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 some cosmopolitan 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 dispersal. 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 slim.
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 Engehnann 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 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.

单选题 The word " cosmopolitan" in the passage is closest in meaning to ______.
A. worldwide B. useful C. well-known D. ancient

单选题 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.

单选题 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.
C. They require open grasslands to survive.
D. They originally lived in deserts.

单选题 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

单选题 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

单选题 The word "slim" in this passage is closest in meaning to ______.
A. unknown B. impossible C. remarkable D. unlikely

单选题 The phrase "these two locations" 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

单选题 Why does the author mention the Amazon River in paragraph 4?
A. To give an example of an important physical barrier.
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.

单选题 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 consists 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.

单选题 In paragraph 8, the author gives New Zealand mud snails as an example of ______.
A. an invasive species that was unintentionally transported to another habitat
B. a native species that has been damaged by an invasive species
C. an invasive species that was intentionally brought to a new environment
D. an animal that spread by means of a sweepstakes route

单选题 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 invasive species from spreading faster than native species.

单选题 Look at the four squares, , , and

填空题 Directions: Below is an introductory sentence for a brief summary of the passage. 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.
·______
·______
·______

A. Organisms that spread by means of sweepstakes routes include species of birds called accidentals that appear in places far from their homes.
B. Biological barriers can be the result of physical features, climate, competition, and behavior.
C. Organisms can cross barriers by means of three types of natural pathways: corridors, filter routes, and sweepstakes routes.
D. Behavioral barriers do not prevent the spread of species from place to place as effectively as physical barriers.
E. Humans may accidentally or intentionally bring some species across natural barriers, and these species may have certain advantages over native species.

F. American bison spread throughout the grasslands of North America.