Population Dynamics - juan

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Transcript Population Dynamics - juan

A Population of Mexican Poppies
• Ecology is the study of how living organisms and the physical
environment interact in a complicated web of relationships
• Biologists call interactions among organisms biotic factors
and interaction between organisms and their nonliving,
physical environment abiotic factors
• Abiotic factors include precipitation, temperature, pH, wind,
and chemical nutrients
Levels of Biological Organization
• Above the level of the individual organism, life is organized
into populations, communities, ecosystems, landscapes, and
the biosphere
• Each level has its own characteristic composition, structure,
and functioning
• An individual belongs to a population, members of the same
species that live together in a specified area at the same time
Features of Populations
• Features that characterize populations include population
density, population dispersion, birth and death rates, growth
rates, survivorship, and age structure
• Because populations share a common gene pool, natural
selection acts directly on allele frequencies to produce
adaptive changes in populations
Features of Populations (cont.)
• Population ecology considers the number of individuals of a
particular species in an area and the dynamics of the
• Population dynamics is the study of changes in populations
– how and why their numbers increase or decrease over time
• Biologists also study populations’ reproductive success or
failure, evolution, genetics, and the way they affect the normal
functioning of communities and ecosystems
Population Density
• Researchers study a large population by sampling part of it
and expressing the population in terms of density
• Population density is the number of individuals of a species
per unit of area or volume at a given time
• Different environments vary in the population density of any
species they can support
• Population density may be determined in large part by biotic
or abiotic factors in the environment
• Individuals in a population may exhibit characteristic patterns
of spacing (dispersion) relative to one another
• Random dispersion occurs when individuals are spaced in a
manner that is unrelated to the presence of others (rare)
• Clumped dispersion occurs when individuals are
concentrated in specific parts of the habitat (most common)
• Uniform dispersion occurs when individuals are more evenly
spaced than a random pattern
Dispersion of Individuals
Within a Population
(a) Random dispersion, illustrated for comparison, rarely, if ever, occurs in
Fig. 53-1a, p. 1155
(b) Clumped dispersion is evident in the schooling behavior of
certain fish species. Shown are bluestripe snappers (Lutjanus
kasmira), photographed in Hawaii. This introduced fish, which
grows to 30 cm (12 in), may be displacing native fish species in
Hawaiian waters.
Fig. 53-1b, p. 1155
(c) Uniform dispersion is characteristic of these nesting Cape gannets
(Morus capensis) on the coast of South Africa. The birds space their
nests more or less evenly.
Fig. 53-1c, p. 1155
Clumped Dispersion
• Clumped dispersion (aggregated distribution or
patchiness) often results from patchy distribution of
resources in the environment
• It also occurs among animals because of the presence of
family groups and pairs, and among plants because of limited
seed dispersal or asexual reproduction
• Clumped dispersion is advantageous when social animals
benefit from their association (e.g. schooling fish)
Uniform Dispersion
• Uniform dispersion occurs:
• when competition among individuals is severe
• when plant roots or leaves that have been shed produce
toxic substances that inhibit growth of nearby plants
• when animals establish feeding or mating territories
• Example: A colony of seabirds nesting in a homogeneous
environment place their nests at approximately equal distance
from one another
Changes in Population Size
• Change in the number of individuals in a population (ΔN),
over a given period of time (Δt ) is ultimately caused by two
factors, expressed on a per capita (per individual) basis:
• Natality (b), the average per capita birth rate
• Mortality (d), the average per capita death rate
ΔN /Δt = N (b − d)
Changes in Population Size (cont.)
• Growth rate (r), or rate of change of a population on a per
capita basis, is the birth rate minus the death rate:
• The rate at which the population is growing at a particular
instant in time (instantaneous growth rate, dN/dt) can be
expressed as:
dN/dt = rN
• Movement of individuals among populations (dispersal) must
be considered when examining changes in populations on a
local scale
• There are two types of dispersal:
• Immigration occurs when individuals enter a population
and increase its size
• Emigration occurs when individuals leave a population
and decrease its size
Dispersal (cont.)
