The Overlapping Pleistocene Ranges of the Beaver (Castor canadensis) and the Alligator (Alligator mississippiensis)

The beaver enjoys an extensive range as the below map shows.  However, it is presently absent in most of penninsular Florida.  The scientific literature offers no reason for this absence.  I propose the beaver can not currently live in south Florida because the alligator population is too dense there.

Top: Current range map of the beaver.  Bottom: Current range map of the alligator.  The ranges of the 2 species overlapped in Florida during the Pleistocene.

Pleistocene-aged fossils of beavers are among those found from the Monkey Jungle Hammock site just outside of Miami, Florida, proving that beavers formerly did live in south Florida.  Presently, beavers do co-exist with alligators in the southern parts of South Carolina, Georgia, Alabama, Mississippi, Texas, and all of Louisiana.  I suspect 2 factors allow for this co-existence:  the population of alligators in these states is not as dense as it is in penninsular Florida, and the existence of major rivers flowing from north to south allows beavers from the north to replace those killed by alligators.  There is a population of beavers in north Florida near the mouth of the Suwannee River.  The river provides a conduit for beaver colonization.  There are no major rivers in south Florida that could facilitate the colonization of beavers from farther north.

Alligator fossils from Florida’s Pleistocene are common,  so alligators were not rare in Ice Age Florida.  Nevertheless, the presence of beavers then in south Florida suggests the alligator population was  not as dense as it is today, especially during stadials.  Though there were some wetlands, the region was much drier then.  The environment was likely dominated by dry longleaf pine savannahs on sandy soils with some oak scrub and open semi-tropical woodlands.  It was harder for alligators to come into contact and breed in large numbers like they do today.  Instead, large male alligators defended their small springs from smaller males, driving them into unsuitable habitat where they were likely to perish.  Moreover, alligators weren’t necessarily at the top of the food chain–jaguars and saber-tooths likely fed upon adults, and a large population of bears gorged on the eggs.  Somewhat cooler summers may have slowed down their reproduction as well.  With the alligator population held in check, beavers could live in a less stressful environment.

My hypothesis is worth studying.  A simple way would be to determine how many alligators (large enough to prey on beavers) live per square mile in south Florida and compare that with alligator per square mile in areas where the 2 species overlap.

6 Responses to “The Overlapping Pleistocene Ranges of the Beaver (Castor canadensis) and the Alligator (Alligator mississippiensis)”

  1. Mark LaRoux Says:

    The alligator map doesn’t reflect some isolated populations along the Tennessee river near Wheeler and Wilson dams that were intruduced in the 40’s to attempt beaver control (didn’t work). Oddly, I think there is a swamp just southwest of Huntsviille airport that holds a large enough gator population to test your hypothesis. Interesting idea, Mark. Maybe someone on field herp forum could help on this. Or TVA herpetologists.

  2. markgelbart Says:

    It wouldn’t work that far north because alligator populations wouldn’t become dense enough.

  3. James Smith Says:

    That sounds like the most logical reason. I did see a lot of beavers in Middle Georgia when I was growing up, but I never saw any where there are lots of alligators.

  4. markgelbart Says:

    They come up about as far as Augusta during summer on the Savannah River.

    I now of one in a Burke County pond, maybe 10 miles south of my house.

  5. Lars Andersen Says:

    Interesting idea. I have long-believed gators have a lot to do with the beavers range in Florida as well. However, I don’t think the north-to-south flow of rivers is a factor as you suggest. Beavers often do spread up rivers in the upstream direction. They are not held back by the downstream flow of water. As a long-time river guide in North Florida, I see beavers and their sign quite a lot and am aware of their movements and fluctuations in their range. The rivers we see them in the most are those with the fewest gators, so I do think there is a correlation.

    • markgelbart Says:

      You misunderstand my point.

      I never wrote that beavers were unable to spread upstream.

      My point was this: Beavers from upstream populations can replace beavers from downstream populations that are taken by alligators. This explains why they are able to co-exist in some parts of their ranges.

      Because Florida has few north-south rivers, beavers from upstream can’t replace beavers taken by alligators in peninsular Florida.

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