The Florida Salt Marsh Vole. Part II: A Glacial Relict

Meadow Vole Range (IUCN)

This map (click to enlarge) shows the current distribution of the meadow vole, Microtus pennsylvanicus, including the dispersed populations in New Mexico, northern Mexico, and the Lower Suwannee and Waccassaca River marshes.

How did the outlier populations get there, and how can the persistence of the Florida population be explained?

The Laurentide Ice Sheet (USGS)

How they got there is attributed to events occurring during the repeated advances of glaciers in North America. Note the map showing the extent of the most recent continental glaciation, which ended about 10,000 years ago. Ice sheets a mile or more in thickness obliterated most of the current range of the meadow vole.

Fossil evidence indicates that the meadow vole, with its habitats and other biota, shifted southward in response to the cooling climate and advancing ice sheet. When the climate warmed and the ice retreated, the vole and its habitats shifted back to the north, eventually occupying their current range. The Florida salt marsh vole and the other scattered southern populations became relicts ("left behinds").

How could most of the southern population of the meadow vole disappear and the tiny remnant Florida population manage to persist? 

In all likelihood the disappearance of the larger population had little to do with temperature tolerance. It is more probable that the south became inhospitable for meadow voles because it became more favorable for their competitors, predators, pathogens, and combinations of these biological agents negatively affecting them. 

Pieces of the Florida puzzle fit together when one recognizes that the edges of salt marshes are extreme environments in which relatively few species can survive. Having adapted to life in this harsh environment, for thousands of years the Florida salt marsh vole has hung on in a thin edge--a narrow habitat too severe for the competitors, predators, or pathogens that would eliminate it.