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The Relationship Between Forest Type and Amphibian Abundance

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Amphibians are cold-blooded vertebrates who undergo fascinating metamorphoses. Once upon a time, they were the top land predators from the Carboniferous to early Permian (Carroll 2009). A diverse group, they occupy many niches and play an invaluable role in the food chain. The assorted amphibian species present in the Morgan Arboretum are the following: Eastern Newt, Blue-spotted Salamander, Eastern Red-back Salamander, Spring Peeper, Gray Treefrog, Wood Frog, and American Toad. Amphibians are diversely populated, meaning small to large populations throughout the globe, and divide their time between aquatic and terrestrial habitats. These features render them an excellent indicator group (Halliday 2005), so the proven decline of amphibians provokes questions regarding the mechanisms of population decline and extinction that can be applied to other species as well (Prairie 2009). Habitat change, resulting from exploitation, invasive species, climate change, pollution disease, fragmentation, logging, and other factors, is the biggest contributor to biodiversity loss (Gardner, Barlow et al. 2007). Because of the rise of anthropogenic driven habitat loss, it is important to study the relationship between amphibians and their surrounding environment.

Vernal pools are temporary shallow wetlands, which typically form from the accumulation of rain or snow melt, only holding water for part of the year. The reduced presence of typical amphibian predators like fish in these ephemeral bodies of water enable many species to effectively compete, breed, and reproduce (Calhoun and DeMaynadier 2008). The majority of vernal pool inhabitants return to the pool where they began life, in order to breed (Colburn, Weeks et al. 2008). When the pool dries up the inhabitants roam upland of the pool, to feed and later overwinter. These temporary ecosystems are necessary for ‘vernal pool obligates’ to survive (Calhoun and DeMaynadier 2008). As vernal pools form primarily in non-porous soils, investigating amongst varying soil types the presence of pools and therefore amphibians in their vicinity is crucial to learn more about this unique intriguing ecosystem.

We chose to focus on two species of amphibians, the Eastern Red-back Salamander (figure 3)  and the Spring Peepers. Eastern Red-back Salamanders have color polymorphism and while some can have orange or yellow backs, these salamander most commonly appear to have a red stripe or a fully darker red back called “lead-back”. Surprisingly, these color polymorphisms can influence their behavior: red-stripe salamanders tend to try to evade predators with speed while the lead-back ones have a tendency to stay immobile (Venesky, Carl, 2007). These salamanders are small, usually 6 to 10 cm and they live hidden in forests, under debris such as rocks, logs or leaf litter. In order to escape predators they can drop their tail, which will regrow (Green, Weir et al. 2014).

Spring Peepers live in forested areas, and while they can climb, they usually are found among the debris on the forest floor: their habitat of preference. They require bodies of water to reproduce since they lay eggs in the water, where tadpoles will hatch and develop. As adults, spring peepers are mostly nocturnal feeders, who eat mainly insects and other small invertebrates. They have the ability to endure a bodily freeze down to negative eight degrees Celsius which may influence their mobility-less of a need to evade cold behaviorally (Constantz 2004).

figure 3

Figure 1: An example of a Red-backed Salamander

The Project

The research question is: Does the forest type impact the amphibian population in vernal pool dominated areas?

The project compares the population of amphibians in vernal pools in different forest types located in the Morgan Arboretum. We used vernal pools as our common factor between sites because the high moisture attracts amphibians (Calhoun, Maynadier 2008). Vernal pools are also ideal locations because there is no lateral water flow and populations change from pool to pool (Zedler 2003). The populations should change depending on other factors such as primary type of trees present and the resulting leaf litter.

The three types of habitats we studied were hardwood, coniferous, and mixed. In the hardwood forest beech is the most common tree. The leaf litter found in this type of forest is quite thick and is high in nutrients (Cote, Fyles, 1993). Hardwood forest are heavily shaded in the summer and in the fall, leaf litter keeps moisture high. A thicker canopy adversely affects a vernal pond’s carrying capacity(Skelly, Bolden et al. 2014). Pine is the most common tree in the coniferous forest (Legare, Bergeron, Leduc, Pare, 2001). It has a lower pH and creates very little leaf litter (Cote, Fyles, 1993). This is because the needles are thin and do not shed at fall. Coniferous forest are therefore more evenly shaded yearlong. (Legare, Bergeron, Leduc, Pare, 2001). It is important to note that coniferous forests tend to have more acidic soils because, pine needles needles falling to the floor contain tannins which acidify the soil. The mixed forest contains a mix of hemlock beech and red maple. This creates a diverse forest floor with medium thickness leaf litter.

