RS:
As to the frequency of snakes on open surfaces (such as roads), I suggest that difference in basic biology can explain these results. Highly secretive species that tend to remain under cover would be less likely to venture onto large open areas. As of a year or so ago, there was a study being conducted at the Savannah Nat. Hist. Reserve (I believe) in which preliminary findings demonstrated behavioral difference between species in their movement onto road surfaces.
At the former Adair army base north of Corvallis and now known as the E. E. Wilson Wildlife area, the Ringneck Snake occurs in much higher densities and far more abundant than the Gopher Snake. During my Rubber Boa field efforts on the wildlife area, I have rarely seen Ringneck Snakes on the surface during daylight hours let alone crossing any of the extensive crisscross roads on the wildlife area. But now and then observing a gopher snake on the surface and on the roads is not unusual. The situation may be somewhat different at night but I suspect that here too, in relation to numerical abundance, the relative frequency of such ventures by secretive snakes is not nearly as high as some of the other 'less secretive' species.
Of all factors that cause species' attrition, I find road mortality no more meaningful than shooting, old age, starvation, predation, dispatching with a stick or hoe, parasites, disease, accidents, freezing, etc. Unless a single agent of mortality, or in combination with all other agents (include removal by collecting), exceeds the capacity of a species to sustain its numbers via reproduction, a species' population will tend to remain relatively static in relation to existing environmental conditions of food, cover, weather, etc.
Where I have encountered a good number of DOR snakes, I have interpreted such observations to indicate a 'healthy'
population of that species occurs in adjacent habitat. When I have observed cottontail rabbits, oppossums, skunks, ground squirrels, raccoons, pheasants, jack rabbits, deer, tree squirrels, spade-foot toads, barn owls, kangaroo rats, etc. road killed with considerable frequency, I view these situations as indicating the populations of such species to be relatively abundant in nearby habitats. This is easily observed with cyclic species such as the Black-tailed Jack Rabbit in which road mortitly is high during the high end of the population cycle and virtually non-existent during the low end of the population cycle.
One also needs to take into consideration the mean home range area of various snake species. Perhaps for some species,
population denisities may be reduced near well traveled roads but such mortality could not possibly adversely affect the species as a whole in any significant manner unless their habitat was unduly fragmented by numerous crisscross roads. This certainly can be the case near human developments. But even here, I have observed a reasonable numbers of snake species in suitable habitat right up to where gravel or paved shoulders occur. I cannot recall a single road or highway in which I have not been able to find snakes of various species along the right of way where suitable habitat exists.
I have also observed a number of species occurring in the median between freeway lanes and populations sustaining themselves on 'islands' created by roads and interstates highways. My son Ryan showed me such a situation when he lived in San Jose in which I believe he found both Gopher Snakes and Racers doing quite well in an isolated triangular island between freeways and a housing development or commercial area. One of my Rubber Boa study sites is on such an island at the south side of Salem, Oregon. And before they widen the I-5 freeway, one of my best sites was next to that freeway south of Salem.
There is the fact that on some roads, over many decades,
road mortality is continuing to occur in decent numbers. If snake populations were seriously reduced, one would expect such road mortality to decline precipitously to the point of almost never occuring after only a short period of time. That some measured declined probably has occurred on some roads can be explained by host of factors only one of which is direct mortality.
That being said, every specimen killed reduces the population of a species one snake at a time. Clearly there is some impact but my point being that such mortality does not have any lasting affect on populations. After all, surely the number of individual specimens per species lost via predation far exceeds, many times over, the mortality that occurs on roads and do we ring our hands over such predation as having a serious, negative impact on snake populations?
On the Field Herpers forum awhile back, it was noted that over collecting has occurred with respect to the Gray-banded Kingsnake on some stretch of road(s) in Texas. Though anecdotal, this scenario does not seem unreasonable. So clearly removal (either by collecting or road mortality) has the potential of reducing a population near the roadway. But that is not the same thing as indicating overall snake populations are seriously impacted in a negative manner by road moratlity (or collecting).
Admittedly, I have not taken the time to fully inform myself on this issue. But when others first mentioned this point of road mortality and used a citation (by Rosen and Lowe) to support their contention, I obtained a reprint of that study thanks to John Gunn (formerly on this site). Depited the author's contention, their data (or lack thereof) clearly does not support their stated position that road mortality impact snake populations in a serious, negative manner. So the results reported by Sullivan on the percent DORs observed and in paricular, the data on numbers of specimens observed during the first and second study, is of no real surprise.
Richard F. Hoyer
