Nature is full of environmental and biological interactions. Some well-known, like how temperature and precipitation determine vegetation in different environments. Others are less obvious, and require keen observation to see. Last month, in the Morgan Arboretum, we looked at one of these subtle changes: how changing soil types affect the presence and abundance of amphibians. Today, we will focus on the Eastern red-backed salamander (Plethodon cinereus). To know how the environment affects the species, we first need to know a little about it!
Red-backed salamanders are one of the most common species found in most deciduous forests of Eastern North America. They are so common that in some systems their biomass is said to reach 93.5% of the total biomass (Burton and Likens, 1975). This species is small and slender, and has two color morphs- a more common copper-backed “red-back” one, and the “lead-back” (Gibbs, 2007).
Like other amphibians, they lay non-amniotic eggs, which are permeable and require moisture to develop. Unlike most amphibians, though, the red-backed salamander’s larvae fully develop inside the egg, making them terrestrial right after birth (Howard, 2003). Amphibians also have bare, permeable skin, which allows them to breathe and absorb water. Since they need to stay moist, amphibians never stray far from water or humid environments. Amphibians’ skin is susceptible to changes in the environment; changes in salt, pH, pollutants, and other contaminants can lead them to be extirpated, making them a good ecological indicator (Green et al. 2014). Their sensitivity also makes them picky when it comes to choosing habitats. Even the lighting can affect a salamander’s abundance (Wyman, 1988).
By feeding on a variety of soil-dwelling invertebrates, Plethodon cinereus plays a big role in its habitat as a predator (Howard, 2003). They are also an important prey item for bigger vertebrates, which helps move nutrients up the trophic levels and keep the ecosystem functional (Wyman, 1998).
Questions & hypotheses
Each species has different environmental needs. We decided to look at different soil factors affecting species abundance, and so, our question is: which soil type does each species of amphibian in the Morgan Arboretum favor? Since amphibians need to be in areas of high moisture content, our hypothesis was that more would live in soils with imperfect or poor drainage.
We first selected three different soils in the Morgan Arboretum. Our choices were St. Amable, St. Bernard, and St. Rosalie, because they have different soil orders, parent material, and drainage speed (figure 1; Table 1). To find each soil, we used a soil map and dug holes to double-check our location. Each site (6 total- 2 for each soil type) had a 25x25m quadrant. Walking parallel to each other, we sampled each area for 20 minutes while listening for rustling leaves and checking under rocks and logs.
As the days got colder, amphibian activity decreased, which reduced the diversity of species we found. It is also possible that disturbances caused by animals or fellow students affected our data.
Our prediction was that we would find more amphibians in poorly drained areas. Research also told us that we should find more salamanders in moist soils (Taub, 1961). That’s why we were surprised when we found that, out of 214 red-backed salamanders, 163 came from our driest sites, St. Bernard. With good drainage, St. Bernard never once had a water puddle, even after rainy days. So why was this place teeming with salamanders?
We came up with our own conclusion on the requirements needed for a good salamander habitat:
- less acidic soil
- rocks and logs to hide under
- loose, porous soil texture
- moist soil with good drainage
Some of these conclusions we got through research. Sugalski and Claussen (1997) state that red-backed salamanders prefer less acidic soils. St. Bernard is by far our least acidic soil, so the salamanders’ preference make sense. We found three times more salamanders in St. Amable (39 individuals) than St. Rosalie (13 individuals), despite St. Amable being more acidic. This means there are other variables in play.
Most of the salamanders we found were hiding underneath a rock, log, or branch. This led us to realize that objects on the ground were required for salamanders to thrive. St. Bernard, a glacial till, had many rocks, while the other sites had many decomposing logs. While this is an important requirement for salamander habitats, it did not help us determine why they favored certain soils.
A red-backed salamander returning to his home under a log
Aside from acidity, we believed soil texture and drainage were important factors. Salamanders burrow themselves in soil, which if impenetrable, will not make a good home. It seems that dry soil is too hard, while wet soil is too sticky. Furthermore, like with worms, poorly drained soil could cause salamanders to drown. St. Rosalie is made of clay, which is known to be compact and have micropores (Hillel 1980). Our St. Rosalie sites were often flooded, making it very difficult not only for sampling, but for salamanders to live. On the contrary, St. Amable and St. Bernard are made of sand and loam, respectively. These are bigger, loose particles, making them easy to bury in. Since St. Amable’s sand is over clay, it did not always drain well, again potentially affecting salamander abundance.
Table 1 : Difference between the soil types used in our project (Agriculture and Agri-Food Canada, 2013)
With more sites and replication, we could figure out if we are right about our variables, or if something else caused the differences in abundance. More well-drained soils with varying soil textures and pH could tell us if red-backed salamanders like all well-drained soils, or if St-Bernard is unique somehow. Further research can also allow us to explore other questions and observations, like why we almost only found juvenile salamanders as it got colder.
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