How do you know the post ice age climate change was the reason for the restriction of Hendersonia's range?
That's a great question, as much of what we think we know about the late Pleistocene climate is based on critters. Primarily, 1) where they are now, and 2) where they were in the late Pleistocene.
So, how do we know that post-glacial climate changes are responsible for the restriction of Hendersonia? I'll briefly go over a few points.
1. H. occulta was once much more widespread throughout the midcontinent in the late Pleistocene. We know this because we find fossils of this snail from Kansas eastward into Ohio, and southward all the way to Louisiana.
2. The climate throughout North America was colder in the late Pleistocene than it is today. We know this through proxy evidence such as tundra plants and animals found in ice age sediments (an interpretation based on the assumption that an organisms current climate tolerances were similar in the past). Isotopic evidence from stalactites in caves, lake sediments, and shelly organisms from lakes and oceans indicates temperatures were colder (isotopic fractionation is temperature dependent). And, studies of orbital variation indicate that there was less solar energy hitting the earth during the late Pleistocene (this is from Milankovitch orbital cycles, a subject for a much longer post sometime later).
3. The current range of H. occulta is limited to the areas discussed previously.
4. We find H. occulta in southwestern Wisconsin today where it inhabits moist hillslopes and valleys. It is currently rare in the state, and one of the few endangered snails found in Wisconsin.
5. We do not find H. occulta in late Pleistocene sediments in southwestern Wisconsin today (e.g. Kuchta et al., 2006, 2007a, 2007b).
Now, absence of evidence is not evidence for absence. However, there are no records of this snail in late Pleistocene sediments anywhere in Wisconsin - and believe me, I've looked. From a sample size of more than 24 sites, and over 10,000 individuals, no H. occulta shells have been found. None.
So, from this, I have concluded that past conditions were not favorable to H. occulta, but are currently sufficient to allow this snail to survive in southwestern Wisconsin. From this pattern, and the fact that it is found in areas that were previously covered by glacial ice, this seems a reasonable explanation.
Science being what it is, however, this is just my hypothesis. This can be tested (and has been tested by me many times) and either supported, or falsified by new evidence. Aydin's question is a great example of the nature of science - and why science is so powerful. I make a statement, he has a question about it (being skeptical is a key part of scientific inquiry and peer review), and I provided specific lines of evidence that led to my conclusion. Further questioning might revolve around particular assumptions, or findings. Then reevaluations of the evidence, new findings, and further review might refine, or even reject, my hypothesis. Voila! Science has been done.