Reptile & Amphibian Forums

Here's a mystery that experts of the genus Nerodia might be able to explain...

I've conducted annual surveys of the snake population of a local urban wilderness area in central OK for several years. This 160-acre area was rife with Diamondback and Plainbelly (Blotched) water snakes (N. rhombifer & N. erythrogaster) in 1999/2000, but populations of both species have dropped in succeding years. They are now at zero! That is, I have not seen a single specimen of either one in this area since 2001!

Other snake genera appear to be unaffected. Rat snakes, ribbon snakes, rough green, even Graham's Crayfish snakes, continue to flourish. But the two Nerodia species, once abundant, are gone!

This is a designated wilderness area, off limits to sport fishing, hunting, swimming, camping, boating (there's a small lake), etc.

Several possibilities come to mind: Disease/plague? Beaver or other predator infestation? Extirpation by humans? A natural cycle? It's just that none of these explanations seem to account for the fact that only Nerodias seem to be affected.

I'd be very interested in ideas from others on this situation.

michaelb

I think first you should assess your survey strategy. By "annual surveys" "for several years" what exactly do you mean? Mark-recapture with an actual population estimate, or opportunistic walks around ponds counting all snakes you saw? (or something else?) How many years do you have data for? Were all surveys done at the same time of year, with generally the same weather conditions? You get the idea. All these issues have bearing on whether your population estimates are comparable from year to year, and whether you can actually say there was a decline.
I would also think about changes in vegetative cover and/or hydrology. Either of these could affect the "detectability" of watersnakes. Successional changes of emergent or streamside vegetation can make watersnakes much more difficult to find.
Lastly, there may have been a reduction in numbers of watersnakes at the site. Perhaps it's a natural phenomenon. Watersnakes emigrate from drying ponds to areas of more permanent water. Perhaps there was a crash in the prey base (i.e. frog or fish dieoff, etc.). Maybe young kids carried snakes off in truckloads. Unfortunately, you'll never be able to find out if any of these things occured if you weren't monitoring them as well.
My advice would be to keep going back and doing the same annual surveys. You should probably take additional notes on other features of the environment, such as relative fish abundance, numbers of crayfish observed, vegetation characteristics, etc. Those additional data may be important later if you find watersnakes again, or if you see another species disappear-
Good luck!
Eric

which is not to say that your observations are inaccurate but possibly insufficient. At least, your "summary" note implys that.

There are just too many unexplained variables to make an intelligent suggestion HOWEVER, and I say this only because it is close to my heart ... with regard to human intervention. If the habitat was not "at risk" from significant alterations due to destruction/construction/toxins etc., it would take both planning and dedication to extirpate all nerodia from such an area in such short time. It is unlikely that you would miss the occurence or not see the signs of such an event. No meteor craters, I expect?
Sorry, no need for sarcasm. It just came to me that a mass extinction theory could rise from this. Well, you presented a mystery, and I'm curious. I'd really like to hear more on this as you research it.
Michael

A little background on this "study." First, I'm a scientist by profession, so I'm quite familiar with the scientific method - testable/refutable hypotheses, repeatability of observation, etc. But I'm not a professional biologist or herpetologist.

The survey is not a rigorous, scientific research project. But neither is it a simple case of infrequent, casual walks through the park. Methodology: repeated walks through the wilderness area (1/4 sq. mi.) - catch and release any snakes encountered, after recording date, time, exact location, confirmation of species, length, and any pertinent remarks (e.g., what it was doing, broken tail or other distinguishing marks, etc.). This solo project typically involves several dozen such walks each season, beginning with the first warm days of March and continuing well into October or early November. (I have the chigger bites to prove it!) There's no tagging, no digging or sweep-and-clear or other activities that would alter the environment, and of course no guarantee that individual specimens will appear more than once in the database. In that sense, I admit that some of the methodology is scientifically flawed, and that these informal survey data would not be suitable for a formal scientific study. But that is not the goal. The goal is simply to gain a subjective idea of the species of snakes present, their relative numbers, and to see if there might be any patterns, cycles, or other trends in population over periods on the order of seasons/years.

Except for the data I've recorded, it is all largely subjective. But even with that being the case, I am confident that my observations are numerous and frequent enough, and involve a sufficiently large sample size, to conclude that there definitely is a downward population trend in Nerodias within this fairly small ecosystem. I'm quite sure it's not a matter of them being harder to find, but rather a case of them simply not being there any more.

