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Critical Habitat, Predator
Pressures, and the
Management of Epicrafes
monensis (Serpentes:
Boidae) on the Puerto Rico
Bank: A Multivariate Analysis1
Abstract.- Epicrates monensis is a endangered
boa endemic to the Puerto Rico Bank. Principal
components analysis, based on data collected
during five years of study and 200 captures of this
species, was used to identify predator, prey, and
habitat variables critical to survival of the snake.
Management recommendations are discussed.
Peter 9. Tolson2
Epicrates monensis is a small (ca. < 1
m snout-vent length) semi-arboreal
boid snake (fig. 1)that exhibits an extremely disjunct distribution on the
Puerto Rico Bank. The Mona boa (E.
m. monensis) is endemic to Isla Mona,
a large island in the Mona Passage
between Hispaniola and Puerto Rico
(Schmidt 1924).The other subspecies,
the Virgin Islands boa (E.m. granti),
is found on scattered islands and
cays from La Cordillera eastward
through the Virgin Islands, including
St. Thomas, Tortola, and Virgin
Gorda (Stull1933; Nellis et al. 1984;
Mayer and Laze11 1988).The boa is
apparently absent from Puerto Rico
and the other large islands on the
bank. Judging from the present distributions, the historical range of Epicra tes monensis encompassed virtually the whole length of the Puerto
Rico Bank. Today, unfortunately, the
snake is endangered (USFWS 1980)
and absent from far more islands on
the bank than it is resident-doubtless the result of a long history of extirpation. It is improbable that the
decline of the boa can be traced to a
single causative factor; more likely
the survival of the snake at certain
localities is due to a complex series of
biotic, environmental, and stochastic
'Paper presented at symposium, Management of Amphibians, Reptiles, and
Small Mammals in North America. (Flagstaff, AZ July 19-21, 1988.)
lPeter J. Tolson is Curator of Amphibians
and Reptiles, Toledo Zoological Society,
2700 Broadway, Toledo, O H 43609.
interactions. The rarity of the snake
has made habitat analysis difficult;
one cannot define critical habitat if
the snake cannot be observed. Prior
to my work, fewer than 13 specimens
of the boa had been encountered, and
habitat descriptions were largely anecdotal with no attempts to quantify
those factors important in determining population levels (Div. of Fish
and Wildlife, USVI 1983; USFWS
1984,1986).
The parameters dictating the distribution and abundance of animal
species within a habitat are often diverse. They include not only the
physical structure of habitat, such as
vegetational composition and spatial
he terogeniety (Rotenberry and Wiens
19801, but also species composition
(Matthews 1985; Moulton, 1985) and
other aspects of community structure
which are less easily defined, such as
competition (Cody 1974) or predation pressure. In the West Indies,
particularly on the Puerto Rico Bank,
utilization of a particular habitat by
the endemic herpetofauna is not only
dependent on the structural attributes of vegetative cover and the composition of the endemic animal communities, but also on the number and
severity of feral and exotic animal
introductions that have occurred.
Colonizations (accidental or otherwise) of the roof rat, Rattus rattus, the
house cat, Felis catus, and the
mongoose, Herpestes auropunctatus,
have profoundly influenced the survival and distribution of endemics on
Figure 1 .- Epicrates monensis granti.
Above-adult female, Cayo Diablo, Puerto
Rico. Below-juveniles born at the Toledo
Zoological Gardens 14 July 87.
the Puerto Rico Bank (Barbour 1917,
1930; USFWS 1986; Div. of Fish and
Wildlife, USVI 1983).
Principal components analysis
(PCA) is a multivariate statistical
technique that has been used by community ecologists to model distributions of animal populations in a
multidimensional habitat space defined by a correlation matrix of habitat variables (See Wiens and Rotenberry 1981 and Matthews 1985).My
current work with Epicrafes monensis
utilizes PCA to correlate the abun-
dance of the boa with certain critical
elements of habitat structure and indices of population densities of preferred prey species and predators.
Compilation of such data is extremely important in establishing the
critical dimensions of the boa niche,
the identification of suitable release
sites for snakes born in captivity, and
the selection of likely search localities
for surveys of previously undescribed populations of the snake. By
using PCA, we also hoped to extract
independent patterns of covariation,
such as the degree of niche overlap
with Alsophis, which might explain
certain distributional anomalies of
the boa populations.
lsla Mona
t------------1
5
km
Methods
Study Areas
1 67'55'
Cayo Icacos
La Cordillera
Cayo Lobos
~
8
-
This study is based on habitat analysis of 24 different localities on the following islands and cays of the Puerto
Rico Bank: Buck Is., Cas Cay, Cayo
Diablo, Cayo Icacos, Cayo Lobos,
Congo Cay, Great St. James Cay, Isla
Mona, Outer Brass Cay, Salt Cay,
Saba Cay, and Steven Cay from February 1986 through April 1988. Some
islands had several plots. Sites were
chosen at random without regards to
presence or absence of boas, but an
attempt was made to select sites so
that sampling included the full spectrum of habitat available to the boa.
Figures 2 and 3 illustrate the location
of
~ sampling
1 ~ plots included in the
shrdy.
Cayo Diablo
Vegetational Profiles of Study Sites
Figure 2.-Location of sampling plots in Puerto Rico. Above-plots on lsla Mona. Below-plots
on La Cordillera.
