Question

How would you expect the number of births to compare to the number of deaths in a population that was changing?

Answer 1

Population Growth - Basic Information
All populations change in size with time
- if births exceed deaths, the population grows
- if deaths exceed births, the population shrinks
- only when births equal deaths does the population stay the same
Other Population Growth Factors
Populations can also change size if organisms move in (immigration) or leave (emigration)

Putting It All Together
We can write a simple equation to show population growth as:

Change in Population Size = (Births + Immigration) - (Deaths + Emigration)

Expressing Population Changes as a Percentage
Suppose we had a population of 100,000 individuals. Suppose in one year there were 1000 births, and 500 deaths.
What percentage of the population were births?
1000/100,000 = 0.01, or in percentage terms, this is 1% of the population.

What percentage of the population were deaths?
500/100,000 = 0.005, or in percentage terms, this is 0.5% of the population.

Assume immigration equals emigration. If so, then they cancel out of our population equation. We'll come back to
this assumption later.

Now, subtract deaths from births but express as a percentage:
1000-500/100,000 = 500/100,000 = 0.005, or 0.5% net growth

Thus, this population would be growing by 0.5% this first year. That means that after one year, there will be 500 more
individuals than the previous year. So, after one year, the population would be 100,500 individuals.

The Net Reproductive Rate
The net reproductive rate (r) is the percentage growth after accounting for births and deaths. In the example above, the population reproductive rate is 0.5%/yr.

Net reproductive rate (r) is calculated as: r = (births-deaths)/population size or to get in percentage terms, just multiply by 100.

Suppose we came back many years later, the net reproductive rate was still the same, but now the population had grown to 1,000,000. How many new individuals would be added each year now? Simply multiply the population by the reproductive rate:
1,000,000 x 0.05 (which is 0.5%) = 50,000

This means that now 50,000 new individuals are added in one year!! The net reproductive rate is the same as before, but because
the population is so much bigger, many more individuals are added.

Exponential Growth
If a population grows by a constant percentage per year, this eventually adds up to what we call exponential growth. In other words, the larger the population grows, the faster it grows!! A curve of exponential growth is an upward sweeping growth curve.