Transcript Ch 53 population Ecology

Chapter 53
Population Ecology
Population Ecology
Population ecology is the study of
factors that affect population:
• Density
• Growth
• A population is a group of
individuals of a single species that
occupy the same general area.
• Population ecology focuses on the
factors that influence a
population’s
• Density
• Structure
• Size
• Growth rate
Population Density
Population density is the number of
individuals of a species per unit of area
or volume. Examples include :
1. The number of largemouth bass
per cubic kilometer (km3) of a
lake
2. The number of oak trees per
square kilometer (km2) in a forest
3. The number of nematodes per
cubic meter (m3) in a forest’s soil
• how to measure population density
• can be impractical or impossible to
count all individuals in a population.
• In some cases, estimated by indirect
indicators, such as number of bird
nests, and rodent burrows.
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
– The dispersion of a population is the pattern
of spacing among individuals within the
geographic boundaries.
• 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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PATTERN OF DISPERSION
UNIFORM
CLUMPED
RANDOM
Uniform Dispersion-(pattern)
Animals that defend territories often show this
Clumped Dispersion-( most
common) usually around a common
resource
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Random Dispersionunpredictable spacing- not
common in nature
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.
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Life Tables and Survivorship
Curves
A life table is an age-specific summary
of the survival pattern of a population.
• Track survivorship
• Help to determine the most
vulnerable stages of the life cycle
• Follow a cohort, of individuals of
the same age throughout their
lifetime.
Survivorship curves
• Graphically represent some of the
data in a life table
• Are classified based upon the rate of
mortality over the life span of an
organism
Type I
• Shows individuals that have a high
probability of surviving through early
and middle life but have a rapid
decline in the number of individuals
surviving into late life.
• Most of the individuals will make it to
adulthood but the proportion surviving
into old age is greatly decreased.
• Is plotted as a convex curve on a
graph.
• Ex. humans
Type II
• Shows a roughly constant
mortality rate for the species
through its entire life.
• This means that the individual's
chance of dying is independent
of their age.
• Are plotted as a diagonal line
going downward on a graph.
• Ex. birds
Type III
• Depicts species where few
individuals will live to
adulthood and die as they
get older because the
greatest mortality for these
individuals is experienced
early in life.
• Curve is drawn as a
concave curve on a graph.
• Ex. o
Life History Traits as
Evolutionary Adaptations
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Age at first reproduction
Number and size of offspring
Reproductive lifespan and ageing
Frequency of reproduction
Number of offspring
Amount of parental care provided
Evolve and represent a compromise of the
competing needs for time, energy, and
nutrients.
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.
Agaves
Population Growth Models
The logistic and the exponential models are theoretical ideals of
population growth.
No natural population fits either one perfectly.
• Population size
fluctuates as
individuals are born
• Immigrate into an
area
• Emigrate away
• Die
– This is also known as semelparity.
• 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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Population Age Structure
• The age structure of a
population is the distribution
of individuals among age
groups.
• The age structure of a
population provides insight
into :
a) The history of a
population’s survival
b) Reproductive success
c) How the population relates
to environmental factors
– 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
The Exponential Growth Model
• Exponential population growth describes the
expansion of a population in an ideal and
unlimited environment.
• Exponential growth explains how a few dozen
rabbits can multiple into millions
• In certain circumstances following disasters,
organisms that have opportunistic life history
patterns can rapidly recolonize a habitat
The Logistic Growth Model
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Limiting factors
Are environmental factors that hold population growth in check
Restrict the number of individuals that can occupy a habitat
The carrying capacity is the maximum population size that a
particular environment can sustain.
Logistic population growth occurs when the growth rate
decreases as the population size approaches carrying capacity.
The carrying capacity for a population varies, depending on the
species
The resources available in the habitat
Organisms exhibiting equilibrial life history patterns occur in
environments where the population size is at or near carrying
capacity.
