Course: BIS 105 Environmental Biology

Unit 4 Learning Module

Populations Through Time

Patterns in Populations Through Time

Although it is not shown in population pyramids, an important factor affecting population size is the age at which individuals become able to reproduce Age at maturity (when reproduction becomes possible) was the factor that even ancient Greeks recognized could affect population growth, when they prohibited marriage for males under the age of 30. We will return to this relationship in a later lesson, but for now, try to grasp it intuitively: if a person delays reproduction until age 30 and then has one child each year for two years, his or her fertility is 2. A person who has two children, one each year, beginning at age 20 also has a fertility of 2. Assume that these four children are born in the same two-year period, and that each offspring reproduces two children at the same age as his/her parent did. Sixty years after the initial four childbirths, the “delayed reproduction” individual will have 2 X 2 X 2 = 8 descendants. However, the early reproducing family will have 2 X 2 X 2 X 2 = 16 offspring – double the population increase of the first family. Do you think this could this be one way to slow human population growth?

Survivorship Curves

The characteristics of populations introduced above – birth rate, death rate, and life expectancy – interact to form several basic strategies for survival. Insurance companies began investigations into life expectancies for various groups of people – males vs. females, for example – and compiled the data in life tables. Biologists plot these patterns through time in survivorship curves, which graph the number of all individuals still living (in powers of ten, on the Y-axis) for each age (on the X-axis). The three basic types of survivorship curves are illustrated below.

Survivorship curves correlate with strategies species use to adapt to various environments. Large organisms in relatively stable environments have few offspring but high levels of parental care; most individuals survive to old age (Type I). Smaller organisms in less stable environments produce many offspring but provide little parental care, and few survive to old age (Type III). Type II species show intermediate characteristics in response to a death rate which remains constant throughout life.

Species showing a Type I pattern have the highest survival rates, with most individuals living to old age. Many large animals, including humans, show this “late loss” pattern of survivorship; few offspring, high levels of parental care, and low “infant” death rates characterize Type I species. As we will see in a later lesson, human populations in rich countries fit this pattern more closely than do those in undeveloped countries.

Species with Type III survivorship patterns experience high death rates among offspring; relatively few survive to old age. Most plants and invertebrates and many fish show this “early loss” pattern. Parents invest most of the reproductive energy in high numbers of offspring to offset the high death rates, and little or no energy remains for parental care.

Species showing intermediate, Type II survivorship curves experience uniform death rates throughout their lives. Some birds and many asexual species show this “constant loss” pattern.