Chapter 8 Understanding Populations

Understanding Populations

Section 1

What Is a Population?

•A population is a group of organisms of the same
species that live in a specific geographical area and
interbreed.

•A population is a reproductive group because
organisms usually breed with members of their own
population.

•The word population refers to the group in general and
also to the size of the population, or the number of
individuals it contains.

7 Billion People Nat Geo

Understanding Populations

Section 1

Properties of Populations

•Density is the number of individuals of the same species in that live in a given unit
of area.

•Dispersion is the pattern of distribution of organisms in a population.

•A population’s dispersion may be even, clumped, or random.

•Size, density, dispersion, and other properties can be used to describe populations
and to predict changes within them.

Population Density and Dispersion via YouTube

Understanding Populations

Section 1

How Does a Population Grow?

• A population gains individuals with each new offspring or birth and loses
them with each death.

• The resulting population change over time can be represented by the
equation below.

Understanding Populations

Section 1

How Does a Population Grow?

• Overtime, the growth rates of populations change
because birth rates and death rates increase or
decrease.

• For this reason, growth rates can be positive,
negative, or zero.

•

Growth rate is an expression of the increase in the size of an organism or
population over a given period of time.

Understanding Populations

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How Does a Population Grow?

• For the growth rate to be zero, the average number of births must

equal the average number of deaths.

• A population would remain the same size if each pair of adults
produced exactly two offspring, and each of those offspring
survived to reproduce.

• If the adults in a population are not replaced by new births, the
growth rate will be negative and the population will shrink.

Understanding Populations

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How Fast Can a Population Grow?

• Populations usually stay about the same
size from year to year because various
factors kill many individuals before they
can reproduce.

• These factors control the sizes of
populations.

• In the long run, the factors also
determine how the population evolves.

Understanding Populations

Section 1

Reproductive Potential

• A species’ biotic potential is the fastest rate at
which its populations can grow.

• This rate is limited by reproductive potential.

• Reproductive potential is the maximum number
of offspring that a given organism can produce.

• Some species have much higher reproductive
potentials than others.

– Examples: Bacteria

Understanding Populations

Section 1

Reproductive Potential

• Reproductive potential increases when individuals produce more
offspring at a time, reproduce more often, and reproduce earlier in
life.

• Reproducing earlier in life has the greatest effect on reproductive
potential.

• Reproducing early shortens the generation time, or the average
time it takes a member of the population to reach the age when it
reproduces.

Understanding Populations

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Reproductive Potential

• Small organisms, such as bacteria and insects, have short
generation times and can reproduce when they are only a few
hours or a few days old.

• As a result, their populations can grow quickly.

• In contrast, large organisms, such as elephants and humans,
become sexually mature after a number of years and therefore
have a much lower reproductive potential than insects.

Biotic Potential

Understanding Populations

Section 1

Exponential Growth

• Exponential growth is logarithmic growth or
growth in which numbers increase by a certain
factor in each successive time period.

• Exponential growth occurs in nature only when
populations have plenty of food and space,
and have no competition or predators.

• For example, population explosions occur when
bacteria or molds grow on a new source of food.

Understanding Populations

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Exponential Growth

• In exponential growth, a large number
of individuals is added to the
population in each succeeding time
period.

Understanding Populations

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What Limits Population Growth?

• Because natural conditions are neither ideal nor constant,
populations cannot grow forever.

• Eventually, resources are used up or the environment changes,
and deaths increase or births decrease.

• Under the forces of natural selection in a given environment,
only some members of any population will survive and reproduce.
Thus, the properties of a population may change over time.

Understanding Populations

Section 1

Carrying Capacity

• Carrying capacity is the largest population that an environment can
support at any given time.

• A population may increase beyond this number but it cannot stay at this
increased size.

• Because ecosystems change, carrying capacity is difficult to predict or
calculate exactly.

• However, it may be estimated by looking at average population sizes or
by observing a population crash after a certain size has been exceeded.

Understanding Populations

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Carrying Capacity

Understanding Populations

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Resource Limits

• A species reaches its carrying capacity when it

consumes a particular natural resource at the
same rate at which the ecosystem produces the
resource.

• That natural resource is then called a limiting
resource.

• The supply of the most severely limited resources
determines the carrying capacity of an environment
for a particular species at a particular time.

Understanding Populations

Section 1

Competition Within a Population

• The members of a population use the same
resources in the same ways, so they will
eventually compete with one another as the
population approaches its carrying capacity.

• Instead of competing for a limiting resource,
members of a species may compete indirectly
for social dominance or for a territory.

• Competition within a population is part of the
pressure of natural selection.

Understanding Populations

Section 1

Competition Within a Population

• A territory is an area defended by one or
more individuals against other individuals.

• The territory is of value not only for the space

but for the shelter, food, or breeding sites it
contains.

• Many organisms expend a large amount of
time and energy competing with members of
the same species for mates, food, or homes
for their families.

Understanding Populations

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Two Types of Population Regulation

• Population size can be limited
in ways that may or may not
depend on the density of the
population.

• Causes of death in a
population may be density
dependent or density
independent.

Understanding Populations

Section 1

Population Regulation

• When a cause of death in a population is density dependent,
deaths occur more quickly in a crowded population than in a
sparse population.

• This type of regulation happens when individuals of a population
are densely packed together.

• Limited resources, predation and disease result in higher rates
of death in dense populations than in sparse populations.

Understanding Populations

Section 1

Population Regulation

• When a cause of death is density independent, a certain
proportion of a population may die regardless of the
population’s density.

