Heather Danson, William Flatley, Jeanne Lumpkin, and Andrew Schmidt
Abstract:
Salamanders breathe through their skin, and therefore are very sensitive
to their surrounding environment. Previous experiments with salamanders
have led to the conclusion that their distribution is dependent primarily
on high moisture, low light intensity, and neutral to basic pH (Sugalski,
1992). After examining these factors at a local cave, it was hypothesized
that the abundance and diversity of salamanders would be greater 30 meters
inside the cave than 30 meters outside the cave. To test this hypothesis,
data was collected on five different days at Wet Cave in Sewanee, Tennessee.
The water depth, light intensity, and water temperature were also recorded
on each of the five days. A kick net sample of insects was taken on one
of the days. The hypothesis was supported by the data collected in this
experiment. Greater species richness was found inside the cave than outside.
The dominant factor seemed to be light intensity. Decreased light intensity
was strongly correlated with increased salamander diversity. The main factor
that had an effect on this distribution was light intensity. There did not
seem to be any correlation between water depth or pH with the salamander
distribution. One factor that could have altered these results was the presence
of leaf litter and soil outside the cave. The rocks inside the cave were
completely bare; therefore, salamanders were easier to find and to capture.
Also, because the five days of data collection were spread out over the
course of about five weeks, the weather outside the cave changed dramatically,
while the environment inside the cave remained constant. The experiment
may have been more conclusive if the data had been collected on five consecutive
days during the late spring or summer. It would have been more likely at
this time of year that conditions outside the cave would be nearly as constant
as those inside the cave.
Introduction:
A glimpse of a salamander is often the highlight of any trip into a cave. Unfortunately, salamanders are becoming increasingly rare around the globe. A recent study estimated that salamander and frog populations around the globe have decreased 60-70% over the past few decades (Monastersky, 2001). Possible explanations for this trend include: habitat loss, disease, and pollution. The influence of pollution is intriguing because of the salamander's possible role as an indicator of the general health of an environment. Salamanders breathe through their skin, which leads to a very low tolerance of environmental pollution. Locally the Tennessee cave salamander is under consideration for listing as an endangered species. Little is known about this species, but it is believed to be a good indicator of the contamination of under-ground aquifers that provide some of the drinking water in Tennessee, Alabama, and Georgia. It is important that information is gathered about salamanders in this region. This trend of depopulation may not only lead to the loss of several unique species, but it may also be an early warning sign of the damage that we are inflicting on our environment.
Sugalski and Claussen's (1997) showed that salamander distribution was primarily influenced by moisture, light intensity, and pH. Moisture is essential to salamanders because they breathe through their permeable skin (Duellman and Trueb, 1994). Their skin must remain moist for gas exchange. Consequently, they are vulnerable to their environment. They lose the majority of their water through the skin, and are very susceptible to dehydration. A higher light intensity will increase this rate of dehydration. A low soil pH increases sodium loss, which also decreases the salamanders ability to osmoregulate (Sugalski and Claussen, 1997). Thus salamanders prefer moist environments with low light intensity and neutral to basic pH: the conditions found within caves.
This knowledge of the salamanders environmental requirements led us to the
hypothesis that the inside of a cave would be much more hospitable to salamanders
than directly outside. The inside of a cave ranges from 95-100% humidity and
light levels drop off to zero within a few meters of the entrance (Barr,1961).
Caves also provide a slightly basic environment as a result of the limestone
that they are carved from. The constant temperature of 13-17 degrees Celsius
(56-59 degrees Fahrenheit) within caves in Tennessee is also ideal. The mean
of the salamander's temperature range is 13.9C (Duellman and Trueb, 1994). Wet
Cave, just north of the Sewanee domain on route 41, provided the perfect
enviornment for studying salamander abundance and diversity. The combination
of these factors led us to our hypothesis that salamander abundance and diversity
will be greater in a 30 meter linear sample of a stream inside of Wet Cave,
compared to a 30 meter linear sample of a stream outside of Wet Cave.
Methods:
This experiment was conducted on five different days at Wet Cave in Sewanee, Tennessee. The data was collected on two days in the morning, two days in the afternoon, and on one day in the evening. We calculated the temperature of each day qualitatively by noting the wind blowing in or out of the cave. The average air temperature inside a cave in Tennessee is 13-17 degrees Celsius or 56-59 degrees Fahrenheit (Barr, 1961). If the temperature is cooler outside of the cave, then the wind will blow into the cave and if the temperature is cooler inside of the cave, then the wind will blow out of the cave. On September 22nd and October 6th, when the salamander survey was carried out in the morning, the temperature was cooler outside the cave than inside the cave. These surveys took place at the beginning of fall when there was not a whole lot of debris and leaf litter on the ground. On October 2nd and 9th, the experiment was carried out in the afternoon and the temperature was cooler inside the cave than outside. Leaf litter was starting to build up on top of the creek and especially along the sides of the creek. On November 1st, the survey took place in the evening around 8pm. The temperature was much cooler outside the cave than inside the cave, and by this time, most of the leaves had fallen off the trees and covered the creek and side banks.
