Download Describing Populations

LESSON
2
Describing Populations
Guiding Question: What are the important characteristics of
populations?
• Explain the usefulness of tracking population size.
• Define population density.
• Describe the three ways populations can be
distributed.
• Explain what age structure diagrams tell you about
a population.
Male resplendent quetzal
4.2 LESSON PLAN PREVIEW
Inquiry Students model
sampling using a “population”
of marbles.
Real World Students explore
local population density.
Differentiated Instruction Support students with a Q&A
on age structure diagrams.
4.2 RESOURCES
Modeling Lab, Using Mark-andRecapture • Lesson 4.2 Worksheets
• Lesson 4.2 Assessment • Chapter 4
Overview Presentation
GUIDING QUESTION
FOCUS Create a class cluster diagram of the phrase Characteristics of
Populations. As you cover the lesson,
add to and revise this diagram.
104 Lesson 2
Reading Strategy As you read, organize the lesson information using a T-chart. Be sure to note each key term and its
definition.
Vocabulary population size, population density, population
distribution, age structure, age structure diagram, sex ratio
Individuals of the same species living in a particular
area make up a population. Species can consist of many populations
that are geographically isolated from one another. This is the case with
the resplendent quetzal (Pharomachrus mocinno), considered one of
the world’s most spectacular birds. These birds are characteristic of the
Monteverde region. Although it ranges from southernmost Mexico to
Panama, the resplendent quetzal lives only in high-elevation tropical
forests. Today, the species exists in many separate populations scattered
across Central America.
In contrast, humans have spread into nearly every corner of the planet.
As a result, it is difficult to define a distinct human population on anything
less than the global scale. Some would maintain that in the ecological sense
of the word, all 6.8 billion of us make up one population.
Whether one is considering humans, or quetzals, or golden toads, all
populations show characteristics that help population ecologists predict
the future of that population. Population size, density, distribution, age
structure, and sex ratio all help the ecologist understand how a population may grow or decline.
Population Size
The overall health of a population can often be monitored by
tracking how its size changes.
Population size describes the number of individual organisms present in a given population at a given time. Population size may increase,
decrease, undergo cyclical change, or remain the same over time. When
population size increases or remains steady, it is often a sign of a healthy
population. When population size declines quickly, however, it can mean
extinction is coming. As late as 1987, scientists reported more than 1500
golden toads in the Monteverde population. In 1988, only 10 toads were
reported. In 1989, scientists found only a single toad. By 1990, the species
had disappeared.
The Decline of the Passenger Pigeon The passen-
ger pigeon (Ectopistes migratorius) illustrates the extremes
of population size. As with the golden toad, dramatic and
rapid changes in population size indicated trouble. Passenger
pigeons, shown in Figure 3, were once the most abundant
bird in North America. Huge flocks literally darkened the
skies. In the early 1800s, an ornithologist (a scientist who
studies birds) named Alexander Wilson described a flock of
2 billion individuals that formed a near-solid mass 390 kilometers (240 miles) long, took 5 hours to fly overhead, and
sounded like a tornado.
Passenger pigeons nested and bred in the forests of the
upper Midwest and southern Canada. Once people began
cutting down the forests, however, hunters had easy access to
the birds. Thousands of pigeons at a time were shot down and
shipped to market as food. By the end of the 1800s, the population size was so small that the pigeons could not form the large
colonies they needed to breed effectively. In 1914, the last passenger pigeon on Earth died in the Cincinnati Zoo in Ohio.
(a)
Determining Population Size Do you
think Alexander Wilson actually counted 2 billion passenger pigeons in the flock? Of course he
didn’t. While simply counting every individual in
a population is the most direct way to determine
population size, it is almost never possible. In
nearly all situations, population size is estimated
using sampling techniques. Here’s how sampling
works: Instead of counting every individual in
a large area, ecologists count the number in a
smaller sample area. Then, they use that information to estimate the number of individuals
in the larger overall area. If there are 100 oak
trees in one square kilometer of a large forest,
for instance, it may be reasonable to estimate
that there are about 1000 oak trees in ten square
kilometers of the same forest. Sampling is particularly helpful for estimating the size of very
large populations and populations that are widely
spread out over an area.
Sometimes it is easier to find signs of organisms rather than the organisms themselves. For
example, it can be very difficult to spot organisms that are rare or that simply prefer to remain
hidden. For these creatures, such as jaguars,
tigers, and tapirs, it is often easier to find and
count animal tracks or droppings than the animals themselves.