• This type of regulation affects all populations in a general
or uniform way.

• Severe weather and natural disasters are often density
independent causes of death.

Understanding Populations

Section 1

Ticket out the Door

1. What is a population?

2. What is density?

3. What is dispersion?

4. What is the equation for calculating a population size?

5. What is growth rate (an equation would be good also)?

6. What is carrying capacity?

Understanding Populations

Section 1

Day one

Chapter 8

Understanding Populations

Section 2: How Species Interact With Each Other

Understanding Populations

Section 1

An Organism’s Niche

• A niche is the unique position occupied by a
species, both in terms of its physical use of its
habitat and its function within an ecological
community.

• A niche is different from a habitat. An organism’s
habitat is a location. However, a niche is an
organism’s pattern of use of its habitat.

• A niche can also be thought of as the functional
role, or job of a particular species in an
ecosystem.

Understanding Populations

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Symbiosis and Coevolution

• Symbiosis is a relationship in which two different organisms live
in close association with each other.

• Symbiosis is most often used to describe a relationship in which

at least one species benefits.

• Overtime, species in close relationships may coevolve.

• These species may evolve adaptations that reduce the harm or
improve the benefit of the relationship.

Understanding Populations

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Ways in Which Species Interact

• Interactions between species are categorized at the level where one
population interacts with another.

• The five major types of species interactions are:

• Competition

• Predation

• Parasitism

• Mutualism

• Commensalism

Understanding Populations

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Species Interactions

Understanding Populations

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Ways in Which Species Interact

• These categories are based on whether each species
causes benefit or harm to the other species in a given
relationships in terms of total effects over time.

• Other types of interactions are possible.

• Many interactions between species are indirect, some
interactions do not fit in a category clearly, and other
types seem possible but are rarely found.

• Therefore, many interactions are neither categorized
nor well studied.

Understanding Populations

Section 1

Competition

• Competition is the relationship between two species (or
individuals) in which both species (or individuals) attempt
to use the same limited resource such that both are
negatively affected by the relationship.

• Members of the same species must compete with each
other because they require the same resources because
they occupy the same niche.

• When members of different species compete, we say that
their niches overlap, which means that each species
uses some of the same resources in a habitat.

Understanding Populations

Section 1

Indirect Competition

• Species can compete even if they never come into
direct contact with each other.

• For example, suppose that one insect feeds on a
certain plant during the day and that another
species feeds on the same plant during the night.

• Because they use the same food source, the two
species are indirect competitors.

• Humans rarely interact with the insects that eat our
food crops, but those insects are still competing
with us for food.

Understanding Populations

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Adaptations to Competition

• When two species with similar niches are placed together in the
same ecosystem, we might expect one species to be more
successful than the other.

• But in the course of evolution, adaptations that decrease
competition will also be advantageous for species whose niches
overlap.

• One way competition can be reduced between species is by

dividing up the niche in time or space.

Understanding Populations

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Adaptations to Competition

•Niche restriction is when each species uses less of the
niche than they are capable of using.

•It is observed in closely related species that use the same
resources within a habitat.

•For example, Chthamalus stellatus, a barnacle species, is
found only in the upper level of the intertidal zone when
another barnacle species is present.

•When the other species is removed, C. stellatus can be
found at deeper levels.

•The actual niche used by a species may be smaller than
the potential niche.

Understanding Populations

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Adaptations to Competition

Understanding Populations

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Predation

• Predation is an interaction between two
species in which one species, the predator,
feeds on the other species, the prey.

• In complex food webs, a predator may also
be the prey of another species.

• Most organisms have evolved some
mechanisms to avoid or defend against
predators.

Understanding Populations

Section 1

Predators

• Some predators eat only specific types of prey.

• In this kind of close relationship, the sizes of each population tend
to increase and decrease in linked patterns, as shown below.

Understanding Populations

Section 1

Parasitism

• An organism that lives in or on another organism
and feeds on the other organism is a parasite.

– Examples include ticks, fleas, tapeworms,
heartworms, and bloodsucking leeches.

• The organism, the parasite, takes its nourishment
from is known as the host.

• Parasitism is a relationship between two species,
the parasite, benefits from the other species, the
host, and usually harms the host.

Understanding Populations

Section 1

Parasitism

• The differences between a parasite and a predator are
that a parasite spends some of its life in or on the host,
and that the parasites do not usually kill their hosts.

• In fact, the parasite has an evolutionary advantage if it
allows its host to live longer.

• However, the host is often weakened or exposed to
disease by the parasite.

Understanding Populations

Section 1

Mutualism

• Many species depend on another species for survival. In
some cases, neither organism can survive alone.

• Mutualism is a relationship between two species in
which both species benefit.

• Certain species of bacteria in your intestines form a
mutualistic relationship with you.

• These bacteria help break down food that you cannot
digest. In return, you give the bacteria a warm, food-rich
habitat.

Understanding Populations

Section 1

Commensalism

• Commensalism is a relationship between two
organisms in which one organism benefits and the
other in unaffected.

• An example is the relationship between sharks and a
type of fish called remoras.

• Remoras attach themselves to sharks and feed on
scraps of food left over from the shark’s meals.

• Even seemingly harmless activity, however, might
have an effect on another species.

Understanding Populations

Section 1

Ticket Out the Door

1. What is the difference between a niche and a habitat?

2. What is symbiosis?

3. What is competition?

4. What is predation?

5. What is mutualism?

6. What is parasitism?

7. What is commensalism?