The Intermediate Disturbance Hypothesis and the Diversity of

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Eukaryon, Vol. 10, March 2014, Lake Forest College
Primary Article
Eukaryon, Vol. 10, March 2014, Lake Forest College
Primary Article
The Intermediate Disturbance Hypothesis and the Diversity of Bacteria on the
Human Body
post-disturbance environment (Jiang & Patel, 2008).
Bacteria on the human body are subjected to many
of the same factors that play into the species richness under
the intermediate disturbance hypothesis, such as exposure to
disturbances of varying intensity and duration, colonization and
degrees of tolerance to different environmental stresses (Fierer
et al., 2008; Grice & Segre, 2011). While bacterial communities
have been shown to differ between sites on the human body, the
factors that determine this diversity are still poorly understood
(Fierer et al., 2008).
In order to test the intermediate disturbance hypothesis,
we studied the bacteria living on similar exposed surfaces of
the human body of male and females under the disturbance of
washing. The maximum disturbance was represented by the
back of the hands, which are subjected to constant scrubbing
and exposure to soap. Minimal disturbance was represented by
the inner forearms, which are usually disturbed when washed
once a day while showering. It was thought that the cheek
lies somewhere in between these two extremes, usually being
subjected to a moderate amount of disturbance of washing just
a few times a day. Therefore, cheeks should show significantly
higher species richness than the back of the hands or forearms.
All three of these surfaces are characterized as dry, external
environments. We also predicted there will be no differences
in diversity between males and females of these surfaces. We
sampled these areas of the body to determine patterns of species
richness, abundance, and species diversity on the human body.
Patrick Hanes
Department of Biology
Lake Forest College
Lake Forest, Illinois 60045
Abstract
The intermediate disturbance hypothesis predicts that the
highest diversity will occur at levels of moderate disturbance.
We measured the species diversity, abundance, and species
diversity of bacteria on the human body in relation to disturbance
by washing. Bacteria were sampled from three surfaces on the
human body with varying disturbances, counted, and classified
into morphospecies. The site with intermediate disturbance had
the highest species richness and species diversity. While these
results match the prediction of the intermediate disturbance
hypothesis, diversity of bacteria may have also been influenced
by physical and behavioral factors.
Introduction
The intermediate disturbance hypothesis predicts that
the greatest diversity occurs at moderate levels of disturbance.
High levels of disturbance will wipe out all species, and low
levels of disturbance allow superior competitors to eventually
drive other species to extinction (Jiang and Patel, 2008; Kadmon
and Bengamini, 2006; Roxburgh et al., 2004; Shea et al., 2004).
Disturbances can include alterations to the environmental
structure, habitat fragmentation, and biotic factors, and can
simply be defined as anything that changes the composition
of niches (Shea et al., 2004). These disturbances increase
the availability of resources to new species by eliminating or
reducing the populations of potential competitors (Jiang & Patel,
2008).
The species richness of a post-disturbance
environment is due to a combination of factors. Disturbance
only promotes greater species richness if it occurs in between
the generation time of the slowest-reproducing species and the
time needed for complete dominance by the best competitor
(Shea et al., 2004). The area of a disturbed environment brings
in the factor of dispersal versus competition (Roxburgh et al.,
2004; Shea et al., 2004). Both the intensity and duration of the
disturbance will have different effects on each species and life
history stage, which may range from no effect to the death of an
individual (Shea et al., 2004). Productivity relative to the severity
of disturbance could affect the species richness of the postdisturbance environment by influencing the population sizes and
competitive advantages for native species (Kadmon & Benjamin,
2006).
Disturbed environments undergo a transition as
recovery proceeds (Roxburgh et al., 2004). The intermediate
disturbance hypothesis assumes that diversity is driven by
species adapting to the trade-off between tolerating fluctuations
in environmental conditions and competing for resources (Jiang
& Patel, 2008; Kadmon & Benjamini, 2006). Due to these
differences in adaptation, the potential coexistence would not
be completely determined by competitive advantages and may
come about by a variety of mechanisms (Roxburgh et al., 2004).
Resulting species composition and richness were also shown to
be influenced by the order in which new species colonized the
*This author wrote the paper as a part of BIOL484 under the direction of Dr.