Reading
Checkpoint
When is sampling necessary?
(b)
FIGURE 3 Population Size (a) The passenger
pigeon was once North America’s most numerous
bird. (b) However, habitat destruction and hunting
drove the species to extinction within a few decades.
ANSWERS
Reading Checkpoint Sampling is
necessary when you cannot directly
count the individuals in a population. Usually, the population is very
numerous and/or very spread out.
Population Ecology 105
What Do
you think?
Some scientists say that extinction is our greatest environmental
problem. Do you think we should
be concerned about species
extinction? What consequences
do you think the loss of golden
toads and harlequin frogs has had
on the wildlife and people of the
Monteverde region?
ANSWERS What Do You Think? Answers will
vary. Check that students’ opinions
include how the loss of the harlequin
frog and golden toad has affected
the Monteverde region.
Reading Checkpoint Population
density is the number of individuals
within a population per unit area;
population distribution describes
how the organisms in an area are
arranged.
Population Density
A population’s density is a measure of how crowded it is.
The huge flocks and breeding colonies of passenger pigeons showed high
population density. Population density describes the number of individuals within a population per unit area. For instance, the 1500 golden
toads counted in 1987 were found within 4 square kilometers (988 acres).
So their population density was 1500 toads/4 square kilometers, or 375
toads/square kilometer. In general, larger organisms, like lions and other
big cats, have lower population densities because they require more
resources, and thus more room, to survive.
Different Densities High population density can make it easier for
organisms to group together and find mates. However, it can also lead to
conflict as individuals compete for resources. Overcrowded organisms may
also become more vulnerable to the predators that feed on them, and close
contact among individuals can increase the transmission of infectious disease. In contrast, at low population densities, organisms benefit from more
space and resources but may find it harder to locate mates and companions.
Density and the Harlequin Frog Overcrowding is thought to have
doomed the harlequin frog (Atelopus varius), which disappeared from the
Monteverde Reserve at the same time as the golden toad. The harlequin
frog, shown in Figure 4, lived in very specific locations called “splash
zones.” Splash zones are areas alongside rivers and streams that receive
spray from waterfalls and rapids. In the 1980s and 1990s, the Monteverde
region of Costa Rica got warmer and
drier. Water flow decreased and many
streams dried up. Soon, there were only
a few small splash zones left.
Scientists recorded frog population
densities up to 4.4 times higher than
normal around the remaining splash
zones. Such overcrowding likely made
the frogs vulnerable to disease transmission, predator attack, and assault
from parasitic flies. From their field
research, researchers concluded that
these factors led to the harlequin frog’s
disappearance from Monteverde. A
very small population is thought to
remain outside the reserve, but scientists worry for its future.
Figure 4 Splashing No Longer
Harlequin frogs like this one are no
longer found in the Monteverde
Reserve. Scientists think that
unusually high population densities,
brought about by changes in climate,
contributed to their decline.
106 Lesson 2
Population Distribution
Populations can be distributed randomly,
uniformly, or in clumps.
Another factor that contributed to the disappearance of harlequin frogs is how individuals
were arranged within the Monteverde ecosystem.
Population distribution, sometimes called population dispersion, describes how organisms are
arranged within an area. Ecologists define three
distribution types: random, uniform, and clumped,
as shown in Figure 5.
In a random distribution,
individual organisms are arranged within a space
in no particular pattern. This type of distribution
can occur when the resources an organism needs
are found throughout an area and other organisms do not strongly influence where members of a
population settle.
▶ Random Distribution A uniform distribution
is one in which individual organisms are evenly
spaced throughout an area. This can occur when
individuals hold territories or otherwise compete
for space. In a desert, where there is little water,
each plant needs a certain amount of room for
its roots to gather adequate moisture. As a result,
there tends to be roughly the same amount of space
between individual plants.
(a) Random
▶ Uniform Distribution In a clumped distribution, individual organisms arrange themselves
according to the availability of the resources they
need to survive. This is the most common pattern
in nature. For example, desert animals may live in
patches around isolated sources of water. During
their mating season, golden toads were found
clumped at seasonal breeding pools. The harlequin
frogs were clumped around splash zones. Humans,
too, show clumped distribution; most people live
and work near large urban centers.
Distributions can depend on the scale at which
you’re looking. For example, ants show a clumped
distribution at a large scale; populations live
together in colonies. Within the colony, however,
individuals may be distributed more evenly.
