Fewer deaths doesn't mean a lower death rate
- The idea
- Predict
- See it
- Why
- Check
The idea
“If fewer animals died this year, the death rate went down.”
100 deaths out of 1,000 animals is a far lower risk than 50 deaths out of 100, even though the count is bigger.
The short version
The mistake: If fewer animals died this year, the death rate went down.
What's actually true: A rate is the change divided by how many there were at the start. Divide first, then compare.
- A per capita rate is a change divided by how many individuals there were at the start. It lets you compare groups of any size.
- In the plot, the same share of the living died each year. The count shrank only because fewer were alive. That constant per capita death rate is Type II survivorship.
- Growth works the same way. 1,000 deer that gain 50 in a year grow at 0.05 per individual. 200 frogs that gain 30 grow at 0.15, three times as fast.
- Trophic efficiency is the same idea. It's the energy at one level divided by the energy at the level below, so a bigger kcal number can still be the smaller percent.
On the exam: Before you compare two changes, divide each one by the starting amount.