The Zoo at the End of Time.

There was a zoo at the end of time. Millions of hungry humans began to encircle it. Breaching the gates they got inside and began to slaughter and eat. A huge swarm of butterflies emerged into the sky and attacked the humans and pushed them back out of the zoo. Out of holes in the ground came a sea of rats that bit and killed many of the humans. The survivors trying to escape ran into a great field of spiders webs and were all trapped there and were eaten by the spiders. A group of small children walked away crying uncontrollably. Birds brought them seeds to eat.

The plot for an animated film about extinction maybe.