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Answers to Mastering Concepts Questions Chapter 39 39.1 1. A habitat is the part of the environment where an organism lives. 2. Population density is the number of individuals of a species in a specific unit area or unit volume of a habitat. 3. The three kinds of dispersion in a habitat are random dispersion (in which a species is dispersed without a discernible pattern), uniform dispersion (in which a species is dispersed in a regular pattern), and clumped dispersion (in which a species is dispersed in groups or clusters). 4. A population will grow when its birth rate is greater than its death rate. It also will grow when there are many more individuals in the reproductive and pre reproductive years than in the post reproductive years. 5. The birth rate of a population is determined by the number of individuals of reproductive age in the population. Birth rate is influenced by how many offspring are produced by each mating, by the age at which individuals are able to mate, and by the number of times an individual reproduces in his or her lifetime. 6. In a Type I survivorship curve, most individuals live long lives and then die. In a Type II survivorship curve, death is constant throughout the life span for the species. In a Type III survivorship curve, most deaths occur early in life and a few individuals survive to old age. 7. If a population has many individuals in pre-reproductive or reproductive years, the population will grow. Conversely, if a population has few individuals in these age groups, the population will not grow. 39.2 1. The per capita rate of increase is the difference between the birth rate and the death rate. 2. Exponential population growth is supported by unlimited space, food, mates, and other environmental factors that promote population growth. 3. In logistic growth, the population experiences limiting factors in the environment that slow growth and cause it to eventually level off. In exponential growth, the population increase continues unchecked and no limiting factors retard it. 4. Environmental resistance is an external force that keeps a population from growing at a maximal rate. It is anything that inhibits reproduction and immigration, or anything that increases mortality or emigration. Environmental resistance helps to determine a habitat’s carrying capacity, the maximum number of individuals of a species that a habitat can support a population. 5. A habitat’s carrying capacity can fluctuate over time, depending upon environmental conditions such as drought, heavy rainfall, other extreme weather conditions, or catastrophes. 6. Density-dependent factors limit population size when a population reaches a certain size. Three examples are infectious diseases, predation, and competition. Density-independent factors limit populations, no matter how large or small they are. Examples are severe weather conditions, geological catastrophes, floods, and oil spills. 39.3 1. K-selected species have adaptations that result in large body size, long lives, fewer young produced, much parental care for young, and late maturation. Humans, elephants, and redwood trees are K-selected species. In contrast, r-selected species have adaptations that result in small body size, short lives, quick maturation, more young produced, little or no parental care for young, and early maturation. Most insects and weeds are r-selected species. 2. Studies with guppies have shown that predators can influence populations of their prey, shifting them towards r-selected traits or, when predation is not a component of the environment, shifting them towards K-selected traits. 39.4 1. Less-developed countries have the greatest rates of human population growth, while more developed countries have the lowest rates of human population growth. 2. As a nation passes through the demographic transition it passes through three general stages. At first, birth and death rates are high so the population does not grow much. Then living conditions improve and the death rate falls, but the birth rate remains high. The population begins to increase. Finally, the death rate stays low as the birth rate declines. Now the rate of population growth slows and may cease. 3. Factors that affect birth and mortality rates are family planning programs, social factors, economic factors, education level of women, and government policies. 4. Some of the environmental consequences of human population growth include increased demands for resources such as food, clean water, and energy. These increased demands result in greater expansion of settlements and farms into wild lands, deforestation, soil erosion, water pollution, introduction of pesticides, increased soil salinity, and loss of biodiversity.