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Distributions of earthworms in the Morgan Arboretum

Figure 1) Walking to our next St. Bernard soil location on a beautiful fall day at the Morgan Arboretum

Figure 1) Walking to our next St. Bernard soil location on a beautiful fall day at the Morgan Arboretum

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?

Methods

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.

Figure 2) Compilation of the soil series map and satellite view of the Morgan Arboretum

Figure 2) Compilation of the soil series map and satellite view of the Morgan Arboretum

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).

Figure 3) An equilateral triangle is formed when the 3 holes are connected

Figure 3) An equilateral triangle is formed when the 3 holes are connected

Figure 4) Fresh leaf litter on the forest floor

Figure 4) Fresh leaf litter on the forest floor

Figure 5) One of our sample points regardless of the amount of obstacles around it

Figure 5) One of our sample points regardless of the amount of obstacles around it

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).

Figure 6) A variety of different earthworms in terms of size

Figure 6) A variety of different earthworms in terms of size

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

Figure 7) A burrow from an earthworm

Figure 7) A burrow from an earthworm

Figure 8) A clew of earthworms in the process of decomposing organic material

Figure 8) A clew of earthworms in the process of decomposing organic material

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).

Additional information: invadingspecies.com, cleaveland.com, npr.org

Figure 9) The amount of earthworms gathered in one of our holes

Figure 9) The amount of earthworms gathered in one of our holes

References

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

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About Crystal Ernst

Hakai Institute postdoctoral scholar at Simon Fraser University (B.C.)

8 comments on “Distributions of earthworms in the Morgan Arboretum

  1. The blog looks awesome! I really like the photographs illustrating the forest and earthworms. You did the earthworm survey in Morgan Arboretum and tried to answer the earthworm functions in this forest ecosystems.

  2. May I ask how many earthworm individual in each sampling plot. Also, I am interested in more information about why you chose the sampling plots. Would you select the plots because their vegetation type? Soil series?

  3. Good work! Great to see undergrads leading innovative natural history work!

    You’ll be interested in a paper just published last week in The Canadian Field-Naturalist, on soil invertebrate biodiversity (including worms) and their environmental predictors.
    http://www.canadianfieldnaturalist.ca/index.php/cfn/article/view/1441

    Jay

    • Hi Jay,

      Thank you for your comment!

      The paper will definitely help with our discussion points.

      Thanks again!

  4. Thanks for your interest!
    We have not analysed our data yet, but the number of earthworms found in our holes varied from approximately 2 to 40! I can say that we always successfully found worms in every holes, and that most of them were found in the first 15cm of soil.
    For how we selected where we would dig, we first localized the St.Bernard soil series present in the Arboretum using the soil survey of the area. After that, the holes were randomly selected. You can look back to the section on our methods for more details on how we did that!
    Don’t hesitate to ask if you have other questions!

  5. Thank you for sharing the results of your study, which is well done. I will be interested in knowing how your results on earthworm population size and earthworm diversity compare with other studies from this region. There were a few earlier census (surveys) of earthworms in the Morgan Arboretum and you might like to include information from the paper

    Whalen, J.K. 2004. Spatial and temporal distribution of earthworm patches in corn field, hayfield and forest systems of southwestern Québec, Canada. Applied Soil Ecology 27:143-151.

    (note that the forest plots sampled in that study were within the Morgan Arboretum).

    Also contact Dr. Benoit Cote because he had a M.Sc. student who did an earthworm survey at the Arboretum… I don’t think they published their survey, but the M.Sc. thesis is available from the McGill library.

    • Thank you very much for your comment!
      When we will be done with data analysis, we will share our results with you for sure.
      We are also very interested in comparing our results with those of other studies. We actually thought we were the first to work with earthworms in the Arboretum, thank you a lot for providing those references. They will be very useful.
      Your interest is very significant for us!

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