Analysis of Earthworm Populations in Soil

A solution is prepared by extracting capsaicin from spicy mustard and then poured directly onto the soil within a sampling quadrat on the ground to sample from each collection site. Collection sites are determined in order to compare three random samples from an area that has been invaded by European buckthorn to three random samples from an uninvaded area. Once poured directly on the ground, the mustard solution can penetrate down through the soil matrix to where earthworms reside. The capsaicin in the mustard causes irritation to mucous membranes. Earthworm bodies exposed to the mustard solution react to the capsaicin irritation by moving away from the mustard solution and coming to the soil surface to expose themselves to oxygen, thereby reducing the irritation. After surfacing, earthworms can be collected and population density analyzed for relationships with European buckthorn. The population means of each collection site is compared with a bar graph to determine if areas with other invasive species have more earthworms, thereby supporting the presence of invasion meltdown. A Student’s T-test is used to determine if the two sites are significantly different enough to support the invasional meltdown hypothesis proposed to exist between European earthworm and European buckthorn.

Sampling site 1 was a managed park, which sees significant disturbances such as aeration and fertilizers.  Sampling site 2 was an unmanaged area, which sees no human interferences.  As shown in Figure 1, site 1 has a higher density of earthworm populations, likely due to the increased hospitability due to human disturbances.  However, site 1 also has higher variability of sampling, indicating the earthworm population may not be as consistently dense as the average suggests.

Figure 1. Bar graph displaying population results from each collection site.

Invasive species are a major threat to biodiversity. Exotic earthworms (eg: Lumbricus terrestris) and European buckthorn (Rhamnus cathartica) have been implicated as part of an “invasional meltdown” occurring in mid-western United States wooded communities. An invasional meltdown is the process where one invasion of a species facilitates the invasion of others. Thus, the rate of loss of ecological health can greatly accelerate as one invasive species makes way for additional ones. As undesired Rhamnus populations currently account for over 90% of vegetative cover in Illinois, the role of Lumbricus populations in landscape management has become critical to understanding and predicting Rhamnus invasion on managed land. Landscape disturbance tends to facilitate Lumbricus invasion and sampling for Lumbricus populations can be an indicator of vulnerability of land areas to likely invasion. Comparing samples of Lumbricus populations can help land management to know where more intensive methods are needed to maintain intended plant diversity and prevent invasion of Rhamnus.