The stream flowing into the cave was divided into six five-meter sections, and marked appropriately. The depth of the stream was taken at every five-meter section in the center of the stream with a meter stick. The light was measured with a LI-Lor LI-250 light meter at every five meters. The light meter was turned on and the light probe was held completely flat in the desired area. The device would then automatically give the correct light reading. The water temperature was recorded at every 15 meters. All of these variables were measured on each of the five days. On October 9th, a kick net sample was taken at the 15-meter mark in the middle of the stream, outside of the cave. This was accomplished by one person holding a net down into the water, directly above the sand and soil of the bottom of the stream, while another person kicked the sand and soil in front of the net with his foot until a large cloud of silt appeared. As the stream flowed down and through the net, any insects that were buried under this layer of sand and soil would be caught in the net. On November 1st, pH of the water was measured with pH paper inside the cave at every 15 meters, and an Oakton electrode pH meter (WD-35615) was used outside the cave at every 15 meters. Two different methods were used because the electronic pH-measuring device required a sample of soil from the water source to measure its pH. Inside the cave, there is no soil, so we obtained pH paper to hold into the stream to find the pH. Outside the cave, enough soil to cover the bottom of the cup was collected and RO water of an equal volume was added to the soil in the cup. This mixture was then swirled, and while swirling, the pH measuring device was inserted into the cup and measured the pH.
After these measurements were recorded on each day of the survey, the
search for salamanders began with two people surveying inside the cave and
two people surveying outside the cave. Both of the groups started at the
30-meter mark and worked their way to the opening of the cave. One person
carefully searched on the right side of the stream or cave, while the other
person searched on the left. The walls of the cave were checked up to eye
level and a one-meter section on each side of the stream was also searched.
If any large rocks were found in the stream, they were gently lifted up
to check for salamanders hiding under these rocks. About ten minutes were
spent looking for the salamanders in each of the five-meter sections. Once
a salamander was spotted, if it was possible to catch, it was closely examined
and the Peterson Field Guide for Reptiles and Amphibians (Conant and Collins,
1991) was used to identify each salamander. In addition, any other types
of animals discovered were recorded. After five days of collecting data,
this data was analyzed and Simpson (1/ [sum of species proportion^2]) and
Shannon-Weiner (-sum [species proportion*natural log of species proportion])
indices were calculated. These indices measure the abundance of diversity
and account for eveness in a particular environment. The higher the value,
the more diverse a particular environment is.
Results:
Four known species of salamanders were found: Dusky (Desmognathus fuscus), Cave (Eurycea lucifuga), Longtail (Eurycea longicauda longicauda) and Northern Slimy (Plethodon glutinosus). More salamanders were found inside the cave than outside the cave (Table 1).
Table 1: Talley of Salamanders Found During Survey
6 |
3 | |
2 |
0 | |
14 |
0 | |
1 |
0 | |
2 |
3 | |
25 |
6 |
There was greater species richness inside (5 Species) as opposed to outside (2 species) the cave. While there was a noticable correlation between light concentration and salamander abundance (Figure 1), there appears to be no correlation between water depth and salamander abundance (Figure 2). The water temperature range inside (14.4±0.5 C) and outside (15.2±0.4 C) the cave overlapped. The pH range inside (6.9±0.7) the cave overlaped with the pH range outside (7.5±0.2) the cave. No insects were found in the kick-net outside or inside the cave. The Simpson (1/ [sum of species proportion^2]) and Shannon-Weiner (-sum [species proportion*natural log of species proportion]) indices were found (Table 2).
Table 2: Mathematical Results of Salamander Survey
Inside Cave |
Outside Cave | |
2.59 |
2.00 | |
1.20 |
0.69 |
Discussion:
The experimental results supported the hypothesis that salamander diversity
and abundance would be higher 30 m within Wet Cave compared to 30 m outside
Wet Cave. Diversity was examined by comparing the species richness inside
the cave (5) compared to outside the cave (2). The Shannon-Weiner and Simpson
indexes were calculated to provide information on the abundance of each
particular species. Both the Shannon-Weiner and Simpson indexes were higher
inside the cave (1.20 and 2.59, respectively) than outside the cave (0.69
and 2.00, respectively). The data supported the hypothesis leading to questions
concerning explanations based on the physical properties of the cave as
to why this distribution was seen.
