The Geographical Distribution of Gliding Animals

Southeast Asia has a unique abundance and diversity of gliding animals: flying squirrels, flying frogs, and flying lizards with wings of skin that enable them to glide through the tropical forest. What could be the explanation for the great diversity in this region and the scarcity of such animals in other tropical forests? Gliding has generally been viewed as either a means of escaping predators, by allowing animals to move between trees without descending to the ground, or as an energetically efficient way of traveling long distances between scattered resources. But what is special about Southeast Asian rain forests?

 

 

Scientists have proposed various theories to explain the diversity of gliding animals in Southeast Asia. The first theory might be called the tall-trees hypothesis. The forests of Southeast Asia are 6 meters taller than forests elsewhere due to the domination of the dipterocarp family: a family of tall tropical hardwood trees. Taller trees could allow for longer glides and the opportunity to build up speed in a dive before gliding. The lower wind speeds in tall-tree forests might also contribute by providing a more advantageous situation for gliding between trees. This argument has several flaws, however. First, gliding animals are found throughout the Southeast Asian region, even in relatively short-stature forests found in the northern range of the rain forest in China, Vietnam, and Thailand. Some gliders also thrive in low secondary forests, plantations, and even city parks. Clearly, gliding animals do not require tall trees for their activities. In addition, many gliding animals begin their glides from the middle of tree trunks, not even ascending to the tops of trees to take off.

 

A second theory, which we might call the broken-forest hypothesis, speculates that the top layer of the forest — the tree canopy — has fewer woody vines connecting tree crowns in Southeast Asian forests than in New World and African forests. As a result,

animals must risk descending to the ground or glide to move between trees. In addition, the tree canopy is presumed to be more uneven in height in Asian forests, due to the presence of the tall dipterocarp trees with lower trees between them, again favoring gliding animals. Yet ecologists who work in different regions of the world observe tremendous local variation in tree height, canopy structure, and abundance of vines, depending on the site conditions of soil, climate, slope elevation, and local disturbance. One can find many locations in Southeast Asia where there are abundant woody vines and numerous connections between trees and similarly many Amazonian forests with few woody vines.

 

A final theory differs from the others in suggesting that it is the presence of dipterocarp trees themselves that is driving the evolution of gliding species. According to this view, dipterocarp forests can be food-deserts for the animals that live in them.

The animals living in dipterocarp forests that have evolved gliding consist of two main feeding groups: leaf eaters and carnivores that eat small prey such as insects and small vertebrates. For leaf-eating gliders the problem is not the absence of any leaves but the desert-like absence of edible leaves. Dipterocarp trees often account for 50 percent or more of the total number of canopy trees in a forest and over 95 percent of the large trees, yet dipterocarp leaves are unavailable to most vertebrate plant eaters because of the high concentration of toxic chemicals in their leaves. Many species of gliding animals

avoid eating dipterocarp leaves and so must travel widely through the forest, bypassing the dipterocarp trees, to find the leaves they need to eat. And gliding is a more efficient manner of traveling between trees than descending to the ground and walking or else jumping between trees.

 

 

Many carnivorous animals also may need to search more widely for food due to the lower abundance of insects and other prey. This is caused by dipterocarps’ irregular flowering and fruiting cycles of two- to seven-year intervals, causing a scarcity of the

flowers, fruits, seeds, and seedlings that are the starting point of so many food chains. The lower abundance of prey in dipterocarp forests forces animals such as lizards and geckos to move between tree crowns in search of food, with gliding being the most efficient means. 

 

【Paragraph 1】

Southeast Asia has a unique abundance and diversity of gliding animals: flying squirrels, flying frogs, and flying lizards with wings of skin that enable them to glide through the tropical forest. What could be the explanation for the great diversity in this region and the scarcity of such animals in other tropical forests? Gliding has generally been viewed as

either a means of escaping predators, by allowing animals to move between trees without descending to the ground, or as an energetically efficient way of traveling long distances between scattered resources. But what is special about Southeast Asian rain forests?

