Oryx Vol 38 No 3 July 2004
© 2004 FFI, Oryx, 38(3), 347–350 DOI: 10.1017/S0030605304000602 Printed in the United Kingdom
Ella Vázquez-Domínguez (Corresponding author), Gerardo Ceballos and
Juan Cruzado Instituto de Ecología, UNAM, CU, Apdo. Postal 70-275,
México D.F. 04510, México. E-mail evazquez@ecologia.unam.mx
Received 14 October 2003. Revision requested 19 January 2004.
Accepted 25 March 2004.
347
Short Communication
Extirpation of an insular subspecies by a single introduced cat:
the case of the endemic deer mouse Peromyscus guardia on Estanque
Island, Mexico
Ella Vázquez-Domínguez, Gerardo Ceballos and Juan Cruzado
Abstract The Angel de la Guarda deer mouse Per-
omyscus guardia on Estanque Island, in the Angel de la
Guarda archipelago of the Gulf of California, was prob-
ably driven to extinction by a single introduced domestic
cat. P. guardia was trapped on the island in October 1995,
at which time the species was still relatively abundant.
In 1998 a domestic cat was spotted on the island; no deer
mice were found at that time nor during subsequent
field work in 1999 and 2001. In 1998, c. 100 cat scats were
collected, 2% of which contained P. guardia bone remains
and 90% P. guardia hair. The cat, which was eradicated
in 1999, was the only introduced predator on the
island. Our results confirm the extreme vulnerability of
island rodent populations to the introduction of alien
mammalian predators. To our knowledge, apart from
the extermination of Stephens Island wren Xenicus
lyalli in New Zealand in 1894 by the lighthouse keeper’s
cat, this is the first report of the destruction of the total
population of an insular species by a single cat. With
two of the three subspecies of P. guardia now extinct the
only potentially extant population is on the larger Angel
de la Guarda Island, where the species was last seen in
1991. A comprehensive survey of the island is required,
with subsequent action for the species recovery and
conservation if it is found to be extant.
Keywords Alien species, Angel de la Guarda deer
mouse, Estanque Island, extinctions, island mammals,
Mexico, Peromyscus guardia.
Islands have the highest extinction rates of vertebrates,
largely as a result of small population sizes, habitat
perturbations and introductions of alien species (Alcover
et al., 1998; MacPhee, 1999). Alien (exotic or non-native)
species have severe negative impacts on native island
species, mainly through the effects of predation, com-
petition, habitat alteration and transmission of diseases
(Dowding & Murphy, 2001; Case et al., 2002). As an
extreme example, the introduction of the brown tree
snake Boiga irregularis to the island of Guam caused a
cascade of vertebrate extinctions (Fritts & Rodda, 1998).
Feral cats Felis silvestris catus have frequently been impli-
cated in the extinction of vertebrates on islands, such as
conilurine rodents in Australia (Smith & Quin, 1996) and
birds and rodents in the Gulf of California (Keitt et al.,
2002; Mellink et al., 2002; Álvarez-Castañeda & Ortega-
Rubio, 2003). In some studies the examination of faecal
pellets has proved that introduced cats eat small rodents
(Smith et al., 1993; Nogales & Medina, 1996).
The islands of the Gulf of California, Mexico, harbour
a diverse and unique biota with a high percentage of
endemic species (Lawlor, 1983; Case et al., 2002) and have
been classified as a priority area for the conservation of
biodiversity in Mexico (Vázquez-Domínguez et al., 1998).
The small Angel de la Guarda deer mouse Peromyscus
guardia (Townsend, 1912) is endemic to the Angel de la
Guarda archipelago in the islands of the central region of
the Gulf of California (Hall, 1981). Different subspecies
have been reported from Angel de la Guarda Island
(P. guardia guardia) and the northern islands of Mejía
(P. g. mejiae) and Granito (P. g. harbisoni) (Lawlor, 1971).
In the 1960s P. guardia was considered abundant but it
has become rare and is currently listed as Endangered
by the Mexican Government (Semarnat, 2002). Mellink
et al. (2002) concluded that P. g. harbisoni is extinct, P. g.
mejiae probably extinct, and P. g. guardia facing severe
conservation threats.
