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The Abundance of Earthworms with Human Activity

Introduction

We are McGill university undergrad students studying environmental biology. In the course St. Lawrence Ecosystem (ENVB 222), we will be conducting a research project geared at evaluating earthworm abundance in the Morgan Arboretum. To expand on the data collected by researchers in last year’s class, we chose to further study earthworms, but in relation to their distribution with compaction. This specific aspect was chosen because the Morgan Arboretum has distinct paths and trails used for both recreation and research. We were interested in viewing the impact of human interactions with the earthworm habitat. This lead to the formation of our research question:

How does earthworm abundance in the Morgan Arboretum change in relation to soil compaction taking into consideration the acidity and temperature of the soil, in two different soil series?

What is an earthworm?

Earthworms are hermaphroditic annelids that are composed of repeating segments called metameres, each of which contains the same internal structures. Externally, earthworms are covered in hair-like structures called setae, which attach to the surface they are digging in and aid in locomotion. These invertebrates feed on dead and decaying plant matter and excrete humus, which is the organic component of soil (Edwards C.A.).

Figure 1: Nutrient cycling - the secretion of humus by an earthworm.

Figure 1: Nutrient cycling – the secretion of humus by an earthworm.

When broken down, the present nutrients become more readily available for plants. In addition, earthworms aerate the soil by their burrowing actions, allowing for more air and water infiltration. These actions can be very advantageous for gardens and other small
and controlled environments, but there is a growing concern about how earthworms (especially the invasive species from Europe) are influencing our North American forest ecosystems.

Why are earthworms considered invasive species?

An invasive species is a plant or animal that is not native to a specific location that has a tendency to outcompete a native species and be detrimental to the environment. There are 180 species of earthworms in North America, 60 of which are invasive (Wikipedia). Our research will be conducted in the Morgan Arboretum, which is a 245 hector forested reserve located on McGill university’s MacDonald campus. Like many-forested regions in North America, the Morgan Arboretum has adapted to having a large amount of leaf-litter on the forest floor, which has influenced production by protecting seedlings and insulating the earth and organisms lying underneath. With the addition of invasive earthworms to North American soils, the composition of the earth is threatened.
One worry is that this will increase soil erosion, as there is less to inhibit the flow of water, which would cause more topsoil to be removed, as well as leach away the vital minerals and chemicals required by the local vegetation (Bohlen, P.J.).

Figure 2: Burrowing action of earthworms seen in St. Bernard soil.

Figure 2: Burrowing action of earthworms seen in St. Bernard soil.

Where do you study earthworms?

Our research project took place over three weeks, where we had three laboratories of four hours to collect our data. Based on last year’s researchers, we chose St. Bernard soil series as our primary area of study as it exhibits desirable living conditions for earthworms. We randomly chose another soil series (St-Amable) in order to compare the results. St. Amable is characterized by sandy conditions, so we expected to find fewer earthworms in this soil.

Figure 3: The Morgan Arboretum forest types, overlain with designated areas of study. The red region represents St. Bernard soil, and the blue shows St. Amable soil.

Figure 3: The Morgan Arboretum forest types, overlain with designated areas of study. The red region represents St. Bernard soil, and the blue shows St. Amable soil.

Methods of study

It was important to ensure that our dig sites were representative of the area, so we chose to use random sampling. During the first two laboratories, our research question was misleading so we hadn’t yet developed a regular method of data collection, so we hadn’t yet determined a proper way to measure compaction. Non-the less, we started by using a measuring tape to dig holes 25cm in diameter and 15cm depth. We used a generic garden shovel to dig these holes. We dug holes near the path at first, then at further distances perpendicular from the path. At this point we had not developed a standardized method of sampling so our intervals varied randomly. As our research question became clearer, we refined our methods and dug holes at 0, 1, 3, 5 and 15 meters away from the path. If there were trees or large rocks in our way we would take three steps to the right (parallel to the path) and continue digging. This ensured that the distance from the path had not been changed. We repeated these steps twice in the two different soil series. Our refined methods allowed us to more accurately determine if earthworm abundance changed with soil compaction at the path.

After digging the hole, we would sift through the earth in search of earthworms. It should be noted that for this research, due to time constraints, it was not feasible to identify specific earthworm species and therefore all earthworms were considered.

A penetrometer was used to measure soil compaction. It was important to measure soil compaction before disrupting the area, to ensure that the soil had not yet been altered by our presence. At each dig site, we took 5 readings from the penetrometer to obtain a representative average. If the soils were too wet, the penetrometer would slip into the ground without producing a reading. In such cases, several steps were taken in each direction of the dig site in order to obtain proper compaction readings.

Worm Fig 4
Temperature and acidity readings were also collected for each hole. Knowing that “[earthworm] density, diversity, and survival are typically low in acidic soils” (Moore, J.-D.), we were expecting less worms in lower pH. Our readings were taken halfway down the hole in order to account for discrepancies in temperate and acidity from the soil surface to the base of the hole. Temperature was an important variable to measure because as we continued our research throughout the months of October and November, the ground temperature continued to decrease and it is a fact that low temperatures limit earthworm distribution (Greiner, H. G.).

