Biological Barriers and Pathways

Virtually all living things have some way of getting from here to there. Animals may
walk, swim, or fly. Plants and their seeds drift on wind or water or are
carried by animals. Therefore, it is reasonable to expect that, in time, all species
might spread to every place on Earth where favorable conditions occur. Indeed,
there are somecokiropolitan species. A good example is the housefly, found almost
everywhere on Earth. However, such broad distribution is the rare
exception. just as barbed wire fences prevent cattle from leaving their pasture,
biological barriers prevent the dispersal of many species.
What constitutes barriers depends on the species and its method of disper-sal. Some are physical barriers. For land animals, bodies of water, chains of
mountains, or deserts are effective. For example, the American bison spread
throughout the open grasslands of North America, but in the southern part of the
continent there are deserts, so the bison could not spread there. For aquatic
creatures, strong currents, differences in salinity, or land areas may serve as
barriers.
Some barriers involve competition with other species. A dandelion seed may be
carried by the wind to bare ground, and, if environmental factors are right, it may
germinate. There is not much chance, however, that any individual
seedling will survive. Most places that are suitable for the growth of dandelions are
already occupied by other types of plants that are well adapted to the area. The
dandelion seedling must compete with these plants for space, water, light, and
nutrients. Facing such stiff competition, the chances of survival are AWL
For animals, some barriers are behavioral. The blue spotted salamander lives
only on mountain slopes in the southern Appalachian Highlands. Although these
creatures could survive in the river valleys, they never venture there. Birds that fly
long distances often remain in very limited areas. Kirkland’s warblers are
found only in a few places in Michigan in the summer and fly to the Bahamas in
winter. No physical barriers restrict the warblers to these two locations, yet they
never spread beyond these boundaries. Brazil’s Amazon River serves as a northern or
southern boundary for many species of birds. They could freely fly over the river, but
they seldom do.
There are three types of natural pathways through which organisms can
overcome barriers. One type is called a corridor. A corridor consists of a single type
of habitat that passes through various other types of habitat. North
America’s Rocky Mountains, which stretch from Alaska to northern Mexico, is an
example. Various types of trees, such as the Engelmann spruce, can be found not
only at the northern end of the corridor in Alaska but also at higher elevations
along the entire length of this corridor.
A second type of natural pathway is known as a filter route. A filter route
consists of a series of habitats that are different from one another but are similar
enough to permit organisms to gradually adapt to new conditions as they
spread from habitat to habitat. The greatest difference between a corridor and a
filter route is that a corridor consists of one type of habitat, while a filter consists of
several similar types.
The third type of natural pathway is called a sweepstakes route. This is dis-persal caused by the chance combination of favorable conditions. Bird watchers
are familiar with “accidentals,” which are birds that appear in places far from
their native areas. Sometimes they may find a habitat with favorable conditions and
“colonize” it. Gardeners are familiar with “volunteers,” cultivated plants that grow
in their gardens although they never planted the seeds for these
plants. Besides birds and plants, insects, fish, and mammals also colonize new areas.
Sweepstakes routes are unlike either corridors or filter routes in that
organisms that travel these routes would not be able to spend their entire lives in
the habitats that they pass through.
Some organisms cross barriers with the intentional or unintentional help of
humans, a process called invasion. An example is the New Zealand mud snail,
which was accidentally brought to North America when trout from New
Zealand were imported to a fish hatchery in the United States. It has caused
extensive environmental damage in streams and rivers. In the invasive species’
native environments, there are typically predators, parasites, and competitors that
keep their numbers down, but in their new habitat, natural checks are left behind,
giving the invaders an advantage over native species. Invasive species may spread
so quickly that they threaten commercial, agricultural, or recreational activities.