Subtropical dry forest is the habitat
where E. monensis is most commonly
observed, particularly on Isla Mona
and St. Thomas. It is characterized by
small (< 5 m) deciduous trees with
small, coriaceous or succulent leaves
and thorns, spines, and secondary
defensive compounds (Ewe1 and
Whitrnore 1973).Examination of the
consisted of CassythialOpuntia
tangles. Ficus-dominated forest was
present on Mona 1 and Congo 2.
Guinea grass, Panicum maximum,
dominated the transect on Buck 2
and Acacia macracantha on Buck 3. A
basic summary of the vegetation of
the smaller cays is given in Heatwole
et al. (1981). Figures 4 through 7 illustrate four typical vegetational
types at transect sites: Coccoloba
grove (Buck 11, mixed palm/shrubland (Diablo 2) OpuntialCassythin
tangles (Diablo 3) and grassland
(Buck 2).
present range of the boa indicates
that it matches the occurrence of dry
subtropical forest on the Puerto Rico
Bank (Ewe1 and Whitmore 1973).
This is most apparent on St. Thomas,
where E. monensis is restricted to the
dry eastern end of the island despite
presumably suitable habitat elsewhere (Nellis et al. 1984).Common
tree species include Burseria
simaruba, Cephalocerekts royenii,
Pictetia aculeata, Bucida buceras,
Guaiacum officinale,Leucaena glnuca,
Tamarindis indica, Melicoccus bijugatus, Acacia ssp., and Capparis cynophallophora (Little and Wadsworth
1964). In addition, on our dry forest
plots ( a s , Icacos 1, Congo 1, Outer
Brass 1, and Gt. St. James I), we encountered many Byrsonima lucida,
Euphorbia petiolaris, and Metopium
toxiferum. On Buck 1, Diablo 1, Gt. St
James 3, and Mona 2 the vegetation
consisted of tree species with compound trunks, primarily Coccoloba
uvifera, Hippomane mancinella, and
Thespesia populnea. Sabal palm groves
were present on Outer Brass 2 and
Salt 2. Salt-tolerant shrublands primarily composed of Suriana and
Tournefortia just above the high tide
line was the dominant vegetation on
Diablo 2, while Diablo 3 primarily
I
Outer Brass Is
Figure 4.-Coccsiobcl uvffera habitat on
Buck 1.
Geornorphology and Topography
of Study Sites
Geomophology of the various islands
and cays studied varied considerably, from the steep-sided metamorphic topography of St. Thomas and
associated cays (Heatwole et al. 1981)
to the cemented dune structure of La
Cordillera (Kaye 1959a). Isla Mona is
composed primarily of a Pleistocene
limestone plateau surrounded by
sheer cliff (Kaye 1959b). In fact, most
islands of the bank have significant
limestone deposits, with varying
amounts of metamorphic rock, in-
Figure 5.-Mixed Cocos and scrubland
habitat on Cayo Diablo. The vegetation at
the center of the island is primarily Cassyfhia vine growing over Opuntia cactus.
2
' 7 )
Congo cay
Figure 6,- Aromatic beachfront shrubland,
primarily Suriaaa and Tourneforfia, near
Diablo 2.
Buck Is
90
Cay
U. S. Virgin Islands
Figure 3.-Location of sampling plots, U.S.Virgin Islands.
Figure 7.-Guinea grass, Panicum maximum, habitat on Buck 2.
cluding gneiss and basalt, present as
well. The cays of La Cordillera are
exceedingly low, with maximum elevations under 15 m. In the Virgin Islands the cays are of moderate elevation with eroded limestone hills approaching 50-300 m in height. An
overview of the geology of the Virgin
Islands is given in Schuchert (1935).
The climate of the Bank is essentially
subtropical to tropical. Temperatures
of the coastal areas range from overnight lows of ca. 15' C to daytime
highs approaching 35 " C. Rainfall,
especially on Puerto Rico, is geographically variable (Briscue 1366).
Areas within the range of E. monensis
typically receive < 750 mm of rainfall
per year.
Sampling Techniques
The presence or probable absence of
the boa on a particular cay was determined by active searching of all
habitat types during surveys (carried
out independently of habitat analysis) from April 1983 to September
1987. Typically 2 weeks or more were
spent searching larger islands and
three to five days for smaller cays.
Only 1 night was spent on Cayo Lobos, as the native vegetation was all
but completely destroyed by human
activity and all densely vegetated areas could be searched repeatedly in a
single night. Our experience with
multiple recaptures of the same individual indicates that the snakes forage every night under most circumstances. Within each 24-hour period
4 hours per night were spent searching likely foraging sites such as vine
tangles, terminal branches of trees,
palm crowns, and beachfront vegetation. During the daylight hours, refugia sites such as debris piles, termite
nests, and palm axils were examined.
After capture, the time, capture
height, habitat description, ambient
temperature, refugium temperature,
and cloaca1 temperature of each
snake were recorded. Later, sex,
body mass, snout-vent length (SVL),
and caudal length (CL) were recorded. The snakes were examined
for reproductive condition, presence
of injuries, and parasite infestation.
Snakes were marked using the technique of Brown and Parker (1976)
and released at the point of capture.
Habitat variables recorded included both physical and biological
parameters (table 1).Predator density estimates include indices of
abundance for likely predators of E.
monensis: the roof rat, Rattus rattus,
the pearlyeyed thrasher, Margarops
fuscatus, and the Puerto Rican racer,
Alsophis portoricensis. Rattus densities
were estimated using removal trapping over a 3-day span on 100-m
transects with Victor snap traps
spaced every 5 m. Presence of Felis
catus was determined by direct observation. Because of the extreme
wariness and trap-shy nature of the
Felis on study plots, only their presence or absence was recorded.
