## Vocabulary

population population density dispersion birth rate death rate life expectancy age structure survivorship curve figure 19-1

A population can be widely distributed, as Earth's human population is, or confined to a small area, as species of fish in a lake are.

figure 19-1

A population can be widely distributed, as Earth's human population is, or confined to a small area, as species of fish in a lake are.

figure 19-2

These migrating wildebeests in East Africa are too numerous and mobile to be counted. Scientists must use sampling methods at several locations to monitor changes in the population size of the animals.

figure 19-2

These migrating wildebeests in East Africa are too numerous and mobile to be counted. Scientists must use sampling methods at several locations to monitor changes in the population size of the animals.

Word Roots and Origins dispersion from the Latin dis-, meaning "out," and spargere, meaning "to scatter"

If the small patch contains 25 oaks, an area 10 times larger would likely contain 10 times as many oak trees. A similar kind of sampling technique might be used to estimate the size of the population shown in Figure 19-2. To use this kind of estimate, the scientist must assume that the distribution of individuals in the entire population is the same as that in the sampled group. Estimates of population size are based on many such assumptions, so all estimates have the potential for error.

### Population Density

Population density measures how crowded a population is. This measurement is always expressed as the number of individuals per unit of area or volume. For example, the population density of humans in the United States is about 30 people per square kilometer. Table 19-1 shows the population sizes and densities of humans in several countries in 2003. These estimates are calculated for the total land area. Some areas of a country may be sparsely populated, while other areas are very densely populated.

### Dispersion

A third population property is dispersion (di-SPUHR-zhuhn). Dispersion is the spatial distribution of individuals within the population. In a clumped distribution, individuals are clustered together. In a uniform distribution, individuals are separated by a fairly consistent distance. In a random distribution, each individual's location is independent of the locations of other individuals in the population. Figure 19-3 illustrates the three possible patterns of dispersion.

Clumped distributions often occur when resources such as food or living space are clumped. Clumped distributions may also occur because of a species' social behavior, such as when animals gather into herds or flocks. Uniform distributions may result from social behavior in which individuals within the same habitat stay as far away from each other as possible. For example, a bird may locate its nest so as to maximize the distance from the nests of other birds.

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## Pregnancy And Childbirth

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