In October 1995 Jesús Ramírez found a previously
unrecorded population of P. guardia on Estanque Island,
a tiny island (0.55 km2) with scarce desert scrub vegeta-
tion in the south of the Angel de la Guarda archipelago
(Fig. 1). He used Sherman live traps for a total of 120
trap-nights and concluded that P. guardia was the only
rodent species on the island; although we do not have
the trapping success data, Ramírez indicated that the

348 E. Vázquez-Domínguez et al.
© 2004 FFI, Oryx, 38(3), 347–350
mouse was ‘relatively abundant and easily trapped’
(pers. comm.). However, later surveys indicated that
P. guardia was extinct (Mellink et al., 2002). From the
animals trapped in 1995, two females and two males
were subsequently kept in captivity until they died
in 1998, after which they were deposited as voucher
specimens in the Mammal Collection at the Instituto de
Ecología, UNAM. Additional fieldwork was carried
out on the island in November 1998 (29 trap nights),
November 1999 (40) and April 2001 (40; Mellink et al.,
2002), but no individuals were trapped. Given the
island’s small size and scarce vegetation, the trapping
effort over a 4-year period is adequate and comparable
with other studies on the California islands (Mellink
1992; Case et al., 2002; Mellink et al., 2002). During the
1998 field trip we collected c. 100 cat scats and saw one
cat. We analysed the contents of the scats to determine
the cat’s prey, including c. 25 scats analysed by Mellink
et al. (2002) and c. 75 that had been stored. In 1999
eradication specialists removed the cat, which was a
female, and confirmed there were no other cats on the
island (B. Tershy, pers. comm.). No other cat was seen or
cat scats found during the subsequent field trip in 2001.
The fact that no individuals could be captured in the
visits of 1998, 1999 or 2001 indicates that the P. guardia
population on Estanque Island is now extinct, and all
available evidence suggests that the single cat, acciden-
tally introduced to the island, was probably responsible.
Our analysis of the cat’s scats revealed that 2% contained
P. guardia bone remains and 93% P. guardia hair, and that
the cat also consumed endemic Baja California fishing
bats Myotis vivesi, birds and lizards (Table 1, Plate 1).
There are several general conservation lessons from
Estanque Island. Firstly, P. guardia is another addition to
the long list of island species that have become extinct or
are Critically Endangered (Smith & Quin, 1996; Alcover
et al., 1998; MacPhee, 1999). At a global scale 83% of all
mammal extinctions are of island species, such as the
Coronados Island packrat Neotoma bunkeri in Mexico,
the Falkland Islands fox Dusicyon australis, and the Sar-
dinian pika Prolagus sardus (Smith et al., 1993; MacPhee,
1999). Considering the mammals exclusively inhabiting
islands, at least 27% of species on the world’s islands
have gone extinct (Alcover et al., 1998). Trends in Mexico
are similar, with 40% of all extinct mammals being
island species (Ceballos & Navarro, 1991). Secondly, this
example further demonstrates the impacts that alien
species have on island animals. Other threatened verte-
brates on the Gulf of California islands include Bryant’s
woodrat Neotoma bryanti, San Lorenzo mouse Peromyscus
interparietalis, black chuckwalla Sauromalus hispidus,
Catalina rattlesnake Crotalus catalinensis, and elegant tern
Sterna elegans (Mellink, 1992; McChesney & Tershy, 1998;
Case et al., 2002). Thirdly, this is one of the most extreme
examples of the susceptibility of island rodent popula-
tions to introduced mammals. To our knowledge there
are no other examples in which the total population of an
Table 1 Material found in the c. 100 cat scats collected on Estanque island during 1998, including species identification (where possible),
percentage of the total scat weight, and description of the corresponding material.
Species or group % of scat weight Material
Angel de la Guarda deer mouse 1.87 16 right inferior mandibles, 13 left inferior,
Peromyscus guardia 24 superior, 115 molar teeth (pieces)
Baja California fishing bat 2.50 16 right inferior mandibles, 7 left inferior, 3
Myotis vivesi complete, 26 superior maxillary teeth
Mammal 93.12 hair (with some pieces of stone)
Bird 2.5 feathers and bones
Reptile insignificant 2 mandibles
Fig. 1 Angel de la Guarda Archipelago, in Baja California, Mexico,
with the location of Estanque Island.