Mammatus clouds over Greeley. Photo by D. Sillman
(Click on the following link to listen to an audio version of this blog … More on Earthworms
This Spring and Summer have been, here in Northern Colorado, quite wet. After our record setting rainfalls of May and June, July and now August have had more than their share of afternoon thunderstorms. A local meteorologist speculated that all of the soil moisture we have accumulated over the past three months is evaporating and triggering these pop-up thunderstorms. There is also a suggestion that our summer monsoon season might be generating many of our storms!
One consequence of these brief (but very welcome) afternoon rains is the almost continuous emergence of earthworms! On almost every sidewalk, driveway, or parking lot a significant number of earthworms of various species are wiggling across the wet surfaces, moving in apparently random directions out from their former burrows in the surrounding grass and flower beds. Why are so many worms emerging all at once? The emergence is probably a behavioral response to moisture and temperature that helps to disperse and expand the boundaries of an earthworm population (no, they are not drowning in the wet soil!). As earthworms reproduce and their clustered cocoons hatch the new worms tend to be clumped together, the rain-triggered dispersion helps to spread their numbers out.
Photo by J. Beaufort, Wikimedia Commons
Our local robins are thriving on these emerging worms. Watching the foraging robins gives you a good idea of how many earthworms are actually slithering along on or just hiding beneath the soil surface. Many other bird species are opportunistically, but with less style and grace than the robins, catching and consuming earthworms, but none of them can match the elegance of the hunting “dance” of the robin. In fact, a number of these other bird species lurk near robins while they are on their worm hunts and try to startle them away from the snagged meals.
I was in Boulder a few weeks ago walking on a semi-urban, hiking path called “The Cottonwood Trail.” There were quite a few old field and also wetland plants in glorious bloom along the trail, but the thing I saw that pleased me the most was a female robin scooping up earthworms from a rain-dampened parking lot. She patiently rolled and folded the worms into tight beak-packs and flew them up to her nest high in a nearby green ash tree. I watched her make four trips. Her nestlings were well fed that morning!
Photo by H. Casselman, Wikimedia Commons
As many of you know, I studied earthworms in my Ph.D. research and in a variety of studies at Penn State back in the 1980’s and early 1990’s. Almost all of the organisms we call “earthworms” are, like house sparrows, European starlings, gypsy moths, garlic mustard plants and Japanese knotweed (and so many other species of plants and animals around us) organisms that were intentionally or inadvertently introduced to North America by human activity. These plants and animals got a foothold (or root-hold)(or tail-hold) on some edge of our continent and then proliferated and spread sometimes from ocean to ocean. Earthworms, then, are a group of alien, invasive species!
Earthworms do many significant things in their soil habitats. They improve the stability of a soil’s structure and its drainage properties, and they accelerate rates of leaf litter decomposition and nutrient cycling. Aristotle called them “the intestines of the Earth,” and Charles Darwin spent many years of his life intensively observing and describing their activities and their extremely positive influences upon soil fertility. My own research described the immense benefits that robust populations of earthworms could have on leaf litter decomposition and nutrient cycling in established forests ecosystems, on re-forested strip mines, and in the cycling and rehabilitation of land-spread sewage sludge.
Recently, however, these beneficial impacts of earthworms on soil are being re-evaluated and re-balanced against a number of deleterious impacts. There are negative consequences of the active shredding and burying of leaf litter in worm-rich soil ecosystems. Earthworm activity leads to the loss of leaf litter habitats that a wide variety of other invertebrates inhabit. It also leads to the loss of the protective, soil covering leaf litter “blanket” and changes the nature of the soil community’s nutrient and energy webs. Earthworm activity also changes the way that organic materials are distributed through the soil profile. The soils of what seem to be undisturbed ecosystems have been, in fact, irrevocably changed from their original conformations by the actions of the introduced earthworms.
Boreal forest(Ontario). Photo by K. Edzardio, Wikimedia Commons
Temperate and Boreal forest soils into which earthworms have been introduced are increasingly facilitating the growth of grasses instead of trees. It is almost as if earthworms are turning tree-supportive soils into garden or meadow soil systems. The invasive earthworms are driving an homogenization of soil physical and chemical properties that some researchers feel could lead to a decline in our forested ecosystems.
Earthworms have recently been found in Arctic and sub-Arctic soils! These soils had been “wormless” since the start of the last Ice Age! There is a fear that these worms will disrupt the equilibria of these ecosystems and alter the plant composition. They might also affect the delicately balanced arctic food chains that already under considerable stress from Climate Change.
Earthworms stimulate decomposing soil bacteria and fungi and may cause the rich, organic, Arctic soils to begin to break down more rapidly. This accelerated decomposition rate would release vast quantities of carbon dioxide into the atmosphere.
Arctic tundar. Photo by USFWS, Public Domain
The local impacts of earthworms are already being observed in the high tundra ecosystems. Worm-worked soils tend to support a higher percentage of grasses and shrubs instead of mosses and lichens. Some of the more generalist consumers (like lemmings and voles) may increase in numbers in these altered vegetative systems. Other consumers, though, that are more specialized in their food choices (like musk oxen, caribou, and reindeer) may decline as their moss and lichen forage decreases.
The earthworms traveled to the far north in ways much like the ways they traveled to North America hundreds of years ago. Human assisted transport both intentional and unintentional are the primary means of dispersal. Transported soil used in gardens and lawns, small amounts of soil clinging to the roots of transplanted plants, even bait worms used for fishing could have served to move the earthworms many thousands of miles into the formerly wormless north. The hermaphroditic nature of most earthworm species (each individual has both male and female reproductive organs) facilitates their rapid reproduction in these new habitats.
Photo by Cosmic Pomegranate, Wikimedia Commons
So, has the invasion and exponential increases in abundance of European earthworms led to increases in American robins and other earthworm eating birds? Have these invasive earthworms changed forest soil properties to favor different tree species or maybe even grasses? Did cleared land plowed into farm fields become more productive because of the swelling numbers of invasive earthworms? Did the earthworms cause the extinction of some litter dwelling beetles and other insects? Did the activities of these extremely active earthworms drive native annelid species into their very restricted present day distributions? Will there soon be earthworms at the North Pole?
All great questions! Stay tuned for as many answers as I can find!