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Digging Dirt Deep—Can Prescribed Fires Control Invasive Earthworm Populations?
By Megan Raisch & Paige Massa
Killer bees. Zebra mussels. Murder hornets. Are any of these ringing a bell? Invasive species like these ones are often talked about on the news: wreaking havoc by exploiting their new environments and harming native species. You may lie awake at night thinking about a swarm of killer bees coming after you, but what about that wriggly or ‘jumping’ earthworm you found in your flower bed? It may be the Amynthas agrestis species, commonly known as the Asian Jumping Worm.
Turns out that not all earthworms are good for the soil. The Amynthas agrestis rapidly decomposes leaf litter in the forest, which in turn affects native plants, microbes, and invertebrate species. Researchers are trying to find a resolution to control the population of invasive earthworms and limit their negative effects on the surrounding ecosystems.
A study published by the Soil Biology and Biochemistry journal and led by Hiroshi Ikeda of Hirosaki University decided to try something new: using fire to combat the effects of the invasive earthworm species. For years, fires started and extinguished by firefighters and foresters (also called prescribed burns) have been used to reduce the occurrence of large wildfires and reduce the growth of invasive plants, promoting overall growth in the forest.
Figure 1. Overview of food and soil preference testing. Figure 1 was created using a BioRender © account.
Ikeda and his team predicted that fire would kill adult earthworms and their cocoons, and also reduce the leaf litter where they live and eat. They began their study with a feeding experiment to find out what food/soil preferences the worms have in various types of leaf litter. Figure 1 displays the basic procedure for testing these preferences. Following this, they performed the prescribed fire experiment, where they burned a small plot of soil with adult worms and cocoons. As shown in Figure 2, they counted the number of viable (or still living) cocoons, living worms, and burned worms.
In the food preference experiment, Ikeda’s team found that partially decomposed litter (fragmented litter) had the highest consumption rate and cocoon count. In the fire experiment, the soil temperature was measured at several depths of soil to see how far the heat of the fire reached. The results showed that the heat did not reach deeper layers of the soil. After collecting data, Ikeda’s team observed a significant decrease in leaf litter in burned plots, but no difference between the numbers of earthworms or cocoons at the burned and unburned plots. This may mean the heat did not kill the adult earthworms because they were able to burrow into cooler soil, avoiding the heat. Cocoon viability, however, was significantly lower in the burned plots than the unburned plots, likely because cocoons are immobile and couldn’t escape the fire.
Figure 2. Overview of the prescribed burn test. Figure 2 was created using a BioRender © account.
Because the cocoon viability was much smaller in burned plots, the researchers suggest that this will decrease the Amynthas agrestis population in the following generation. These results indicate that prescribed burns can be an effective method for controlling the invasive worm species.
Sudden changes in ecosystems such as the introduction of invasive species can often have a domino effect, leading to damages that we could never anticipate. The purpose of this experiment was to evaluate the life cycle of Amynthas agrestis populations and then conduct tests to find the best way to prevent their spread. To successfully limit population size, Ikeda’s team found that prescribed burns should be applied after the worms reproduce, but before the cocoons hatch. This way, the cocoons will be less viable due to the fire, and the future generations will decline.
So the next time you hear a newsreel on invading species, or you dig up an earthworm in your garden, take a moment to appreciate researchers like Ikeda, who work hard to find solutions to preserve balance in our ecosystem.
Literature references:
Ikeda, H., Callaham, M. A., O’Brien, J. J., Hornsby, B. S., & Wenk, E. S. (2015). Can the
invasive earthworm, Amynthas agrestis, be controlled with prescribed fire? Soil Biology and Biochemistry, 82, 21–27. https://doi.org/10.1016/j.soilbio.2014.12.011
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