The sentence is fine. The current generation of butterflies find (present tense = at the moment of speaking, or "now") the same forests that
their predecessors had found generations ago.
had found is the past perfect. It is used to denote an activity that was going on in the past and continued up to a certain point of time in the past, after which it stopped.
This time span (within which the predecessors were finding) was totally in the past, and therefore not still happening at the moment of speaking = in the present = now.
So you have the predecessors finding forests during some past time period that came to an end at a certain past time point. And after this past time point the predecessors were no longer finding forests. Why? Because by that time they had died out. (This specific reason is given in the article from which your sentence is extracted and which can be found by Binging or Googling it.)
The simple past would speak about this past activity of finding with no sense of duration, no sense of some ending point B.
A comparable sentence is
Today's farmers plow the same fields that their predecessors had plowed in the past.
The predecessors plowed during some time period or time span, totally in the past, and which stopped at some point in the past, namely when the predecessors retired from plowing or died at the plow or whatever.
Again, the use of simple past plowed instead of had plowed would convey no sense of a past time span with a discrete ending point.
The past time period can be mentioned explicitly:
Today's farmers plow the same fields that their predecessors had plowed between 1888 and 1929, when the great depression hit and no one could afford to plow any longer.
(Ignore any historical or practical errors, we are talking about the grammatical meaning here; this is not a history lesson.)
Here, the past perfect coincides with the time period beginning in 1888 and stopping at the past time point of 1929.
You could still use the simple past plowed:
Today's farmers plow the same fields that their predecessors plowed between 1888 and 1929.
But now the verb tense itself (simple past) does not indicate a period of time in the past after which the plowing stopped. The time period between 1888 and 1929 is actually seen as one past point of time, as far as the verb goes.