What is this?
A survivorship curve
What does it show?
The pattern of survival of species across their lifespans can be divided into 3 types.
What species are depicted?
A blue whale, a squirrel and a dandelion
What are the approximate maximum lifespans of these species?
| Species | Maximum lifespan |
| Blue whale | a century |
| Squirrel | a few years |
| Dandelion | 1 year |
What scale is used on the x-axis to compare lifespans?
Percent of maximum lifespan
What unit is used along the y-axis?
Individual organisms
This shows what happens to 1000 organisms born. This is an arbitrary number to start with.
Why are there dips in the early years for type I and type II species?
This indicates the relatively high risk of infant mortality for all species, not just type III species.
What is the shape of the survivorship curve for type II species and what form of equation would represent this?
Because this is a semi-log plot, a straight line represents an exponential relationship. Because the slope of the line is negative, the exponent would be negative.
What kind of plot is this?
This is a semi-log plot, which a logarithmic scale on the y-axis and a linear scale on the x-axis.
What is wrong with the labels on the y-axis?
There is no 0 on a logarithmic scale. (This is a very common mistake and the mistake that most clearly separates those who understand logarithmic plots and those who do not.)
It is appropriate for the age to be 0; however, the 0 and the tick mark are clearly both being associated with the y-axis.
The 0 should be a 1.
What units could be used for the y-axis to avoid arbitrary numbers and what numbers would be shown on the y-axis for these units?
There are a couple of options.
| Original numbers | Percent of organisms born | Fraction of organisms born |
| 1000 | 100% | 1 |
| 100 | 10% | 0.1 |
| 10 | 1% | 0.01 |
| 0 (should be 1) | 0.1% | 0.001 |
What characteristics define type I, II and III species and what are some examples?
| Type | Characteristics | Examples |
| I | Most live to adulthood, have relatively few offspring, risk of death highest in old age | Mammals |
| II | Risk of death is constant throughout life | Birds Reptiles |
| III | Have a huge number of offspring, risk of death is highest early in life | Trees Insects Fish |
In a stable ecosystem, on average, of all the eggs produced by a single salmon, how many hatch and survive to reproduce?
2
In a stable ecosystem, on average, of all the acorns produced by a single oak, how many sprout and survive to reproduce?
1
2 eggs must reach maturity and produce their own eggs in order to replace the male and female salmon. Each oak can produce acorns, so only one acorn must produce an oak that replaces the original. This is on average, in a stable ecosystem.
CoMo Science 
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