Wednesday, June 30, 2010

Alligators in Virginia.....The Science!

Gator habitat? Let's find out!
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Dream of being a habitat ecologist?! Then get used to doing math!
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(8-8-11) - Note to readers: Click HERE for more detailed analysis on where in the Tidewater area that alligators might move in and not be forcibly moved out in the next decade or so......


So, I've been following my web traffic with my little buddy Sitemeter. I've seen some interesting trends develop over just the last month or two since I installed it. The most unexpected has been the volume in web traffic I receive from people searching for "alligators in Virginia" and finding my first post on the topic. Since apparently this is a hot topic among internet users (who knew?) I thought I'd look into the topic a little further (as I've been meaning to do for over a decade). Remember that I mentioned the USFWS Habitat Suitability Index Model for Alligators? Well, I downloaded it, read it, and figured out the model. It's an interesting one. The results are in the excel worksheet above, but what fun is that? Here's what I learned....
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First of all, as I sort of mentioned before, we have to blissfully ignore and/or accept three admissions made by the creators of this habitat model. First, though I suspect they were tasked by USFWS to create this model for the entire country, the authors created a habitat model only for the northern Gulf of Mexico (TX, MS, LA, FL panhandle). Second, a lot of geographic assumptions in their text are based on a 1958 field guide to reptiles and amphibians. That's more embarassing than the first thing, for sure. Third, the authors admit (I missed this on my first read) that they never actually field tested the model. And as a habitat ecologist, I can say that that's incredibly pathetic. But you know what? It's better than my model, because I don't have one.
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So I entered all of their variables for these sites into their mathematical model for 15 sites near the NC/VA border where alligators are not recognized to exist naturally, 2 sites in NC where they are recognized to be reproducing successfully, and 1 site on the Gulf in TX that is supposedly renowned for its gator population. I did not use 5 sites out of the 18 because once I started working with the HSI model, it was clear that they would score very poorly. As the authors of the HSI recommended, I did my analysis of each habitat "factor" through a combination of memory of some of these areas, photographs I could find, and aerial photos (I used Google Earth). So, are there gator-compatible habitats in Virginia, based on a mathematical model created by the USFWS, who propose that alligators do not live in Virginia?
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Absolutely. But I was surprised by the results. You'll recall (links above) that the two closest reproducing populations are at Alligator River NWR (on Albemarle Sound, 60 miles south of the VA border) and Merchant Millpond State Park (in the inland forested swamps, less than 25 miles south of the VA border, with gators occasionally - and officially - spotted within 12 miles of the border at Dismal Swamp State Park).
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So what? Well, according to the HSI scores (see below), the highest quality alligator habitats near the VA-NC border are where Back Bay meets the Currituck Sound.....60 miles north of Alligator River NWR. Inland, this is not true, as marginal, patchy habitat occurs just over the VA line within the Dismal Swamp. This probably goes a long way toward explaining why alligators have not moved north faster. A pretty basic conclusion is this: Alligators expanding northward on the coastal sounds are likely to settle on other territories far south of the VA line - they need to go no further. Alligators expanding northward in the interior swamps are likely to find low-quality habitat, and any net progress northward is haphazard.
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The HSI scores backed up my analysis...the three reference sites (known gator habitat) scored between 0.57 and 0.75 (out of a possible 1.00 for optimal habitat). 6 out of 7 sites in the far-flung coastal area of Virginia Beach & Chesapeake met or exceeded that range. 0 out of 4 sites in the much closer interior flooded swamp (and rivers like the Nottoway) met that range, although 3 sites were close.
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A marketing analogy is easy - would you be more likely to drive 1 mile for a cheap, grizzly hamburger, or an unknown but longer distance to get a delicious steak dinner, cooked just the way you like it? That's exactly the type of decision that sub-adult male alligators are having to make when they get kicked out of their territory and consider a swim north.
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I'll polish up this analysis soon with some final predictions for where-exactly, and when-approximately, I believe alligators are likely to first naturally settle and reproduce in Virginia. As a teaser, here are the habitat suitability rankings of the wetlands I analyzed (this is assuming that alligators were air dropped on each site at the same time). Remember that I omitted 99% of the actual area of southeastern VA, concentrating only on "reasonably possible" habitats. These all represent marginal (ranks 14-8), good (ranks 7-2) , or near-optimal (#1) habitat.
19 - 15 Omitted
14. Impoundment at Cavalier WMA
13. Great Dismal Swamp @ Dismal Road
12. Mackay NWR Impoundments (NC)
11. Great Dismal Swamp (Lake Drummond)
10. Camp Mill Impoundments
9. Great Dismal Swamp (NC) (Existing gators - Reference Site)
8. North Landing River - lower reach
7. Aransas NWR Sloughs (TX) (Reference Site)
6. Northwest River - Hog Island (NC)
5. Northwest River - Irrigation Ponds
4. Merchants Millpond (NC) (Reference Site)
2. Ponds & marshes along the Albemarle & Chesapeake Canal
2. Large stormwater ponds along West Neck Ditch
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and the #1 ranked site is......
#1 - Knotts Island Causeway, marsh ponds, and impoundment. If alligators establish here, they will easily migrate up into, and through, Back Bay.

