Habitat Preferences of
terrestrial Isopods: a study of comparing moist and dry substrates.
Emmalee H. Phipps
Tennessee Technological University
(TTU) Cookeville, Tennessee 38501
4/22/09
Habitat Preferences of
terrestrial Isopods: a study of comparing moist and dry substrates.
Emmalee H. Phipps
Tennessee Technological University
(TTU) Cookeville, Tennessee 38501
4/22/09
Abstract: This study was conducted to compare
substrate preferences of terrestrial isopods. Because isopods are crustaceans
and possess gills, it is expected that they would prefer substrates that are
moist. It is also expected that because they are detritivores, they would
prefer substrates with more organic matter. Hypothesis: Terrestrial
Isopods prefer moist soil over other substrates, e.g. there is a significant
difference in the numbers of Isopods found on either substrate. Null
Hypothesis: Terrestrial Isopods do not prefer moist soil over other
substrates, e.g. there is no difference in the numbers of isopods found on
either substrate. The substrates to be used in this experiment were soil, sand,
and gravel. This experiment was divided into two separate parts: the first
experiment compared the three substrates against one another in three separate
covered Petri dishes, and the second part of the experiment compared the same
substrates, only the sand and gravel were moistened. The purpose of the two
separate parts is to determine (a) whether or not the isopods prefer moist,
organic soil over dry sand and dry gravel and (b) whether they prefer moist
soil (as found in their natural habitat) over moist sand and moist gravel.
Another byproduct of this study is to determine the survival rates of the
isopods on each substrate. In all trials soil was the preferred substrate over
both sand and gravel, gravel was preferred over sand, and the isopods housed in
the dry sand and gravel substrates had a 100% mortality rate. The results for
this study were as predicted, with moist soil being the preferred substrate
over all other substrates, and gravel preferred over sand. Moist gravel and
moist soil had about the same level of preference. Based on these results we
can conclude that Isopods do indeed have strong preferences for different
substrate types, thus our null hypothesis can be rejected.
Key Words: arthropods, Isopods, Oniscus, Pill Bugs, Porcellio, Armadillium vulagre, behavior, environment, moisture,
substrates, bias, habitat preferences
Introduction
Based on
general knowledge and observations we know that terrestrial Isopods are
crustaceans and possess gills for respiration. Because of this they require
moist habitats to survive. Isopods are also detritivores, and actually improve
soil quality wherever they are found (Tripathi 2006). This characteristic makes
them useful for restoring perturbed soils (Snyder 2008). While they typically break down leaves
and other organic matter, some studies have suggested that Isopods will also
consume grains (Saska 2008). Because of their eating habits Isopods are
generally found in soil habitats. Isopods also provide a food source for a
variety of larger organisms, and in one study it was found that they actually
collect the mineral Cd from the foods they eat and make it available to their
predators (Monteiro 2008). The purpose of this study is to determine whether or
not Isopods truly prefer soil over other substrates. In previous studies
substrate size influences Isopod movement and behavior (Davis 2007).
For this
study I wanted to determine whether terrestrial isopods display a habitat bias
for soil over other substrates (sand and gravel), and whether the presence of
moisture has an impact on that bias. Based on current knowledge the isopods
should demonstrate a habitat bias for soil over sand and gravel, and there will
be a difference in the numbers counted on either substrate. The null hypothesis
then is that there will be no difference in the numbers of isopods counted on
either side.
Materials and Methods
Research for
this study was conducted at the researcher’s private residence. A total of 70
terrestrial isopods (Armadillium vulgare) were used for this study, and collected from a wooded area
adjacent to the testing facility. This experiment was carried out in two parts.
Part one was primarily a moisture comparison measuring preferences of moist
soil over dry sand, moist soil over dry pebbles and dry pebbles vs. dry sand. Part
two was set up the same as part one, but was designed to see if the isopods
showed different preferences if pebbles and sand were moistened. The materials
for this experiment consisted of three covered 14.5cm Petri dishes, moist soil,
sand and gravel. The Petri dishes were
loaned to me from the Biology Department at Tennessee Technological University.
