Looking at the human body’s microclimate in a biogeographic context
Introduction
Species diversity varies across the globe and different environmental conditions. Despite the differences in the number of species between locations, there are measurable patterns relating to species diversity. For example, moving from the poles to the equator, there is an increasing gradient of the number of species present (MacArthur 1965). Typically, the tropics are believed to contain the greatest species diversity because they are composed of many habitats in a relatively uniform climate. The stability of the tropics, along with high habitat complexity, high productivity, and greater competition, are thought to be the driving forces behind the increase in diversity (Pianka 1966). On top of this, it is hypothesized that more uniform environments will have more effective barriers to dispersal than environments with greater fluctuation. This is because organisms that exist in fluctuating environments are more likely to adapt to a greater range of conditions. They are also less likely to encounter unbearable conditions than their counterparts from a stable environment with a smaller range of conditions present (Jazen 1967). As such, organisms from tropical environments are more restricted in their ability to migrate than organisms from temperate climates, allowing diversity to build over time.
Establishment of a species in a new location is a rare and difficult phenomenon that gets increasingly unlikely as the distance between locations grows. Dispersal, especially over long distances, is characterized by difficulties involving isolation, ecological opportunities, and climatic differences. Typically, species that are good dispersalists are less competitive and weedier than their counterparts (Carlquist 1966). These species have adapted for movement rather than being able to outcompete other individuals and species. Dispersalist species would be unlikely to successfully establish a new population in a location already inhabited by other species. Rather, they would need to find an empty or sparsely populated area to colonize, which are few and far between. Meanwhile, species that are adapted for survival rather than dispersal are unlikely to survive the journey to a new location and will not migrate or expand from their original range without some form of intervention.
On average, the human body is home to trillions of individual microbes and thousands of species (Hulcr et al. 2012). The human body can be considered a planet with each individual harboring unique microbial communities that live in a range of physiologically and topographically distinct niches. Much like how the Earth is broken down into different biomes based on environmental conditions, human skin is broken down in different regions based on physiological characteristics. Skin is generally characterized as one of three types: sebaceous, wet, or dry (Byrd et al. 2018). Therefore, the human body can be thought of as at least three different biomes. These biomes can then be compared to preexisting environments on Earth. Moist, hairy underarms can be considered analogous to tropical rainforests, while smooth, dry forearms are comparable to desert areas (Grice et al. 2009). Known patterns about Earth’s species diversity can then be applied to the human body to hypothesize how microbe richness and abundance may vary between each body part.
To explore and test the dynamics of microbes on the surface of the human body, species presence and amounts were recorded on three different parts of the human body: the armpit, the elbow crease, and the back of the knee. We hypothesize that the armpit is a warmer and more constant environment compared to the knee and the crease of the elbow. Since the armpit is a more constant environment, we predict that there will be greater species diversity. This hypothesis is based on the tropics being a highly diverse ecosystem with which the armpit shares many qualities, such as high humidity, complex environment, and relatively constant conditions. Our second hypothesis is that isolation between body parts would decrease similarity in microbe species. Therefore, we expect the elbow to have more similar microbes to the armpit because they are closer in proximity to each other than the back of the knee is to either of them.