Later Alligators!

18 comments:

Tovar said...

Interesting stuff! I'll be curious to see how this gator migration plays out.

Elizabeth | The Natural Capital said...

As a gator fan originally from FL I would love to see them move farther north. Even after 14 years of living in the DC area (with, admittedly, many trips home to FL) I'm so conditioned to expect them that occasionally when I'm canoeing up here I'll see a log or something and my first reaction is to think it's a gator! A few questions come to mind for your continuing research:

1. Does the model include average temperatures? And therefore will potential habitat expand due to rising temperature?

2. Are there records of gators further north before they were widely hunted -- say, in the natural history observations of some of the first European settlers?

3. In general is the gator population on the increase or decline -- i.e., if it's increasing, will there be pressure for gators to make that extra trek north?

Evan said...

Great post. As a Virginia native, I would be very interested to see a natural migration of this species.

I have a love/hate view of the prospect of gators. The photographer side of me is thrilled. As a scuba diver, though, it's one thing I wouldn't like to share the water with. I think the photo-side of me would win out though and I'd be grateful to see one locally in the wild.

Swamp Thing said...

Elizabeth - thanks for stopping by!
In my first post, I looked at temperature - namely, January temperatures, and could not find a difference of more than a few degrees between the gators' existing habitat and a few miles north into VA.

Also in the first post, a commentor mentioned that his professor once said that gators existed as far north as the James River (25-35 miles north of the VA/NC border), which makes a certain amount of sense. I can't find any documentation of that, though.

The gator population is increasing, and everything I write about gators assumes a continued increase. Gators have no natural predators once they reach about 18" in length, and they are protected from hunting (except for domesticated "farm" gators). As a result, habitat pressures have and will make them move out of their current range...how fast and where is exactly the question (I'll hypothesis about this in detail in my next gator post).

Swamp Thing said...

Evan, I've had "close encounters" with a big female gator in the kayak in northeastern Georgia....no bueno, mi amigo!

I am trying not to pass a value judgment like that...but sure, it means that human behavior around ponds and wetlands might have to change.

T. Brook Smith said...

Great analysis. I've been waiting for spotted bass to begin the northern migration toward Chicago and Lake Michigan, but I haven't taken it as far as you did here with alligators.

There are now armadillos in Illinois...how much longer can it be?

Swamp Thing said...

TBS - why don't you try (in all your spare time, ha ha!):

http://el.erdc.usace.army.mil/emrrp/emris/EMRIS_PDF/SpottedBass.pdf

There's the model!

Frank Watrous said...

A fascinating presentation. That alligators once occurred in Virginia seems highly probable. To the James River? Maybe. The populations at the northern extremities of this species range (as with many species) were probably not dense and may have been distributed sporadically and somewhat disjunctly, to the extent that local microclimates provided survivable conditions. These populations would have been vulnerable to even limited hunting pressures (Indians and early European settlers) and to habitat alterations. As a result, they may have 'quietly' slipped out of existence at an early date, undocumented in any formal sense. Further, exactly where these populations and individuals resided in swampy country relative to the Virginia-North Carolina boundary may have been difficult to ascertain and of limited interest in any case. A similar case study would be the shadowy documentation of jaguars in the southeastern US (including the Carolinas) in earliest Colonial days. That alligators can manage very competently in cold (freezing) weather conditions, even with ice coverage of waterways, is well documented in numerous herpetological articles. Adults and sub-adults have been found far to the north of their natural range (e.g., Pennsylvania), where they have successfully survived several (or many) winters. The major delimiter - for self-sustaining populations - as various authors have suggested, is more probably northern regions where the duration of summer warmth is too short for at least occasional completion of egg incubation, or, even if that can occur, the onset of winter comes too early for the newly hatched young to survive winter rigors. Now, back to southeastern Virginia: Is it too cold there, relative to adjacent North Carolina, for 'gators to survive? This is most improbable. A suggestion that SE Virgina is not such a bad place to live can be obtained by studying the ranges of large southeastern watersnakes, species and subspecies(living in habitats roughly similar to alligators). Their ranges, one after another, extend right across the border into SE VA -- though generally not much further. Sometimes place names can be a hint as to where species previously (or currently) existed. Wilfred T. Neill in his exhaustively researched The Last of the Ruling Reptiles: Alligators, Crocodiles, and Their Kin (Columbia Univ. Press, 1971)includes SE Virginia within the historical range of the alligator (map, page 189), noting (page 183) that the northeasternmost alligator place name (Alligator Sink) is in the Dismal Swamp of Virginia. Frank W.

hoqiez said...

Honestly, I am speechless. By trade I spend my days tinkering with databases and Excel spreadsheets, and Alligators in Virginia have been a longtime obsession with me. This was like Christmas in August! GREAT GREAT work and thank you!

