Download Populations respond to pressures..

KEY CONCEPT
Populations respond
to pressures.
.
Sunshine State
STANDARDS
SC.G.1.3.2: The student knows that
biological adaptations
include changes in
structures, behaviors,
or physiology that
enhance reproductive
success in a particular
environment.
SC.G.2.3.3: The student knows that a
brief change in the
limited resources of an
ecosystem may alter
the size of a population or the average
size of individual
organisms and that
long-term change may
result in the elimination of animal and
plant populations
inhabiting the Earth.
BEFORE, you learned
NOW, you will learn
• Four characteristics are used to
describe a population
• Scientists study these four
characteristics to predict population change
• About limits to population
growth
• How population density affects
limiting factors
• About two reproductive
strategies found within
populations
EXPLORE Population Density
How does population density vary?
PROCEDURE
1
Choose three different locations in your
school where you can observe how many
people enter and leave an area during a
specific time period.
MATERIALS
• stopwatch
• notebook
2 Position three people at each location a
counter, a timekeeper, and a recorder.
3 Count the number of people who pass
through the area for at least 2 minutes.
Record the number.
VOCABULARY
immigration p. 537
emigration p. 537
limiting factor p. 538
opportunist p. 541
competitor p. 542
CHOOSE YOUR OWN
STRATEGY
Use a strategy from an
earlier chapter or one of
your own to take notes on
the main idea: Population
growth is limited.
536 Unit 4: Life Over Time
4 Compare your data with the data collected
by other groups.
WHAT DO YOU THINK?
• Where was the number of people the highest?
the lowest?
• Explain what may have affected population
density at each location.
Population growth is limited.
No population can grow forever. Every population has a limit to its
growth. For example, the cockroach has been around for more than
300 million years. This insect has outlived the dinosaurs and may
persist long after humans have become extinct. Yet even if cockroaches
became the only species on the planet, several factors would limit
their population size.
Birth, Death, Immigration, and Emigration
When scientists study how a population changes, they must consider
four things: birth, death, immigration, and emigration. There is even a
simple formula to help scientists track population change.
Population change (birth immigration) (death emigration)
It is too simple to say that a high birth rate means population
growth, or that many deaths mean population decline. Immigration is
the movement of individuals into a population. For example, if a
strong wind blows the seeds of a plant from one area into another, the
new plant would be said to immigrate into the new area. Immigration
can increase a population or help stabilize a declining population.
Birth and immigration introduce individuals into a population.
Emigration is
the movement of individuals out of a population.
If resources become scarce within a habitat, some of the individuals
might move to areas with greater supplies. Others may even die.
Death and emigration remove individuals from a population.
Check Your Reading
List two factors that lead to population growth and two that
lead to population decline.
Consider, for example, a flock of seagulls that flies inland during
a storm. They stop at a city dump, where food is plentiful. These
incoming seagulls become part of the seagull population that is
already living at the dump. A raccoon population living in the same
area has been eating the seagulls’ eggs, causing the number of seagull
births to decrease. If enough seagulls immigrate to the dump, the
seagull population would increase, making up for the decrease in
births. Immigration would help keep the population stable. The
seagull population would also increase if part of the raccoon population moved away.
537
Limiting Factors
When a population is growing at a rapid rate, the birth rate is much
higher than the death rate. That means that more individuals are
being born than are dying during a particular time period. There are
plenty of resources available, and the population size is increasing
rapidly. Eventually, however, the population will stop growing, because
a habitat can support only a limited number of organisms.
A limiting factor is a factor that prevents the continuing growth
of a population in an ecosystem. Abiotic, or nonliving, limiting factors
include air, light, and water. Other limiting factors can be living
things, such as other organisms in the same population or individuals
belonging to different species within the same area.
Check Your Reading
What are two limiting factors?
Competition can occur between different populations sharing the
same habitat. Competition can also occur among individuals of the
same population. Suppose, for example, that a population of deer in a
forest preserve were to increase, either through births or immigration.
Population density at the forest preserve would go up. More and more
deer in that area would be competing for the same amount of food.
Density-Dependent Factors
Density-dependent factors have a greater effect on
populations with many individuals in a small area.
Factors may include
•
•
•
•
538 Unit 4: Life Over Time
Competition
Disease
Parasitism
Predation
Effects of Population Density
In the situation described above, the seagull population could decrease
as a result of competition for food. Competition is an example of a
density-dependent factor—that is, a limiting factor that affects a population when density is high. Disease is another density-dependent
factor. The more crowded an area becomes, the easier it is for disease to
spread, so more individuals are affected. If population density is low,
there is less contact between individuals, which means that disease will
spread more slowly. Density-dependent factors have a greater effect on
the population as it grows. They can bring a population under control,
because they apply more pressure to a growing population.
