March is a time of transition. As the hours of day and night balance temperatures diverge. Snow and cold still reign, but warm spells are increasingly common, slimming the snowpack and bathing streets in meltwater. Snow still covers Yellowstone, though much of it has metamorphized into a condensed icy mass. Brown vegetation begins to appear bit by bit, with small patches of bare ground opening across the Northern Range. The first migrants will appear soon: Mountain Bluebirds returning from the southwest and Red-winged Blackbirds flying up from their lower elevation wintering grounds. But while park visitors wait for their colorful flashes or joyful trills, there are other subtler signs of what is to come.

One of the earliest signs of spring is the easiest to miss. To the hasty eye it might look like dirty snow. A second glance reveals black dots, crawling, in the thousands. They fill footprints in the snow and cover snowbanks like sprinklings of pepper Closer inspection sends many springing away in powerful leaps. Those that remain crawl on six tiny legs, their black exoskeletons just 1.5mm long. They are Snow fleas, a common creature that is rarely seen except in late winter when these critical decomposers congregate in view on melting snow.

Snow fleas are not actually fleas. They are not even insects. They belong to a class of wingless arthropods known as Collembola. Collembola, more commonly referred to as springtails, were originally classified as insects, but with internal mouthparts and morphology that more closely resembles crustations, Collembola now merit their own class under the subphylum Hexapoda. There are a few different species commonly called Snow fleas belonging to the Hypoigastruridae family. Both common names refer to their furcula, a ‘little fork’ appendage in latin, that tucks beneath their abdomen. The furcula is literally spring loaded with a mechanical advantage. As it snaps open it propels the Snow fleas several inches – or several dozen times their body length – through the air toward new habitat or away from predators, similar to how true fleas jump hosts using their back legs.

Snow fleas are small and easy to miss, but their ecological shadow looms large. Springtails are one of the most abundant macroscopic animals on the planet, accounting for an incredible amount of biomass despite their small body size. As decomposers, Snow fleas consume leaf litter, fungi, alge, and bacteria breaking down organic matter and recycling nutrients through the ecosystem. They also distribute spores from the various types of fungi that make up a healthy soil community. Their movements, digestive tracks, and even decomposing bodies provide this important ecosystem function. For example, the vast majority of plant life of earth survives through a symbiotic relationship with mycorrhizial fungi. Mycorrhizial fungi extend plant root networks, helping their hosts absorb nutrients and water from the soil in exchange for carbon and sugars. When nitrogen – one of the three most important elements for plant growth – is low, these fungi first release pheromones to attract springtails and then toxins to paralyze them and ultimately break down their carcasses for this critical nutrient.

This recycling and redistribution of nutrients occurs year-round, generally with Snow fleas and other springtails hidden beneath the leaf litter and humus that covers forest floors. Gardeners might get a glimpse of Snow fleas as they shift through soil, but they are easy to miss. In late winter, however, Snow fleas congregate on top of the compressing snowpack, spreading out in search of new habitat and food, perhaps even feeding on the mold and bacteria that concentrates in partially melted snow. Similar to true insects, Snow fleas produce their own antifreeze: glycine-rich proteins that prevent ice crystals from forming inside their exoskeletons and causing damage. This allows them to survive and even thrive in the cold as their predators – beetles, ants, centipedes, mites, and others – are largely dormant.

As the snow melts, Snow fleas will fleas will return to the leaf litter to lay eggs and complete another season maintaining healthy soil. Their appearance in March is brief and subtle, but it heralds the burst of life that will soon come.

by Sam Archibald, Lead Field Educator