Prey density data includes of
population densities for Anolis cristatellus and Ameiva exsul. Anolis, Alsophis, Ameiva, and Margarops were
counted by having two observers
slowly walk the transects and counting the individuals of each species
observed within a 5 m distance on
each side of the transect line. On
Cayo Diablo, independent estimates
of Ameiva and Anolis cristatellus
populations were gathered by surveys of 5 m2 quatrats. Anolis cristatellus perch heights were measured
with a metric tape except on Cayo
L o b s and Salt Cay. Canopy height
was estimated for each habitat with
the help of a metric tape. Vegetative
composition was determined by subjective stratified sampling using 10
m2quadrat plots (Clarke 1986);plant
samples were taken for species identification from each island. Vegetation coverage data indicates the percentage composition of five different
classes of vegetation: trees (trunk circumference at shoulder height > 25
cm), palms, Qpuntia cactus, shrubs
and small trees (trunk circumferences < 25 cm), and grasses. Vegetation structural data includes the
number of dominant plant species,
the height of the canopy, and the
continuity of the vegetation (a measure of the difficulty for the boa to
crawl from one plant to another
without going to the ground). Plants
were identified by David W. Nellis
and the author.
I attempted to use continuously
distributed standarized environmental variables whenever possible.
Absence of a particular predator or
prey species on a given a sample plot
did not always indicate its absence
from the island on which the plot
was situated. Only male Anolis perch
heights were used for the statistical
analysis, as female and juvenile A.
cristatellus tend to frequent the
ground under all circumstances (Kiester et al. 1975).Mean male Anolis
perch height data were pooled for
each island for character 16 of the
PCA data matrix, as some plots were
completely devoid of Anolis.
Statistical Analysis
Principal components analysis was
performed using the Statistical
Analysis System "SAS" release 5.16
(SAS Institute 1985).Significant habitat components, which included both
biotic and structural variables of the
collecting localities (e.g. those which
accounted for > 10%of the total variance in the data), were clustered on
the basis of their association within
the PCA data matrix. The second
step of the analysis compared the
relative abundance of E. monensis at
each collecting locality with habitats
described by the significant axes of
the principal components. Regression
analysis, ANOVA, and descriptive
statistics (mean, standard deviation,
etc.) were performed using Statview
512) on an Apple Macintosh Plus.
Results
tributes of vegetation are all important contributors to variance in the
PCA patterns. Factor patterns for the
first six principal components are
given in table 1. Principal component
I accounts for 23.4% of the variance.
This component clusters habitats
with high shrub and palm densities,
low numbers of single trees, vegetational continuity, and low canopy
heights. Important biotic charasteristics of this space include Felis presence and low Ratfus, Margarops, and
Alsophis densities with high Anolis
perch heights. Principal component II
accounts for an additional 17.0%of
the variance observed. This axis describes sites having low grass density, high compound tree densities,
canopy height > 3 m ,and high
Ameiva densities. Principal component I11 accounted for '3.1.2%of the
variance and suggested an association between low Ameiva density,
low compound tree density, low
grass density, high shrub density,
and canopy height > 3 m. Factor IV
accounted for another 10.7%of the
variance and clustered high Alsophis
and Anolis densities with Felis absence and low palm density. Components V-VI were less significant in
the PCA (e.g. each accounted for < 10
% of the variance) but added some
interesting ecological information to
the habitat analysis. Principal component V clustered high Rattus density
with low Mmgarops density; principal component VI grouped high Margarops density with low Anolis density.
Habitat Utilization by Epicrates
monensis
Multivariate Analysis of Habitats
The PCA indicates that biotic factors,
plant composition, and structural at3Theuse of trade and company names
is for the benefit of the reader; such use
does not constitute an official endorsement
or approval of any service or product b y
the U. S. Department of Agricutture to the
exclusion of others that may be suitable.
The vegetational profiles of climax
plant communities (and E. monensis
collection localities) in the dry forest
may differ considerably depending
on island size, geology, geomorphology, rainfall, and history of human or
feral mammal disturbance. However,
most dry forest habitats on the Bank
are structurally simple, with usually
only two to five dominant plant species (table 2). Captures and sightings
of the Mona boa have been limited to
three distinct localities: dry plateau
forest adjacent to Uvero and Pajaros
(Campbell and Thompson 1978; Rivers et al. 1982) Coccoloba uvifera
groves of Pajaros (M. Frontera, Pers.
Cornm.), and Cwos groves and
nearby vegetation adjacent to Playa
Sardinera (C. Rodriguez pers.
comm.). The Virgin Islands boa has
been encountered repeatedly on only
two islands: St. 'Thomas and &yo
Diablo. All specimens from St. Thomas were captured on the east end sf
the island near Red Hook. Two specimens were found beneath a limestone slab during construction of the
Vessup Bay Estates housing subdivision, another was taken from a stone
wall, and a third was found as a
roadkill near Smith Bay. R. Thomas
captured a specimen crawling in a
viney tangle ca. 2.4 rn high (Sheplan
and Sshwartz 197'4).
'The Red Hook area is dominated
by xeric forest composed primarily
of Burseria, Crofon, and Acacia. No
habitat data is available for E. m.
granfi on Tortola. I have received reports that the boa was present in the
palm forest of Outer Brass Island (J.