Materials and Methods

To construct the three metre square quadrants, we cut four red ropes, each three metres long. We then made loops on both sides of the rope and put the flags through the loops to form each corner of the quadrant. This process allowed us to save time and to set an accurate quadrant that would remain the same throughout the data collection, thus increasing our efficiency and reducing errors. Additionally, the bright color of the rope made the boundary of quadrant very clear. (Figure 2)

figure 1

Figure 2: Example of a 3m^2 quadrant. This quadrant marking system worked efficiently and clearly marked out the section of study.

On each data collection day, we walked through the Morgan Arboretum and located a vernal pool area in our previously selected forest types: deciduous forest, coniferous forest or mixed forest. Upon locating a proper vernal pool, our vernal pool criteria being the existence of a clear moist depression although some ambiguity exists in the precise vernal pool definition(Zedler 2003), we marked down the precise location of every vernal pool using GPS coordinates and then labeled it’s position on the map so that we could ensure in the future to study another area. After recording the location, we marked out a three metres by three metres square quadrant in the vernal pool area with the ropes and flags, and took photos of the square quadrant and the surrounding forest area. Information such as number of rocks and logs within the quadrant we recorded on a premade data sheet, this information is important as rocks and logs influence the microhabitat(Scheffers, Edwards et al. 2014). Starting from the outer corners and working inwards all group members turned over the leaf litter, rocks and logs until the naked soil was visible. We knew we would find more amphibians, specifically salamanders, under the rocks and logs so we took special care when moving them(Basile, Romano et al. 2017). Every amphibian found we photographed in situ next to a ruler to serve as length reference and we recorded species name, microhabitat, and quantity of discovered amphibians.We made up an amphibian identification sheet so that we could efficiently and accurately record the species we encountered (Figure 3). After thoroughly turning over the entire quadrant with no more discoveries we removed the flags and ropes, making sure we left nothing behind that could negatively influence the wildlife more than we already had. The method above was repeated for all three different forest types for each day of data collection days over a three week period.

figure 2

Figure 3: Homemade Identification Guide (with species letter code)

Expected results

The observations we made on the first data collection day is a good example of the data we expect to observe throughout the entire experiment. We hypothesised that the acidic soil typical of a coniferous forest is not a good support for amphibian compared to the soil of a deciduous forest soil, which is less acidic. Indeed the conifers creates an acidic litter that is not as suited to many invertebrates that the salamanders feed on. Whereas the rich litter from the deciduous trees (Cote, Fyles, 1993) favor the presence of decomposers the salamander preys upon.

We also know that the temperature is an important variable for salamanders who go into hibernation when temperature of soil falls at approximately 4 degree celsius beneath the soil, not in logs (F J Vernberg, 1953). In the coniferous forest, there is a lack of leaf litter covering the ground. Indeed the uncovered soil gets colder at the surface and deeper down below, while the airy leaf litter in deciduous forest insulates the soil and keeps it warmer for longer. For that reason, we expect to see more salamanders on a covered deciduous soil, even as it gets colder.

We expect a high population density of amphibians in the deciduous forest, a lower one in the mixed forest and the lowest population density in the coniferous area. However, we are aware that salamanders don’t only hide beneath debris on the forest floor but also deeper in the soil, (Taub, 1961) especially as the season gets colder. We only collected data on three different days, each a week apart and did not dig in the soil for our experiment. For that reason, we expect the varied temperatures to impact our results.


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Calhoun, A. J. K. and P. G. DeMaynadier (2008). Science and conservation of vernal pools in Northeastern North America.

Carroll, R. L. (2009). The rise of amphibians : 365 million years of evolution. Baltimore, Johns Hopkins University Press.

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Spring Peeper Profile. National Geographic Society.

Taub, F. B. (1961). The distribution of the red‐backed salamander, Plethodon c. cinereus, within the soil. Ecology 42(4): 681-698.

Venesky, M. D. and C. D. Anthony (2007). Antipredator adaptations and predator avoidance by two color morphs of the eastern red-backed salamander, Plethodon cinereus. Herpetologica 63(4): 450-458.

Zedler, P. H. (2003). Vernal pools and the concept of “isolated wetlands”. Wetlands 23(3): 597-607.

Vernberg, F. J. (1953). Hibernation Studies of Two Species of Salamanders, Plethodon Cinereus Cinereus and Eurycea Bislineata Bislineata.



One comment on “The Relationship Between Forest Type and Amphibian Abundance

  1. Holy Guacamole!

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