Glossary
salamander: a type of amphibian animal resembling a lizard
sweepstakes: a game of chance; a lottery

1 The word pop*
in the passage is closest in meaning to
A  worldwide
B  useful
C  well-known
D  ancient
2  How does the author explain the concept of biological barriers in paragraph 1?
A  By providing several examples of biological barriers
B  By describing the process by which barriers are formed
C By comparing biological barriers with a familiar man-made barrier
D By explaining how houseflies have been affected by biological barriers
3  What does the author suggest about American bison in paragraph 2?
A They spread to North America from South America.
B A body of water stopped them from spreading south. O
C They require open grasslands to survive.
D  They originally lived in deserts.
4 According to the passage, very few dandelion seedlings survive because of
A the danger of strong winds
B competition from other dandelions
C the lack of a suitable habitat
D competition from other species

5  In this passage, the author does NOT provide a specific example of
A a bird that is affected by behavioral barriers
B an aquatic animal that is blocked by physical barriers
C a land animal that is affected by behavioral barriers
D a tree that has spread by means of a corridor
6 The word slittlin this passage is closest in meaning to
A unknown
B impossible
C remarkable
D unlikely
7  The phrase theft, Wit ki(i iiits in paragraph 4 refers to
A Michigan and the Appalachian Highlands
B Brazil and the Bahamas
C the Appalachian Highlands and Brazil
D  the Bahamas and a few places in Michigan
8  Why does the author mention the Amazon River in paragraph 4?
A  To give an example of an important physical barrier O
B To point out that many migrating birds fly across it
C To provide an example of a behavioral barrier
D  To describe a barrier that affects aquatic animals
9  According to paragraph 6, how does the author distinguish a filter route from a
corridor ?
A A corridor consists of one habitat for its entire length, but a filter route con-sists of more than one.
B Organisms cannot live all of their lives in some parts of a filter route, but
they can in a corridor.
C The distance from one end of a filter route to the other end is longer than
the distance from one end of a corridor to the other.
D Plants spread through a corridor, while animals spread through a filter
route.
10 of 26 In paragraph 8, the author gives New Zealand mud snails as an example of
0 an invasive species that was unintentionally transported to another habitat
0 a native species that has been damaged by an invasive species
0 an invasive species that was intentionally brought to a new environment
0 an animal that spread by means of a sweepstakes route
11 Which of the following sentences best expresses the essential information in
the sentence below? (Incorrect answer choices omit important information or
change the meaning of the original sentence in an important way.)
In the invasive species’ native environments, there are typically predators,
parasites, and competitors that keep their numbers down, but in their new
habitat, natural checks are left behind, giving the invaders an advantage over
native species.

A Invasive species are organisms that leave their native environments behind
and move to a new environment.
B Native species are at a disadvantage compared to invasive species because
they face environmental dangers that invasive species have left behind.

C The  greatest danger from invasive species is that they may spread parasites
among native species.
D In a new environment, predators, parasites, and competitors prevent inva -sive species from spreading faster than native species.
12 Look at the four squares [■] that indicate where the following sentence could
be added to the passage.
They may be blown off course by storms or may be escaping population
pressures in their home areas.
The third type of natural pathway is called a sweepstakes route. This is dispersal
caused by the chance combination of favorable conditions. ■ Bird watchers are
familiar with “accidentals,” which are birds that appear in places far from their
native areas. • Sometimes they may find a habitat with favorable conditions and
“colonize” it.■ Gardeners are familiar with “volunteers,” cultivated plants that grow
in their gardens although they never planted the seeds for these
plants. ■ Besides birds and plants, insects, fish, and mammals also colonize new
areas. Sweepstakes routes are unlike either corridors or filter routes in that
organisms that travel these routes would not be able to spend their entire lives in the
habitats that they pass through.
Circle the square [■] that indicates the best place to add the sentence.
13  DIRECTIONS: Below is an introductory sentence for a brief summary of the pas -sage. Complete the summary by writing the letters of three of the answer   choices that express the most important ideas of the passage. Some of the
answer choices are incorrect because they express ideas that are not given in the
passage or because they express only details from the passage.
Biological barriers prevent organisms from spreading to all habitats where
conditions are suitable.
Answer Choices
A. Organisms that spread by means of                                                                           D. Behavioral barriers do not
sweepstakes routes include species of                                                                               prevent the spread of species
birds called accidentals that appear in                                                                               from place to place as effectively
places far from their homes.                                                                                                  as physical barriers.
B. Biological barriers can be the result                                                                          E. Humans may accidentally or
of physical features, climate, compe-                                                                                intentionally bring some species
tition, and behavior.                                                                                                                 across natural barriers, and
C. Organisms can cross barriers by                                                                                     these species may have certain
means of three types of natural                                                                                             advantages over native species.
pathways: corridors, filter routes,                                                                                     F. American bison spread
and sweepstakes routes.                                                                                                          throughout the grasslands of
North America

