When mountains go into deep freeze things survive in warm pockets of decay under the frozen crust. Mushrooms keep feeding, as do molds, worms and voles. Tiny, wingless insects barely large enough to be seen wriggle under rotting logs, collembola, one of the oldest insects.
Modern insects have wings; collembola do not. They diverged from insects somewhere around silverfish, before mayfly. All those squirmy, ancient things lived in muck. When life existed in the seas and land was barren, there was muck, and it was squirmy, burrowing things that first crawled from swamps into the freezing hills. Colonization of land by plants and animals took hundreds of millions of years, and something like collembola, tiny, prolific guts, crawled through every minute of it.
Even when a crust freezes on soil, a microecosystem percolates away where heat from rotting leaves stands against the thirsty cold. As a pond freezes only on its surface, a muck world lingers in pockets and layers under frozen forest soil. Summer has its garish leaves and crickets, but winter is not death. It is a retreat to a weak, small world.
Primitive things mastered weak worlds long before there were rich, complex worlds, and they remain masters to this day. Their abundance is the rich world's sustenance. Their weak percolation brews a nutritious stew for spring's roots and another year's garish feast.