Life tables, survivorship, & age-sex structure
- To predict if a population will grow or shrink, ecologists need to know birth and death rates for organisms at different ages as well as the current age and sex makeup of the population.
- Life tables summarize birth and death rates for organisms at different stages of their lives.
- Survivorship curves are graphs that show what fraction of a population survives from one age to the next.
- An age-sex pyramid is a "snapshot" of a population in time showing how its members are distributed among age and sex categories.
|Age interval in years||Number surviving at beginning of age interval out of 1000 born||Number dying in age interval out of 1000 born||Age-specific mortality rate—fraction of individuals alive at beginning of interval that die during the interval|
- 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.
- 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.
- Countries with rapid population growth have a sharp pyramid shape in their age structure diagrams. That is, they have a large fraction of younger people, many of whom are of reproductive age or will be soon. This pattern often shows up for countries that are economically less developed, where lifespan is limited by access to medical care and other resources.
- Areas with slow growth, including more economically developed countries like the United States, still have age-sex structures with a pyramid shape. However, the pyramid is not as sharp, meaning that there are fewer young and reproductive-aged people and more old people relative to rapidly growing countries.
- Other developed countries, such as Italy, have zero population growth. The age structure of these populations has a dome or silo shape, with an even greater percentage of middle-aged and old people than in the slow-growing example.
- Finally, some developed countries actually have shrinking populations. This is the case for Japan. The population pyramid for these countries typically pinches inward towards its base, reflecting that young people are a small fraction of the population.