Snakes Are Declining Around the World, But Why?

© 2010 by Linda Moulton Howe

“Our data revealed an alarming trend.
The majority of snake populations had declined sharply ....”    

- C. J. Reading, Ph.D., U. K. Centre for Ecology and Hydrology

June 2010, Biology Letters.

June 23, 2010  Hammond, Louisiana - When the research paper above was first made public in Biology Letters the first week of June 2010, I was startled to learn that the European science team reported that beginning in the year 1998, 64% of the seventeen snake populations around the world that they studied have declined - some as severely as 90% loss - “and none (of decliners) have shown any sign of recovery over nearly a decade since the crash.” Some of those species in decline include the asp, smooth snake from Europe, the Gabon viper, the rhinoceros viper of West Africa and the royal python.

The royal python (Python regius) is a non-venomous
python species found in Africa. This is the smallest of the African
pythons and is popular in the pet trade. Image Wikipedia.

Rhinoceros-horned Viper  (Bitis nasicornis)  is a large, heavy- bodied viper
with a prominent nasal horn reminiscent of a rhinoceros that inhabits forests
in equatorial Africa. Its beautiful color and pattern are also excellent camouflage
for invisibility on the forest floor. Image Wikipedia.

The researchers are not certain why some snake populations are getting smaller quickly, but 1998 was the warmest year on record making some wonder if global climate change could be a significant factor.

I took that question and the Biology Letters report to the President of the American Society for the Study of Amphibians and Reptiles (SSAR), Dr. Brian Crother, Ph.D.  Dr. Crother is Assistant Dean in the College of Science and Technology and Prof. of Biology in the Department of Biological Sciences at Southeastern Louisiana University in Hammond, Louisiana. Prof. Crother is also on the Board of Governors for the American Society of Ichthyologists and Herpetologists and on the Executive Committee of the World Congress of Herpetology.

Interview:

Brian I. Crother, Ph.D., Asst. Dean, College of Science and Technology and Prof. of Biology, Dept. of Biological Sciences, Southeastern Louisiana University, Hammond, Louisiana, and President, Society for the Study of Amphibians and Reptiles (SSAR), Board of Governors for the American Society of Ichthyology and Herpetology and Executive Committee of the World Congress of Herpetology: 

“Those authors really do show a remarkable precipitous decline of snake populations of the ones they looked at anyway over about a two to four year span (1998 to 2002). I mean a really remarkable drop.

WHAT ABOUT YOUR OWN RESEARCH?

What we have found – we looked at over 500 species of snakes and somewhere between 15% to 20% of the species we looked at we would call ‘threatened.’ – on the edge of blinking out. We know there are these several thousand species of reptiles. And what we want to do is take a random sample of 1500 reptile species and then looked at their status – are they doing fine? Or are they threatened with extinction?

On the family level, things like colubridae snakes – these are typically harmless, non-venomous snakes - for the species that we looked at that was over 200 species, there is an expected threat level that is very high. And what they found in the U. K. study is that in the colubrids that they looked at, two species were very high in threat level. So two of them dropped precipitously and one of them did not. The U. K. study is a little complicated because they had multiple populations of single species.

Caspian whipsnake (Coluber caspius) in the Family Colubridae. Image Wikipedia.

[ Editor’s Note:  Wikipedia – “A colubrid (from Latin coluber, snake) is a snake that is a member of the family Colubridae. It is a broad classification of snakes that includes about two-thirds of all snake species on earth. Colubrid species are found on every continent, except Antarctica. ]

For example, one of them, a rat snake type, in one of the populations, it dropped really fast and in another population, it did not drop. So that makes the explanation for the precipitous drop hard.

Rat Snakes (Zamenis longissimus) are medium to large constrictors that can be found
through a great portion of the northern hemisphere. They feed primarily on rodents
and birds and some species exceed 10-feet-long. Image Wikipedia.

THE FACT THAT YOU ALL IN THE UNITED STATES ARE COMING UP WITHSOME 20% OF THE SNAKES YOU HAVE LOOKED AT IN AT LEAST A ‘THREATENED’ CATEGORY, WHAT DO YOU SPECULATE MIGHT HAVE HAPPENED FROM 1998 ONWARD THAT WOULD TAKE OUT CERTAIN POPULATIONS OF SNAKES?

People are left with looking at some kind of very large scale issue and the single large scale issue that continues to pop up is global climate change. How it works with reptiles is a harder thing to understand.

