Population Ecology Lecture by Salman Saeed

Course Title:
Plant Physiology and
Ecology
Course Instructor:
SALMAN SAEED
Botany department
UNIVERSITY college of management & Sciences, Khanewal,
PAKSITAN

 Population ecology is the study of
populations in relation to the environment. It
environmental
includes
population density and
influences on
distribution, age
structure, and variations in population size.
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Salman Saeed (Botanist)

 Population size
 Population density
 Dispersion
patterns
 Demographics
 Survivorship curves
 Population growth
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 “In population genetics and population
ecology, population size (usually denoted N)
is the number of individual organisms in a
population”.
Factors that Govern Population
Size
1. Crude Birth Rate (CBR)
2. Crude Death Rate (CDR)
3. Immigration
4. Emigration
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Natality
 The birthrate, which is the ratio of total live
births to total population in a particular area
over a specified period of time
Mortality
 The death rate, which is also the ratio of the
total number of deaths to the total population.
Immigration
 The number of organisms moving into
occupied by the population is
area
called
immigration.
Emigration
 The number of organisms moving out of the area
occupied by the population is called emigration.
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 Factors that increase population size
1. Natality is recruitment to a population
through reproduction.
2. Immigration from external populations e.g.
Bird migration.
 Factor reducing population size
1. Mortality which is the death rate from any
source e.g. predation.
2. Emigration, where individuals leave the
population for another habitat.
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Population Change = (births +
immigration) – (deaths + emigration)
Parameters that effect size or density of a
population
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 “Population density is a measurement of
the number of people in an area. It is an
average number”.
 It is usually shown as the number of
people per square kilometer.
Density = Population/ Area
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1. How to quantify nature – ecologist
role
2. Estimates are allows for
comparisons between different
populations in terms of space and
time measure.
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2 attributes
Mobility
Based on movements of these
organisms
Size
Small animals/plants are usually
more abundant than large
animals/plants
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3 types uniform
clumped
 The dispersion of a population is the pattern of
spacing among individuals within the
geographic boundaries.
random
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 Clumped is a pattern when individuals are
aggregated in patches.
 Most frequent pattern of distribution in a
population
Reasons:
 Some area of habitat are more suitable than
others
 Heterogeneous environment with resources
concentrated in patches
 Tendency of offspring to remain with parents
 Mating or social behavior of the individuals
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spaced in which
 Evenly
members of the
distributions,
population maintain a
minimum distance from one another.
 In plants due to competition for water
,
sunlight, or available nutrients
Example
Creosote bushes in the Mojave desert
 In animals due to strong territoriality
Example
The desert lizard Uta sp
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total
 It is a spacing pattern based on
unpredictability.
 Least common pattern of distribution
Reasons
 Members of a species do not frequently
interact with one another
 Not heavily influenced by the
microenvironments within their habitat
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Density Independent
Floods
Hurricanes
Unseasonable
Weather
Fire
Clear Cutting
Pesticide Spraying
Density Dependant
Competition for
Resources
Predation
Parasitism
Infectious Disease
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 Demography is the study of the vital
statistics of a population and how they
change over time
 Two statistics that are of particular import
are a population's age structure and a
population's sex ratio.
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 It is the ratio of males to females in a
population.
Primary sex ratio
Secondary sex ratio
Tertiary sex ratio
 The human sex ratio is of particular interest
to anthropologists and demographers.
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 The age structure describes the number of
individuals in each age class as a ratio of one
class to another.
 Age classes can be specific categories, such
as people in the same age range.
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 Age structure diagram
 Graphical illustration that shows the
distribution of various age groups & sex ratio
in a population.
 Three age categories:
1. Prereproductive (ages 0-14)
2. Reproductive (ages 15-44)
3. Postreproductive (ages 45 and up)
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 A life table (mortality table ) is a table which
shows, for each age, what the probability is
that a person of that age will die before his or
her next birthday.
 From this starting point, a number of
inferences can be derived.
1. The probability of surviving any particular
year of age
2. Remaining life expectancy for people at
different ages
 Separately for men and for women because of
their substantially different mortality rates.
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 Graph showing the number or proportion of
individuals surviving at each age for a given
species or group (e.g. males/females).
 Constructed for a given cohort (a group of
individuals of roughly the same age) based on
a life table.
 Three types
1. Type I
2. Type II
3. Type III
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 Type I survivorship curves are characterized
by high survival in early and middle life,
followed a rapid decline in survivorship in
later life.
Example: Humans
 Type II curves are an intermediate between
Type I and III, where roughly constant
mortality rate is experienced regardless of
age.
Example: Some birds
 In Type III curves, the greatest mortality is
experienced early on in life, with relatively
low rates of death for those surviving this
bottleneck.
Example: Octopus
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 Refers to how the number of individuals in a
population increases (or decreases) with
time.
 Controlled by the rate at which new
individuals are added to the population -- the
birth rate, and the rate at which individuals
leave the population -- the death rate.
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2 types of pop growth
Exponential
population growth
dN = rmaxN
dt
Logistic population
growth
dN = rmaxN (K-N)
dt K
Population Growth
Mathematically
Defined
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N=K/2
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 If a population has a constant birth rate
through time and is never limited by food or
disease, it has what is known as exponential
growth.
 With exponential growth the birth rate alone
controls how fast (or slow) the population
grows.
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 Continuous population growth in an unlimited
environment can be modeled exponentially.
dN / dt = rmax N
rate of
 As population size (N) increases,
population increase (dN/dt) gets larger.
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 For an exponentially growing population, size at
any time can be calculated as:
Nt = Noert
 Nt = number individuals at time t.
 N0 = initial number of individuals.
 e = base of natural logarithms.
 r (rmax ) = per capita rate of increase.
 t = number of time intervals.
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 As resources are depleted, population
rate slows and
growth
eventually stops: logistic
population growth.
 Sigmoid (S-shaped) curve
 Carrying capacity (K) is the number of individuals
of a population the environment can support.
 Finite amount of resources can only support a
finite number of individuals.
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dN/dt = rmaxN(1-N/K)
 rmax = Maximum per capita rate of increase under
ideal conditions.
 When N nears K, the right side of the equation
nears zero.
 As population size increases, logistic growth rate
becomes a small fraction of growth rate.
 Highest when N=K/2
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Population Ecology Lecture by Salman Saeed

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