LaFlace pers. comm.) but I was unable to find ~t there even after five
trips to the island. Virgin Islands
residents also report the boa as inhabiting Great St. James Is. (D. Neilis
pers. comm.), Great Carnanoe:
Necker Is., and Virgin Gorda, (Mayer
and Lazell 19881, but these sightings
have not been confirmed by biologists. Grant (1932) mentioned anecdotally (he did not capture the
holotype himself) that "the boa is
found on rocky cliffs on Tortola and
Guana Islands."
On Cayo Diablo, Coccoloba uvifera
is the habitat most commonly associated with foraging E. monensis. Of the
79 active snakes we captured, 51
were found in Coccoloba, ten in Caesalpinea, nine on Cassythia, seven in
Suriunn, and two in Opuntia. Twentythree percent of the snakes were ac-
tive at heights > 2 m. Of these, 67%
had SVLs > 400 mm. Seventy-five
percent of juvenile snakes (under 300
rnm SVL) foraged at heights < 1.5 m,
but regression analysis indicated that
these differences were not statistically significant. Of the 149 inactive
snakes taken from refugia, 43% were
in Cocos or Sabal a d s , 36% were in
termite nests, and 21% were under
rocks or debris. Fifty-one percent of
snakes taken from termite nests were
females; over half of these were
gravid. Gravid females use termite
nests or sun-baked debris to thermoregulate and may elevate their
body temperatures to over 33"C.
Prey Density and Epicrates
monensis Distributions
The greatest concentrations of Epicrates monensis are in areas
(particularly Coccoloba groves) with
Anolis densities > 60 Anolis/100 m2.
This Anolis/Epicrates association is
reinforced by PCA (see below). My
field logs indicate that the greatest
success in finding foraging Epicrates
occurs when observations of sleeping
Anolis are > 12 lizards/h. Numerical
counts of sleeping Anolis and the
times between sightings are regularly
noted in my field book as a rough
guide to potential hunting success in
a study locality.
Anolis cristatellus is the primary
prey species of E. monensis, and the
mean foraging height of the snake (x
= 1.356, SD = 1.079 N = 54) is close to
the mean perch height of sleeping
Anolis (x males = 1.816 m, SD = 0.993,
N = 17; x females = 1.323 m, SD =
,681, N = 14; x juveniles = 1.417 m,
SD = 0.169, N = 5).
High Ameiva densities are also a
common component of localities
with high boa densities, although I
observed only one instance of a boa
feeding on Ameiva, which are
strongly diurnal.
Feral Mammal Abundance and
Epicrates msnensis Distribu8isns
Of the 10 islands surveyed for this
study, only three were completely
devoid of rats: Cayo Diablo, Cayo
Icacos, and Steven Cay. These islands
have high Ameiva and Anolis densities, but only Diablo Cay harbors a
population of the boa. It also has the
highest densities of Epicrates monensis
found anywhere on the bank, > 100
snakes/hc at some localities. Those
islands with heavy rat densities (ca.
20 rats/hectare~-Buck Is., Cas Cay,
and Salt Cay-have lower Ameiva and
Analis densities and apparently no
boa populations, despite suitable
habitat. Rat densities are not always
correlated with low Anolis densities,
however. Some islands, such as
Outer Brass and Congo, have Anolis
densities apparently high enough to
support populations of the boa, but
their perch heights (table 2) are significantly different from those Anolis
inhabiting rat free islands. ANOVA
performed on the regression line (y =
-5.548~+ 1.127) which plots Anolis
perch height vs. rat density on my
study islands (Tolson and Campbell
in prep) shows a negative correlation
(p = .0137) between rat density and
Anolis perch height. This is not
surprising. Anolis cristatellus resident
on rat-infested islands exhibit a typical escape behavior. Male Puerto
Rican A. cristatellus escape to the canopy when threatened (Heatwole
1968),but those on Congo Cay, Outer
Brass, and Salt Cay all run to the
ground when disturbed, even when
suitable cover on the ground is lacking. At night, the Anolis are not usually found sleeping exposed on vegetation, but rather under rocks. This is
extremely unusual behavior for A.
cristatellus (E. Williams pers. comm.).
Although one does not often
discover E. monensis on islands
which are infested with rats, some
sympatry does occur. Isla Mona and
St. Thomas are islands with moderate rat densities and extant (although
apparently dwindling) populations
of E. monensis. Interestingly, at localities where Epicrates coexists with Rattus, there are also significant numbers of introduced mammalian
predators such as Felis and Herpestes
(table 2).
Discussion
PCA and E. monensis Habitat
Utilization
The Puerto Rico Bank encompasses a
total land area in excess of 9,300 km2,
of which 1700+ km2 (or 17.6%)is covered with subtropical dry forest
(Ewe1 and Whitmore, 1973). This
xeric forest is widely distributed
throughout the range of Epicrates
monensis, yet the boa, as far as we
know, occupies only seven islands of
the 243 that make up the banks-effectively exploiting only 0.04% of the
land area available to it. PCA helped
to identify those factors which seem
to define critical boa habitat. Several
vegetative parameters which cluster
together in the PCA are descriptive
of habitat where I or others have
encountered E. monensis repeatedly.
These include areas with high shrub
and palm densities coupled with a
low canopy and vegetational continuity. These values describe plot
habitat on Diablo 2, Icacos 2, and certain sites within the Red Hook area
of St. Thomas. Either high shrub or
high palm densities coupled with
vegetational continuity and lower
canopy are found on Diablo 3, Icacos
3, and Mona 1. Of these two subsets
of PC I, boas occur on Diablo 2 and 3,
Mona 1, St. Thomas, and almost certainly inhabited Icacos 1 and 3 at one
time.