But with reptiles, it could be because they have pretty narrow physiological requirements associated with temperature because they are ectotherms. Their temperature fluctuates with the environment.  It could be what is being affected by global climate change.

[ Editor’s Note:  Wikipedia – “Ectothermic, from the Greek ‘outside hot,’ - sometimes confusingly known as ‘cold-blooded’ - refers to creatures that have relatively low metabolic rates, control body temperature through external means and are dependent on environmental heat sources. For example, many reptiles regulate their body temperature by basking in the sun.]

Could Snake Decline Be Linked
to Top Predator Decline?

In ecology, we think of animals existing in sort of this hierarchy of predator and prey relationships. We have things at the bottom and things at the top. Large, mammal carnivores would be sitting at the top and grass would be sitting at the bottom, if you will. You have an herbivore eating the grass and then something eating the herbivore. And then something eating the thing that is eating the herbivore.

Well, snakes tend to fit right in the middle of all this structure in predator-prey interactions. One of the things that we do know is that if you do knock out the top predators like the large mammals of wolves and mountain lions and animals like that, then the medium level of animal predators begins to proliferate. Nothing is eating them, so that would mean things like skunks and opossums and raccoons and all of those kinds of animals would be eating snakes.

So if you increase the medium level of animal predators (that eat snakes), you will see a decline in snakes. And anywhere that humans are encroaching, the first thing that goes are the top predators. We don’t get along very well with things that can eat us! (laughs)

So, that’s a possible explanation. As human populations expand, we knock out the top predators and the obvious result is the loss of snakes because of the increase of middle predators.

But the timing in the U. K. paper is so interesting because it all happens in that short window (1998 to about 2004). And it happens in protected areas as well as unprotected areas, so I don’t know if that explanation can be generalized.

Could Increasing Pesticides Hurt Snakes?

HAS ANYONE LOOKED AT 1998 GOING FORWARD ABOUT WHETHER OR NOT THERE WAS AN INCREASE IN NEW PESTICIDES SUCH AS THE NICOTINE-BASED ONES THAT HAVE COME UP AS SUSPICIOUS IN THE DECLINE OF HONEY BEES AND POLLINATORS?

The possibility is certainly there for some populations of snakes. But going back to the U. K. study, most of the populations looked at were in protected, natural parks and preserves where there should not be that level of spraying. Now whether such pesticide and chemicals leak into protected areas is another question.

Biggest Decline In Female Snakes

The snake declines were asymmetrical with regard to sex. In every case except one, it was the females that declined significantly more than the males. And if you are knocking out the females, you can imagine then that there goes the population. So it might be something targeting female physiology just by chance.

AS A SNAKE EXPERT, WHAT WOULD YOU SPECULATE COULD TAKE OUT THE FEMALE POPULATION OF SNAKES AND NOT AFFECT THE MALES?

One of the things that females do more so than males is they store up a lot of fat in preparation for breeding. So if there were some kind of secondary compound that was being stored in the fat, maybe that could be a cause of the asymmetry of death between the females and males.

THAT GOES BACK TO PERHAPS THE ISSUE OF CHEMICAL ASSAULTS IN THE ENVIRONMENT?

Right, that is what I am implying. But it seems weird when you’re in the very well protected habitats ( studied by the U. K. scientists). Unless there is leakage, it implies that the chemical stuff from pesticides gets spread out a lot farther than we thought it did.

BEYOND PESTICIDES, HERBICIDES AND CHEMICAL ASSAULTS, IS THERE ANYTHING ELSE THAT YOU COULD SPECULATE ABOUT THAT WOULD KILL THE FEMALE SNAKES AND NOT THE MALE SNAKES?

No, not really. It should be a random issue if we think about habitat loss or degradation, disease, prey decline – all of these things should sort out randomly between males and females. It’s unusual! It’s a very unusual pattern.

Snake Decline, Even Extinctions,
Mean More Rodents and Disease

IF FEMALE POPULATION DECLINES PERSIST, DOESN'T IT MEAN SOME SPECIES ARE HEADED FOR EXTINCTIONS?  WHAT HAPPENS TO THE BALANCE OF NATURE ON THIS PLANET IF SNAKE POPULATIONS DECLINE AND DISAPPEAR?

If they went, you could expect a very rapid increase in rodent population specifically. And associated with increased rodent populations would be increased disease to other mammals such as us humans. Think of the cause of bubonic plague, for example.

Snakes are THE best natural rodent control on the planet in most parts of the world. I think that alone would be the biggest effect for humans – uncontrolled rodent populations. If you lose the females, there goes all the reproductive output. So, yes, they would face extinction.