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
K-Selected Species
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Poor colonizers
Slow maturity
Long-lived
Low fecundity
High investment in care for the
young
• Specialist
• Good competitors
r-Selected Species
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Good colonizers
Reach sexual maturity rapidly
Short-lived
High fecundity
Low investment in care for the
young
• Generalists
• Poor competitors
Regulation of Population Growth
Density-Dependent Factors
• The logistic model is a description of intraspecific
competition, competition between individuals of the
same species for the same limited resources.
• As population size increases competition becomes
more intense
• The growth rate declines in proportion to the intensity
of competition
• A density-dependent factor is a population-limiting
factor whose effects intensify as the population
increases in density.
• Density-dependent factors may include accumulation
of toxic wastes, stress, predation, limited food supply,
limited territory, infectious diseases
Density-Independent Factors
• Are population-limiting factors whose intensity is unrelated to
population density
• Include abiotic factors such as fires, floods, sudden
unpredictable severe cold spells, earthquakes and volcanoes and
catastrophic meteorite impacts
• In many natural populations, density-independent factors limit
population size before density-dependent factors become
important.
• Over the long term, most populations are probably regulated by
a mixture of both Density-independent and -dependent factors
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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Population Cycles
• A well studied example of boom and bust
cycles are the cycles of Snowshoe hares
• One of the hares’ predators, the lynx
• The cause of these hare and lynx cycles
may be winter food shortages for the hares
• Overexploitation of hares by lynx
• A combination of both of these mechanisms
• 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.
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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.
Some populations fluctuate erratically, based different
factors.
•Dungeness crab populations fluctuated hugely over a 40year period.
•One key factor causing these fluctuations is cannibalism.
•Large numbers of juveniles are eaten by older juveniles and
older crabs.
Water temperatures and ocean currents.
•Small changes in these variables cause large fluctuations in
crab population numbers.
Fig. 52.18
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• Other populations have regular boom-andbust cycles.
– There are populations that fluctuate
greatly.
– A good example involves the lynx and
snowshoe hare that cycle on a ten year
basis.
Applications of Population
Ecology
• Population ecology is used to
• Increase populations of organisms we wish
to harvest
• Decrease populations of pests
• Save populations of organisms threatened
with extinction
• Conservation of Endangered Species
Example of application
• A major factor in population
decline is habitat destruction or
modification.
• The red-cockaded woodpecker
requires longleaf pine forests with
clear flight paths between trees
• Suffered from fire suppression,
increasing the height of the
vegetation on the forest floor
• Recovered from near-extinction to
sustainable populations due to
controlled burning and other
management methods
The human population has been
growing almost exponentially for three
centuries but cannot do so indefinitely
• The human population increased relatively
slowly
until about 1650 when the Plague took an
untold number of lives.
– Ever since, human population numbers
have doubled twice
• How might this population increase stop?
Age Structures
• Help predict a population’s
future growth.
• Population momentum is the
continuation of population
growth as girls in the prereproductive age group reach
their reproductive years.
• Age structure diagrams may
also indicate social conditions.
An expanding population needs
• Schools
• Employment
• Infrastructure
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Fig. 52.22
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Estimating Earth’s carrying capacity
for humans is a complex problem
• Predictions of the human population vary
from 7.3 to 10.7 billion people by the year
2050.
– Will the earth be overpopulated by this
time?
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• Wide range of estimates for carrying
capacity.
– What is the carrying capacity of Earth for
humans?
– This question is difficult to answer.
• Estimates are usually based on food, but
human agriculture limits assumptions on
available amounts.
• Ecological footprint.
– Humans have multiple constraints
besides food.
– The concept an of ecological footprint
uses the idea of multiple constraints.
Our Ecological Footprint
• An ecological footprint is an estimate of the
amount of land required to provide the raw
materials an individual or a population
consumes, including:
• Food
• Fuel
• Water
• Housing
• Waste disposal
• For each nation, we can calculate the
aggregate land and water area in various
ecosystem categories.
• Six types of ecologically productive areas
are distinguished in calculating the
ecological footprint:
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Land suitable for crops.
Pasture.
Forest.
Ocean.
Built-up land.
Fossil energy land.
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– We may never know Earth’s carrying
capacity for humans, but we have the
unique responsibility to decide our fate
and the fate of the rest of the biosphere.
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