• The growth rate of a local population must take into account
birth rate (b), death rate (d), immigration rate (i),and
emigration rate (e) on a per capita basis
• The per capita growth rate equals the birth rate minus the
death rate, plus the immigration rate minus emigration rate:
r = (b − d) + (i − e)
Exponential Population Growth
A plot of population size versus time (under optimal
conditions) has a J shape characteristic of exponential
population growth
• Accelerating growth rate occurs when optimal conditions
allow constant per capita growth rate – the larger the
population gets, the faster it grows
• Populations will increase exponentially as long as their per
capita growth rates remain constant
Exponential Population Growth
Environmental Limits
• Organisms cannot reproduce indefinitely at their intrinsic rate
of increase because of environmental limits
• Environmental limits include limited availability of food, water,
shelter, and other essential resources (resulting in increased
competition), and limits imposed by disease and predation
• At or near the limits of the environment to support the
population, population growth rate may decrease to nearly
Carrying Capacity
• Carrying capacity (K) is the largest population that can be
maintained for an indefinite period by a particular
environment, assuming no changes in the environment
• In nature, carrying capacity is dynamic and changes in
response to environmental changes
Logistic Population Growth
• When a population regulated by environmental limits is
graphed over longer periods, the curve has a characteristic S
shape (logistic population growth)
• Initial exponential increase is followed by a leveling out as
carrying capacity of the environment is approached
The Logistic Model
• The logistic model describes a population increasing from a
small number of individuals to a larger number of individuals
that are ultimately limited by the environment
• The logistic equation takes into account the carrying capacity
of the environment:
dN/dt = rN [(K − N)/K]
• The element [(K − N)/K] reflects decline in growth as
population size approaches its carrying capacity
Carrying Capacity and
Logistic Population Growth
Population Crash
• A population rarely stabilizes at K (carrying capacity) but may
temporarily rise higher, then drop back to, or below K
• Sometimes a population that overshoots K will experience a
population crash, an abrupt decline from high to low
population density
• Population crashes are observed in bacterial cultures,
zooplankton, and other populations whose resources have
been exhausted
• Changes in population size are caused by natality, mortality,
immigration, and emigration
Density-Dependent Factors
• Factors that affect population size fall into two categories:
density-dependent factors and density-independent factors
• If a change in population density alters how an environmental
factor affects that population, then the environmental factor is
• The effects of density-dependent factors on population growth
increase as the population density increases
Density-Dependent Factors (cont.)
• Density-dependent factors act as negative feedback
• As population density increases, density-dependent factors
tend to slow population growth by causing an increase in
death rate and/or a decrease in birth rate
• Population density declines
Density-Dependent Factors
and Negative Feedback
Density-Dependent Factors (cont.)
• Predation, disease, and competition are examples of densitydependent factors
• As the density of a population increases:
• Predators are more likely to find prey
• The chance of transmitting parasites and infectious
disease organisms increases
• Competition for resources such as living space, food,
cover, water, minerals, and sunlight increases
Cyclic Changes in Density
• Lemming populations in the arctic tundra have a three- to
four-year cyclical oscillation (“boom or bust”)
• Possible explanations:
• Dense lemming populations overgraze the food supply;
reduced food reduces the population
• Population density of predators that eat lemmings
increases in response to increasing density of prey –
reducing lemming populations
• Lemming populations decrease in years when climate
restricts growth of food
Fig. 53-5, p. 1159
• Competition is an interaction among two or more individuals
that attempt to use the same essential resource (food, water,
sunlight, or living space) that is in limited supply
• Use of the resource by one individual reduces the availability
of that resource for other individuals
• Competition occurs within a population (intraspecific
competition) and among populations of different species
(interspecific competition)
Intraspecific Competition
• Interference competition (contest competition)
• Dominant individuals obtain an adequate supply of the
limited resource at the expense of other individuals
• Example: red grouse
• Exploitation competition (scramble competition)
• All individuals in a population share the limited resource
equally – at high population densities none of them
obtains an adequate amount
• Example: moose population on Isle Royale
Effects of Density-Dependent Factors
• In natural communities, it is difficult to evaluate the relative
effects of different density-dependent factors
• Example: Effects of lizards on spider populations
