The Effects of Urbanization on Earthworm Abundance

Source: Tractor Supply Co.

Ecology is the study of living organisms and the world around them, and its applications are especially focused on keeping the environment healthy. This has historically involved studying “natural” areas, but recent urbanization has shifted the attention of many ecologists towards the impact of human development on organisms in metropolitan areas. When cities expand, it makes sense that some organisms are simply driven out of the area, but many more stay and inhabit the parks, gardens, and trees that remain. The benefits of studying these organisms are diverse, from helping maintain the populations themselves to gaining a better understanding of how their ecological role can be harnessed to improve the city. Earthworms, members of the phylum Annelida that are found across the world, are one example of an organism whose urban presence has been of interest to some ecologists. Their roles include decomposing organic matter, enhancing the physical structure of soil, and they can even act as a natural fertilizer. Essentially, earthworms are both creators and indicators of healthy soil and play crucial roles in keeping city natural environments healthy and pretty.

Figure 1 of the paper showing variation in earthworm
density across different soil types.

With these ideas in mind, a research group based at the University of Idaho set out to investigate the presence of worms in urban environments. Specifically, they looked at how earthworm density and diversity varied across different levels of age and management intensity in urban soils. The findings of this study were published in a paper titled “Earthworm Population density and diversity in different-aged urban systems” by Smetak, Johnson-Maynard, and Lloyd. They conducted field research in both residential lawns and urban parks in a small city in Northwest Idaho. At each site, they collected data on irrigation, fertilization, age, and aeration before digging up small pits and recording the earthworm biomass. The results indicated a clear difference in worm density among study areas. Old urban parks were home to the most density, while old residential lawns were more dense with earthworms than younger ones. These findings led the authors to conclude that increased landscape age, decreased soil density, and increased fertilizer presence were all associated with increased earthworm density. These conclusions are especially important in the context that increased earthworm density has been shown to come with a healthier surrounding ecological environment.

While the University of Idaho study explored a number of potential factors influencing earthworm diversity in grassy soils, the impact of management intensity was especially intriguing to me. Accordingly, Kyle Buse and I will be conducting a similar experiment that focuses on earthworm abundance in non-grassy soils on the campus of Grinnell College. The college dedicates a great deal of resources towards planting and maintaining small gardens in an effort to make the campus more attractive and clean. Some areas, however, receive more attention than others, and this gradient of management level presents an intriguing opportunity for investigating urban earthworm abundance. This can be measured without digging up soil using established techniques of of mustard irritation to bring worms in a specific area to the surface to be counted. We hope that the findings of this study will build upon those presented by Smetak et al. and further understanding of how maintenance levels affect earthworm diversity in urban areas.

           

References

Milner, T. (2007). Earthworms - A Gardener's Best Friend: Summer 2007 Out Here Magazine:

Tractor Supply Co. Retrieved October 18, 2019, from https://www.tractorsupply.com/out-here_issues_2007-summer_earthworms-a-gardeners-best-friend.

Smetak, K. M., Johnson-Maynard, J. L., & Lloyd, J. E. (2007). Earthworm population density

and diversity in different-aged urban systems. Applied Soil Ecology, 37(1-2), 161-168.