Biotic homogenization and urban soil microbial communities
Biotic Homogenization (BH) is defined as the process by which disparate communities converge with respect to a given taxonomic, genetic, or functional characteristic; it is an emergent process dependent on the balance of invasion, extirpation and speciation. Because humans drastically reduce the barriers to geographic dispersion while simultaneously vacating vast areas of their indigenous ecological communities, the concept of BH has been particularly applied to the anthropic homogenization of ecological communities on a global scale, and cited as an underlying process driving biodiversity loss. Many studies document the BH process among plant and animal communities, but to our knowledge only one applies the concept to microbial communities. Urban centers are areas of intensified human impact and are particularly vulnerable to BH; no study has been conducted to test whether BH is taking place among the microbial community as a result of urbanization. We hypothesized that urban soil microbial communities are converging on a global scale via a combination of invasion of human-associated microbes and local extirpation due to a shift in the local environmental parameters. We sampled five cities with an global distribution. Within each city, we sampled according to four management schemes: Reference, Remnant, Turf, and Ruderal.
We generated bacterial and fungal sequence data using the MiSeq Illumina sequencing platform (giving us over 2 million reads for the global dataset). Quality assurance and bioinformatics data processing was conducted in the QIIME pipeline, in R, and in PCORD. To test the BH hypothesis, we compared Jaccard dissimilarity indexes among the categories in each city and globally. Our preliminary data supports a distinct difference in bacterial and fungal response to urbanization. Our data suggests that while the fungal communities are converging, the bacterial communities are not. Instead our data shows that at a global scale, bacterial communities are structured mainly by geographic distance. In fact community composition of each site category are divergent, rather than convergent. This suggests that among bacteria, the rate of speciation is higher than the rate of anthropic homogenization. The fungal community composition is much more closely dependent on plant community structure; the plant community is one prominent example in which BH has been shown to occur, and it makes sense that the fungal community would show a similar pattern. Our preliminary results support the view that the geographic scale and taxonomic resolution at which we study BH has an impact on the results.