Elusive Terrestrial Tigers in the Underground Scene
By Kyle Welsh
When the shallow sloughs and ponds of central Alberta freeze and become blanketed with snow, oxygen levels beneath the ice plummet to levels that are lethal to most fish. Consequently, these systems lack fish as an apex aquatic predator. But, as always occurs in nature, another life form arises and fills this ecological niche. In the pothole wetlands of central Alberta, this organism is the western tiger salamander (Ambystoma mavortium melanostictum).
Salamander vs. pike
These salamanders come to the wetlands in the spring as the ice melts. They breed and lay their eggs, which eventually hatch into aquatic larvae that remain in the pond until late summer or early fall. While in the ponds, both adult and larval salamanders consume a myriad of invertebrates and even some vertebrates such as tadpoles and frogs, and thus fill the role of the top aquatic predator. Tiger salamanders increase the productivity of wetlands by asserting a top-down effect on the food chain in much the same way that northern pike do in a larger lake. As such an influential member of the ecosystem, the importance of this organism to sloughs and wetlands is obvious. But, unlike fish, salamanders can leave the ponds before they freeze over and venture out onto land where they will spend most of their life foraging and overwintering, typically only returning to the ponds to breed.
However, much of what the adults actually do on land remains a mystery. This is because they live up to their moniker of “mole salamander” and spend most of their life underground, occasionally surfacing at night after a big rain. Due to their secretive nature, many aspects of their foraging, movement, and overwintering behaviour are unknown, including the specific habitats that they frequent while on land. They remain underground largely because they need a relatively narrow range of environmental conditions to survive. As ectotherms, salamanders cannot directly control their body temperature and must therefore avoid temperature extremes. In our northern climate, it is important that they get deep enough underground to avoid freezing in winter. They also need a moist environment to avoid desiccation.
The goal of my research is to examine the specific components of the terrestrial environment these salamanders use and need to survive. For example, throughout much of their range, the western tiger salamander is a denizen of semi-arid prairie grassland and shrubland habitats. However, in central Alberta the tiger salamander is found in the Aspen Parkland ecoregion, which has a higher rainfall than the prairies farther south and is thus more forested. Within the Aspen Parkland ecoregion are grazing and agricultural lands that are closer in structure to a prairie grassland. This overall setting is unique within the range of this species because it presents distinct terrestrial options from which the salamanders can choose the ideal, if they do in fact have a preference.
My question is: Do they have a preference? More specifically, are these tiger salamanders strictly associated with open grassland type habitat? Or will they also use forested areas adjacent to ponds, so long as the aquatic habitat is suitable? Furthermore, if there is an “ideal” terrestrial habitat, what components of this habitat make it so crucial?
Given their somewhat picky range of conditions and the climate of central Alberta, it’s no wonder they spend so much time underground to avoid becoming popsicles in winter and sun-dried in summer. Throughout their continental range, tiger salamanders often find suitable conditions in small mammal burrows, such as those of northern pocket gophers (locally referred to as “moles”), where they can get below the frost line and survive the winter. However, in warmer regions of the southern United States, they do not need to burrow as deep into the ground to avoid freezing, and can find suitable refugia in a variety of habitats in the form of stump holes, root networks, and leaf litter.
In the harsh climate of central Alberta, they are likely more selective and their distribution could be limited by the availability of suitable overwintering refugia much like other northern amphibians and reptiles. My hypothesis is that tiger salamanders will be dependent on open terrestrial habitat such as grazing and agricultural lands (instead of forested lands) where they potentially rely on the presence of pocket gopher tunnel networks to forage, avoid desiccation, and provide necessary access below the frost line to survive the winter.
My specific study area encompasses the Beaver Hills, directly east of Edmonton where there are hundreds of potential ponds within a patchwork of native aspen/poplar forest, grazing, and agriculture on private and public lands. If you are a resident of this area or the greater Edmonton area and have salamanders on your property, you can submit observations to Alberta Volunteer Amphibian Monitoring Program. Observations contribute to studies like mine, and to general knowledge about the species’ distribution and ecology.
Why does it matter what a salamander needs?
The prairie populations of the western tiger salamander, including all within Alberta, were recently elevated to “special concern” by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC). Threats to this species include increased urbanization resulting in habitat loss, road mortality, emergent disease, and in some places, fish stocking, although the severity of the threat is unknown because there are significant knowledge gaps regarding the natural history of this species. To conserve our native salamanders, we have to protect where they live as well. To do so, we need to know exactly what they need to thrive in order to make effective and informed management decisions.
Central Alberta marks the northernmost occurrence of the western tiger salamander (Ambystoma mavortium). This salamander is one of only two species found in Alberta and is the largest terrestrial salamander in North America. It is a member of a group of pond-breeding terrestrial amphibians called the mole salamanders.
If you happen to have any questions about my study, please feel free to email me at: firstname.lastname@example.org