UNIT 3.

POPULATIONS
Chapter 8: Understanding Populations
Chapter 9: The Human Populations
Chapter 10: Biodiversity
Chapter 8: Understanding Populations

Section 1. How Populations Change in Size

Learning Objectives

1. Describe the three main properties of a population.

2. Describe exponential population growth.
3. Describe how the reproductive behavior of individuals can
affect the growth rate of their population.
4. Explain how population sizes in nature are regulated.
 What Is a Population?
´A population is the set of individuals within a species
living in the same place at the same time.
´Figure 1.1 shows examples of a population and part of
a population.
´The adults within a population form a reproductive
group because, by definition, organisms breed with
members of their own population.
 Properties of Populations
´Populations may be described in terms of size,
density, and dispersion, as shown in Figure 1.2.
´Population size is the total number of individuals,
whereas density is the number of individuals per unit
area or volume, such as the number of tilapia per
cubic meter of water in a Laguna lake.
´A population’s dispersion describes the arrangement
of its individuals in space.
´A population’s dispersion may be even, clumped, or
random.
 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 in succeeding pages.
´The percentage change in the size of a population
over a given period of time is that population’s growth
rate.
´The growth rate is the birth rate minus the death rate.
´ Over time, the growth rate of a population changes
because birth rates and death rates increase or
decrease.
´ “Growth” rates can be positive, negative, or zero.
´ For a population’s 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 decrease.
 How Fast Can a Population Grow?
´A female sea turtle may lay 2,000 eggs in her
lifetime.
´Figure 1.3 shows newly hatched sea turtles leaving
their nests for the ocean.
´If all of them survived, the turtle population would
grow rapidly. But many young turtles are eaten by
crabs or fish, and others starve.
´All populations experience deaths, but death rates
can differ among species and populations.
 Reproductive Potential
´ A species’ biotic potential is the fastest rate at which its populations can grow. This rate is
limited by the maximum number of offspring that each member of the population can
produce, which is called its reproductive potential.
´ Some species have much higher reproductive potentials than others.
´ A bacterium can produce 19 million descendants in a few days or weeks.
´ Reproductive potential is higher when individuals produce more offspring at one 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, the average time it takes a member of the
population to reach the age when it reproduces.
´ Small organisms, such as bacteria, have short generation times.
´ Some bacteria can reproduce when they are only twenty minutes old.
´ As a result, their populations can grow quickly. In contrast, large organisms, such as
elephants and humans, become sexually mature only after a number of years.
´ The human generation time is about 20 years, so humans have a much lower reproductive
potential than bacteria.
 Exponential Growth
´Populations sometimes undergo exponential growth,
which means they grow faster and faster.
´For example, if a pair of dogs gives birth to 6 puppies,
there will be 6 dogs in one generation. If each dog in
that generation mates and has a litter of 6 puppies,
there will be 36 dogs in the next generation. The
following generation will contain 216 dogs, and so on.
If the number of dogs is plotted on a graph versus time,
the graph will have the shape shown in Figure 1.4.
´Exponential growth occurs in nature only when
populations have plenty of food and space, and have
little or no competition or predators.
 What Limits Population Growth?
´Because natural conditions are neither ideal nor
constant, populations cannot grow forever and rarely
grow at their reproductive potential.
´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 tend to change
over time.
 Carrying Capacity
´ Carrying capacity is the population size where birth rates and
death rates are equal.
´ Another definition of carrying capacity for a particular species is
the maximum population that its ecosystem can support
indefinitely.
´ A population may increase beyond its carrying capacity, but it
cannot stay at an increased size for long.
´ If a population is larger than the carrying capacity, it may use up its
resources, and fewer individuals will survive to reproduce.
´ Carrying capacity is difficult to predict or calculate. However, it
can be estimated by looking at average population sizes or by
observing a population crash after a certain size has been
exceeded.
 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
for the species in that area.
´For example, plant growth is limited by supplies of
water, sunlight, and mineral nutrients.
´The supply of the most severely limited resources
determines the carrying capacity of an environment
for a particular species at a particular time.
 Competition Within a Population
´ The members of a population tend to 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 directly for a limiting resource, members of a
species may compete indirectly for a resource by competing for
social dominance or for a territory.
´ 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 also 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.
´ Some examples of competition within species are shown in Figure 1.6.
 Patterns of Population Change
´ Rates of birth or death in a population may be density dependent or density independent.
´ Density-dependent deaths occur more quickly in a crowded population than in a sparse
population.
´ Limited resources, predation, and disease often result in higher rates of death in dense
populations than in sparse populations.
´ The pine trees in Figure 1.7 are infected with a disease that is spreading in a density-
dependent pattern. Many of the same kind of pine tree are growing close to each other,
so a disease-carrying beetle easily spreads the disease from one tree to another.
´ When a cause of death is density independent, a certain proportion of a population dies
regardless of the population’s density.
´ This type of regulation affects all members of a population in a general or uniform way.
´ Severe weather and natural disasters are often density-independent causes of death.
´ Populations can show alternating periods of exponential growth and population crashes
with density-independent death rates.
´ Many species of animals in unpredictable environments show this pattern of population
change.
 Section 2. How Species Interact with Each Other