• Spider population densities were higher in lizard-free
enclosures than in enclosures with lizards
• Enclosures without lizards had more species of spiders
• In this experiment, effects of two density-dependent
factors (predation and interspecific competition) cannot be
evaluated separately
Density-Independent Factors
• Any environmental factor that affects the size of a population
but is not influenced by changes in population density is a
density-independent factor
• Density-independent factors are typically abiotic, such as
random weather events (e.g. blizzards, hurricanes)
• Example: Mosquito populations in arctic environments
• No adult mosquito survives winter; the entire population
grows in summer from eggs and hibernating larvae
r Selection
• r selected populations have traits that contribute to a high
population growth rate (r)
• r-selected species have small body size, early maturity,
short lifespan, large broods, and little or no parental care
• r-strategists are opportunists in variable, temporary, or
unpredictable environments where long-term survival is low
• Examples: Insects such as mosquitoes; plants such as desert
annuals that rapidly grow, flower, and die
K Selection
• K selected populations have traits that maximize survival in
an environment near the carrying capacity (K)
• K-selected species produce few offspring, have long
lifespans, late reproduction, and large body size
• K strategists tend to be found in relatively constant or stable
environments, where they have a high competitive ability
• Examples: Redwood trees; tawny owls, which pair-bond for
life and invest in parental care of their young
Survivorship Curves
• Survivorship is the probability that a given individual in a
population or cohort will survive to a particular age
• Plotting the logarithm (base 10) of the number of surviving
individuals against age, from birth to the maximum age
reached by any individual, produces a survivorship curve
• Ecologists recognize three main types of survivorship curves:
Type I, Type II, and Type III
Survivorship Curves (cont.)
• In Type I survivorship, probability of survival decreases more
rapidly with increasing age; mortality is greatest later in life
• Example: humans
• In Type II survivorship, probability of survival does not change
with age; death is equally likely across all age groups
• Example: some lizards
• In Type III survivorship, probability of survival increases with
increasing age; young are most likely to die
• Example: oysters
Survivorship Curves
Human Population Growth
• It took thousands of years for the human population to reach
one billion, around 1800
• It took 130 years to reach two billion (1930), 30 years to reach
three billion (1960), 15 years to reach four billion (1975), 12
years to reach five billion (1987), and 12 years to reach six
billion (1999)
• The United Nations projects that the human population will
reach about seven billion by 2012
Human Population Growth
Human Population Growth (cont.)
• Recent increase in human population is due to a decrease in
death rate (d), not an increase in birth rate (b)
• Greater food production, better medical care, and improved
sanitation practices have increased life expectancies
• Example: From 1920 to 2000, the death rate in Mexico fell
from 40 per 1000 individuals to 4 per 1000 – the birth rate
dropped from 40 per 1000 individuals to 24 per 1000
Birth and Death Rates: Mexico
Human Demographics
• The science of human demographics deals with human
population statistics such as size, density, and distribution,
and provides information on populations of various countries
• Countries can be classified into two groups based on their
rates of population growth, degrees of industrialization, and
relative prosperity: highly developed and developing
• Developing countries have two subcategories: moderately
developed and less developed
Highly Developed Countries
• Highly developed countries (such as the US)
• Highly industrialized
• Low rates of population growth
• Low birth rates
• Low infant mortality rates
• Long life expectancies
• High average GNI PPP per capita
Developing Countries
• Moderately developed countries (such as Mexico)
• Birth rates and infant mortality rates generally higher than
in highly developed countries, but declining
• Medium level of industrialization
• Lower average GNI PPP per capita
• Less developed countries (such as Bangladesh)
• Highest birth rates and infant mortality rates
• Lowest life expectancies
• Lowest average GNI PPP per capita
Fertility Changes
Table 53-4, p. 1168
Age Structure
• Age structure is the number and proportion of people at
each age in a population
• An age structure diagram represents the number of males
and females at each age, from birth to death
• The overall shape of an age structure diagram indicates
whether the population is increasing, stationary, or shrinking
Age Structure (cont.)
• For highly developed countries, age structure diagrams have
more tapered bases – a smaller proportion of the population
is prereproductive
• The age structure diagram of a stable population shows
approximately the same number of people at prereproductive
and reproductive ages
• In a population that is shrinking, the prereproductive age
group is smaller than either the reproductive or
postreproductive group
Age Structure Diagrams