In PC 11, habitat correlates include
high compound tree density, high
canopy height, vegetational continuity, and low grass density. This is a
perfect structural and compositional
description of Diablo 1, which has
the highest population of E. monensis
I have ever encountered, and Mona
2-another locality where E. monensis
has been observed (Campbell and
Thompson 1978).It seems clear from
these data that the unifying variable
which causes an intersection of these
two differinghabitat types is vegetational continuity-an interlockingof
the branches of shrubs or the tree
canopy. I believe this vegetational
characteristic is essential to E. monensis foraging success and survival. It
probably not only decreases the
search time between encounters with
sleeping Anolis while foraging, but it
also potentially limits the encounters
between the boa and Felis and Herpestes. Fortunately, at least some
tracts of subtropical dry forest and
Coccoloba have remained relatively
undisturbed on the Virgin Islands,
Isla Mona, and Puerto Rico and its
offshore satellites. Much suitable
habitat does exist-even near popula ted areas.
While habitats throughout the
Bank are presumably underutilized
by E. monensis, and suitable areas for
reintroduction apparently exist in a
number of localities, the extant boa
populations are so fragmented and
reduced in numbers that it is crucial
to protect those areas now supporting the boa. This may be difficult.
Historically, vegetation on Puerto
Rico and the Virgin Islands has been
severely disrupted, and 17th-18th
century land use patterns on the U.S.
Virgin Islands may partially explain
the limited distribution of the boa on
the east end of St. Thomas and its
absence from St. John. Even now
enormous pressures exist for continued development on the east end of
St. Thomas. Construction around
Red Hook seems to have accelerated
in recent months, perhaps in response to the decline of interest rates
in the United States, and three relatively undeveloped areas on the east
end-Red Hook Mountain, Cabrita
Point, and Water Point-all have projects in progress that do not involve
federal funding. The management
authority on St. Thomas, U.S. Virgin
Islands-the Division of Fish and
Wildlifehas no control over such
development.
In contrast, Puerto Rican islands
with populations of Epicrates monensis are in no imminent danger of development. Cayo Diablo is part of the
Reserva Forestal de La Cordillera,
and Isla Mona is likewise a Forest
Preserve (although it was once proposed to develop the island as a
deep-water oil port). A problem does
exist, however, with habitat destruction on isolated cays caused by
campers and fishermen (Heatwole
and Mackenzie 1967). Coccoloba trees
in the larger groves-areas where the
greatest densities of E. monensis are
found-are often used as firewood by
visitors. A survey done in 1987 of
damage to Coccoloba stands on Cayo
Diablo showed that many trees sustained some sort of damage caused
by human activity, primarily machete cuts and burns from fires
started at the bases of the trees.
Effects of Feral Mammals
My analysis shows that Rattus and
Felis are a primary influence on community composition on the Puerto
Rico Bank. Felis presence is associated with low Alsophis, Margarops,
and Rattus density (table 1: PC I); Felis absence is associated high Anolis
and Alsophis densities in PC IV (table
1). Clearly the presence of Fdis in E.
monensis habitat is a mixed blessing.
Cats present a great danger to Epicrates because they hunt at night.
Several instances of cat predation of
Epicrates have been reported on St.
Thomas (D. Nellis pers. comm.) In
fact, in April and May of 1988 two E.
monensis were rescued from cats on
St. Thomas and were incorporated
into the captive breeding program at
the Toledo Zoological Gardens. In
contrast, however, on islands where
boas and rats coexist-Isla Mona and
St. Thomas-there are also significant
populations of Felis. Cats feed on
Rattus and may keep rat populations
at levels low enough to permit survival of the boa. Their apparent adverse affect on Alsophis and Margarops density-two potential predators
of E. monensis-may also be of some
small benefit in certain circumstances. Weiwandt's (1977) observation of cat predation of Alsophis on
Isla Mona corroborate the PC I linkage of cat presence with low Alsophis
density.
I cannot be certain whether Rattus
affectboa populations by acting primarily as a constraint on their resource levels or by direct predation.
Although I have been unable to demonstrate that rats forage on boas, I
have every reason to suspect that
they do. Rattus is known to prey on
lizards (Whitaker 1978).While surveying for boa populations on the
Bank I found habitat (Congo Cay,
Outer Brass Cay) which provides optimal foraging opportunities for the
boa (e.g. vegetation associated with
population densities of > 60 Anolis/
100 m2on rat-free islands) but had no
or few boas and were virtually over-
run with rats at night. Rats may also
affect boa populations by preying on
Anolis directly or by influencing their
perching behavior, (indicated by the
negative correlation between rat density and Anolis perch height (table 1:
PC I) or selection of sleeping sites. If
lizards rarely rest in the canopy at
night but rather seek refuge sites on
the ground, there would be potentially disastrous consequences for
boa foraging success. Rattus also apparently affect Margarops density
(table 1: PC V).
There can be little doubt that the
Indian mongoose, Herpestes auropunctatus, threatens Epicrates mmonensis directly as well, but I believe the risk to
Epicrates is sometimes exaggerated.