 

1. According to paragraph 1, it is generally thought that the ability to glide is useful to forest-dwelling species because gliding

A. allows them to adapt to a wide variety of forest conditions

B. eliminates the need to travel long distances in search of food

C. provides a rapid, energy-efficient way of descending from the top of a tree to the ground

D. enables them to move through the forest without being exposed to predators on the ground

 

 

【Paragraph 2】

Scientists have proposed various theories to explain the diversity of gliding animals in Southeast Asia. The first theory might be called the tall-trees hypothesis. The forests of Southeast Asia are 6 meters taller than forests elsewhere due to the domination of the dipterocarp family: a family of tall tropical hardwood trees. Taller trees could allow for longer glides and the opportunity to build up speed in a dive before gliding. The lower wind speeds in tall-tree forests might also contribute by providing a more advantageous situation for gliding between trees. This argument has several flaws, however.

First, gliding animals are found throughout the Southeast Asian region, even in relatively short-stature forests found in the northern range of the rain forest in China, Vietnam, and Thailand. Some gliders also thrive in low secondary forests, plantations, and even city parks. Clearly, gliding animals do not require tall trees for their activities. In addition, many gliding animals begin their glides from the middle of tree trunks, not even ascending to the tops of trees to take off.

 

2. All of the following are mentioned in paragraph 2 in support of the tall-trees hypothesis EXCEPT

A. Tall trees make longer glides possible.

B. Tall trees make building up speed in a dive possible.

C. Tall trees make gliding from the middle of tree trunks possible.

D. Tall-tree forests have lower wind speeds.

 

3. Select the TWO answer choices that point to flaws in the tall-trees hypothesis, according to paragraph 2. To receive credit, you must select TWO answers.

A. Many gliding animals are unable to ascend to the tops of tall trees.

B. Gliding animals are not evenly distributed throughout the forests of the Southeast Asian region.

C. In Southeast Asia, many gliding animals are found in places where trees tend to be relatively short.

D. Many gliding animals begin their glides from positions midway up the trunks of trees.

 

【Paragraph 3】

A second theory, which we might call the broken-forest hypothesis, speculates that the top layer of the forest — the tree canopy — has fewer woody vines connecting tree crowns in Southeast Asian forests than in New World and African forests. As a result, animals must risk descending to the ground or glide to move between trees. In addition, the tree canopy is presumed to be more uneven in height in Asian forests, due to the presence of the tall dipterocarp trees with lower trees between them, again favoring gliding animals. Yet ecologists who work in different regions of the world observe tremendous local variation in tree height, canopy structure, and abundance of vines, depending on the site conditions of soil, climate, slope elevation, and local disturbance. One can find many locations in Southeast Asia where there are abundant woody vines and numerous connections between trees and similarly many Amazonian forests with few woody vines.

 

4. Paragraph 3 implies which of the following ideas about forests in which there are abundant woody vines connecting tree crowns?

A. The tree canopy is more even than it is in other forests.

B. In such forests, animals can move between trees by traveling on vines.

C. Such forests generally contain a wider diversity of animals than other forests do.

D. There are likely to be fewer predators on the ground in such forests than in other forests.

 

 

5. Paragraph 3 supports the idea that one problem with the broken-forest hypothesis is that

A. ecologists have found gliding animals in areas of Southeast Asia where trees are connected by vines and not found them

in Amazonian forests where trees are not connected by vines

B. in Southeast Asia, the forests with the fewest woody vines connecting the tops of trees turn out to have the most gliding animals

C. according to ecologists in different regions of the world, gliding animals are as abundant and varied in some forests of Africa and the New World as they are in Southeast Asian forests

D. gliding is no easier in broken forests with an uneven canopy structure than it is in forests where the trees are all about the same height

 

 

【Paragraph 4】

A final theory differs from the others in suggesting that it is the presence of dipterocarp trees themselves that is driving the evolution of gliding species. According to this view, dipterocarp forests can be food-deserts for the animals that live in them. The animals living in dipterocarp forests that have evolved gliding consist of two main feeding groups: leaf eaters and carnivores that eat small prey such as insects and small vertebrates. For leaf-eating gliders the problem is not the absence of any leaves but the desert-like absence of edible leaves. Dipterocarp trees often account for 50 percent or more of

the total number of canopy trees in a forest and over 95 percent of the large trees, yet dipterocarp leaves are unavailable to most vertebrate plant eaters because of the high concentration of toxic chemicals in their leaves. Many species of gliding animals avoid eating dipterocarp leaves and so must travel widely through the forest, bypassing the dipterocarp trees, to find the leaves they need to eat. And gliding is a more efficient manner of traveling between trees than descending to the ground and walking or else jumping between trees.

