Rise of the Earthworm

All right, there are two inches of snow out there right now and this morning when I woke up the radio announced a minus four degree wind chill. Maybe I started these “Signs of Spring” too early this year. No good deed ever goes unpunished. There are, though, some “spring-things” going on outside.

Like the male tufted titmice’s singing and fighting for mating territory (my front yard bird feeder and fancy, “ice-free” bird bath are highly contested resources!). Like the house finches (even on cold snowy mornings like today) greeting the day with their chattering, group songs. But, the big observation of the week came just before the snow and cold hit…the rise of the earthworms!On Wednesday night it was in the 50’s and drizzling. Deborah and I were walking out of the Conference Center here on campus after our Galapagos Trip pot-luck dinner and were surprised to see a seven inch long, mature, Lumbricus terrestris (“nightcrawler”) working his/her way (they are hermaphrodites) down the wet sidewalk.
Now much of my research for my Ph.D. involved the ecology of earthworms. I studied how these organisms interacted with leaves and soil, and how they stimulated bacterial and fungal decomposition and the cycling of important plant nutrients. I developed ways to quantify their influences on soil structure and stability, and did a number of studies that explored both their mutalistic and anatagonistic interactions with other soil invertebrates. For all of this knowledge and expertise, though, what people wanted to know after they found out that I studied earthworms (after asking “why? You seem so normal otherwise.”) was an explanation of why earthworms crawl up out of soil en mass in the spring  and litter themselves across yards, and sidewalks, and streets, and tennis courts, and golf courses, etc.

First of all, no they are not drowning.

Earthworms are very prolific organisms. Their abundant progeny hatch from cocoons deposited around the surface openings of the adult worms’ burrows and then begin to rapidly grow. There is a logical ecological “need” for these clumped-together worms to spread themselves out more evenly across their habitats. Studies on re-forested strip mines in southeastern Ohio have shown that nightcrawlers (just like the one on the sidewalk Wednesday night) can disperse themselves over many acres after only a few years of time.

Early spring evenings, with their soaking rains, warm air temperatures, and lack of earthworm predators is a perfect time for this dispersion to occur. Worms in thawing soil are stimulated to follow the moisture gradient upward and emerge out onto the surface of the soil. They then crawl off in random directions. Some end up on sidewalks or streets frequently meeting unpleasant ends. Most find themselves in some new section of their habitat area and quickly re-burrow themselves down into the soft, moist soil before dawn. It is not known how many times an individual worm will make one of these nocturnal forays or exactly how far an individual worm might travel in a season. The energy expense for the trip and the environmental dangers involved would seem to be quite serious stresses on the worm. The end results, though, distinctly benefit the population as a whole: the clumped concentrations of individuals are reduced, new habitats are colonized, and, quite possibly, worms that are less fit fail to survive and, so, their genes are not passed on to the next generation.

Just beneath the snow layer, the soil is still warm and thawed. It melts the snow that is in direct contact and generates a narrow air space called the “subnivia.”  There is an essay out on the Virtual Nature Trail about this subnivian space. The covering snow is a very efficient insulator and throughout the winter even on the coldest days and nights both invertebrates and small vertebrates are active here. For now, some of the first spiders, and beetles, and even earthworms that have awakened with the early hints of spring might just be hunkered down in the subnivia, waiting.

More spring, for them and all of us, is on the way.     

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