predation
 Populations in nature are often controlled by
predation
 The regulation of a population by predation takes
place within the predator-prey relationship (one
of the best known mechanisms of population
control)
Predation Can Affect Population Sizes
Parasitism & Disease
 Parasites can limit the growth of a population
because they take nourishment at the expense of
their hosts
 This often weakens the host and can lead to disease
or death
Density-Independent Factors
 Density-independent limiting factors affect all
populations in similar ways, regardless of the
population size




Weather
Natural disasters
Seasonal cycles
Human activities

Damming rivers, cutting forests
Human Population Growth
 Like the populations of many other living organisms,
the size of the human population tends to increase
with time


Began growing more rapidly ~500 years ago because
agriculture and industry made life easier
Improved sanitation, medicine and health care
 But…earth’s resources are limited!
Human Population Growth
Patterns of Population Growth
 The scientific study of human populations is called
demography

Examines the characteristics of human populations and
attempts to explain how those populations will change over
time
 Birthrates, death rates, and the age structure of a
population help predict why some countries have
high growth rates while other countries grow more
slowly
The Demographic Transition
The Demographic Transition
 Over the past century, population growth in the
US, Japan, and much of Europe has slowed
dramatically. One hypothesis as to why is that
these countries have completed the demographic
transition (a dramatic change in birth and death
rates).
 As countries modernize, advances in nutrition,
medicine, and sanitation result in more children
surviving into adulthood and more adults living to
old age. These changes lower the death rate and
begin the demographic transition.
Age Structure
 Population growth depends in large part on how
many people of different ages make up a given
population
 We can predict future growth using models called
age-structure diagrams

Graph the numbers of people in different age groups in the
population
Age Structure Diagrams
 Age structure diagram- a population profile, graphs
the numbers of people in different age groups in the
population