If we looked at the spacing among individuals in our herd of bison, we’d notice that the animals are pretty much grouped together. In contrast to the creosote bushes in the desert, each bison doesn’t graze at exactly the same distance from its neighbors, and the bison don’t completely fill the grassland valleys on which they graze.

The bison have a clumped distribution, whereas the creosote bushes have a uniform distribution. The distribution of weeds in a lawn is much more haphazard, and this type of distribution is called a random distribution.

A clumped distribution may be caused by behavioral factors.

For example, the bison group together for protection and to form families. Clumped distribution may also be caused by the availability of food.

A uniform distribution, as in the creosote bush population, results from competition for nutrients and water. The bushes are spaced far enough apart that they don’t compete for scarce resources.

Random distributions are found where these kind of factors don’t exist. If birds or the wind scatter weed seeds on a lawn, any part of the lawn is fair game for the weeds, resulting in a random spatial pattern.

In a population, the number of individuals per unit area is called the population density. Population density gives information about population size. In order to fully study the population's demographics, its statistical characteristics, we also need to know the distribution of ages. .. If there are no young organisms and the population consists mainly of old organisms past their reproductive age, then the population is destined for extinction. If there are many young organisms, the birth rate is high and the population will grow.

Most populations have overlapping generations. The average time from the birth of an individual to the birth of its offspring is called the generation time. In general, this time is related to the size of the organism.

Larger organisms usually have longer generation times. For example, an elephant reproduces every 12 to 30 years, whereas a bacterium can reproduce three times in a single hour.

Scientists construct life tables to determine the length of time that individuals in a population are expected to survive. Life tables include data about the birth and deaths of cohorts, groups of individuals in a population that are the same age.

Data from a life table are often used to plot a survivorship curve. The data shown here are for female domestic sheep. You can see that about half the sheep are expected to survive for 7 years

The curve for the sheep drops slowly until later in life. This shape of curve is typical for animals that receive care during their lifetimes, indicating that most deaths are caused by old age.

Now look at the second curve, based on a population of trout in a creek. In this case, most individuals die early in life, as they are eaten by predators.

Another way of analyzing the age distribution of a population is to construct an age pyramid, which shows the proportion of the population in each cohort.

This age pyramid is for the United States, a country with a relatively high standard of living. You can see that many people live well into their 80s, and the death rate is relatively low for children.

This second age pyramid is for Pakistan, a developing country with high levels of poverty and inadequate medical care. The shape of the pyramid is very different. The death rate is highest for young children, and few people live past the age of 60.

Now that we’ve looked at the distribution of populations, we’ll move on to see how scientists model population growth.

Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education