And, it validates my personal hunches that the most continuous, suitable habitat for migration was Back Bay.

Swamp Thing said...

Frank & Hoqiez thanks for the comments & compliments.

Frank - when I teach wildlife or ecology courses, I use the "place name" history analogy quite a bit - often using the "bison" and "elk" place names on the east coast as a discussion starter. Thanks for the reference to Neill's work - I need to get a peek at that before I publish my next section on this topic.

Hoqiez - data is very exciting - as ecologists, we're trained to rely on the best data available to make decisions (when politics and budgets don't interfere)...of course there's some subjective data entry, but one hopes that it washes out in a quantitative system like this. It's been a "decent" tool for USFWS over the last 20 years or so.

wxgeektim said...

Guilty! I searched "Alligators in VA" and hit your site. Wonderful data that I will begin to look at. I am of two minds: scared and relieved. Scared because we live within a mile of the Potomac River and have many tidal creeks and wetlands near our neighborhood. The thought of one of those bubbas lurking in the shallows is not something I think about when I go to the river, having been raised a bona fide city slicker in Alexandria. Relieved because I'm in my 50's and can clearly recall the pollution of the Potomac being so bad you wouldn't put your hands in it. To think the water is considered to be clean enough to even hypothesize an alligator presence is a miracle. I doubt the gators will make it up as far as Prince William, and I worry more about Cottonmouths, but its fabulous to think of the possibilities. Thanks for the info!

dmason390 said...

Just following up, went for a day trip to Merchants mill pond which is about 25 miles from Suffolk Virginia, 15 miles from Whaleyville and probaby 10-12 miles (less as the crow flies) from the VA border in near Eason, NC. I can tell you that if you want to see some nice gators take a canoe trip in the morning. We saw two about 6 feet long and several slides. Too hot during much of the day. As I metioned, this swamp is connected to the Nanesmond River where serveral gators has been spotted on the VA side over the years. This population is well documented, reproducing. See links. Google it, less than 15 miles from the border with sparse development and lots of water. Surprised it did not come up on your analysis. Can't wait to see the conclusion

http://www.ncparks.gov/Visit/parks/memi/main.php

Swamp Thing said...

DM - I hear you! I used MMPSP as a reference site for this analysis and I mentioned it a little deeper in the text of both this and the first post.

Over the winter, I'll be posting up a time sequence map of where/when they're likely to head...somehow I didn't know that Merchants Mill connected to the Nansemond?

If you have any pictures of your trip, near the areas you saw gators, I would love to see what that habitat looks like.

Thanks!

dmason390 said...

Go to the NC parks link they have better pics than I have including some of the Merchant Mill Pond gators. The area is pretty much like Dismal swamp, mature cyress and white oaks. I did not see as much spanish moss as you might see in some of the more coastal areas such as around Virginia Beach (False Cape or lower Chickahominy/James), but it was very dry and hot for this time of year. We are near drought in Central NC and I believe Ne/NcNC (including Merchnat Mill Pond) is in "Moderate Drought". We are also having well above normal temps (broke records for most 90+ days in a year and not done it may make 90 on saturday!!!). I say that to say conditions are not ideal for herpin' or getting a real idea of the "normal" fauna. I have seen very few snakes and none fo the larger rat species. Only small worm snakes, ring necks or red bellies. A ranger told us that most reptiles are still nocturnal becuase of the heat. Best viewing would be early morning pre dawn, but the park is closed during that time.

d. nash said...

That is a great read. I'm moving to virginia (norfolk area) in a few months, so this was a fun way to learn about the diverse habitats and how they are changing. It is exciting to think I could be living near gator habitat.

Thanks!

Anonymous said...

This is a very interesting read indeed. Have been to M. Mill Pond and seen the gators. However, I have no desire to see these things moving any further north. As a parent in a water loving family and community who spend many hours boating/skiing in and/or near some of the proposed likely migration sites...hell no to the gators.

River Mud said...

Anon - we have a 2 year and I totally understand....but suburban gators are a symptom, not the problem. Even if the climate warms significantly, VA is unlikely to see gator populations even as dense as southern South Carolina...I'd guess, anyway.

Tracker said...

Alligators were reported in the Dismal Swamp Canal (ICW) at South Mills, N.C. a couple of years ago. (Article in the Virginian Pilot). I personally spoke w/attendant at the South Mills, N. C. rest area, a few miles south of the Va line and was told that people were feeding the gators, so the N.C. wildlife personnel removed them south to the Alligator River WMA. The attendant informed me that they did not capture all of them (family) and that boaters coming south out of Va reported seeing gators at the locks in Deep Creek, Va. She said the ones they missed moved on up toward Va which is only a short distance. I have not heard of anyone reporting any gators since in the Deep Creek area of Va. Fishers have reported seeing gators between Eliz City & Va border in this same stretch of water. Again reported via Virginian Pilot News.
Tracker
Va Beach