Herpestes predation on endemic West
Indian snakes is well documented
(Maclean 1982), but the mongoose is
a stictly diurnal, terrestrial predator;
Epicrates monensis is nocturnal and
arboreal. Herpestes poses the greatest
danger to the diurnal West Indian
racers, genus Alsophis, and are directly responsible for the extinction
of Alsophis sancticrucis on St Croix
and the extirpation of A. portoricensis
from St. Thomas and St. John. In contrast, I have found Epicrates monensis
abroad during the daylight hours on
only two occasions over a period of
several years. It seems that Herpestes
would have the greatest chance of
capturing Epicrates when the latter is
resting in some moderately accessible location during the day-in
loose sections of termite nests, for
example. Feral pigs G u s scrofa) may
also threaten the Mona boa to some
degree, either by eating them or by
destroying vegetation, such as terrestrial bromeliads, that may act as
snake refugia. I have no data on the
magnitude of this threat.
Natural Predators
The Puerto Rico Bank has no extant
species of native mammalian predators, but two nocturnal avian predatory species may pose a limited
threat to Epicrates monensis. The yellow-crowned night heron, Nyctanassa
violacea, and the Puerto Rican screech
owl, Otus nudipes, are two potential
predators of the boa. While populations of Otus are declining on the
bank (IUCN 1981) those of the heron
seem quite stable. I have repeatedly
observed herons foraging at night in
boa habitat on both Isla Mona and
Cayo Diablo. Examination of the debris beneath heron rookeries on Cayo
Diablo has revealed numerous fragments of Anolis and Ameiva skin and
skeletal materials, usually ribs, vertebrae, and jaw elements. No snake
remains have been found, but my coworkers and I are continuing to investigate this potential problem. I
also found that Anolis densities and
perch heights are reduced (table 2)
on plots with high pearly-eyed
thrasher densities. In PC I (table 1)
high Anolis perch heights are associated with low thrasher density.
These birds also prey on Anolis, and
are so common in some areas they
could easily depress Anolis population numbers. Principal component
VI (table 1) couples high thrasher
density with low Anolis density.
Two arthropods are potential
predators of E. monensis: the land
crab Gecarcinus and the hermit crab
Caenobita clypeatus. Searches of terrestrial refugia for Epicrates have revealed that these snakes are nearly
always absent from areas occupied
by Gecarcinus and Caen~bita.This is
especially true in termite nests.
Snakes only occupy areas of the nest
that are inaccessible to crabs. If
weathering or disturbance causes a
section of termite nest to become
habitable for crabs it is abandoned by
Epicrates, despite their prior use of
the refugium for several past field
seasons. In hundreds of examinations
of refugia over the past five field seasons, I found Epicrates in association
with Caenobita on only one occasion: I
found a gravid female thermoregula ting under a discarded tarpaulin in
the midst of several Caenobita on 7
September 1987. Evidence for preda-
tion by the aforementioned species is
strictly circumstantial, but the fact
remains that over 17%of the Epicrates captured have obvious
wounds, scars, or partially amputated tails. This is strong evidence
that some form of natural predation
is occurring.
Climatic/Stochastic Events
The apparent extirpation of the snake
from the majority of the islands on
the Bank relate not only to the arrival
of European man on the Bank and
the habitat destruction which followed, but also to climatic, eustatic,
and stochastic events, many of which
had profound influences on habitat.
During the late Pleistocene several
climatic and eustatic events occurred
that apparently set the stage for the
decline of E. monensis on the Bank.
Foremost among these was a dramatic change in the climate of Puerto
Rico. From a relatively xeric climate,
Puerto Rico became progressively
more mesic during the late Pleistocene. Today, over 81% of Puerto
Rico's vegetation is classified as
moist or wet forest (Ewe1 and Whitmore 1973). Pregill(1981) and Pregill
and Olson (1982) describe the effect
this climatic change had on the xericadapted Puerto Rican herpetofauna.
This extreme climatic shift may have
resulted in the extirpation of E.
monensis on Puerto R i ~ oIn
. ~addition, sea levels rose nearly 100 m
about 8,00040,000 years ago and
separated the Virgin Islands from
one another and from Puerto Rico,
transforming what was a contiguous
land mass into a scattered series of
islets and cays spread over nearly
400 km. Many of these cays now
have extremely low elevations (Heatwole and Mackenzie 1967).
41t is unclear why E. monensis is absent
from the dry forest in southwestern Puerto
Rico. Habitat in the Guanica forest seems
quite suitable for the boa: perhaps brtf-~er
survey work will result in its discovery there.
The fragmentation of E. monensis
into several small demes may have
left several populations without the
genetic resources to survive changing
environments, and doubtless allowed
stochastic processes such as disease,
prey fluctuations, or storms to extirpate many isolated populations. I assume that the influences of random
events on the present distribution of
the native herpetofauna complicates
the multivariate analysis by introducing more variance into the correlation matrix. These factors may explain the absence of snakes from islets with suitable habitat, as some of
these islands may have inadequate
food resources or lower probabilities
of recolonization.
Management Recommendations
The forces threatening Epicrates
monensis are complex. Solutions for
the recovery of the boa will not be
simple, but I am optimistic about the
chances of success. My management
recommendations are summarized
below.
Saving Boa Habitat
This may be impossible on St. Thomas, but with luck the boa may coexist with man (as it now does) at some
relatively developed localities. Continued protection of Isla Mona and
La Cordillera are absolutely necessary.
Continued protection and management should be extended to those
cays now protected by the Division
of Fish and Wildlife, U.S. Virgin Islands-particularly Congo Cay, Outer
Brass Cay, Salt Cay, Savana Island
and Steven Cay-as these sites might
eventually be utilized for reintroduction programs. The smaller islands
should be off limits to casual visitors
to prevent habitat damage and human persecution of the snakes.
Predator Eradication on Suitable
Offshore Islets
Rat control programs should be initiated immediately on those islands
with habitat suitable for E. monensis.
