45.1D: The Study of Population Dynamics
- Distinguish between life tables and survivorship curves as used in demography
Demography
Population size, density, and distribution patterns describe a population at a fixed point in time. To study how a population changes over time, scientists must use the tools of demography: the statistical study of population changes over time. The key statistics demographers use are birth rates, death rates, and life expectancies; although, in practice, scientists also study immigration and emigration rates, which also affect populations.
These measures, especially birth rates, may be related to the population characteristics described in prior sections. For example, a large population would have a relatively-high birth rate if it has more reproductive individuals. Alternatively, a large population may also have a high death rate because of competition, disease, or waste accumulation. A high population density may lead to more reproductive encounters between individuals, as would a clumped distribution pattern. Such conditions would increase the birth rate.
Biological features of the population also affect population changes over time. Birth rates will be higher in a population with the ratio of males to females biased towards females, or in a population composed of relatively more individuals of reproductive age.
The demographic characteristics of a population are the basic determinants of how the population changes over time. If birth and death rates are equal, the population remains stable. The population will increase if birth rates exceed death rates, but will decrease if birth rates are lower than death rates. Life expectancy, another important factor, is the length of time individuals remain in the population. It is impacted by local resources, reproduction, and the overall health of the population. These demographic characteristics are often displayed in the form of a life table.
Life tables
Life tables, which provide important information about the life history of an organism, divide the population into age groups and often sexes; they show how long a member of that group will probably live. The tables are modeled after actuarial tables used by the insurance industry for estimating human life expectancy. Life tables may include:
- the probability of individuals dying before their next birthday (i.e., mortality rate )
- the percentage of surviving individuals at a particular age interval
- the life expectancy at each interval
The life table shown is from a study of Dall mountain sheep, a species native to northwestern North America. The population is divided into age intervals, as seen in the leftmost column. The mortality rate per 1,000 individuals is calculated by dividing the number of individuals dying during an age interval by the number of individuals surviving at the beginning of the interval, multiplied by 1,000.
For example, between ages three and four, 12 individuals die out of the 776 that were remaining from the original 1,000 sheep. This number is then multiplied by 1,000 to get the mortality rate per thousand.
As can be seen from the mortality rate data (column D), a high death rate occurred when the sheep were between 6 and 12 months old, which then increased even more from 8 to 12 years old, after which there were few survivors. The data indicate that if a sheep in this population were to survive to age one, it could be expected to live another 7.7 years on average, as shown by the life expectancy numbers in column E.
Survivorship curves
Another tool used by population ecologists is a survivorship curve, which is a graph of the number of individuals surviving at each age interval plotted versus time (usually with data compiled from a life table). These curves allow comparison of life histories of different populations.
Humans and most primates exhibit a Type I survivorship curve because a high percentage of offspring survive early and middle years; death occurs predominantly in older individuals. These species have few offspring as they invest in parental care to increase survival.
Birds show the Type II survivorship curve because equal numbers of birds tend to die at each age interval. These species may also have relatively-few offspring and provide significant parental care.
Trees, marine invertebrates, and most fishes exhibit a Type III survivorship curve. Very few individuals survive the younger years; however, those that live to old age are likely to survive for a relatively-long period. Organisms in this category usually have large numbers of offspring and provide little parental care. Such offspring are “on their own” and suffer high mortality due to predation or starvation; however, their abundance ensures that enough individuals survive to the next generation, perpetuating the population.
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Key Points
- The key statistics used in demography are birth rates, death rates, and life expectancies which may be influenced by population characteristics and biological factors.
- Birth rates, death rates, and life expectancies are the basic determinants of how a population changes over time.
- Life tables are demographic tools which shows a population’s life expectancy and mortality within age groups.
- A survivorship curve is a graph of the number of individuals surviving at each age interval plotted versus time.
- The characteristics and behavior of a species, such as number of offspring produced, its percentage of surviving offspring, and degree of parental care, determine the shape of its survivorship curve.
Key Terms
- life table : a demographic tool which shows a population’s life expectancy and mortality within age groups
- survivorship curve : a graph of the number of individuals surviving at each age interval plotted versus time
- mortality rate : the number of deaths per given unit of population over a given period of time