I'm getting too long here, but I want to share some other observations that may address some of your other questions and comments. I'll do that in another post. Thanks for your responses! michaelb

Excellent response. Education without pain (other than my pride) but no ruler across the knuckles. I appreciate that.
Definitely then, "the game is afoot"! Assuming no human intervention (lack of habitat destruction/discarded waste);
1) have water levels risen, reducing shallow shoreline?
2) are there subtle indications of native predator population?
3) has a "new" predator (been) introduced?
4) has a more favorable habitat been created nearby?

human involvement;
a) even at some distance from the site has there been any heavy industry; high/low frequency work such as earth compacting or drilling even temporarily?

I suspect from your last response, these where undoubtedly considered and I'm spitting into the wind. I believe you're right to be concerned since for one, erythrogaster are a very broad ranging species with multiple habitat and food selections common even to individuals. Except in the extreme, natural changes, even variations in habitat should'nt bother them as long as they have food, water, cover and more cover.
Now here's a thing; erythrogaster are viewed as perhaps the most intelligent of the nerodia with I believe, rhombifer a close second. I'm not suggesting that they may have calculated the risk and crossed the road but, they are very aware of their surroundings and (is it possible) left, due to vastly more frequent human visitors? I think you mentioned a refuge previously? Are people invited to go there?
If this post comes across as "child like", it's because it is. My life has been spent in industrial inspection labs and sites with extremely sparse herping field experience.
Michael

You're certainly not spitting into the wind. In fact, you've probably hit the nail on the head, because the answers to your questions are yes, yes, yes (possibly), no, and yes!

Actually water levels haven't risen. They've fallen, tho!! Recent dry summers have dropped the lake level several feet, effectively moving the water line well away from where the shoreline normally is. There now is up to 20 feet of open flat dry land between the water and what used to be shoreline vegetation. This is not good if you're a water snake that likes to hide out near the water. The lake is large enough that it will never dry up completely, but this certainly is an environmental change that might give semi-aquatic snakes a cause to move elsewhere.

Subtle indications of native predator population? You bet there are!! Two come to mind. Beavers - a most definite increase in beaver activity over the past few years. Their handiwork (gnawed trees, beaver dams) has been much more evident lately. The other candidate is carp. The lake is home to what seems to be a growing number of carp, or at least some type of large fish (3-4 feet or more in length). They apparently have nests in the shallow water just off shore; if you walk by close enough to startle them, they suddenly swim off - and create a sizeable commotion in doing so. (It's a wonder that there aren't stories of a lake monster!)

I don't know if the lake is stocked. It could be that these large fish were introduced recently, or at least their numbers increased through a stocking program. Or, their numbers may be on a natural upswing (thanks in part to the consumption of large numbers of water snakes!!)

As for human involvement, yes, there is a fair amount of activity going on in areas surrounding the park. Mostly residential construction. That could, I suppose, force predators out of surrounding areas and into the park. That might explain the (apparent) increase in beaver activity.

There aren't a whole lot of human visitors, at least not a substantial increase therein over the years. Recreational fishing (catch and release) is allowed, but otherwise it's simply a wilderness area with a lot of trails for jogging, hiking, etc. No camping, no hunting, no boating, no fishing, no motor vehicles. (...not a single luxury, like Robinson Crusoe, as primitive as can be!) Nature lovers go there to get away, but visitors' numbers generally are kept down by ticks, chiggers, and the heat of summer.

The bottom line is that there are several possible explanations, and the answer may involve a combination of factors. I think I'd bet on the carp. Large predatory fish certainly would explain why only semi-aquatic snakes are disappearing!

Now, that carp factor MAY have a much more significant impact than simple predation as well! Thiaminase "poisoning" is documented (and repeatedly discussed on this forum) such that cyprinids, including the infamous goldfish and common carp, carry an enzyme which causes a lethal nervous disfunction in nerodia, if sufficient volume is eaten and the displaced(?) thiamin is not replenished. On a goldfish only diet, the damage has been observed in less than a year.
The fecund nature of the carp with it's tendency to "push out" other species of fish would reduce a nerodias' feeding options.
Again, just a thought.
Michael