Mountain snowpack is usually discussed as a water supply issue, and it is one. For the animals living under and around it, snow does several other jobs that are lost at the same time.
Snow as insulation
A deep snowpack traps a layer of relatively stable air at ground level, and the space beneath it stays close to freezing even when the air above drops far lower.
Voles, shrews, insects and plant roots overwinter in that space. Their survival depends on the snow arriving early and staying, not on how cold the winter is overall.
Thin or intermittent snow removes the buffer, exposing soil to hard freezing. Small mammal populations crash in those winters, and the predators relying on them follow a year later.
Timing of melt and plant growth
Alpine plants begin growth when snow clears, so melt date sets the start of the growing season more directly than temperature does.
Herbivores time their reproduction to that green-up, aiming to nurse young when forage quality peaks rather than when it is merely abundant.
An early melt shifts the peak forward. Animals whose breeding cues come from daylength rather than weather cannot adjust as quickly, and arrive after the best forage has passed.
The camouflage mismatch
Several mountain species, including some hares, weasels and ptarmigan, moult to white coats or plumage on a schedule driven mainly by daylength.
That schedule was reliable when snow arrival and departure were consistent. A white animal on brown ground is conspicuous to every predator in the area.
Populations do carry variation in moult timing, so selection can act on it, but the shift required is large and the species involved are not fast to adapt.
Water released through the summer
Snowpack functions as a reservoir that releases water slowly through the warm months, sustaining streams long after precipitation has stopped.
Precipitation falling as rain instead runs off immediately, producing higher winter flows and much lower late-summer ones.
Cold-water fish feel this most directly, as shrinking summer streams warm faster and fragment into disconnected pools well before autumn.
Why moving upslope stops working
The standard response to warming is to shift to higher ground, and mountain species have been recorded doing so across many ranges.
Mountains narrow as they rise, so each step upward offers less area. Populations become smaller and more isolated as they climb.
Species already near the summit have nowhere further to go, which is why high-elevation specialists are among the hardest groups to conserve in place.