There are about 3,000 species of snakes and they are found on every continent, except Antarctica. They play a role in every ecosystem we know about. To remove snakes from these systems would probably create a collapse that we don’t understand. Mid-level predators would no longer have snakes to eat and this includes things like owls and raptors besides skunks and coyotes and opossums and raccoons and weasels and things like that. It would be a big cascade!

ANOTHER ECOLOGICAL DISASTER LIKE THE GULF.

Aaaaagh! Yes, and we don’t understand the repercussions of that yet either. You just have to keep coming back to the over-arching explanation that seems to be a general change in the climate of the Earth. The general change in the climate is opening up new opportunities for organisms that used to be kept down at a very low frequency. We’ve known about Chytrid fungus for a long time, but something allowed it, something released it, to become as virulent and as reproductively capable as it has become to do the damage to the amphibians that it did.

Suppressed Immune Systems?

I suspect in the end, if I can go out on a limb, that that’s what it’s going to come to with snakes and everything else where we see these mass, precipitous declines. So, there has to be an additional trigger, I would say, and that is the host organisms like the bat, amphibians and snakes, they have  to be losing some immuno-response capability. They are no longer able to fend off these organisms that they have been living with for millennia.

ALL OF THESE SPECIES HAVE BEEN ON EARTH MUCH LONGER THAN HOMO SAPIENS.

Yes.

WHAT SUDDENLY COULD TRIGGER THE WEAKENING OF THE IMMUNE SYSTEM IN SO MANY DIFFERENT TYPES OF LIFE RANGING FROM THE AMPHIBIANS AND THE SNAKES TO THE HONEY BEES AND POLLINATORS AND BATS THAT WOULD WEAKEN ALL OF THOSE IMMUNE SYSTEMS AND MAKE THEM VULNERABLE TO SOMETHING LIKE FUNGI?

Yes. That’s a good question. I hate to target us humans, but our pollution – all the chemicals we put out, whether pesticides, herbicides or waste. These things are all getting into the water systems and the atmosphere. They are cycling and I think they are slowly becoming concentrated enough maybe to cause these effects. So we are reaching some kind of tipping point and all these different animals might have different tipping points in how they respond to all these chemicals.”

More Information:

For further related information about species extinctions, please see other Earthfiles reports from the Archive below:

