Most niche of an organism is comprised of both biotic and abiotic factors. Biotic factors are the living elements, such as food resources, predatory animals, or a mutual relationship with other organisms. Abiotic factors are the non-living elements of the environment, such as temperature, moisture, and refuge. All of these living and non-living factors comprise the niche requirements of every organism on earth. These organisms are capable of choosing the microhabitat that best meets their niche requirements.
Plethodon cinereus is the organism studied in this project, nick named the red-backed salamander, and along with the four-toed salamander. Plethodon cinereus is perhaps the most abundant and widespread salamander in North America. It mainly lives in forested habitats. On a micro scale, the red-backed salamander burrow under logs, rocks, leaf litter, and choose to live in underground burrows made by other organisms, or in naturally occurring crevices. Its entire life cycle takes place on land as an amphibian, no stage in its life it enters an aquatic environment. Plethodon cinereus is a predaceous creature feeding on a variety of small invertebrate arthropods in the forest soils. As an amphibian a certain amount of moisture is needed within its niche for the prevention of dehydration and decreased performance, which could ultimately result in its death, therefore moisture is considered as one aspect of important niche to salamander.
In this lab, three particular aspects of the red-backed salamander’s niche have been examined in relation to microhabitat selection. The three aspects are moisture, refuge, and abundance of food resources. These aspects were interpreted to find any preference in their niche; under logs or in leaf litter for soil moisture and in resource abundance. Following is the discussions of the method used to test the three aspects of niche, and the statistical analysis used to test the findings and validate the results.
The study took place at the Dune Acres area of the Indiana Dunes National Lakeshore. Sampling sites featured two main microhabitats, under logs or in forest litter, beside a hiking trail. In forest litter a 1 x 1 meter plot was measured with a meter stick, and then boundaries were marked with flags. Leaf litter within the boundary was carefully removed from the plot, and any salamanders found in the plot were captured, and carefully handled. Measurement of the surface soil moisture was taken with a soil moisture probe. The measurement was taken in a sideways approach instead or probing straight down, thereby obtaining moisture at near surface soil levels of roughly three inches. For more accurate moisture reading pressure was applied with a foot to surface area directly above the moisture probe by standing on the soil. Leaf litter was placed back and any salamanders captured were also returned.
Similar procedures were applied to log microhabitats. Logs were gently rolled over, and any salamanders present were captured. The soil surface area underneath the log was measured with a meter stick. Surface soil moisture was also taken in the same procedures as in leaf litter plots. Logs were placed back carefully, salamanders were returned, and leaf litter surrounding the log was also placed back.
In measuring the abundance of resources, sticky traps were used in both of the microhabitats. Sticky traps consisted of the same size cardboard pieces with “Tangle Trap Coating” applied at the site. Sixteen sticky traps were placed in each microhabitat, but only retrieved fifteen sticky traps, and one was missing perhaps lost to animal such as deer or raccoon walking away with sticky boards sticking to the leg or reasons unknown. The sticky traps were placed under leaf litter, and sticky traps were also placed partly underneath the log. Flags were placed near the traps for later identification. The sticky traps remained in the field for more than one week, about 10 days. The number and the volume of each invertebrate caught was recorded and calculated, which will make an estimate for the salamander’s resource abundances.
In analyzing the data, a t-test was calculated on each of the three variables. A t-test on the density of red-backed salamanders was compared between those found under a log and those found in the leaf litter plots; and another t-test was done on four-toed salamander in the same manner. Next, t-test was calculated on food volume comparing the density of food under log to density of food under leaf. Then, a t-test was done on soil moisture comparing the moisture under log to moisture under leaf. Since that the salamanders were not found in most of the plots the natural log of the density plus one was taken for the density to reduce a skew in the data, and also the natural log of zero is undefined. Therefore, a total of four t-tests were done to compare the three aspects of niche, which are moisture, refuge, and abundance of food resources.
Out of all 46 plots, there are 27 from log and 19 from leaf litter microhabitats. A total of 17 red-backed salamanders were found and captured. Out of the 17 red-backed salamanders 16 were found under log microhabitats, and one was found under leaf microhabitat. Seventeen red-backed salamanders were underneath log microhabitats with an average of 0.72 salamanders per meter square; and one red-backed salamander was underneath leaf microhabitat with an average of 0.04 salamanders per meter square. There were eighteen times as many salamanders that refuge underneath logs than in the leaf litter. Results from the t-test reject the null hypothesis with a t-value of 3.26 falling in the critical region, beyond the critical value of 2.10 with 18 degrees of freedom. This accepts the alternate hypothesis that there is a difference in red-backed salamander density between log and leaf litter microhabitats.
From the same 46 plots, a total of 12 four-toed salamanders were found and captured. All were found under log microhabitats, and none was found under leaf microhabitat. The average density of four-toed salamander under the log was 0.63 per meter square, and none was found under the leaf litter. From the t-test, the null hypothesis was rejected with a t-value of 3.32 falling in the critical region, beyond the critical value of 2.10 with 18 degrees of freedom. This accepts the alternate hypothesis that there is a difference in four-toed salamander density between log and leaf litter microhabitats.
In resource abundance there was an average invertebrate volume of 131.31 millimeter cube in log plots, and an average invertebrate volume of 22.73 millimeter cube in leaf plots. A result from the t-test accepts the null hypothesis with a t-value of 1.19 falling in the non-critical region, below the critical value of 2.13 with 15 degrees of freedom. This accepts the null hypothesis that there is no difference in between volume of food found near logs and volume of food near leaf litter.
Out of the 35 log plots there was a mean surface soil moisture level of 45.46, and out of the 19 leaf plots there was a mean surface soil moisture level of 55.96. In a calculated result from the t-test accepts the null hypothesis with a t-value of -1.69 falling in the non-critical region, between the critical values of +/- 2.10 with 18 degrees of freedom. This accepts the null hypothesis that there is no difference in soil moisture under logs to soil moisture under leaf litter.
The t-test shows that red-backed salamander density was greater, 18 times as great, underneath log microhabitats than in leaf litter microhabitats; and for four-toed salamander all were found under the log micro habitat. Perhaps, the results suggest that in the selections of microhabitat between log and leaf litter, log is preferred over leaf litter.
Another aspect of the niche is the resource abundance, and had been tested. The t-test shows no difference in the volume of invertebrates in either log or leaf litter microhabitat. The salamander can choose whichever microhabitat, and there will still be the same level of resources. We only found that food abundance was the same level, however, we did not test whether the food level was sufficient.
The t-test for moisture shows that there is no difference in the amount of moisture in underneath the log and leaf litter microhabitats. Water was not a problem for the salamanders, since the moisture level was found to be adequate in both log and leaf litter microhabitats. This shows that moisture does not play a role in the selections of the microhabitat niches by the salamanders, since we found that there is no difference in soil moisture under logs to soil moisture under leaf litter.
The results show that, having a log microhabitat may be more beneficial than living in the leaf litter. Underneath a log, it offers a much more protective shelter especially from predators, but also the scolding sun and wind, since we found that water and food as resource abundance was not a problem. The leaf litter can offer many hiding places, but it is not as reliable. Wind can ruin the leaf litter environment, predators can easily move in it to forage, and the sun may cause the leaf litter to raise to higher temperatures in a short time than if the salamander was underneath a log, therefore, clearly shows that the log microhabitat provide a better environment for the salamanders. Possibly but not certainly, because may be there are various other unknown reasons and variables that are hard to control, in comparing the three niches it clearly shows that the niche of refuge is the greatest need in the selection of the log or leaf litter habitat by reflections of the results we computed.