The transition between the two moods could be more fluid or dramatic, but you have done well at conveying the individual moods clearly, and you have natural melodic flow. I really like that you are trying to tell an emotional story.
I'm not an expert on form, but I think a nice trick you could try to employ is to let some recognizable elements (motifs/themes/rhythms/harmonic devices/whatever) carry over from mood 1 to mood 2 to give the piece coherency despite mood changes.
Given your described inspiration source, I hear the wilderness before and after, but I hear all the destructive wildlife arriving at once with no foreshadowing, and all the chirping birds and butterflies disappearing at once.
That's not necessarily wrong in itself. If the story was that you were hiking in the woods one day, went home, then returned next week and saw that everything had changed, then we wouldn't observe a gradual transformation from the hiker's POV, but in that case we are maybe missing a pause from leaving the woods, and anticipation building as the hiker returns, followed by the dramatic discovery of the change.
For example, you could have an intro to the cheery section (the hiker approaches the wilderness), then, following a short departure after that section (the hiker goes home - a brief silence or some restive transition), repeat the intro (the hiker returns) in order to falsely promise the return of the butterflies, but then betray the listener with discordant shock (all the birds and butterflies are dead).
If your story should rather use gradual transformation (omniscient narrator - we see the transformation happening before our floating eyes), I think a nice trick to go from idyllic to dark is to play a dissonant, maybe off-key pedal point (a sustained bass note that juxtaposes the harmony), and then gradually pervert the music above into something more sinister.
There are many ways to skin a cat.
It's a very nice first piece all in all.