The "Natural Rate" of Population Growth?

The population growth rate graph, again:

Graph #1: A Natural Rate of Population Growth?

I put a line on there in red, suggesting what looks to me to be a "natural" rate of population growth since the 1960s, or anyway a rate from which the actual rate is sometimes disturbed and to which it apparently desires to return.

Don't be outraged; it's just an impression.

Anyway, when I look at the blue line after 1960 I see what looks like one particularly large and long-lasting disturbance:

Graph #2: A Large Disturbance beginning around 1990

For some reason, I had in mind that this large disturbance was a result of immigration as opposed to the other thing. I don't know why I thought immigration, but I did. For the longest time.

Maybe I read it somewhere, and then forgot. Anyway, I finally tracked it down and yes, it looks like immigration caused that disturbance. The next graph is part of an image presented by weforum:

Graph #3: from World Economic Forum

There looks to be a pretty consistent uptrend since about 1950 on this graph. But just around 1990 there is one particularly large spike. It shows a tripling, from about 600 thousand to about 1800 thousand immigrants, and it attributes the spike to "IRCA legislation".

I looked it up. IRCA is the Immigration Reform and Control Act of 1986. So yeah, immigration in this case. But I still see it as a disturbance or deviation from the "natural" tendencies which arise as an outgrowth of economic conditions.