1 Population ecology is the science that measures changes in population size and composition and identifies the causes of these fluctuations. Population ecology is not concerned solely with the human population. In ecological terms, a population consists of the individuals of one species that simultaneously occupy the same general area, rely on the same resources, and are affected by similar environmental factors. The characteristics of a population are shaped by its size and by the interactions among individuals and between individuals and their environment.
2 Population size is a balance between factors that increase numbers and factors that decrease numbers. Some factors that increase populations are favorable light and temperature, adequate food supply, suitable habitat, ability to compete for resources, and ability to adapt to environmental change. Factors that decrease populations are insufficient or excessive light and temperature, inadequate food supply, unsuitable or destroyed habitat, too many competitors for resources, and inability to adapt to environmental change.
3 An important characteristic of any population is its density. Population density is the number of individuals per unit, such as the number of maple trees per square kilometer in a county. Ecologists can rarely determine population size by actually counting all individuals within geographical boundaries. Instead, they often use a variety of sampling techniques to estimate densities and total population sizes. In some cases, they estimate population size through indirect indicators, such as the number of nests or burrows, or signs such as tracks or droppings.
4 Another important population characteristic, dispersion, is the pattern of spacing among individuals within the population"s geographical boundaries. Various species are distributed in their habitats in different ways to take better advantage of food supplies and shelter, and to avoid predators or find prey. Within a population"s range, densities may vary greatly because not all areas provide equally suitable habitat, and also because individuals space themselves in relation to other members of the population.
5 Individuals may be spread across the environment in three possible dispersion patterns: clumped, uniform, or random. A clumped dispersion means that individuals are gathered in patches throughout their habitat. Clumping often results from the irregular distribution of resources needed for survival and reproduction. For example, fallen trees keep the forest floor moist, and many forest insects are clumped under logs where the humidity is to their liking. Clumping may also be associated with mating, safety, or other social behavior. Crane flies, for example, swarm in great numbers, a behavior that increases mating chances, and some fish swim in large schools so they are less likely to be eaten by predators.
6 A uniform dispersion means that the members of a population are evenly spread throughout their habitat. Uniform spacing occurs when individuals are able to survive anywhere in their habitat, but must also interact directly with other individuals in the population. For example, regular spacing of plants may result from shading and competition for water. In animal populations, uniform distribution is usually caused by competition for some resource or by social interactions that set up individual territories for feeding, breeding, or resting. Individuals use the resources found immediately around them and spread out to use all of the available resources.
7 While clumping and uniform patterns occur where there are direct interactions among individuals, random spacing may be seen where neither strong attraction nor strong repulsion exists among individuals in a population. In random dispersion, individuals are spread throughout their habitat in an unpredictable, irregular manner. Each individual"s location is independent of the locations of other individuals. In tropical rain forests, trees of the same species can be randomly dispersed. Overall, random patterns are rare in nature, with most populations showing a tendency toward either clumped or uniform distribution.
Populations change in size, structure, and distribution as they respond to changes in environmental conditions. Four main variables—births, deaths, immigration, and emigration—determine the rate of change in the size of the population over time. A change in the birth rate or death rate is the major way that most populations respond to changes in resource availability. Members of some animal species can avoid or reduce the effects of environmental stress by emigrating from one area and immigrating to another with more favorable environmental conditions, thus altering the population"s dispersion.

单选题 Which sentence below best expresses the essential information in the highlighted sentence in paragraph 1? Incorrect choices change the meaning in important ways or leave out essential information.

单选题 According to the passage, which factor might cause the population of a species to decrease in size?

单选题 Which of the following is an indirect indicator of a population"s density?

单选题 The distribution pattern of individuals within a population"s geographical boundaries is known as

单选题 The word range in paragraph 4 is closest in meaning to

单选题 The word patches in paragraph 5 is closest in meaning to

单选题 All of the following are given as reasons for clumping EXCEPT

单选题 The phrase set up in paragraph 6 is closest in meaning to

单选题 Which of the following situations would be most likely to result in a uniform dispersion pattern?

单选题 Random spacing in a population differs from other dispersion patterns in that it

单选题 The word repulsion in paragraph 7 is closest in meaning to

单选题 Why does the author mention immigration and emigration in paragraph 8?

填空题 Look at the four squares, A, B, C, and D, which indicate where the following sentence could be added to the passage. Where would the sentence best fit?
For example, they might estimate the number of black bears in a national park by counting Individuals in a few sample plots representative of the whole park.
An important characteristic of any population is its density. A Population density is the number of individuals per unit, such as the number of maple trees per square kilometer in a county. B Ecologists Can rarely determine population size by actually counting all individuals within geographical boundaries. C Instead, they often use a variety of sampling techniques to estimate densities and total population sizes. D In some cases, they estimate population size through indirect indicators, such as the number of nests or burrows, or signs such as tracks or droppings.

填空题 An introductory sentence for a brief summary of the passage is provided below. 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.

Population ecology is the science of populations and the factors that influence them.

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A. A population is affected by its size, density, and dispersion, as well as the relationships among its members.
B. Dispersion of individuals in a population may be clumped, uniform, or random, as determined by environmental and social factors.
C. Clumping is related to safety; for example, fish swim in large schools to reduce their chances of being eaten by predators.
D. Most populations have either clumped or uniform distribution because random spacing is rare in nature.
E. Populations change in density and dispersion as they respond to changes in environmental conditions and resource availability.
F. If a population has an abundant food supply and a suitable habitat, it will not undergo evolutionary change.