Survivorship and Live table

Survivorship and Live table

Survivorship curves

• Type I. Humans and most primates have a Type I survivorship curve. In a Type I curve, organisms tend not to die when they are young or middle-aged but, instead, die when they become elderly. Species with Type I curves usually have small numbers of offspring and provide lots of parental care to make sure those offspring survive.
• Type II. Many bird species have a Type II survivorship curve. In a Type II curve, organisms die more or less equally at each age interval. Organisms with this type of survivorship curve may also have relatively few offspring and provide significant parental care.
• Type III. Trees, marine invertebrates, and most fish have a Type III survivorship curve. In a Type III curve, very few organisms survive their younger years. However, the lucky ones that make it through youth are likely to have pretty long lives after that. Species with this type of curve usually have lots of offspring at once—such as a tree releasing thousands of seeds—but don't provide much care for the offspring.

Age-sex structure

How can we use the birth and death rates from a life table to predict if a population will grow or shrink? To do this effectively, we need a "snapshot" of the population in its present state.

For instance, suppose we have two populations of bears: one made up mostly of young, reproductive-aged female bears and one made up mostly of male bears past their reproductive years. Even if these populations are the same size and share a life table—have the same reproduction and survival rates at a given age—they are likely to follow different paths.

• The first population is likely to grow because it has many bears that are in prime position to produce baby bears, cubs.
• The second population is likely to shrink because it has many bears that are close to death and can no longer reproduce.

So, who's currently in a population makes a big difference when we are thinking about future population growth! Information about the age-sex structure of a population is often shown as a population pyramid. The x-axis shows the percent of the population in each category, with males to the left and females to the right. The y-axis shows age groups from birth to old age.

Governments around the world keep records of human birth and death rates—not just for the overall population of a country but also for specific groups within it, broken down by age and sex. Often, this data is arranged in summary tables called life tables. Enterprising insurance companies make good use of these life tables, taking the probability of death at a given age and using it to calculate insurance rates that, statistically, guarantee a tidy profit.

Ecologists often collect similar information for the species they study, but they don't do it to maximize profits! They do it to gain knowledge and, often, to help protect species. Take, for example, ecologists concerned about the endangered red panda. They might follow a group of red pandas from birth to death. Each year, they would record how many pandas had survived and how many cubs had been born. From this data, they could better understand the life history, or typical survival and reproduction pattern, of their red panda group.

What's the use of a life history? In some cases, ecologists are just plain curious about how organisms live, reproduce, and die. But there is also a practical reason to collect life history data. By combining birth and death rates with a "snapshot" of the current population—how many old and young organisms there are and whether they are male or female—ecologists can predict how a population is likely to grow or shrink in the future. This is particularly important in the case of an endangered species, like the red pandas in our example.

Life tables

A life table records matters of life and death for a population—literally! It summarizes the likelihood that organisms in a population will live, die, and/or reproduce at different stages of their lives.

Let's start simply by taking a look at a basic life table that just shows survival—rather than survival and reproduction. Specifically, we'll focus on the animal below: the Dall mountain sheep, a wild sheep of northwestern North America.