With earthworms getting more attention as they make their way into once earthworm-free forests, it was surprising to find that there had been no past research done on the distribution of earthworms in the Morgan Arboretum. Due to the limited amount of time, we took this opportunity to target the St.Bernard soil series (one of the most common) as our research location. It is important to note that
for this natural history project, we will not be discriminating between specific species of earthworms but for earthworms as a whole. In a period of three field excursions, our goal is to answer the following research question: What is the earthworm distribution in the St. Bernard soil series of the Morgan Arboretum?
Three different locations of the St. Bernard soil series were selected to see if there was a difference in the distribution of earthworms. One location being beside a trail often used, one deeper in the forest, and the last on the edges of the agricultural field. A correlation between those specific locations and the presence of earthworms will be established, as well as a statement on the distribution of earthworms in the St. Bernard soil series of the arboretum.
To do so, a specific method is followed at each location on every lab session day. To locate the St. Bernard soil, a compilation of the soil series map and the arboretum map is used. As soon as we enter each location, an object is blindly thrown to identify the place where the first of three holes will be dug. To determine the location of the second hole, a team member spins around and points in a direction and walks 6 meters from the first hole. The third and last hole is situated at 6 meters from the second to make an equilateral triangle between the three holes. Blue tape is attached to a tree next to the first hole to ensure no repeats for the next lab session.
For each hole, we are limiting an area of about 1m2 to randomly take 10 measures of the height of leaf litter. After that, a hole of 30cm³ is dug. The soil is separated into two layers of 15cm each, and is put on two different plastic bags. The earthworms from each layer are put into a tray and counted. During this process, observations are made on moisture and the rocks in the soil. It is also important to note the temperature of the day and of the previous days to see if weather such as rain, had an effect on the distribution. This kind of research method is called the Handle Sample (Great Lakes Worm Watch 2011).
General information on earthworms
The behavior and anatomy of earthworms are indicative of how well adapted they are to living in soil. Earthworms excrete a fluid that lubricates their skin, making it easier for them to tunnel through the soil. Each segment on their body (except for the mouth and anus) has a pair of setae, hair like structures that anchor parts of their body during movement (typical feature of all oligochaetes). Earthworms have no eyes but are sensible enough to light to distinguish between day and night; this is helpful since most of their predators are diurnal and explains why they are most active at night (Darwin 1881).
Earthworm burrows are typically found near the topsoil, closest to their main sources of food: decaying organic matter and leaf litter. However, they are known to tunnel as deep as 2m during periods of dryness or in winter (Encyclopaedia Britannica 2013). Their digestive systems run through the entire length of their tube-like bodies. The food they eat is passed through their bodies and left in their burrows making the nutrients more readily available to plant life, thus accelerating the natural process of nutrient cycling (Encyclopaedia Britannica 2013).
Additional information: bbc.co.uk
Earthworms and the St. Bernard soils
St. Bernard soils have good drainage and are very stony due to their development on glacial till (Lajoie 1960). The A horizon is characterized as having a thick first layer of soil under a large accumulation of organic matter. The natural vegetation on this soil consist of sugar maples, yellow birch and beech trees which do not acidify the A horizon (Lajoie 1960). It is because of these reasons that invasive earthworm species would likely be found in the A horizon of St. Bernard soils.
Earthworms greatly affect the soil and the surrounding vegetation. Earthworms create tunnels in the soil, aerating it out and speeding up erosion and weathering (Wironen and Moore 2006). Also, an increased number of earthworms can lead to an increased amount of carbon and nitrogen in the soil (Wironen and Moore 2006). Earthworms also contribute to the decomposition of organic matter at the surface of the soil and it is believed that this “may lead to changes in plant species” (Wironen and Moore 2006).
Additional information: soilsofcanada.ca
Earthworms as invasive species
The invasion of the European earthworm, Lumbricus terrestris to North America in the 1700s has many ecologists worried for the future of mixed deciduous-conifer trees of North America (Frelich et al. 2006). It may be hard to digest the fact that earthworms can be harmful in the soil due to their range of beneficial attributes to the home of gardens, but in a forest, these small soil engineers disrupt the ecosystem greatly. The thick carpet layer of leaf litter on forest floors has structured the soil underneath, such that bulk density is much lower than soil cultivated by humans (Frelich et al. 2006). With the earthworms slowly moving towards these forests, they increase the bulk density by aggregating soil particles together during decomposition. Thick leaf litters that have been accumulated over the years are also crucial in nutrient cycling and promotion of root growth for plant species (Frelich et al. 2006). For example, worm invasion in the sugar maple trees (Acer saccharum) has reduced the amount of tree seedlings and plant cover (Frelich et al. 2006). Seeds and seedlings are exposed once leaf litter is removed, leaving it more prone to freezing, predators and other organisms (Frelich et al. 2006). Such changes to the forest floor are bound to have a negative impact on plants that have adapted to thick forest floors. Although L. terrestris is a slow moving earthworm, its soil engineering impact is huge and must be carefully looked after for the future of North America’s forests (Frelich et al. 2006).
Darwin C. 1881. The formation of vegetable mould, through the action of worms, with observations on their habits. First edition. London: Murray.
Encyclopaedia Britannica [Internet]. 2013. United Kingdom: Encyclopaedia Britannica; [updated 2013; cited 2013 Oct 24]. Available from: http://www.britannica.com/EBchecked/topic/176371/earthworm
Frelich LE, Hale CM, Scheu S, Holdsworth AR, Heneghan L, Bohlen PJ, Reich PB. 2006. Earthworm invasion into previously earthworm-free temperate and boreal forests. Biological Invasions. 8(6): 1235-1245. DOI: 10.1007/s10530-006-9019-3
Great Lakes Worm Watch [Internet]. 1999-2011. Duluth, Minnesota: University of Minnesota; [updated 2011; cited 2013 Oct 24]. Available from: http://www.nrri.umn.edu/worms/research/methods_worms.html
Lajoie PG. 1960. Soil Survey of Argenteuil, Two Mountains and Terrebonne Counties, Quebec. Ottawa: Research Branch, Canada Dept. of Agriculture.
Wironen M, Moore TR. 2006. Exotic earthworm invasion increases soil carbon and nitrogen in an old-growth forest in southern Quebec. Canadian Journal of Forest Research. 36: 845–854. DOI:10.1139/X06-016