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Chapter-
Population Ecology
Population- an interbreeding group of
individuals of a single species that occupy the
same general area
Community-the assemblage of interacting
populations that inhabit the same area.
Ecosystem- comprised of 1 or more
communities and the abiotic environment within
an area.
Major Characteristics of a
Population
• Population dynamics is the study of how
populations change in:
– Size – ( total # of individuals)
– Density ( # individuals in a certain space)
– Age distribution ( the proportion of individuals
in each age in a population) in response to
changes in environmental conditions.
The characteristics of populations are
shaped by the interactions between
individuals and their environment
• Populations have size and geographical
boundaries.
– The density of a population is measured as the
number of individuals per unit area.
– The dispersion of a population is the pattern of
spacing among individuals within the geographic
boundaries.
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MEASURING DENSITY
Density – Number of individuals per unit
of area.
•Determination of Density
•Counting Individuals
•Estimates By Counting Individuals
•Estimates By Indirect Indicators
•Mark-recapture Method
N = (Number Marked) X (Catch Second Time)
Number Of Marked Recaptures
• Measuring density of populations is a
difficult task.
– We can count individuals; we can estimate
population numbers.
Fig. 52.1
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PATTERN OF DISPERSION
UNIFORM
CLUMPED
RANDOM
• Patterns of dispersion.
– Within a population’s geographic range,
local densities may vary considerably.
– Different dispersion patterns result within
the range.
– Overall, dispersion depends on resource
distribution.
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Clumped Dispersion
• Resources a species
needs varies greatly
place to place
• Living in herds/flocks
for protection
• Hunting packs
increases survival of
gaining food
• Temporary groups for
mating
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Uniformed
Uniform
Dispersion
• Species maintain
consistent distance
between individuals
• Access to scarce
sources and less
competition
Random Dispersion
• Organisms with
random distribution is
rare. Even within
forest you find clumps
of species, or
vegetation around
light sources, water ,
or another resource.
Fig. 52.2c
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Demography is the study of factors
that affect the growth and decline of
populations
• Additions occur through birth, and
subtractions occur through death.
– Demography studies the vital statistics
that affect population size.
• Life tables and survivorship curves.
– A life table is an age-specific summary of
the survival pattern of a population.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Population Dynamics
•Characteristics of Dynamics
•Size
•Density
•Dispersal
•Immigration
•Emigration
•Births
•Deaths
•Survivorship
Parameters that effect size or density of a population:
Immigration
Birth
Population (N)
Death
Emigration
Figure 1. The size of a population is determined by
a balance between births, immigration, deaths and
emigration
Age Structure: the proportion of individuals in each
age class of a population
Age Pyramid
Female
Male
Age Interval ~ y
8-9
6-7
4-5
2-3
0-1
-10.0
-5.0
0.0
5.0
10.0
Percent of Population
Figure 2. Age pyramid. Notice that it is split into two halves for male and female members of
the population.
• The best way to construct life table is to follow
a cohort, a group of individuals of the same
age throughout their lifetime.
Table 52.1
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Survivorship Curves
–A graphic way of representing
the data is a survivorship
curve.
• This is a plot of the number of
individuals in a cohort still alive at
each age.
–A Type I curve shows a low death
rate early in life (humans).
–The Type II curve shows constant
mortality (squirrels).
–Type III curve shows a high death
rate early in life (oysters).
Survivorship Curve
• Reproductive rates.
– Demographers that study populations
usually ignore males, and focus on
females because only females give birth
to offspring.
– A reproductive table is an age-specific
summary of the reproductive rates in a
population.
• For sexual species, the table tallies the
number of female offspring produced by
each age group.
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Reproductive Table
Table 52.2
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Life History
• The traits that affect an organism’s schedule
of reproduction and survival make up its life
history.
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Life histories are very diverse, but they
exhibit patterns in their variability
• Life histories are a result of
natural selection, and often
parallel environmental factors.
• Some organisms, such as the
agave plant,exhibit what is
known as big-bang
reproduction, where large
numbers of offspring are
produced in each reproduction,
after which the individual
often dies.
• This is also known as
semelparity
Agaves
• By contrast, some organisms produce only
a few eggs during repeated reproductive
episodes.
– This is also known as iteroparity.
• What factors contribute to the evolution of
semelparity and iteroparity?
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Limited resources mandate trade-offs
between investments in reproduction
and survival
• The life-histories represent an evolutionary
resolution of several conflicting demands.
– Sometimes we see trade-offs between survival
and reproduction when resources are limited.
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• For example, red deer show a higher mortality
rate in winters following reproductive
episodes.
Fig. 52.5
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• Variations also occur in seed crop size
in plants.
– The number of offspring produced at each
reproductive episode exhibits a trade-off
between number and quality of offspring.
dandelion
Coconut palm
The exponential model of population
describes an idealized population in an
unlimited environment
• We define a change in population size based on
the following verbal equation.
Change in population
=
size during time interval
Births during
time interval
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–
Deaths during
time interval
• Using mathematical notation we can
express this relationship as follows:
– If N represents population size, and t
represents time, then N is the change is
population size and t represents the
change in time, then:
• N/t = B-D
• Where B is the number of births and D is the
number of deaths
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– We can simplify the equation and use r to
represent the difference in per capita birth
and death rates.