The moist soil was collected from the woods adjacent to the testing facility,
which is also were the isopods were collected. The sand was also collected from
an outdoor location adjacent to the testing facility, dried overnight in the
oven and then sifted the following morning. The pebbles used for this study was
white fish gravel I had on hand. Also included for the experiment were cover
shelters (plastic lids) placed on either substrate in each dish and a digital
camera for photo documentation. For the experiment itself the substrates to be
compared were placed on either side of the Petri dishes and the three groups of
ten isopods were placed in their respective dishes on the dividing line of the
substrates. The isopods in the Control group were housed in a Petri dish
containing only soil, to measure their behaviors and mortality. They were then
monitored over a 12 hour period and every hour the numbers on each substrate
were counted and any interesting behaviors noted. The statistical method used
to analyze and compare the count data was the G-test (Brown 2008).
Results
In
experiment 1 isopods in groups A and B demonstrated a significant bias towards
soil over dry sand and dry gravel (Figure 1). Group C isopods housed in dry
sand vs. dry gravel had a 100% mortality rate, all perishing within 7 hours
(Figure 2). In experiment 2 isopods demonstrated a strong bias for moist soil
over moist sand and for gravel over moist sand. Interestingly, in Group B,
isopods demonstrated a similar preference for both moist gravel and moist soil
(Figure 3). Isopods housed in soil groups tended to bury in and remain
stationary, with any wandering behaviors at a minimum. However isopods housed
in the gravel vs. sand runs exhibited a significant amount of wandering,
especially when the substrates were dry. The results for the G-tests suggested a significant
difference in preferences of the compared substrates with the exception of
Group B in experiment 2(moist soil vs. moist gravel) and Group C in experiment
1 (data invalid due to mortality). The critical value was set to 3.84, which is
from the G-test table of critical
values with the p-value set to 0.05
and df = 1. A G-value greater than 3.84 suggests a significant difference in
preferences for the compared substrates, and thus a rejection of the Null
Hypothesis (Table 1). The Null Hypothesis was then rejected for all groups
except again for Groups B in experiment 2, as there was not a significant
difference in the two groups.
Figure 1 In figure 1 Groups
A and B demonstrated a strong bias for soil. Group C data omitted due to
100% mortality
Figure 2 In figure 2 isopods
wandered constantly until they began to perish around 1:00pm. All 10 had
died within 7 hours.
Figure 3 In figure 3 Group
A demonstrated a strong bias for soil over sand, and Group C demonstrated a
strong bias for pebbles over sand. In Group B soil and pebbles had about
the same level of preference.
|
G-values for Isopod Count Data |
||
|
Experiment
1 |
Experiment
2 |
|
|
Group A
(Soil vs. Sand) |
12.08 |
8.9 |
|
Group B (Soil
vs. Pebbles) |
6.92 |
-0.44 |
|
Group C (Sand
vs. Pebbles) |
------- |
9.06 |
Table 1 In table 1 all
G-values except for those from Group B in Experiment 2 were higher than
critical value 3.84 (p-value = 0.05 & df =1) suggesting a significant
difference in substrate preference.
Discussion
Most of the
results documented for this experiment were as predicted. Isopods did indeed
demonstrate a strong bias towards soil over the other two substrates. The
second most preferred substrate was gravel. Sand was the least preferred substrate.
Isopods never lingered on sand for very long. The most probable reason for
their soil preference is the moisture content. Since isopods are crustaceans
and possess gills, they would naturally prefer a substrate with a higher
moisture content. Another reason they would likely prefer soil is the presence
of detritus or organic matter, which is their natural food source. Their
preference for gravel over sand is possibly due to the fact that the spaces
between the rocks provided hiding as well as allowed for oxygen circulation.
Conclusion
Based on the
cumulative results for this study it can confidently be concluded that
terrestrial isopods (Armadillium vulgare) do indeed exhibit a habitat bias towards soil over gravel and sand,
and a preference for gravel over sand. It can also be concluded that moist soil
and moist gravel have about the same level of preference. Based on the
mortality of all 10 isopods in Group C of experiment 1 it can safely be concluded
that terrestrial isopods require a moist substrate for survival. Thus, in
conclusion there is a significant difference in isopod preference for certain
substrates over others and our Null Hypothesis can be rejected.
Acknowledgements
I would
like to thank Dr. Chris Brown of the TTU Biology department for all his
guidance and assistance throughout my project. I would also like to thank the
TTU Biology lab for use of materials used for this experiment.
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