Answer Explanation
1.A The word cosmopolitan means “found in most
places in the world” rather than in a limited range.
It is often used about people to mean “worldly and
sophisticated,” but here it is used to describe ani-mals that live all over the world. The example of
the housefly provides a clue to the meaning of
the word.
2.C The author compares the concept of biological
barriers with a fence, a familiar type of man-made
barrier: “Just as barbed wire fences prevent cattle
from leaving their pasture, biological barriers pre-vent the dispersal of many species.”
3.C The author says, “the American bison spread
throughout the open grasslands of North America,
but in the southern part of the continent there are
deserts, so the bison could not spread there.” We
can infer from this sentence that bison can live
only in open grasslands.
4.D The author says that “Most places that are suitable
for the growth of dandelions are already occupied
by other plants that are well adapted to the area.
The dandelion seedling must compete with these
plants for space, water, light, and nutrients. Facing
such stiff competition, the chances of survival are
slim.” Clearly, it is the competition with other
species of plants that causes so few dandelion
seedlings to survive.
5.B The author does give an example of A in paragraph
4 (the Kirkland’s warbler). There is an example of C
in paragraph 4 (the blue spotted salamander) and
of D in paragraph 5 (the Engelmann spruce).
However, there is no example of B, an aquatic ani-mal that is stopped by physical barriers.
6.D In many cases, the word slim means “thin,” but in
this case it is used with the word chances to mean
“unlikely possibilities.”
7.D The two locations that the Kirkland’s warbler is
restricted to by behavioral borders are “a few
places in Michigan in the summer and . . . the
Bahamas in winter.”
8.C The author states, “Brazil’s Amazon River serves as
a northern or southern boundary for many species
of birds. They could freely fly over the river, but
they seldom do.” This indicates that the Amazon is
an example of a behavioral barrier rather than a
physical one.
9.A In paragraph 6, the author says, “The greatest dif-ference between a corridor and a filter route is that
a corridor consists of one type of habitat, while a
filter consists of several similar types.”
10.A The New Zealand mud snail is an example of an
invasive species that was carried unintentionally
to its new environment. (“An example is the new

Zealand mud snail, which was accidentally
brought to North America . . .”)
11.B This choice best restates the original sentence.
Although this choice does not give the examples
mentioned in the original sentence (predators,
parasites, and competitors) and although it uses
different grammar and vocabulary, this choice is
closest in meaning to the sentence from the pas-sage. Choice A leaves out some important infor-mation from the original sentence, and choices C
and D are not accurate.
12.You should circle the second square. The word they in the
new sentence refers back to birds,and the sentence
explains why birds appear in places far from their homes.
The third type of natural pathway is called a
sweepstakes route.This is dispersal caused by the
chance combination of favorable conditions. ■
Bird watchers are familiar with “accidentals,”
which are birds that appear in places far from their
native areas. ■They may be blown off course by
storms or may be escaping population pressures
in their home areas.Sometimes they may find a
habitat with favorable conditions and “colonize” it.
■Gardeners are familiar with “volunteers,” culti-vated plants that grow in their gardens although
they never planted the seeds for these plants.
■Besides birds and plants, insects, fish, and mam-mals also colonize new areas. Sweepstakes routes
are unlike either corridors or filter routes in that
organisms that travel these routes would not be
able to spend their entire lives in the habitats that
they pass through.
13.B, C, Choice B summarizes the information in
E paragraphs 2, 3, and 4 of the passage. Choice C
summarizes the information in paragraphs 5 and
6, and choice E summarizes the information in
paragraph 7. Choices A and F are only details in
the passage. There is nothing in the passage to
indicate that behavioral boundaries are not as
effective as physical or climatic barriers, so choice
D is not a valid answer