• N/t = rN OR dN/dt = rN
– If B = D then there is zero population
growth (ZPG).
– Under ideal conditions, a population grows
rapidly.
• Exponential population growth is said to be
happening
• Under these conditions, we may assume the
maximum growth rate for the population (rmax) to give
us the following exponential growth
• dN/dt = rmaxN
Fig. 52.9
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The logistic model of
population growth incorporates
the concept of carrying capacity
• Typically, unlimited resources are
rare.
–Population growth is therefore
regulated by carrying capacity (K),
which is the maximum stable
population size a particular
environment can support.
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Example of Exponential Growth
Kruger National Park, South Africa
POPULATION GROWTH RATE
LOGISTIC GROWTH RATE
Assumes that the rate of population
growth slows as the population size
approaches carrying capacity, leveling
to a constant level. S-shaped curve
CARRYING CAPACITY
The maximum sustainable population
a particular environment can support
over a long period of time.
Figure 52.11 Population growth predicted by the logistic model
• How well does the logistic model fit the
growth of real populations?
– The growth of laboratory populations of
some animals fits the S-shaped curves
fairly well.
Stable population
Seasonal increase
– Some of the assumptions built into the
logistic model do not apply to all
populations.
• It is a model which provides a basis from
which we can compare real populations.
Severe Environmental Impact
• The logistic population growth model and life
histories.
– This model predicts different growth rates for
different populations, relative to carrying
capacity.
• Resource availability depends on the situation.
• The life history traits that natural selection favors may
vary with population density and environmental
conditions.
• In K-selection, organisms live and reproduce around K,
and are sensitive to population density.
• In r-selection, organisms exhibit high rates of
reproduction and occur in variable environments in
which population densities fluctuate well below K.
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Species Reproductive Patterns
• r-Selected species, opportunists – species with a
capacity for a high rate of population increase
– Many small offspring
– Little to no parental care or protection
– Reproductive opportunists
• K-selected species, competitors – reproduce later
in life and have a small number of offspring with
fairly long life spans
– Few large offspring
– High parental care
K-Selected Species
•
•
•
•
•
Poor colonizers
Slow maturity
Long-lived
Low fecundity (reproductive rate)
High investment in care for the
young
• Specialist
• Good competitors
r-Selected Species
•
•
•
•
•
Good colonizers
Reach sexual maturity rapidly
Short-lived
High fecundity
Low investment in care for the
young
• Generalists
• Poor competitors
Positions of r- and K-Selected Species on the S-Shaped
Population Growth Curve
Transitioning between J and S curves…
• Carry capacity isn’t fixed
– Varies depending on climate and season
– Unpredictable changes can be devastating to the species
AND the habitat
• Reproductive time lag – period needed for the
birth rate to fall and the death rate to rise in
response to resource overconsumption
– May lead to overshoot
– Dieback (crash)
Types of Population Change
• S- Stable – population fluctuates slightly
above and below its carrying capacity
– Characteristic of undisturbed rain forests
– Late loss curve
• R- Irruptive – short-lived rapidly reproducing
species
– Linked to seasonal changes in weather or
nutrient availability
– Algal Blooms
– Early loss curves
r-Curve Fluctuations
S-Curve Fluctuations
Types of Population Change
• Cyclic fluctuations, boom-and-bust cycles
– Top-down population regulation
• Controlled by predation
– Bottom-up population regulation
• Controlled by scarcity of one or more resources
• Irregular – changes in population size with no
recurring pattern
– chaos
Introduction
• Why do all populations eventually stop
growing?
• What environmental factors stop a population
from growing?
• The first step to answering these questions is
to examine the effects of increased
population density.
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Density-Dependent Factors
•
•
•
•
•
•
limiting resources (e.g., food & shelter)
production of toxic wastes
infectious diseases
predation
stress
emigration
Density-Independent Factors
•
•
•
•
severe storms and flooding
sudden unpredictable severe cold spells
earthquakes and volcanoes
catastrophic meteorite impacts
• Density-dependent factors
increase their affect on a
population as population
density increases.
– This is a type of negative
feedback.
• Density-independent
factors
are unrelated to population
density, and there is no
feedback to slow population
growth.
Fig. 52.13
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Negative feedback prevents unlimited
population growth
• A variety of factors can cause negative feedback.
– Resource limitation in crowded populations can
stop population growth by reducing reproduction.
• Intraspecific competition for food can
also cause density-dependent behavior of
populations.
– Territoriality.
– Predation.
– Waste accumulation is another
component that can regulate population
size.
• In wine, as yeast populations increase, they
make more alcohol during fermentation.
• However, yeast can only withstand an
alcohol percentage of approximately 13%
before they begin to die.
– Disease can also regulate population
growth, because it spreads more rapidly
in dense populations.
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings
Population dynamics reflect a complex
interaction of biotic and abiotic influences
• Carrying capacity can vary.
• Year-to-year data can be helpful in analyzing
population growth.