Seeking Common Ground…Using Earthworms

My only experience with earthworms is running outside after a rain, picking them up off the sidewalk and throwing them back into the garden. I did this as a child and still do it today. My neighbor used to cut them in half to see if they would grow back as I watched in horror. I do not condone doing this. But the great thing about earthworms is that I am not the only person that has stories like these. Earthworms are everywhere! And all over the world there are people picking them off the sidewalk and throwing them in the garden. I have moved more times than I can count and earthworms are always there, wriggling on the pavement after a drenching rain. 

Now, a group of researchers are determined to find out if we are seeing the same types of earthworms in cities around the world. 

In a new study led by Dr. Toth of the University of Veterinary Medicine in Budapest, Hungary, the researchers looked at how earthworm species are distributed across urban habitats around the world. There are approximately 3,500 species of earthworms around the world, but only about 3% of these species are widely distributed and these species are often associated with human activity. Earthworms burrow in the soil which creates more space for water and nutrients to accumulate which are then used by other soil organisms. Humans interact with earthworms, particularly in urban areas, whether we are aware of it or not, because we disturb the soil where the earthworms live. Urbanization, the process of converting wildland or agricultural land into a mosaic of pavement and buildings, is a major cause of local species extinction and reduces biodiversity. In cities, humans manage to create an environment that has less variability than wildlands which leads to a less diverse group of species living in urban environments. This trend has been seen in plants, microbes, and insects but until now, has not been addressed for earthworms on a global scale. 

Do earthworms species converge in urban areas? 

In other words, do we see the same types of earthworms in urban environments regardless of how far away the cities are from each other?

The authors looked at the species of earthworms in five cities: Baltimore, USA; Budapest, Hungary; Helsinki and Lahti, Finland; and Potchefstroom, South Africa. These cities were chosen because they represent a gradient of climate, geology, and human population density. The study also included an undisturbed site outside of each city that has not been urbanized. Within each city, the authors took samples from areas that experienced low disturbance and low management such as a forest or grassland, high disturbance and medium management such as a turf of a public green space, and finally, high disturbance and low management such as soil mounds used for construction. 

Across all the cities in this study, the authors found 19 species of earthworms. Baltimore, USA had 16 species, 11 of which were found in green space/lawn plots. In fact, public green spaces, lawns, and turf are critical in maintaining a high number of earthworm species in all cities. However, European cities had much lower species richness with only 5-6 total species. Despite this, the researchers found significant overlap in the species of earthworms present across all the cities, meaning there are the same species of earthworms in US cities as there are in cities on the other side of the world! 

All of the earthworm species identified are species known to be associated with human activity and this may be why the communities are so similar even across oceans. Urban areas are dominated by human activity, we dig up soil, move soil, plant things in the soil, and much of that biological material moves with humans as we travel around the world. Imagine a tiny earthworm hitching a ride on your muddy boots as you traipse from city to city. Additionally, soil in cities have similar nutrient levels due to human activity and we tend to plant similar ornamental plant species creating similar habitats across the world! All these little actions add up and we eventually get what the researchers call “biotic homogenization.” This is basically the idea that in urban areas, due to human activity causing local extinction and the introduction of non-native species, these communities start to look identical despite the distance between them. This is what happened with the earthworms. 

So is this good…or bad…something in between?

There are two ways to approach this result. The first is that as humans continue to manipulate the landscape, we may cause more similarities between ecosystems. We know that humans are a major cause of local species extinction and we have a habit of changing the environment and displacing many species. With this in mind, we must try to protect the local diversity we have in our own backyards by limiting nonnative introductions and local extinctions in order to preserve the greater biodiversity on earth.  

However, in the spring of 2020, when we are confined to our homes and facing social isolation due to COVID-19, it is nice to think about the similarities we have — both as human beings and as organisms in our urban ecosystems. In a way, it is comforting knowing that as the spring rains drench our soils, I can go outside, pick up an earthworm from the sidewalk and move it to the soil and there might be someone down the street or across the world picking up the same species of earthworm and moving it to safety. So as you are practicing social distancing, consider looking at the ground and wave to your local earthworm community, marvel at the biodiversity in your own neighborhood, and think about people all over the world waving at their own community of earthworms. 

Citation: Toth, Z. K. et al. Earthworm assemblages in urban habitats across biogeographical regions. 2020. Applied Soil Ecology 151: 103530.

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Brianne Palmer

I am a PhD candidate at San Diego State University and the University of California, Davis studying how biological soil crusts respond and recover from fire. Most of my research is in coastal grasslands and sage scrub. We use DNA and field measurements to understand how cyanobacteria within biological soil crusts help ecosystems recover after low severity fires. I am also involved with local K-12 outreach within the Greater San Diego Metro Area.

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