(b) Uniform
▶ Clumped Distribution Reading
Checkpoint
hat is the difference between popuW
lation density and distribution?
(c) Clumped
Figure 5 Population Distribution There are three
fundamental population distribution patterns seen in nature:
(a) random, (b) uniform, and (c) clumped.
Population Ecology 107
How do changes in population
size relate to environmental
conditions?
Application As a class, discuss how
each of the populations pictured in
Figure 6 will likely grow. Then, have
small groups of students talk about
how the different types of population growth might affect the environment in which each population
lives. Make sure students discuss
present and future effects. Then, ask
a few volunteers to summarize their
group’s discussion for the class.
ANSWERS Lesson 2 Assessment
1. A population that stays the same
size or grows is likely healthy. A
rapidly decreasing population size
can mean that the population is
going extinct.
2. 40 flamingos in a 10-m2 area
3. In random distribution, individuals are arranged in no particular
pattern. A uniform distribution is
one in which individuals are evenly
spaced. In clumped distribution,
organisms are gathered into
groups; this is the most common
distribution in nature.
4. The “mostly young” population. It
has many pre-reproductive individuals that can have offspring in
the near future.
5. Population ecologists might be
interested in what caused mass
extinctions so that they can
recognize extinction when it is
happening today and possibly
avoid future extinctions caused by
human activity.
Figure 6 Age Structure Diagrams
A population’s age structure diagram
shows relative frequencies of males
and females in different age groups.
Populations with more young, prereproductive individuals tend to grow
quickly. Populations with more older,
post-reproductive individuals tend to
decline.
108 Lesson 2
Age Structure and Sex Ratios
Age structure diagrams show the number of males and females
in different age groups within a population.
Different populations have different mixes of ages. Likewise, populations
can vary in the proportion of males and females present. These differences can affect how a population grows.
Age Structure Populations almost always include individuals of
different ages. Age structure, or age distribution, describes the relative
numbers of organisms of each age within a population. Age pyramids,
or age structure diagrams, are visual tools scientists use to show the age
structure of populations. As shown in Figure 6, the width of each horizontal bar in the diagram represents the relative size of each age group.
Individuals capable of having offspring make up the reproductive group.
Young individuals who have not yet reached the age where they can have
offspring are called pre-reproductive. Older individuals past the age of
having offspring are called post-reproductive.
Age structure diagrams can be effective tools when predicting population growth. A population with an even age distribution will likely remain
stable as births keep pace with deaths. A population made up mostly of
individuals past reproductive age will tend to decline over time. In contrast, a population of mostly reproductive or pre-reproductive individuals
is likely to increase over time. Populations that are “bottom heavy” are
capable of rapid growth. In this respect, the wide base of an age pyramid
is like an oversized engine in a sports car—the bigger the engine, the
faster it accelerates.
Age Structure Diagrams
Pre-reproductive age
Male
Female
Reproductive age
Male
Post-reproductive age
Female
Male
Female
Age
BIG QUESTION
Mostly young
(growing)
Mix of young and old
(stable)
Mostly old
(declining)
Sex Ratios A population’s sex ratio is its proportion of males to
females. Notice that age structure diagrams also give information about
sex ratio by providing the relative numbers of males and females in each
age group. Sex ratio is an important characteristic for populations of
organisms that reproduce sexually and have distinct male and female
individuals. For example, in monogamous species (in which each sex
takes a single mate), a 50:50 sex ratio is often ideal. If there are too many
males or females, called an unbalanced sex ratio, many individuals would
be left without a mate—making it much harder for any given individual
to reproduce and pass on genes.
Figure 7 Sex Ratio The relative
number of males and females
determines a population’s sex ratio.
In this South African population of
springboks, the ratio of males to
females is about 50:50.
2
1. Relate Cause and Effect How is a population’s size
related to its well-being?
2. Calculate Which population of flamingos is more
dense: 15 flamingos in a 5-square-meter area, or 40
flamingos in a 10-square-meter area?
3. Apply Concepts Describe the three patterns of
population distribution. Which of these is the most
common distribution in nature?
4. Infer Which of the populations shown in
Figure 6 do you think is most likely to get bigger in
size? Explain your reasoning.
5. Explore the BIGQUESTION Paleontologists use
fossils to study past life on Earth. Paleontologists
have documented several times in Earth’s history
when massive numbers of species have declined
and then disappeared from the fossil record all
at once. Why do you think population ecologists
studying living organisms might be interested in
these mass extinction events?
Population Ecology 109