Why Do Species Live Where They Do and Not Where They Don’t?

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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, I thought 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 might not have spent much time looking. 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 in an area, you are unlikely to find them there. The bottomlands of the Champlain Valley simply don’t fit the bill for a Spring Salamander. 

G. porphyriticus adult dorsal in hand close

So why do some species live in certain places, but not others? Take Timber Rattlesnakes as an example. In Vermont, rattlesnakes are limited only to northwestern Rutland County (though they used to also occur farther north in the Champlain Valley, as well as the Southern Connecticut River Valley). 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 and escaped pets. 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 reptile and amphibian species (sea turtles being the most notorious exceptions). More importantly, there are usually specific reasons why each species lives in one place, but not in another. Not only does a species need to get to a spot, they need to be able to survive there.

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crotalus_horridus

In the case of Timber Rattlesnakes, it’s pretty straight forward. Timber Rattlesnake populations depend on warmer temperatures than most of Vermont has to offer and they require 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 south and west facing low elevation rocky ledges that provide the basking habitat needed for females to reproduce. Far fewer locations meet both criteria, and an even smaller number were historically close enough to other rattlesnake populations for the snakes to have ever gotten there in the first place. 

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, and the Eastern Ratsnake , and a pattern begins to emerge. The Southern Champlain Valley has things these animals need that the rest of the state doesn’t.

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pantherophis_alleghaniensis

All animals have the same basic requirements to survive and reproduce. They need to eat, drink, find shelter, and find mates. 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 (the new lack of competition is how coyotes expanded their range east after the decline 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 Vermont except in what we call the “Northeast Kingdom”, where cooler temperatures lead to forests having a more boreal (northern) species composition. I think most people reading this will be familiar with the USDA plant hardiness zones (the number and letter on the back of seed packets indicating how far north or south a plant can be grown). Hardiness zones are determined by the lowest expected annual temperature, which is also a good indicator for the length of the growing season and 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.

L. triangulum close up K. Briggs

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 where annual temperatures typically bottom out around negative 25 degrees Fahrenheit or so, there just aren’t many places where that is possible. The farther north you go, the larger proportion of snake species found in an area will give birth to live young. Live-bearing species, such as Gartersnakes, can follow the warmth throughout the day while gravid (pregnant), making up for the fact that no single spot is warm enough for the development of young. Out of Vermont’s 12 species of snake, only one egg-laying species is common in the Northeast Kingdom.

hardiness milksnake

Circling back to the Spring Salamander, that species belongs to a group of salamanders known as “Plethodontids”, or “lungless” salamanders, which absorb oxygen directly through the roofs of their mouths and through 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 typically found in cool, well-oxygenated mountain brooks. Cooler water has more oxygen, and brooks running down steep slopes with lots of ripples are constantly aerated (and have fewer fish, which eat salamanders). They also need large flat rocks in the stream under which to hide and lay eggs. 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 is where the Green Mountains are located. These two 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.

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gyirnophilus_porphyriticus

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 also 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.