 

6. According to paragraph 4, what special difficulty do leaf-eating animals face in a dipterocarp forest

A. Dipterocarp trees are less leafy than other canopy trees.

B. There is no efficient method of getting from one tree to another.

C. Most trees are very tall with leaves that are difficult to reach.

D. There is a large distance between trees that have edible leaves.

 

 

【Paragraph 5】

Many carnivorous animals also may need to search more widely for food due to the lower abundance of insects and other prey. This is caused by dipterocarps’ irregular flowering and fruiting cycles of two- to seven-year intervals, causing a scarcity of the flowers, fruits, seeds, and seedlings that are the starting point of so many food chains. The lower

abundance of prey in dipterocarp forests forces animals such as lizards and geckos to move between tree crowns in search of food, with gliding being the most efficient means. 

 

 

7. How does paragraph 5 relate to paragraph 4

A. Paragraph 5 shows that the food-desert theory introduced in paragraph 4 can account for only part of what needs to be explained.

B. Paragraph 5 explains why the author calls the theory set out in paragraph 4 the food-desert theory.

C. Paragraph 5 completes the account of the food-desert theory begun in paragraph 4.

D. Paragraph 5 outlines an alternative to the food-desert theory described in paragraph 4

 

 

8. According to paragraph 5, what is responsible for the relative scarcity of insects and other prey in dipterocarp forests

A. The inability of insects and other prey to eat the toxic seeds, flowers, and fruits of dipterocarp trees

B. The efficiency with which lizards and geckos hunt their prey

C. The abundance of carnivorous animals in dipterocarp forests

D. Dipterocarps’ irregular flowering and fruiting cycles

 

 

【Paragraph 4】

A final theory differs from the others in suggesting that it is the presence of dipterocarp trees themselves that is driving the evolution of gliding species. ■According to this view, dipterocarp forests can be food-deserts for the animals that live in them. ■The animals living in dipterocarp forests that have evolved gliding consist of two main feeding groups:

leaf eaters and carnivores that eat small prey such as insects and small vertebrates. ■For leaf-eating gliders the problem is not the absence of any leaves but the desert-like absence of edible leaves. Dipterocarp trees often account for 50 percent or more

of the total number of canopy trees in a forest and over 95 percent of the large trees, yet dipterocarp leaves are unavailable to most vertebrate plant eaters because of the high concentration of toxic chemicals in their leaves. ■Many species of gliding animals avoid eating dipterocarp leaves and so must travel widely through the forest, bypassing the dipterocarp trees, to find the leaves they need to eat. And gliding is a more efficient manner of traveling between trees than descending to the ground

and walking or else jumping between trees.

 

9. Look at the four squares【■】that indicate where the following sentence could be added to the passage.

For each group, a dipterocarp forest is like a desert in that food resources are few and far apart.

Where would the sentence best fit?

 

10.【Directions】Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some sentences do not belong in the summary because they express ideas that are not presented in the passage

or are minor ideas in the passage. This question is worth 2 points.

Various theories have been proposed to explain the unique abundance and diversity of gliding animals in the rain forests of Southeast Asia.

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Answer Choices

A. One theory is that so many gliding species evolved in Southeast Asia because the forests are exceptionally tall, but there is evidence that calls that theory into question.

B. The fact that gliding animals are most abundant in the short-stature forests of China, Vietnam, and Thailand shows that gliding did not evolve as an adaptation to an environment of tall trees.

C. Ecologists have shown that the abundance of gliding animals in different regions of the world corresponds to variations in tree height, canopy structure, and abundance of vines.

D. The hypothesis that gliding evolved to compensate for a scarcity of vines linking tree canopies overlooks problematic evidence from both Southeast Asian and Amazonian forests.

E. In forests that are dominated by tall trees, jumping from tree to tree or descending to the ground may be a more efficient way of traveling through the forest than gliding.

F. Dipterocarp trees create an environment in which many species must travel widely to find food, and gliding may have evolved as a rapid and efficient way of moving between tree crowns.

 

 

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