2.4: Life Tables and Survivorship

Population ecologists use life tables to study species and identify the most vulnerable stages of organisms’ lives to develop effective measures for maintaining viable populations. Life tables, like Table \(\PageIndex{1}\), track survivorship, the chance of an individual in a given population surviving to various ages. Life tables were invented by the insurance industry to predict how long, on average, a person will live. Biologists use a life table as a quick window into the lives of the individuals of a population, showing how long they are likely to live, when they’ll reproduce, and how many offspring they’ll produce. Life tables are used to construct survivorship curves, which are graphs showing the proportion of individuals of a particular age that are now alive in a population. Survivorship (chance of surviving to a particular age) is plotted on the y-axis as a function of age or time on the x-axis. However, if the percent of maximum lifespan is used on the x-axis instead of actual ages, it is possible to compare survivorship curves for different types of organisms (Figure \(\PageIndex{1}\)). All survivorship curves start along the y-axis intercept with all of the individuals in the population (or 100% of the individuals surviving). As the population ages, individuals die and the curves goes down. A survivorship curve never goes up.

Table \(\PageIndex{1}\): Life Table for the U.S. population in 2011 showing the number who are expected to be alive at the beginning of each age interval based on the death rates in 2011. For example, 95,816 people out of 100,000 are expected to live to age 50 (0.983 chance of survival). The chance of surviving to age 60 is 0.964 but the chance of surviving to age 90 is only 0.570.

Age (years)	Number Living at Start of Age Interval	Number Dying During Interval	Chance of Surviving Interval	Chance of Dying During Interval
0-1	100000	606	0.993942	0.006058
1-5	99394	105	0.998946	0.001054
5-10	99289	60	0.999397	0.000603
10-15	99230	70	0.999291	0.000709
15-20	99159	242	0.997562	0.002438
20-25	98917	425	0.995704	0.004296
25-30	98493	475	0.995176	0.004824
30-35	98017	553	0.994362	0.005638
35-40	97465	681	0.993015	0.006985
40-45	96784	968	0.989994	0.010006
45-50	95816	1535	0.983982	0.016018
50-55	94281	2306	0.975541	0.024459
55-60	91975	3229	0.964895	0.035105
60-65	88746	4378	0.950668	0.049332
65-70	84368	6184	0.926698	0.073302
70-75	87184	8670	0.889101	0.110899
75-80	69513	12021	0.827073	0.172927
80-85	57493	15760	0.725879	0.274121
85-90	41733	17935	0.570241	0.429759
90-95	23798	14701	0.382258	0.617742
95-100	9097	7169	0.211924	0.788076
100 and over	1928	1928	0	1.000000

SOURCE: CDC/NCHS, National Vital Statistics System.

Survivorship curves reveal a huge amount of information about a population, such as whether most offspring die shortly after birth or whether most survive to adulthood and likely to live long lives. They generally fall into one of three typical shapes, Types I, II and III (Figure \(\PageIndex{1}\)a). Organisms that exhibit Type I survivorship curves have the highest probability of surviving every age interval until old age, then the risk of dying increases dramatically. Humans are an example of a species with a Type I survivorship curve. Others include the giant tortoise and most large mammals such as elephants. These organisms have few natural predators and are, therefore, likely to live long lives. They tend to produce only a few offspring at a time and invest significant time and effort in each offspring, which increases survival.

In the Type III survivorship curve most of the deaths occur in the youngest age groups. Juvenile survivorship is very low and many individuals die young but individuals lucky enough to survive the first few age intervals are likely to live a much longer time. Most plants species, insect species, frogs as well as marine species such as oysters and fishes have a Type III survivorship curve. A female frog may lay hundreds of eggs in a pond and these eggs produce hundreds of tadpoles. However, predators eat many of the young tadpoles and competition for food also means that many tadpoles don’t survive. But the few tadpoles that do survive and metamorphose into adults then live for a relatively long time (for a frog). The mackerel fish, a female is capable of producing a million eggs and on average only about 2 survive to adulthood. Organisms with this type of survivorship curve tend to produce very large numbers of offspring because most will not survive. They also tend not to provide much parental care, if any.

Type II survivorship is intermediate between the others and suggests that such species have an even chance of dying at any age. Many birds, small mammals such as squirrels, and small reptiles, like lizards, have a Type II survivorship curve. The straight line indicates that the proportion alive in each age interval drops at a steady, regular pace. The likelihood of dying in any age interval is the same.

In reality, most species don’t have survivorship curves that are definitively type I, II, or III. They may be anywhere in between. These three, though, represent the extremes and help us make predictions about reproductive rates and parental investment without extensive observations of individual behavior. For example, humans in less industrialized countries tend to have higher mortality rates in all age intervals, particularly in the earliest intervals when compared to individuals in industrialized countries. Looking at the population of the United States in 1900 (Figure \(\PageIndex{1}\)b), you can see that mortality was much higher in the earliest intervals and throughout, the population seemed to exhibit a type II survivorship curve, similar to what might be seen in less industrialized countries or amongst the poorest populations.

Figure \(\PageIndex{1}\): (a) Survivorship curves show the distribution of individuals in a population according to age. Humans and most large mammals have a Type I survivorship curve because most death occurs in the older years. Birds have a Type II survivorship curve, as death at any age is equally probable. Trees have a Type III survivorship curve because very few survive the younger years, but after a certain age, individuals are much more likely to survive. (b) Survivoship curves for the US population for 1900, 1950, 2000, 2050, 2100