There are also density-independent factors. These limiting factors have the same effect on a population, whether it has a high
density or a low density. Freezing temperatures could be considered
a density-independent factor. A freeze might kill all of the flowering
plants in an area, whether or not the population density is high.
A natural event such as a wildfire is another example of a densityindependent factor. When a wildfire occurs in a forest, it can wipe
out an entire ecosystem.
Check Your Reading
How are limiting factors that are density-dependent different
from limiting factors that are density-independent?
Density-Independent Factors
Density-independent factors are typically changes
in weather. These factors affect low-density and
high-density populations equally.
Factors may include
•
•
•
•
•
Drought
Hurricanes
Tornados
Fires
Floods
Chapter 15: Population Dynamics 539
Limiting factors include nonliving factors in the environment and
natural events such as earthquakes, fires, and storms. During times of
drought, there may not be enough food to meet the needs of all the
organisms in an area. The quality of the food declines as well. For
example, a lack of water may cause a population of trees to produce
fewer pieces of fruit, and the fruit itself may be smaller. If there is little
food available, a condition called famine arises. If the famine is severe,
and if death rates exceed birth rates, then the population size will fall
dramatically.
Check Your Reading
How do limiting factors affect populations? Remember: a
summary includes only the most important information.
Limiting factors affect human populations as well. However,
humans have found different ways to help overcome many of these
limits. In Section 15.3 you will read about how the human response to
limits differs from that of other biological populations.
Limiting Factors
What limiting factors
determine plant growth?
SKILL FOCUS
Designing
experiments
Using the materials below, design an experiment to
test how limiting factors such as sunlight or water
can determine how well a plant population will grow.
PROCEDURE
1
Decide how to use the beans, soil, and water.
2 Write up your experimental procedure. Include any safety tips.
WHAT DO YOU THINK?
• What variables did you use in your experiment?
• What evidence do you expect to see to support the goal of your experiment?
CHALLENGE Conduct your experiment. Be sure
to observe your beans daily and note which ones
are most healthy. Make a chart and record your
observations. The beans should grow for at
least two weeks before you make
your conclusion.
540 Unit 4: Life Over Time
MATERIALS
•
•
•
•
6 paper cups
potting soil
beans
water
TIME
20 minutes
Populations have distinct reproductive
survival strategies.
Although reproduction of offspring is not necessary for the survival of
an individual organism, it is necessary for the survival of a species.
Scientists studying populations observe patterns in the reproductive
strategies used among species. There are two main strategies that
many species use. There are also many species whose strategies
fit somewhere in between.
Strategies of Opportunists
Opportunists are species that reproduce rapidly if their population
falls below carrying capacity. They share many characteristics, including a short life span and the ability to reproduce large quantities of
offspring. Their population size tends to change often, and opportunists live across many areas. Opportunists include algae, dandelions,
bacteria, and insects. These species can reproduce and move across an
area quickly. In addition, they can adapt quickly to environmental
changes. Populations of opportunists often grow rapidly.
FLORIDA
Content Preview
reminder
Species reproduce either
asexually or sexually. Read
more about this on page
817.
VOCABULARY
Remember to make a
word magnet for the
term opportunist.
Include examples in
your diagram.
Opportunists
Pine trees are opportunists that
can spread across an area quickly.
Pine cones release
huge amounts of
pollen into the air.
Chapter 15: Population Dynamics 541
Competitors
Wolves are examples of competitor species. These cubs
will be cared for by adults until they are able to hunt.
Strategies of Competitors
You might be familiar with the term competitor as meaning an organism that struggles with another to get resources. Scientists who study
population growth use the term competitor in another way.
Competitors are species with adaptations that allow them to remain at
or near their carrying capacity for long periods of time. Competitors
have many characteristics that differ from those of opportunists.
Species that have a competitive reproductive strategy often live
longer and have fewer offspring. Elephants and saguaro cacti are two
examples of competitors. The offspring of competitors take longer to
develop than those of opportunists. Also, animals with this strategy
tend to take care of their young for a longer period of time.
Competitors are not distributed across areas as widely as opportunists,
but greater numbers of their offspring survive to reproductive age.
KEY CONCEPTS
CRITICAL THINKING
1. What four factors do scientists
consider when they measure
population change?
4. Analyze Why would it be a
mistake to predict population
growth based on birth rate
alone?
2. Give two examples of densitydependent factors and two
examples of densityindependent factors.
3. Other than life span, how do
opportunists and competitors
differ?
542 Unit 4: Life Over Time
5. Apply Give an example of a
factor that limits a population
near you.
CHALLENGE
6. Synthesize There has been
an oil spill along a waterway
famous for its populations of
seals, dolphins, and sea birds.
Six months later, all populations show a decline. Explain
what factors might have
caused such a change and
whether the oil spill was a
density-dependent or densityindependent factor or both.