Big Snake, Small Island: Monitoring Eastern Indigo Snakes on North Captiva

Co-authored by: Javan Bauder and Chris Lechowicz

As we depart by boat from Captiva Island on a calm blue-green
sea, our smiles are as broad as the horizon. Fishing dolphins surface close by,
and a snow-white tern air-surfs above us. Scanning the southern tip of our
island destination we see dozens of twisted tree skeletons embedded in the
beach and adjacent dunes—carcasses of Australian pines killed by salt water
intrusion or tossed asunder by Hurricane Charley in 2004. Quiet water lagoons fringing the island are forested with the jungle-thick growth of red mangroves.

Let me assure you, this isn’t the standard experience for
the Indigo Snake biologist while enroute to a study site.

Located in the Gulf of Mexico off the coast of southwestern Florida, North Captiva Island supports a population of Eastern Indigo Snakes that has
captivated the interest of the island’s 130 residents. Along with neighboring
Cayo Costa, Captiva, and Sanibel Islands, North Captiva is part of the Pine Island
Sound chain of barrier islands. These islands support natural communities that
include open beaches, mangroves, salt marsh, coastal scrub, cabbage palmetto and
tropical hammocks. Although such a “beach front” setting seems like
an unusual place to find Eastern Indigo Snakes, both Indigo Snakes and Gopher
Tortoises still occur on North Captiva and Cayo Costa Islands.

Unfortunately, Indigo Snakes no longer occur on Captiva or Sanibel Islands, presumably due to human development. However, North Captiva’s
conservation-minded development has allowed the Indigo Snakes to persist. The
island is only lightly developed (just over 300 residential buildings) and
roads are absent from North Captiva, transportation being provided through
battery-powered golf carts traversing a network of narrow sand and crushed
shell paths, shaded by the canopy of cabbage palms and gumbo limbo trees. The
southern portion of the island is protected by North Captiva State Park. Access to the island is only by boat or plane.

Even though Indigo Snakes on North Captiva do not face the
same threats from development as their mainland counterparts, this population
may be isolated from populations on the mainland or other barrier islands. Isolated
populations, especially smaller ones, are more at risk from loss of genetic
diversity, changes to the environment, and random catastrophes (such as disease
or wildfire). For example, in a small population, it is possible that all the
individuals that died of natural causes could be females, which could have a
huge impact on the breeding potential of the population. Monitoring small,
isolated populations is important to detect declines caused by these factors.

The population’s island status and presence in an
interesting habitat, combined with the support of island residents, makes it
important to study and monitor the status of eastern Indigo Snakes on North
Captiva. As a result, The Orianne Society and The Sanibel Captiva Conservation
Foundation (SCCF; represented today by herpetologist and lead investigator
Chris Lechowicz), along with the cooperation of the local residents, will begin
a long-term Indigo Snake population study on the island. This project is also
made possible with the support and permission of the Florida Fish and Wildlife
Conservation Commission.

After our boat reaches dock, we all pile onto a cart and Chris
and Joel, both of SCCF, lead us (myself and wife) on a combination ecological
tour and snake hunt. Soon I’m a child again, emotions bouncing from elated to
awestruck to curious as we encounter the sites and sounds of North Captiva. On
the state park trail we marvel at the native tropical flora (coco-plum,
myrsine, and the bonsai forms of ancient joewood). And the tortoises, oh my, Gopher
Tortoises every bit as large as sizeable snapping turtles seem to be
everywhere, crossing paths with the regal nonchalance that only a tortoise can
pull off…One massive fellow engaged in housekeeping (fyi – tortoises have a
gene for fastidiousness and regularly sweep debris and litter lodged deep in
their tunnels to the surface) pivots workman-like at the mouth of his burrow as
he tosses sand and shell fragments skyward.

We have a marvelous lunch at Jane’s house before Chris and I
speak to the island’s residents about their indigos snakes, what makes them
tick, the conservation threats they face, and the goals of our study. Because
data from marking and recapturing Indigo Snakes comes slowly this will be a
long-term study. But over time, we hope that our mark-recapture data will yield estimates of population size, survival,
population growth, movement patterns, sex ratios, and age structure. We will
also take scale clips from each Indigo Snake for genetic analysis which
can tell us just how isolated this population is from the mainland.

Both Chris and I are delighted by the turnout, and by the
passion and interest these folks evince for Drymarchon couperi. Many
residents have seen indigos on the island, including monster-sized male snakes
close to 7 feet long; snakes are more than occasionally seen slinking through
the sea grape and ornamental plants that decorate the lawn-less yards; cell
phones are pulled from purses and pockets as several folks exhibit photos they
have snapped of indigos swallowing hatchling tortoises or rat snakes.

One of the residents asks an excellent question, “If
tortoise burrow shelters are valued by the snakes it seems, well, counterintuitive
they would eat baby gophers?” In fact, Indigos do frequently consume very
small tortoises but also eat a diverse array of other animals. And on a site
like North Captiva that has a large tortoise population, with adult tortoise
females nesting every year, and potentially living in excess of 50 years, well,
Indigos don’t get them all.

As we carted along earlier we had pondered what adult
indigos, big snakes with big appetites, are
eating on the island. Unlike many sites that support Indigo Snakes, according
to Chris, the island fauna is home to a depauperate complement of amphibians
(only southern toads, greenhouse frogs, Cuban and green treefrogs) and
relatively few species of snakes (interestingly, the island is “viperless”:
rattlesnakes and cottonmouths are absent from North Captiva). Although snake
diversity may be low, based on the countless fresh snake tracks that we
observed crossing the sandy cart paths, snake numbers are high. We captured several
racers and eastern coachwhips, and we suspect their ranks are regularly diminished
via predation by King Indigo.

Now at an
interesting station (my 50^th year of life on earth) I am prone to
wax nostalgic. At one point on our wonderful field day with SCCF and the North
Captiva residents I harkened back to a special trip I made to a nearby island. Twenty-five
years ago I camped on Cayo Costa, just a mere 0.5 miles from the northern tip
of North Captiva. I awakened to a beautiful sea, a beach decorated with
colorful shells, and watched in awe as a solitary oystercatcher worked an
exposed bar in the dawn light. This picturesque scene was rudely interrupted by
a swarm of “no-see-ums” that, to put it mildly, had their way with me.

This trip was a great
encouragement to those of us at The Orianne Society as it highlighted many
important components of our mission to save the Eastern Indigo Snake. We have
established a great partnership with Chris and the SCCF. We were able to
continue educating the public about the importance of this amazing reptile and
encouraged them to become involved as citizen scientists by reporting valid
sightings (with a digital photograph) of Eastern Indigo Snakes on North Captiva
to a designated email account: indigo(at)sccf.org. Finally, this project will
allow us to use sound science to determine the status of a very interesting
population of Indigo Snakes and provide information that can be used to ensure
the viability of that population. I was reminded of the importance of our
mission, and realized that we are making strides in the right direction.

Stay tuned!