• 05/27/2010 — New Fungus Strain Killing People and Animals in Northwest
• 05/05/2010 — Updated: U. S. Honey Bee Industry Struggles with 34% Colonies Loss
• 02/25/2010 — Lionfish Invaders Are Eating Up Other Marine Life in Florida Keys, Bahamas and Bermuda
• 01/14/2010 — Death Stars: Supernovae and Gamma-Ray Bursts
• 11/20/2009 — Red List of Earth Life Facing Extinction Keeps Growing
• 10/06/2009 — Cosmic Rays Reaching Earth At Highest Level in 50 Years
• 06/26/2009 — Mysterious Northeast Bat Deaths Now in 9 States and Headed Toward Kentucky
• 03/30/2009 — European Honey Bee Decline Continues While Aggressive Africanized Honey Bees Attack in Southern U. S.
• 03/06/2009 — Unexplained Stranding of 200 Pilot Whales and Dolphins
• 03/06/2009 — Unexplained Stranding of 200 Pilot Whales and Dolphins
• 02/26/2009 — Unprecedented Northeast Bat Die-off Spreading Rapidly
• 01/29/2009 — Part 1: Nanodiamonds Link Outer Space Impactors to Earth Extinctions 12,900 Years Ago
• 01/29/2009 — Part 2: Nanodiamonds Link Outer Space Impactors to Earth Extinctions 12,900 Years Ago
• 12/21/2008 — Mystery of Missing East Coast Acorns
• 10/24/2008 — Rapidly Changing Earth
• 09/26/2008 — NRDC Sues EPA for Honey Bee Lab Data and EPA Approves Another Bee-Killing Pesticide
• 08/15/2008 — Amphibian Warning Bell of Mass Extinctions
• 06/21/2008 — Updated June 25, 2008: Increasingly Acidic Pacific Coast Waters Threaten Marine Life
• 02/29/2008 — Mysterious Bat Deaths in New York, Vermont and Massachusetts
• 02/05/2008 — Federal Court Rejects Bush Navy Sonar Exemption
• 01/18/2008 — Amphibians Dying Out At Alarming Rate
• 08/08/2007 — 2007's Warm, Erratic Global Weather
• 07/11/2007 — Mystery of Night Shining Clouds - Another Global Warming Change?
• 06/28/2007 — Hackenberg Apiary, Pennsylvania - 75-80% Honey Bee Loss in 2007. What Happens If Colony Collapse Disorder Returns?
• 06/21/2007 — Large Lake in Southern Chile Has Disappeared
• 06/01/2007 — Is Earth Close to Dangerous Tipping Point in Global Warming?
• 05/29/2007 — Deadly VHS Fish Virus Has Spread to Lake Michigan
• 04/12/2007 — First 2007 Crop Pattern in United States
• 03/17/2007 — Honey Bee Disappearances Continue: Could Pesticides Play A Role?
• 02/23/2007 — Scientists Hope "Amphibian Arks" Can Save Frogs and Toads
• 02/23/2007 — Part 1: Earth Life Threats - Alarming Disappearance of Honey Bees
• 02/02/2007 — Updated: New U. N. Global Climate Change Report: Earth Could Warm Up 3.2 to 11.52 Degrees Fahrenheit by 2100
• 01/10/2007 — 2006: USA's Warmest Year On Record
• 12/16/2006 — Updated: Unprecedented Die-Off of 2,500 Mallard Ducks in Idaho
• 12/08/2006 — NASA Wants Permanent Moon Base by 2024
• 12/07/2006 — Earth Headed for Warmest Period in 55 Million Years?
• 09/23/2006 — E. coli O157:H7 - Why Can't It Be Washed Off Contaminated Spinach?
• 09/09/2006 — Methane - Another Threat in Global Warming
• 08/19/2006 — Repair of Earth's Ozone Layer Has Slowed
• 07/18/2006 — 2006 - Hottest Year So Far in U. S. History
• 02/20/2006 — Mysterious Deaths of Whales in Mexico
• 09/23/2005 — Phenomenon of "Instant" Hurricanes in 2005
• 08/26/2005 — What Is Killing Amphibians Around the World?
• 08/05/2005 — Scientists Puzzled by "Bizarre" Pacific Coast Die-offs in 2005
• 05/07/2005 — Did Milky Way Gas and Dust Turn Earth Into Icy Snowball Four Times?
• 04/20/2005 — Outer Space Impact At Serpent Mound, Ohio, 256 Million Years Ago
• 04/01/2005 — What's Killing Off Marine Life Every 62 Million Years?
• 03/20/2005 — Astronaut John Young: "The Moon Can Save Earth's Civilization."
• 02/26/2005 — Collapse of Societies: From Easter Island to Iraq - to Western World?
• 02/03/2005 — Kyoto Protocol Goes Into Effect February 16, 2005. British Scientists Warn Global Temperatures Could Climb Higher Than Earlier Estimates.
• 08/27/2004 — Global Warming Impact On Birds - More Extinctions Expected
• 09/02/2003 — Updated - Astronomers Don't Think Asteroid Will Hit Earth in 2014
• 07/19/2003 — Update - Defiance, Missouri T-Pattern Cut in Saplings
• 11/14/2002 — What Happened 12,000 Years Ago That Killed So Many Animals?
• 07/20/2002 — Extinctions of Earth Life Are Accelerating Rapidly
• 04/27/2002 — Earth's Magnetic Anomalies - Could the Poles Flip?
• 01/05/2002 — Global Warming Update - Could Increasing Carbon Dioxide Gas Be Transformed Into Limestone?
• 12/22/2001 — Scientists Warn That Climate and Earth Life Can Change Rapidly
• 06/09/2001 — Environmental Updates and Colt Mutilated in Leitchfield, Kentucky
• 02/25/2001 — Environmental Updates
• 02/07/2001 — 94% Decline In Aleutian Islands Sea Otter Population

Websites:

Biology Letters:  Are snake populations in widespread decline?  http://rsbl.royalsocietypublishing.org/content/early/2010/06/03/rsbl.2010.0373.full.pdf+html

Society for the Study of Amphibians and Reptiles:  http://www.ssarherps.org/

American Society of Ichthyologists and Herpetologists:  http://www.asih.org/

World Congress of Herpetology:  http://www.worldcongressofherpetology.org/index.php?section=11

Royal Python, Wikipedia:  http://en.wikipedia.org/wiki/Python_regius

Colubrid Snakes, Wikipedia:  http://en.wikipedia.org/wiki/Colubrid

Rhinoceros-horned Viper, Wikipedia:  http://en.wikipedia.org/wiki/Bitis_nasicornis