Growing up in the southern Champlain Valley of Vermont, I knew there were Spring Salamanders in the state, and always hoped to find one in the one of the many small streams scattered across the landscape. Topping off around 7 inches in length and with bright salmon-orange skin, you’d think they’d be easy to find, but I never managed to turn one up in the valley. Had I known then what I know now, I wouldn’t have even looked. Unlike Spotted Salamanders, which are considered habitat generalists, Spring Salamanders are habitat specialists. They have very particular habitat requirements and if those requirements are not met, you won’t find them. The Champlain Valley simply doesn’t fit the bill.
So why do some species live in certain places, but not others? Take Timber Rattlesnakes as an example. Here in Vermont rattlesnakes are limited only to northwestern Rutland County (and historically the southern Connecticut River Valley as well). If someone tells me they saw a rattlesnake outside of that area, part of my response includes informing them of where you would expect to see rattlesnakes and that anywhere else the most likely explanation will involve mistaken identity. People sometimes ask about migration as a possibility, and to be fair, migration happens, especially when you consider human assisted migrations. But, reptiles are not great at dispersing to new places on their own and for most species, a “large” migration would mean anything over a few miles. More than ten miles, and you’re talking about a nearly impossible journey for all but a handful of species (sea turtles being the most notorious exceptions). More importantly, there are usually specific reasons why each species live in one place, but not in another.
In the case of Timber Rattlesnakes, it’s pretty straight forward. Timber Rattlesnake populations depend on warm temperatures and an abundance of woodland rodents. Most Vermont forests don’t have the tree composition (lots mast-producing species such as hickory) to support rodent populations large enough for the rattlesnakes to live on. Even fewer places have the rocky/ledgy cliff faces low enough in elevation and pointed the right direction for temperatures the rattlesnakes need. Far fewer locations meet both criteria, and an even smaller number are close enough to other rattlesnake populations for the snakes to have ever gotten there.
Rattlesnakes are not alone in their habitat requirements here in VT. Compare their range map to that of our only lizard, the Five-lined Skink. Toss in Eastern Ratsnakes too, and a pattern begins to emerge.
All animals have the same basic requirements to survive. They need to eat, drink, find shelter, and mate. On top of all that, they need to compete with other species for those resources. In some cases, environmental conditions alone dictate where a species can live, but in others competition plays a really big role (that is why coyotes have been able to expand their range east in the absence of mountain lions and wolves).
One of my favorite examples of a species range being linked directly to environmental conditions is that of the Eastern Milksnake. The Milksnake is common everywhere in the state except in what we call the “Northeast Kingdom”, where it’s colder and the forests have more boreal (northern) species. I think most people reading this will be familiar with the USDA plant hardiness zones (every seed packet you buy will say which zones a plant can be grown in, and each zone is determined by the minimum expected annual temperature). Plants usually don’t care about what the coldest temperature is because they are dormant in the cold season, but zones are a good indicator for the length of the growing season as well as the expected warm season conditions. If you look at the Milksnake range and compare it to the boundary between zones 4a and 4b, there is an almost perfect match, and it’s not a coincidence.
One reason for this is that Milksnakes lay eggs. Egg-laying snakes need to find a place to deposit their eggs that will remain warm enough for the young to develop and hatch before winter, and once you get up to zone 4a (an area where the minimum expected temperature is between negative 25 and 30 F), there just aren’t many places where that is possible. The farther north you go, the larger proportion of snake species that occur in an area are going to give birth to live young because live-bearing species such as Gartersnakes can follow the warmth throughout the day while gravid (pregnant), making up for the fact that no one spot is warm enough for the development of young. Out of Vermont’s 12 species of snake, only one egg-laying species lives up in the Kingdom.
Now let’s go back to the Spring Salamander. They belong to a group of salamanders known as “Plethodontids”, or “lungless” salamanders, which absorb oxygen directly through the roofs of their mouths and their moist skin. The Spring Salamander’s large size means they have a smaller respiratory surface compared to their overall volume than other lungless salamanders, which may partly explain why Spring Salamanders are only found in cool, well-oxygenated bubbling mountain brooks. Cooler water has more oxygen, and brooks running down steep slopes with lots of ripples have even more oxygen (and fewer fish, which eat salamanders). They also need large flat rocks along the stream under which to hide. If you look at their range map for VT, you’ll see that the bulk of the Spring Salamanders are found running north to south right down the center of the state, which happens to be where the Green Mountains are located. The below maps compare the Spring Salamander range to towns with changes in elevation greater than 1300 feet, excluding any town dominated by limestone and carbonaceous bedrock (which does not produce the flat rocks necessary for Spring Salamander shelter). While the match is not perfect, it’s very close. The places they don’t occur are the Lake Champlain, Lake Memphremagog, and Connecticut River lowlands, which are flatter and warmer, as well as uplands with the wrong kind of bedrock.
Sometimes range maps are wrong. That is either because they are outdated or because data were incomplete when the maps were drawn. Species ranges do change, but unless humans are involved, herps aren’t likely to colonize new territory hundreds of miles away from the rest of their species. Instead what you get are ranges slowly moving over time as populations gradually expand into new territories or contract from old ones. And when ranges contract, remnant populations sometimes get left behind, which is a common reason why populations of some species are discovered in unexpected places. The bottom line here is that species ranges are not arbitrary, and if you dig a little deeper, you’ll usually find a biological reason explaining why a species range is where it is, and why it isn’t somewhere else.