Urbanization is a pretty broad term that I will generalize here as a shift in the biotic, chemical, and physical environment as a result of human encroachment. As we change a natural ecosystem into human, urban habitats, we quite clearly alter how that location functions as an ecosystem. Many studies addressing how communities change with the corrosive effects of urbanization find that species diversity decreases. Though, most of these studies focus on larger species such as trees, mammals, birds, insects, amphibians and fish. The communities of microscopic organisms like bacteria, archaea, and fungi are only recently becoming addressed, with the advancement of genetic technologies.
A terrific paper published in Nature Ecology & Evolution earlier this year sheds light on how these microorganisms respond to urbanization. A main threat of urbanization is called biotic homogenization. This occurs when one species, or a few similar functioning species dominate and displace more natural communities. Many times, biotic homogenization is a result of an invasive, widespread species that makes it to a new urban environment through a human vector. We tend to introduce non-native species all over the world, both accidentally and on purpose. It also occurs when species extinctions increase. In both cases, the genetic, taxonomic or functional similarity increases between two or more locations over time. Using novel genetic tools, Dietrich Schmidt and his team inform us how these communities shift with the impending advancement of urbanization.
These researchers wanted to examine how three different kingdoms of organisms (fungi, archaea, and bacteria) respond to urban erosion. If they only used soil samples from just one urban center, they would not have been published in such a high impact journal. They instead collected several soil samples from five cities: Baltimore, USA; Helsinki and Lahti, Finland; Budapest, Hungary; and Potchefstroom, South Africa. There is a bit of a latitudinal bias, as most cities here are temperate or boreal, besides the South African location. They acknowledge this fact, but this is the case in most studies attempting to gain insight from a global scale. Universities where most research is carried out are predominantly distributed across temperate regions. So, this paper better describes how urbanization erodes temperate and boreal ecosystems, with a brief glance showing how a warmer location responds.
The most compelling pattern found was the loss of diversity in ectomycorrhizal fungi in boreal and temperate regions. Upon the establishment of urban centers, the number of nutrient sequestering mycorrhizal species is greatly reduced. This is most likely a deforestation response, because with the removal of their tree host, no photosynthate is reaching the community of fungi that depend on the sugar reward. In these places, the community of fungi showed great functional convergence, with overall abundance and richness being greatly reduced. This is supported by several studies, and it truly does make sense. In a previous blog entry, I mention how temperate and boreal ecosystems have more diverse assemblages of mycorrhizal fungi than locations closer to the equator. With an initial high diversity, any disturbance including urbanization will show up as statistically significant.
This study which spanned five cities and three continents revealed that archaea and bacteria are not susceptible to biodiversity loss with the advancement of urban centers. However, it did disclose how ectomycorrhizal species respond; it’s not good. This functional guild of forest floor inhabitants is vital to the health of forest ecosystems across Earth’s landscape. This publication helps us understand that some microscopic organisms are better adapted to existing in the modern, human age, while others will fall by the wayside if we aren’t proactive. To protect ectomycorrhizal communities as we develop future cities, we should leave patches of remnant ecosystems, and plant native flora that can be colonized by native fungal assemblages. I can’t stress it enough; mycorrhizal fungi are keystone species involved in an abundant amount of ecological feedbacks. It is up to us to understand what is happening beneath the forest floor, so we can protect the ecosystems we love-so we aren’t the last ones to love them.