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49IGUANAJune 2003

Only rarely is a population so thoroughly known
that every member is individually recognized and a
full census possible. Because of this, a variety of sta-
tistical methods, some of them extremely sophisti-
cated, are normally employed to estimate population
sizes. As with all statistical methods, population esti-
mates have limitations. The amount of available data
is one crucial factor: the more, the better. In addition,
certain assumptions, some of them unique to one set
of estimators or another, apply.

Unfortunately, studies on critically endangered
species tend to run afoul of one or both issues, mak-
ing reliable population estimates difficult. This is the
case with Stout Iguanas. Not only are the populations
small, they are unreliable subjects (a captured adult
iguana may retreat into its burrow for up to three
weeks in response to the stress), hard to spot in the
field (the terrain is rough and the animals are crypti-
cally colored), and often not accessible (work on
Guana normally can be conducted only during the
month of October). These limitations render any esti-
mate of population size less reliable than one would
like and therefore much less satisfying to a wildlife
manager. Nonetheless, work with endangered
species requires that their population sizes be esti-
mated, so that one can determine if the population
is stable, growing, or — in a worst-case scenario —
declining. A preliminary estimate, known to be
imperfect, is better than none, as long as the limita-
tions of the estimate are kept firmly in mind when
management decisions are being made.

Several attempts to monitor the Guana iguana
population were made over the years. Combined
with the evidently growing numbers of hatchlings
every year, they suggested that the population was
doing well and growing at a steady pace. In October
2002, a more concerted effort was made by marking
several individuals with water-based, white, exterior
latex paint (which quickly dries and is waterproof)
squirted from a two-ounce (60 cc) syringe barrel.
Marking began on 4 October and continued until 13
October. Twenty-three individuals were marked, but
on most days some sighted iguanas escaped
unmarked. Beginning on 15 October, we did six

“round-up” counts of marked and unmarked indi-
viduals, finishing on 29 October. These provided six
population estimates, based on the Petersen Index:
69, 115, 138, 138, 138, and 207. The numbers are
relatively close to one another, suggesting that they
probably represent a fairly robust estimate. A calcu-
lated mean (134) is probably not too far from the real
number. A calculated standard deviation (45 in this
case) allows us to say that we are 95% confident that
the true population size is between 44 and 224: two
standard deviations from the mean in each direction.
Unfortunately, this method does not meet all of the
assumptions of the test, which renders the numbers
uncertain.

A second method for calculating population sizes
was proposed by Z.E. Schnabel in 1938. Using this
method, we can add to the six Petersen Index tallies
the data from 4–13 October. This method generates
remarkably similar numbers, estimating the popula-
tion size at 95 individuals and the 95% confidence
interval at 58–185. Once again, not all of the
assumptions are met, but the fact that two very dif-
ferent methods with different assumptions led to sim-
ilar estimates gives us more confidence that the pop-
ulation size really is about 100 individuals.

We arrived on Necker Island on the afternoon of
14 October 2002 and quickly marked five Stout
Iguanas. The next day before our departure, we saw
six iguanas, only one of which was marked. The
implication is that one-sixth of the population was
marked; so five times six is 30. No statistics can be
done on a single estimate, but this observation con-
forms remarkably well with the opinion of resident
naturalist and caretaker, Brian Andrews, who believes
that about 20 individuals are present, not counting
the current year’s hatchlings.

References
Ricker, W.E. 1975. Computation and interpretation of biological

statistics of fish populations. Fisheries Research Board of
Canada Bulletin No. 191.

Schnabel, Z.E. 1938. The estimation of the total fish population
of a lake. American Mathematical Monthly 45:348–352.

Guana and Necker Island 
Population Assessments 2002

Gad Perry and Numi Mitchell
The Conservation Agency


