The Invasion of Iguana iguana (Jul/Aug 1996)

On September 5, 1995, Hurricane Luis moved slowly through the northern Lesser Antilles,
causing devastation in its path. With sustained winds in excess of 140 mph, almost
nothing was left untouched. Barely a week later, Hurricane Marilyn hit with torrential
rains. As the inhabitants of Anguilla, the northernmost island, began to make sense
of their lives one month later, they saw on their beaches iguanas that looked different
from the locally known species. Another species had invaded the island.
News of the vagrant iguanas reached me almost immediately through friends on Anguilla.
For the past 10 years, I have been conducting research on lizards on the island. Because
there is no resident biologist on Anguilla, I am often contacted when a biological
issue arises. A photo of one individual was quickly dispatched to me, and I identified
it as Iguana iguana, a species that occurs on islands much further south, such as
Guadeloupe and Monserrat. Seeing this as a wonderful opportunity to document colonization
in the Lesser Antilles, I booked an immediate flight to Anguilla.
Upon my arrival on the island, I was greeted with all the stories of the rafting
iguanas. Over a period of three days, a mat of hundreds of logs and trees had come
floating ashore on the northeastern beaches, having been uprooted from another island
and cast out to sea as a result of the hurricanes’ torrential winds and rain. Iguanas
had apparently been in some of these trees when the storms struck, and they were washed
out to sea with the trees, eventually washing ashore on Anguilla, still clinging to
those trees.
Clues such as the types of trees, the known distribution of Iguana iguana in the
Lesser Antilles, the path of the storm, and the ocean currents — not to mention a
sign that washed up bearing the inscription “Parque Nacional de Guadeloupe”
— all led me to believe that these iguanas came from the island of Guadeloupe, which
lies 175 air miles southeast of Anguilla. Adrift at sea for more than a month, and
at the mercy of the ocean currents, these animals most probably travelled much further
than 175 miles to reach Anguilla’s shores.
Joined by a few enthusiastic helpers, I conducted a survey that turned up 12 individual
iguanas. We obtained tissue samples from four of them and are awaiting the results
of DNA analysis to verify our theory about their origin.
Scientists have long debated how animals got to the islands of the Lesser Antilles.
There are those who believe that a land bridge existed between the mainland of South
America and the islands of the Lesser Antilles and that colonization occurred across
the land bridge. It is hypothesized that as the sea level rose in the late Pleistocene
the land bridge was submerged. Other scientists suggest that colonization was accomplished
by overwater dispersal of an individual or group of individuals on rafts such as logs
or vegetation mats. Both theories have their doubters. The land bridge opponents note
that there is no evidence for a connection between the islands and continent. The
opponents of overwater dispersal suggest that it is doubtful that a pregnant female
or that a male and female could float over large distances, resist drying out, dehydration
and starvation, and arrive on foreign shores to establish a population.
While evidence of Iguana iguana rafting in the Lesser Antilles does not disprove
the land bridge theory, it certainly supports the overwater dispersal theory. While
there are a few records of overwater dispersal, they have been either insects in vegetation
mats, or single animals. This is the first documentation of relatively large animals
rafting en masse. It is evident that these events, while rare, do occur with major
phenomena such as massive hurricanes. We will continue to monitor the invaders, to
determine whether the population survives, and its impact on the local species Iguana
delicatissima.
Ellen J. Censky, Assistant Curator Section of Amphibians and Reptiles Carnegie
Museum of Natural History
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