Demography
Demography is the study of growth rate, age structure, and other characteristics of populations.
Life Tables
Ecologists study populations to determine life expectancies and lifespans of different species. Demographers mark a cohort of individuals born around the same time and monitor their survival until all of them die.
A life table summarizes the demographic characteristics of a population. It also summarizes the proportion of the cohort that survived to a particular age.
Age specific mortality rate (equation= # died during interval/#alive at start of interval) is the proportion of individuals that were alive at the start of an age interval but died during the age interval. Age specific survivorship (equation= # still alive at end/# alive at start of interval) is the proportion of individuals that were alive at the start of an age interval and survived until the start of the next age interval. Together for an age interval, the sum of both must equal 1.
Life table
Survivorship
Survivorship curve is a graphic display of the rate of survival of individuals over the lifespan of a species.
Ecologists have identified three general survivorship curves: type 1, type 2, and type 3.
Survivorship curves give a base line for other population monitoring and act as a reference point for new data
Type 1
Type 1 curves are relatively flat at first, showing a low death rate in the beginning and middle years, and suddenly drop steeply, as death rate increases in older age groups.
These curves are usually used for large animals that produce few young and provide their young with extended care, which decreases the death mortality and younger ages.
Examples: moose, deer, humans, sheep and goats
Type 2
Type 2 curves reflect a relatively constant rate of mortality in all age groups. This pattern makes a steadily declining survivorship. Generally have a short gestation period.
Many small animals and mammals face a constant mortality due to predation, disease, and starvation.
Examples: lizards, five-lined skink, rodents, turtles
Type 3
Type 3 curves drop rapidly at the beginning, which reflects a very high death rate early in life, then flatten out as the death rate declines, due to there being very few individuals that survive the critical stage.
Example: the black crappie, trees, marine invertebrates, and most fish
Fecundity
Fecundity is the potential for a species to produce offspring in a lifetime. It's the potential reproductive capacity of an individual or population.
Fecundity can increase or decrease depending on many factors. When there is plenty of food and the climate is optimal, species will have higher reproduction rates, and vice versa.
Fecundity is also influenced by generation time. Generation time is the average time between the birth of an organism and the birth of it's offspring.
Fecundity is also influenced by a population's sex ratio. Sex ratio is the relative proportion of males and females.
Life tables include data for fecundity. Fecundity provides relevant information for ecologists conserving rare or endangered species.