Menke
Methods
We sampled 20 young adults, 10 males and 10
females, at Lake Forest College in late March and early April of
2013. Q-tips were rubbed thoroughly on areas of interest (back of
the hand, inner forearm, and cheek) for approximately 5 seconds
in order to collect bacteria. It was noted when the participants
last showered or if they wore make up or had facial hair on the
sampled surface, as these factors may affect the growth and
community composition of bacteria. Samples were collected
from the same side of the body. We swirled the contaminated
end of the Q-tip in a microfuge tube filled with 0.5 mL of PBS
solution and chilled the tubes in a refrigerator for 24 hours. Each
tube was then shaken and 50 μL of solution were pipetted out
and diluted into a second microfuge tube filled with 0.95 mL of
solution. We pipetted 50 μL of solution from the second tube and
applied it to the surface of a TSA plate. The mixture was evenly
spread over the plate surface with 5-7 sterile glass beads and
left to incubate at 36°C for 6 days.
We counted and classified the grown bacteria into
morphospecies by the color and texture of the colonies. We used
a 2-way Anova to test for differences between areas for species
richness, abundance, and species diversity. Species diversity
was calculated by using the Shannon-Weiner Diversity Index.
82
Results
We observed 621 colonies comprised of 15
morphospecies (Fig. 1). Two morphospecies classified as
“white, shiny” and “grey, dull” were the most abundant,
comprising 17.7% and 60.3% of colonies respectively. Only
one species was represented by just one colony. 14 of the
60 plates exhibited no growth of bacterial colonies. Of our
participants, seven of the females were wearing makeup,
four men had facial hair, and 13 showered the morning
of sampling, while five showered the morning before.
respectively (Fig. 4). Species diversity was significant for the
model (F5, 90=2.551, P=0.34), sex (P=0.049), and surface
(P=0.017). There was no significance for interaction of the sex
and surface (P=0.898).
Figure 1: A sample of cultures. Clockwise from the top
left: hand, forearm, cheek, and no culture present.
The average for the species richness was what would
be expected if they were driven by the intermediate disturbance
hypothesis. The check had the highest average number of
species per sample (2.75±0.29 species) compared to the back
on the hand and the forearm (Fig. 2). Males with facial hair had
higher mean species richness on the cheek (3.75±1.43). Females
without make up had higher mean species richness (6±1.21).
Participants who showered the morning before being sampled had
higher mean species richness (2±0.75) compared to those who
showered the morning of sampling (1.95±0.55). Species richness
exhibited a significant difference for the model (F5, 60=2.55,
P=0.049), sex (P=0.019), and surface (P=0.045). There was no
significance for the interaction of the sex and surface (P=0.875).
Figure 3: The mean abundance of colonies per sample for surface
and sex. There was no observable trend.
Figure 4: The species diversity for surface and sex. The cheek
exhibited the highest species diversity. Males had higher species
diversity for every surface.
Figure 2: The mean species richness per sample for
surface and sex. The cheek exhibited the greatest species richness
for both sexes.
The mean abundance of bacterial colonies was
greatest on the female hand (30.4±26.05) (Fig. 3). However, this
data was skewed by an outlier. When the outlier was removed,
the average abundance of colonies on the hand was reduced to
1.78 per sample, and the male cheek had the highest average
abundance (12.5±5.23). Males with facial hair had higher mean
abundance on the cheek (21.25±12.45). Females wearing
makeup had a slightly higher mean abundance on the cheek
(3.71±1.69). Participants who showered the morning of sampling
had greater mean abundance of colonies (12.90±12.70) than
those who showered the morning before sampling (6.36±6.74).
Abundance was insignificant for the model (F5, 60=0.720,
P=0.611), sex (P=0.550), surface (P=0.586), and the interaction
of the sex and surface (P=0.347).
Species diversity was greatest on the cheeks of
both males and females. The H values of the Shannon-Weiner
Index for males and females were (2.41±0.04) and (1.91±0.03)
83
Discussion
Our predictions were supported for species richness
and species diversity, both of which follow the pattern for the
intermediate disturbance hypothesis. We fail to reject the null
hypothesis for the abundance of colonies for each surface.
We also fail to reject the null hypothesis for the abundance
differences by sex. The average for all measurements showed
initial differences among the three surfaces. While the cheek
was highest in average species richness and species diversity,
the variation the variation of measurements between the hand
and forearm could be due to environmental differences other
than the level of disturbance. Differences between sexes
in these measurements were apparent, and may be due to
physical or behavioral factors. At intermediate disturbance, the
gap in measurements between males and females closes, which
means disturbance at this level may cancel out any effects that
contribute to differences in sex.
The removal of the outlier for abundance made
the made the male cheek have the greatest abundance of
Eukaryon, Vol. 10, March 2014, Lake Forest College
bacterial colonies. This inconsistency and lack of significance
between surfaces for abundance may be due to several factors.
Disturbances have a unique effect on each species, of which
some will suffer greater reduction in their populations than
others (Roxburgh et al., 2004; Shea et al., 2004). Therefore, the
insignificant differences in the bacterial colonies could be due
to the abilities of a dominant species being compromised by
disturbance, leading to a more equal distribution of populations.
