It is just a quirk of the English language.
The difference in spelling likely comes from convention rather than a specific reason. Over time, words and phrases can evolve in their structure, and in this case, "Birth Rate" has become commonly written as one word. It is similar to other language inconsistencies that have developed over time.
Approximately 0-1% of babies say their own name as their first word. Most infants tend to say simple words like "mama," "dada," or "ball" as their first words.
The word "subjugated" is similar in meaning to "oppressed." It also conveys the idea of being under the control or authority of another in a negative or unjust way.
Excess or abundance are other words for surplus in social studies.
The synonym of "isolate" is "seclude" or "separate," both words indicate the act of being alone or separated from others.
The word population does not have an adverb.If you were to make "population" an adverb, it would be "populationly" or "populationally". But neither of those words exist nor make sense in the English language.
One option is the word birthrate.
Example sentence - The birth rate declined over the last two decades.
The birthrate in the United States has been declining.
The birthrate has changed and gone much higher than before.
The death rate of stupid people is astounding.
The death rate in a country can be high or low. there is your sentence.
Due to an increasing population, the death-rate is also increasing.
ZPG
Is the crude birth rate minus the crude death rate of a population
It is a rate of processing word.
Words you could use are speed, distance, rate, and miles per hour.
2 words a minute. One word every thirty seconds.