Preliminary studies of rat eradication
using anticoagulant poisons on some
small cays near St. Thomas have produced promising results (Division of
Fish and Wildlife, USVI 1983).It is
critical, however, that time and funding be committed for follow up studies on any islands made the subject
for a rat control program. This must
be done to ensure that immunity to
poisons has not evolved or that
populations are being replenished by
recolonization from St. Thomas.
It is unlikely that Felis or Herpeste
will ever be eradicated from larger
islands such as Isla Mona or St. Th
mas, but Felis control programs nr
in force on Mona should be conti~
ued to further reduce populations
and should be expanded to inch ' - I
Cayo Icacos. It is important to c o ~
vince management authorities tha
feral mammal control measures on
the Bank must be increased, and
quickly.
It is a credit to the evolutionary re
silience of this little snake that it has
survived at all. Few endangered species have been exposed to such a
wide range of adverse effects and
have still survived. It is my fervent
hope that this, and other endemic
species of the Caribbean, will not be
exterminated in the wake of the living human debris, such as Rattus rattus, that we have allowed to pollute
the islands of the West Indies.
Captive Breeding for
Reintroduction Purposes
Captive propagation can figure significantly in the recovery of this
snake (USFWS 1986)The current cooperative breeding plan for E. monensis should be expanded to more
American Associa tion of Zoological
Parks and Aquarium member institu-
tions, and Species Survival Plan designation should be sought for the
snake immediately to facilitate genetic management of the captive
population.
For the present, until genetic
analysis has been completed, the
strategy of deme integrity maintenance should be continued, with St.
Thomas founders and La Cordillera
founders managed as separate populations. Continuous outcrossing
within demes facilitated by a random
pair mating scheme should be encouraged. Fortunately, the first captive breeding has already taken
place, the proximate factors critical
to reproduction have been identified
(Tolson and Tuebner 1987), and there
is no reason why the captive population cannot be expanded quickly for
reintroduction attempts within five
years.
I firmly believe that we are finally
at the point where we can look forward to augmenting boa populations, rather than helplessly watch
them decline.
Acknowledgments
This research was conducted as part
of USFWS recovery activities under
the support of the Institute of Museum %rvices conservation program
(Grant IC-70095-87) and the Toledo
Zoological Society. I am extremely
grateful to Dr. David W. Nellis, Division of Fish and Wildlife, U.S Virgin
Islands, Drs. Eduardo R. Cardona
and Jose A. Vivaldi, Departments de
Recursos Naturals, Commonwealth
of Puerto Rico, and to Hilda DiazSoltero and Robert Pace of the
USFWS Caribbean Field Office for
their counsel and logistical support
during the execution of this project.
I thank Earl W. Campbell 111, Jorge
L. Pinero, and Carlos Diez for their
assistance in the field, which was often given under difficult conditions.
C. Ray Chandler and Earl W.
Campbell I11 aided in the statistical
analysis.
Literature Cited
graphy of the Puerto Rican Bank.
Barbour, Thomas 1917. Notes on the
herpetology of the Virgin Islands.
Proceedings of the Biological Society of Washington 30:97-104.
Barbour, Thomas 1930. Some faunistic changes in the Lesser Antilles.
Proceedings of the New England
Zoological Club 11:73-$5.
Briscoe, C.B. 1966. Weather in the
Luquillo Mountains of Puerto
Rico. U.S.D.A. Forest Service Research Paper ITF-3,250 p. Institute
of Tropical Forestry, Rio Piedras,
Puerto Rics.
Brown, William S. and William S.
Parker 1976. A ventral scale clipping system for permanently
marking snakes (Repiilia, Seventes). Journal of Herpetology
lO(3)247-249.
Campbell, Howard W. and Frederick
G. Thompson 1978. Observations
on a captive Mona Island boa, Epicrates monensis monensis Zenneck.
Bulletin of the Maryland Herpetological Society 14(2)98-99.
Clarke, Robert 1986. The handbook
of ecological monitoring. 298 p.
Clarendon Press, Oxford.
Cody, Martin L. 1974. Competition
and the structure of bird communities. Monographs in Population
Biology 7. Princeton University
Press, Princeton, New Jersey.
Division of Fish and Wildlife, U.S.
Virgin Islands 1983. Rat control on
small tropical cays. 26 p. Unpublished report.
Ewel, Jacob J. and John .L. Whitmore
1973. The ecological life zones of
Puerto Rico and the U.S. Virgin
Islands. U.S.D.A. Forest Service
Res. Paper ITF18.71 p. Institute
of Tropical Forestry, Rio Piedras,
Puerto Rico.
Grant, Chapman 1932. Herpetology
of Tortola; notes on Anegada and
Virgin Gorda, British Virgin Islands. Journal of the Department
Agriculture of Puerto Rico 16:327329.
Heatwole, Harold, Levins, Robert,
and Michael D. Byer 1981. Biogeo-
Atoll Research Bulletin No. 251:l5%.
Heatwole, Harold. and Frank Mackenzie 1967. Heqxtsgeography of
Pale~ge~graphy,
Puerto Rita 3'faunal sirniliaritgr, and endemism,
Evolution 21A29-438.
Kaye, Clifford A. 1959a. Shoreline
features and quaternary shoreline
changes in Puerto Rics. US.Geological Survey Professional Paper
327-B:49-240.
Kaye, Clifford A. 1955%. Geology of
Isla Mona, Fuerto Rico, and notes
on the age of the Mona Passage.