Our experiment had a dissimilar finding to that experiment of Sugalski and
Claussen (1997). Their experiment found that light intensity was a stronger
influence on salamander distribution than was soil moisture, but less
so than soil pH (Sugalski and Claussen, 1997). Our findings are inconsistent
with those in that light was more important than pH. Our findings are supported
by the findings of Petranka in that Cave salamanders, although restricted
to limestone, are not dependent on high-pH substrates (Petranka, 1998).
A possible explanation for this is related to the fact that the pH in our
study area remained constant both inside and outside the cave. Light was
the factor that varied greatly, and therefore had a stronger influence than
soil pH.
Another interesting point of topic in Petranka's findings is a negative correlation between Cave and Longtail salamanders. We noticed a similar correlation. In our research area, the number of Cave salamanders was much greater than the number of Longtail salamanders (Table 1). Petranka explains this by noting that Cave salamanders appear to function as top predators in many cave communities, possibly out competing Longtail salamanders (Petranka, 1998). This may very well be the case in our research area. An alternate explanation, however, is that both Dusky and Longtail salamanders tend to retreat into hibernation in early October (Petranka, 1998). Our data partially supports this, where we did not find any Longtail salamanders after October 1 (we did however find seven dusky salamanders after October 1). This would lead me to believe that Longtail salamanders are less resistant to habitat variation (i.e. water level changes)(Figure 2).
The method in which we collected data was helpful to our experiment in two ways. First, using a relatively small sample size, 60 meters total, it was much easier to obtain and analyze our data. If we had a larger sample area, we might have warned off salamanders that were deep in our area. Petranka noted a correlation between human disturbance and low salamander counts (Petranka, 1998). By repeating precise processes in two different habitats, we were able to cut down on substantial error and were better able to contain a controlled setting. Second, by collecting data on several occasions (mornings, afternoons, and evenings), we were able to observe a more natural habitat, covering all opportunities when salamanders might be active and present.
Several factors may have biased the outcome of our experiment. The difference of sub-aquatic groundcover, such as leaves, in the study area outside the cave may have contributed to the lack finding as many salamanders in that area. The study area inside the cave had no leaf litter or surrounding plants, thus making it easier to spot and capture salamanders. Supposing that there was no sub-aquatic ground cover outside the cave, the outcome of our experiment may have been different. In addition, because the habitat outside the cave is comprised mostly of soil, rather than rock, the salamanders could have been buried in the soil, making them harder to spot.
One weakness in the design of our experiment is in the fact that we drew
samples in the fall, when the weather and temperature varied outside the
cave, but remained constant inside the cave. A more accurate experiment
may have been designed to draw samples at a time when temperatures outside
the cave were equally as constant as they are inside the cave, particularly
in the late spring or summer.
Works Cited:
Photos of the Dusky, Longtailed, and Northern Slimy salamanders were found online at www.npwrc.usgs.gov/narcam
Barr, T.C. 1961. Caves of Tennessee. Nashville, Tennessee: State of Tennessee Department of Conservation and Commerce Division of Geology.
Conant, R., Collins, J.T. 1991. Peterson Field Guide: Reptiles and Amphibians, Eastern/Central North America. 3rd ed. Boston: Houghton Mifflin Company.
Duellman, W.E, Trueb, L. 1994. Biology of Amphibians. Baltimore: The Johns Hopkins University Press.
Monastersky, R. April 20, 2001. Studies uncover complex reasons why amphibians are vanishing around the world. The Cronicle of Higher Education. A28.
Petranka, J.W. 1998. Salamanders of the United States and Canada. Washington: Smithsonian Institution Press.
Sugalski, M.T., Claussen, D.L. 1997. Preference for Soil Moisture, Soil pH, and Light Intensity by the Salamander, Plethodon cinereus. Journal of Herpetology. 31. 245-250.
We'd like to thank Mr. Harry Clark and Ms. Cathy Clark for generously allowing us to do research on their property, and their patience through our many scheduling conflicts. Many thanks to Christy Morgan for her help with collecting techniques and general salamander information. We'd also like to thank Dr. Haskell for providing us with supplies and information. On behalf of the salamander survey we'd like to give a shout-out to Krystin Krause for getting up early on a cold Saturday morning to help us collect data. We'd also like to thank the anonymous artist of the cave salamander used at the top of the page.
