My theory is that most animal and plant life was entirely wiped out. The hardy species that did manage to cling to life mutated rapidly. The zone is now dominated by tough, low lying plants that have developed varying levels of toxins and tough leaves to prevent themselves from being eaten. Most now have carrion flowers to attract flesh flies and beetles because the typical pollinators like butterflies were too sensitive to survive the change. Those that did now have carnivorous caterpillars or toxic ones adapted to eating the nasty plants. Many of the larger plants are now carnivorous in nature.
Fungi took advantage and rapidly evolved. Some are benign and even attractive, but some parasitic varieties adapted to hosting on higher animals, including people, affecting the mind and eventually bursting from the host. Others have highly toxic spoors.
Rats are still with us in a variety of shapes and sizes. Their naturally adaptive and aggressive nature served them well. Don’t sleep alone.
Insects are still themselves for the most part. Others are large and unpleasant. You may need a bigger shoe. - Urran
Credits to Urran for most of these
Bestiary