U.S. Geological Survey Prafessianal Paper 3I?-E:'H4'f -198.
Kiester, A. Ross, George C. Gorman,
and David Colon Arroyo 1975.
Habitat sePection behavior of three
species of Anolis lizards. Ecology
56:220-225.
Little, Elbert E. Jr. and Frank H.
Wadswoath 11964. Common trees
of Fuerto Rico and the Virgin islands. U.S. 1[4epar6mentof Agriculture, Agriculture Handbook 249.
548 p. Washington, D.C.
MacLean, William I?. 1982. Reptiles
and amphibians of the virgin Islands. 54 g. roPnacmillan Caribbean,
London.
Mayer, Gregory C. and James D,
Lazell 1988. Distributional records
for reptiles and amphibians from
the Puerto Rico Rank. Herpetological Review 1961):23-24.
Matthews, William J. 1985. Distribution of midwestem fishes on
mu1tivariake environmental gradients, with emphasis on Notropis
lu trensis. American Midland
Naturalist 113(2):225-237.
Moulton, Michael P. 1985. Morphological similarity and coexistence
of congeners: an experimental test
with introduced Hawaiian birds.
Oikos 44:301-305.
Nellis, David W., Norton, Robert L.,
and William P. MacLean 1984. On
the biogeography of the Virgin Islands boa, Epicrafesmonensis
granti. Journal of Herpetology,
17(4):413-417.
Pregill, Gregory K. 1981. Pleistocene
herpetofaunas from Puerto Rico.
72 p. Miscellaneous Publications
Museum of Zoology, University of
Michigan, No. 71.
Pregill, Gregory K. and Storrs L.
Olson 1981. Zoogeography of
West Indian land vertebrates in
relation to Pleistocene climaticcycles. Annual Review of
Ecology and Systematics 1275-98.
Rivero, Juan A., Rafael Joglar, and
Idabella Vazquez. 1982. Cinco
nuevos ejemplares del culebron de
La Mona Epicrates m. monensis
(0phidia:Boidae). Caribbean Journal of Science 17(1-07-13.
Rotenberry, John T. and John A.
Wiens 1980. Habitat structure,
patchiness, and avian communities in North American shrubsteppe vegetation: a multivariate
analysis. Ecology 61:1228-1250.
§AS Institute 1985. SAS users guide:
statistics Version 5.957 p. SAS Institute Inc. Cary, North Carolina.
Schmidt, Karl P. 1926. The amphibians and reptiles of Mona Island,
West Indies. Field Museum of
Natural History Publications
12(12):149-163.
Sheplan, Bruce R. and Albert
Schwartz. 1974. Hispaniolan boas
of the genus Epicrates and their
Antillean relationships. Annals of
the Carnegie Museum 45(5):57143.
Stull, Olive G. 1933. Two new subspecies of the family boidae. 4 p.
Occasional Papers of the Museum
of Zoology, University of Michigan, Number 267.
Tolson, Peter J. 1987. Phylogenetics
of the boid snake genus Epicrates
and Caribbean vicariance theory.
68 p. Occasional Papers of the Museum of Zoology, University of
Michigan, Number 115.
Tolson, Peter J. and Victoria A.
Tuebner 1987. The role of social
manipulation and environmental
cycling in propagation of the boid
genus Epicrates: Lessons from the
field and laboratory. American
Association of Zoological Parks
and Aquariums Regional Conference Proceedings 1987:606-613.
U.S. Fish and Wildlife Service 1980.
Status of Virgin Islands boa clarified. Endangered Species Technical Bulletin, Department of Interior, U.S. Fish and Wildlife Service, Endangered Species Program,
Washington, D.C. 5:12.
U.S. Fish and Wildlife Service 1984.
Mona boa recovery plan. 14 p.
U.S. Fish and Wildlife Service, Atlanta, Ga.
U.S. Fish and Wildlife Service 1986.
Virgin Island tree boa recovery
plan. 23 p. U.S. Fish and Wildlife
Service, Atlanta, Ga.
Weiwandt, Thomas A. 1977. Behavior, ecology, and management of
the Mona ground iguana, Cyclura
stegnegen. Ph. D. dissertation, Cornell University, Ithaca, New York.
Whitaker, A. H. 1973. Lizard populations on islands with and without
Polynesian rats, Rattus exulans
(Peale). Proceedings of the New
Zealand Ecological Society 20:121130.
Wiens, John A. and John T. Rotenberry 1981. Habitat associations
and community structure of birds
in shrubsteppe environments. Ecological Monographs 5(l)21-41.
Appendix A.
PCA Variables Measured on Island Study Plots.
Variable
Predator
Ratfus density
Felis presence
Alsophis density
Margarops density
Prey
Anolis density
Ameiva density
Anolis perch height
Coverage
Percent cover C trees
Percent cover S trees
Percent cover palms
Percent cover Opuntia
Percent cover grasses
Structural
Vegetational continuity
Canopy height
Plant diversity
Description
Rats captured/trap hour
Present = 1, absent = 0
Mean no. Alsophis observed/day on transect
Mean no. Margarops observed/day on transect
Mean no. Anolis/5 m of transect
Mean no. Ameiva/5 m of transect
Mean perch height in m of male Anolis
No.compound trees/no. woody plants
No. single trees/no. woody plants
No. palms/no. woody plants
No. Opuntia/no. woody plants
Grassland area/total area
Contiguous = 1, high = .75, Moderate = .5
low = .25, absent = 0
>3 m = 1,l-2 m = .5, <1 m = 0
No. of dominant plant species on plot