Figure 5: Various species of earthworms found in St. Bernard soil.

Figure 5: Various species of earthworms found in St. Bernard soil.

References:

Clive A. Edwards and P.J. Bohlen (1996) Biology and Ecology of Earthworms, Volume 3.Champlain & Hall, London, 433 pages.

Great Lakes Worm Watch. (n.d.). Retrieved November 7, 2014, from http://www.nrri.umn.edu/worms/default.htm

Holly G. Greiner, Andrew M. T. Stonehouse, and Scott D. Tiegs (2011) Cold Tolerance among Composting Earthworm Species to Evaluate Invasion Potential. The America Midland Naturalist 166 (2) : 349-357.

Invasive earthworms of North America. (n.d.). Retrieved November 7, 2014, from http://en.wikipedia.org/wiki/Invasive_earthworms_of_North_America

Jean-David Moore, Rock Ouimeta and Patrick J. Bohlenb (2013) Effects of liming on survival and reproduction of two potentially invasive earthworm species in a northern forest Podzol. Soil Biology and Biochemistry 64 : 174-180.

Patrick J Bohlen et al. (2004) Non-native invasive earthworms as agents of change in northern temperate forests. Frontiers in the Ecology and the Environment 2 (8): 427-435.

Salamanders: Their natural history and our research. (2013, November 11). Retrieved November 7, 2014, from https://stlawrencelowlands.wordpress.com/2013/11/25/salamanders-their-natural-history-and-our-research/

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About Crystal Ernst

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

10 comments on “The Abundance of Earthworms with Human Activity

  1. Thanks for the post, students! You have an interesting project. Although you didn’t direclty test “diversity”, do you have a sense of how many species you might have collected? Do you think the different species might respond differently to soil compaction? (Why or why not?)

    • We definitely noticed a high diversity of earthworms in the arboretum, mostly because of their varying size. In soils with a higher level of organic matter, such as St. Bernard we noticed a higher species diversity. In the sandier, St. Amable soil, I think we noticed more larger species. I think this is because they have an easier time manoeuvring through the rough soil than smaller annelids.

  2. I realized I didn’t entirely answer your question. In relation to compaction, different species will react differently to compaction. Larger species will have a harder time burrowing through the earth than smaller species.

  3. Hi Students, it sounds like you have a very interesting project here. The impacts of invasive earthworms are poorly studied in most of Canada (Alberta is the exception, see some rather excellent papers by Erin Cameron here: http://scholar.google.ca/citations?user=Ks4J6EYAAAAJ&hl=en ).

    in your methods you state that you took 5 pentrometer readings to get an average value for the site along your transect, but only dig one hole to sample for earthworms. Had you dug multiple holes, do you think you would have had much variability in the number of worms you found in each site?

    • We really appreciate you taking the time to read our blog and provide us with feedback. Also, thanks for sharing Erin’s papers!

      I hope this answers your question. We dug 5 holes per transect and took 5 penetrometer readings at each hole, which was just done to give us a better average reading considering that one reading was not accurate enough. During each visit to the arboretum we took 3 transect readings, relatively spaced out to have a better representation of the area. I think that eliminated the possibility of us distorting the compaction readings. However, I wish we had more time to dig multiple holes and collect more data, but we think that we have enough to have an overview of the relative abundance from the path to further in the forest.

  4. This was really interesting, didn’t know that worms were so diverse! Considering that they are invasive species, have you found any indications on how we might control or predict their population growth?

    • Thanks for your insight and comment!

      During my research, I didn’t come across many methods of stoping the spread of these invasive species. Basically I found that introduced earthworms have a positive correlation between their presence and proximity to roads and boat launch areas (Cameron et al., 2007). So introduced earthworms are mainly spread by humans and it’s important to be cautious when transferring plants to limit their spread. I predict that their populations will continue to grow especially with increasing temperatures.

  5. Cool project!! Great methods to evaluate your research. As you mentioned that as temperatures decreased so did earthworm density. It must have been tricky especially when one of our lab days was a lot colder than the other field days. As a group did, was there a great decline in earth worms when the air temperature had changed or did the soil temperature remain quite steady since the air temperature wasn’t such a drastic change?

    • Actually its interesting that you bring this up. As I was going through our data, I noticed that there was slight correlation between earthworm abundance and soil temperature. Our results did not show any significant change, and could be easily left up to chance. However it is known that earthworms are less “active” in colder temperatures. I think the reason why we found such little correlation was because the air temperature during our lab periods did not change drastically from week to week, and the soil temperature still remained comfortably above freezing, between 6-10 degrees Celsius.

  6. Hi guys!
    I’m glad you decided to continue with my project from last year. It looks nice and good job. In the end I hope you were actually digging in St. Amable soil and not the ‘unknown’ soil I was worried about!

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