The opposite trends in the abundance for males and females
among surfaces are possibly due any differences in the
environment dependent on sex.
Subtle differences in the physical traits of areas of the
human body could affect the factors that shape the intermediate
disturbance hypothesis. The skin of a human can be categorized
into habitats by the presence of hair follicles, secretion of salt
and water, temperature, humidity, and topography (Costello
et al., 2009; Grice & Segre, 2011). These factors were not
accounted for in our study and almost certainly have influence
on bacterial communities. The dry surfaces of the human body,
including the forearm, tended to be more diverse (Grice & Segre,
2011). The diversity of the forearm was usually higher than the
hand, especially for men. Men tend to have more acidic skin
and produce more sebum, both of which tend to lower bacterial
diversity (Fierer et al., 2008). However, males were higher in
our measurements of species diversity, so this doesn’t seem to
be a viable cause. Yet, other sex-specific factors, such as the
production of sweat and hormones, may be possible factors
(Fierer et al., 2011).
The presence of either facial hair or makeup over the
cheek may have created a different microhabitat than the arm
or hand and could alter the effects of disturbance. Facial hair
could act as a barrier to minimize the disturbance of washing
and present more of a heterogeneous environment. Stability
and structural heterogeneity have been attributed to an increase
in species richness by allowing organisms to evolve into new
forms with little extinction and specialize within a structurally
varied environment (Pianka, 1966). Makeup could act as a
barrier that traps heat and moisture to the underlying skin. High
temperatures and humidity are said to promote diversity, and
environments of the human body with these traits were found to
have greater quantities of bacteria (Grice & Segre, 2011; Pianka,
1966).
Behavior of participants could affect the diversity of
bacteria at the time of sampling. Time between disturbances
is a factor in the ability of a community to persist through a
disturbance (Shea et al., 2004). This factor is represented by
the how long it had been since the participants showered before
sampling. The time since hand washing has a significant effect
on the community composition of bacteria, but not diversity,
and females were more likely to have washed their hands
recently (Fierer et al., 2008). The mean species richness of
our participants increased with time since they last showered,
while greater abundance was observed from participants who
showered the morning of sampling. However, it is unknown what
effects hand washing had on the hands of our participants, as
they all sampled at least an hour after washing their hands.
Frequencies of hand washing have been observed to differ by
sex and may explain the differences in diversity between males
and females. We assumed the surfaces sampled for bacteria
came into negligible instances of contact with foreign surfaces.
Any increased contact with foreign surfaces would offer more
opportunities for bacteria to colonize a post-disturbance
environment and establish themselves in the new location
(Fierer et al., 2008; Jiang & Patel, 2008).
Understanding the factors that drive patterns of
Primary Article
diversity for bacteria on human skin may hold the potential
for the treatment and prevention of skin disease. Alterations
to the composition of bacterial communities on the skin have
been known to cause infections and disorders (Grice & Segre,
2011). Washing as a means of hygiene has been shown to be
a possible mechanism for the propagation of bacterial diversity.
This resulting diversity may influence the ability of a pathogen
to infect a host. The opening of niche space that arises out of
a disturbance facilitates the establishment of new colonizers
to a community, and it is possible that this may also apply to
pathogens (Jiang & Patel, 2008). However, greater diversity from
intermediate disturbance may also suppress species that lead to
disease (Grice & Segre, 2011). If the effects of high skin bacteria
on pathogen colonization can be determined, it may be possible
to use our skin bacteria as a form of biological control that can
evolve with diseases. In order to truly further our understanding
of skin disease, normal states of bacterial communities must first
be ascertained (Costello et al., 2009; Fierer et al., 2008).
A more accurate pattern of bacterial diversity on the human
body may be determined in future studies. A larger sample size
would compensate for any outliers and could provide a more
complete collection of bacterial species that occur on the human
body. Other variables, such as facial hair or makeup, sex, and
time since showering should be held constant to eliminate their
possible effects on the response of bacteria to disturbance.
Comparisons between variants of these factors would make a
great study for the types of environment that lead to diversity and
if diversity is more likely to be the result of stability or disturbance.
Understanding these factors may explain why surfaces with the
most and least disturbance still showed differences in diversity.
Washing seemed to act as an adequate disturbance, but it is
worth investigating what other types of disturbances, such as
chemical or abrasive, could result in the highest diversity at
intermediate levels.
Note: Eukaryon is published by students at Lake Forest
College, who are solely responsible for its content. The views
expressed in Eukaryon do not necessarily reflect those of the
College. Articles published within Eukaryon should not be cited
in bibliographies. Material contained herein should be treated as
personal communication and should be cited as such only with
the consent of the author.
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