Learning Objectives

1. Explain the difference between niche and habitat.

2. Give examples of parts of a niche.
3. Describe the five major types of interactions between
species.
4. Explain the difference between parasitism and predation.
5. Explain how symbiotic relationships may evolve.
 An Organism’s Niche
´Many ecologists are interested in understanding
species’ role in ecosystems and requirements for
survival.
´To do this they measure a species’ niche.
´There are several ways to define a niche.
´A niche can be the range of conditions in which a
species can survive.
´For example, a plant may only be able to survive in a
particular range of temperatures, with a certain
amount of rainfall, and with access to enough
nutrients.
´Other scientists are interested in the ecological role of
a species.
´These scientists define a species’ niche by the
resources they use, or the species they feed on.
´For example, zebras are large herbivores on African
grasslands. Kangaroos occupy a similar niche in
Australia. Herbivores often interact with carnivores, like
lions, if they both exist in the same habitat.
´A niche is different from a habitat. An organism’s
habitat is a location. However, an organism’s pattern
of use of its habitat is part of its niche.
 Ways in Which Species Interact
´The five major types of species interactions, summarized
in Figure 2.2, are competition, predation, parasitism,
mutualism, and commensalism.
´These categories are based on whether each species
causes benefit or harm to the other species in a given
relationship.
´Not all interactions occur directly.
´For example, a tiger shark may cause sea turtles to not
use certain areas. Sea grasses in these areas are not
grazed as heavily. Tiger sharks have a positive indirect
interaction with the seagrass!
 Competition

´For most organisms, competition is part of daily

life. Seed-eating birds compete with each
other for seeds under a bush, and the bush
competes with the tree next to it for nutrients in
the soil.
´Competition is a relationship in which different
individuals or populations attempt to use the
same limited resource. Each individual has less
access to the resource and is harmed by the
competition.
´Competition can occur both within and between
species.
´Members of the same species compete with each
other because they require the same resources—
they occupy the same niche.
´When members of different species have niches
that overlap they may compete for some
resources.
´If two species have requirements that are too
similar, one species may eliminate the other from
a habitat. This is called competitive exclusion.
Types of Competition
´ Species may compete in different ways. Sometimes,
individuals try to get as many resources as they can but
do not fight over resources or get in each other’s way.
´ In this type of scramble competition, the winner is the
individual or species that gets the most resources the
fastest.
´ Some examples of scramble competition are fish feeding
on plankton or sea turtles and sea cows feeding on
seagrass.
´ In interference competition, individuals fight over
resources or get in each others’ way when feeding. For
example, lions will steal kills from wild dogs and hyenas.
 Adaptations to Competition
´ If two species have identical resource needs in the same
ecosystem, the more successful species might drive out the less
successful species.
´ The individuals that do best and leave the most offspring will be
those that either feed on slightly different resources or use
resources in different ways.
´ One way competition can be reduced between species is by
dividing up the niche.
´ Niche restriction occurs when each species uses less of the niche
than it is capable of using.
´ Niche restriction is observed in closely related species that use the
same limited resources within a habitat.
 Predation
´ An organism that feeds on another organism is called a predator,
and the organism that is fed upon is the prey. This kind of
interaction is called predation.
´ Examples of predation include sharks eating fish, bats eating
insects, or wolves eating deer.
´ Predators come in many shapes and sizes.
´ Most predators are themselves prey of other predators.
´ For example, lizards are predators of insects, but are prey of
hawks. Because almost all species (including blue whales) have
predators, most organisms have evolved some mechanisms to
avoid or defend against predators.
 Parasitism
´ An organism that lives in or on another organism and feeds on it is
a parasite.
´ The organism the parasite takes its nourishment from is known as the
host.
´ The relationship between the parasite and its host is called parasitism.
´ Examples of parasites are ticks, fleas, tapeworms, bloodsucking
leeches, and mistletoe.
´ Unlike predators, parasites usually do not kill their hosts. Therefore,
some people consider vampire bats to be parasites. In fact, a
parasite can have an evolutionary advantage if it allows its host to
live longer. However, the host is often weakened or exposed to
disease by the parasite.
 Mutualism
´Many species depend on another species for survival.
In some cases, neither organism can survive alone.
´A close relationship between two species in which
each species provides a benefit to the other is called
mutualism.
´Certain species of bacteria in your intestines form a
mutualistic relationship with you. These bacteria help
break down food that you could not otherwise digest
or produce vitamins that your body cannot make. In
return, you give the bacteria a warm, food-rich habitat.
 Commensalism
´A relationship in which one species
benefits and the other species is neither
harmed nor helped is called
commensalism.
´An example is the relationship between
certain orchids and trees, shown in Figure
2.8. The orchid grows around the tree’s
branches without harming the tree. The
height exposes the orchid to rain and
sunlight.
 Symbiosis and Coevolution

´ A relationship in which two species live in close association is called

symbiosis.
´ Many types of species interactions are considered symbiotic. In some
cases a symbiotic relationship is parasitic. In some cases, it is
commensalism or mutualism.
´ For example, the symbiotic mutualism between corals and the small
algae living inside them that make food for corals allows huge reefs to
form!
´ Over time, species in close relationships may coevolve. These species
may evolve adaptations in response to one another.
´ For example, coevolution can be seen in the relationships of flowering
plants and their pollinators. Many types of flowers seem to match the
feeding habits of certain species of insects or other animals that spread
pollen. Predators and prey also can coevolve, with prey evolving better
ways to escape and predators evolving to be better hunters.