Which of the following sentences uses a semicolon correctly?

The monarch butterfly population used to be a million strong; now, it has drastically declined to only a fraction of that number.
The monarch butterfly population; used to be a million strong now it has drastically declined to only a fraction of that number.
The monarch butterfly population used to be a million strong, now it has drastically declined; to only a fraction of that number.
The monarch butterfly population used to be a million; strong now it has drastically declined to only a fraction of that number.

The monarch butterfly population used to be a million strong; now, it has drastically declined to only a fraction of that number.