The Winter: Hemlocks and Snow Fleas

Last Saturday, Deborah and I drove up to Laurel Hill State Park to go for a snow hike with Rob and Michele Bridges. We started on the Hemlock Trail and did a steep climb up to the ridge over Laurel Hill Creek and then circled back down into a four acre preserve of virgin hemlocks. The trails were snow covered and slippery underfoot, but the sun regularly broke through the light cloud cover and warmed up both the snow surface (making the slopes of the trail even slipperier) and us (causing us to shed layers of clothing, hats, scarves, and gloves as we trudged along).

It was a beautiful day for a hike! (Photo by D. Sillman)

We spent about 2 hours on the Hemlock Trail loop and then drove over to the Pump House trail and did another hour or so. We stopped at the CCC-built dam and crossed the sequence of the narrow, snow-covered, log, foot- bridges that spanned the winding track of the fast-running Jones Mill Run. The run was bordered and, in places, covered with a thick layer of ice. The rapidly flowing water under the ice sheets made hypnotic patterns of light and shadow on the surfaces.

Over the three hours we were hiking, we saw no birds (although we did hear one woodpecker pounding on a distant tree). We also saw no squirrels, no deer and very few other people. We did see a great population of young hemlock seedlings filling in the understory along the edges of the old hemlock stand (Photo by D. Sillman). Any light gaps that occur here will stimulate a growth race in these saplings. It may take many decades or centuries but this hemlock “sapling-front” will expand steadily outward and begin to spread these long-lived hemlocks out through the transient secondary forest of red oak, black cherry, and ash. The relic forest of the once locally dominant hemlocks will slowly take over its former territory.

Along with the hemlock saplings we also came upon some insects, but we had to look very carefully to see them. They were snow fleas! (Microscope Photo below by D. Sillman (at 35X))

For most of us, the word “flea” carries a lot of negative connotations. “Snow fleas,” though, have nothing to do with those fleas that can infest our cats and dogs during the summer months. A snow flea is a collembolan, a very “primitive” and very small insect that lacks wings. Collembola can be found in any soil and leaf litter system in staggeringly huge numbers. They are important members of the decomposer community and through their direct shredding of plant debris and their continuous ingestion of fungi and bacteria and nematodes accelerate the breakdown of organic materials and the recycling of important nutrients in our soil ecosystems. Collembola move about on their six tiny legs (slow and steady) but can also employ their spring-loaded “furculae” (rod-like structures on their abdomens) to suddenly and spectacularly leap several centimeters up and across their surface habitats. This ability, which is just short of flying, is a very effective avoidance maneuver to evade a predator or to get away from almost any type of disturbance that might arise. These furculae-propelled, leaping collembolans, in fact, are commonly referred to as “springtails.”

We saw snow fleas in the millions dusted across the surface of the sun dappled, warming snow all along both of the Laurel Hill trails.

My guess is, though, that if there had been other people out on the trails that afternoon, most of them would not have even noticed this amazing abundance of insects! Each snow flea is less than a sixteenth of an inch long, and, in spite of its dark blue (although they do look black) coloration (which really stands out against the white of the snow) they are only barely noticeable unless you get your eyes quite close to the surface of the snow!

But, why are they out here in the snow? How do they survive the freezing cold temperatures?
I have found a few facts about these insects, but I will add some of Deborah’s and my speculations to give you a rounded off picture of their life cycles and ecological activities.

The scientific name of this organism is Hypogastrura nivicola. The species name (“nivicola”) is derived from the Latin “niveus” and means “snow.” Of the hundreds (thousands?) of species of collembolan that have been described and named this species has been recognized for their unusual association with snow! Snow flea eggs hatch into immature life forms called nymphs down in the soil in the late spring. These nymphs feed on fungi and grow and molt into larger and larger individuals and finally mature into adults in the fall or early winter. For most of the cold months of the winter these adults remain quiet and inactive in the soil and litter system. On warm, sunny days, though, these adults are stimulated to dig their way up onto the snow surface where they can be found, as we found them last Saturday, in the millions! A very unusual protein in their bodies that was just described in 2007 enables them to tolerate the freezing temperatures of the snow cover and not freeze solid as they dig up to surface.

Why do they come into the light? Deborah and I speculate that they come up to find mates. They warm up in the sunny microclimate of the snow surface and crawl and hop about vigorously and engage in obvious interactive behaviors.

Some of these collembolans get stranded up on the snow cover and die, but most dig their way back to the soil and litter microhabitat and wait for the spring thaw. In the spring, the mated females lay their eggs which will then hatch into nymphs and begin the cycle all over again.

By the way, the protein that keeps these collembolans from freezing is being explored as a possible agent to help in the preservation of human organs being stored or transported for transplant surgeries. It is also being researched as a possible additive to help make better, smoother ice creams!

I wonder what Snow Flea ice cream will taste like?