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MICROHABITAT SPATIAL UTILIZATION BY THE SOCORRO ISLAND LIZARD
UROSAURUS AURICULATUS (COPE)
ALFREDO ORTEGA-RUBIO,SERGIO ALVAREZ-CÁRDENAS,
PATRICIA GALINA-TESSARO and GUSTAVO ARNAUD-FRANCO
Centro de Investigaciones Biologicas
de Baja California Sur, A.C.
Apdo. Postal 128
La Paz 23000
BCS, Mexico
ABSTRACT
Natural vegetation of Socorro Island, in the Revillagigedo Archipelago in Mexico, has been damaged to a
significant extent by overgrazing by introduced sheep. For this reason and also because of predation by feral
cats, also introduced by humans, the only reptile of the island, the lizard Urosaurus auriculatus, is threatened.
In this work we analyze the use of the habitat and substrata by the lizard. We have found that a greater diversity of microhabitats are used in the grassland than in the Boumelia-Guetarda forest, two of the main floristic
associations of the island inhabited by the lizard. Also, we have found that this lizard shows preference for the
use of the medium-sized rocks in the grassland and for medium-sized trees in the forest. U . auriculatus can use
different kinds of microhabitats and thus be successful in severa1 habitats, probably because of the absence of
interspecific competition. However, in spite of its ecological plasticity, this species is senously threatened by species
introduced to the island. Key Words: Lizard, Mexico, Revillagigedo Archipelago; Socorro Island, Urosaurus
aruiculatus.
INTRODUCTION
MATERIALS AND METHODS
Urosaurus auriculatus is the only native terrestrial
reptile on one of the most remote places of Mexico,
Socorro Island. This endemic lizard reaches, at present,
high densities in the natural ecosystems of the island.
This species is now threatened by two exotic species
introduced by the man to this oceanic island: cats and
sheep.
To the present, approximately one-third of Socorro
Island has been deprived of its natural vegetation by
the sheep, which lack any natural enemy. The places
where the vegetation has been overgrazed have also
lost their soil because violent tropical storms cross this
island each year. In the places where natural vegetation remains, Urosaurus auriculatus must cope with
predation by the feral cats.
Because most works concerning this endemic and
threatened lizard have focused mainly on taxonomic
aspects, while others only secondarily consider
ecological aspects of this species (Brattstrom 1955,1982,
1990, Cope 1871, Mittleman 1942, Slevin 1926), the
main objective of this work is to provide additional
ecological information regarding this lizard.
STUDY SITE. Socorro Island is part of the Revillagigedo Archipelago which also includes San Benedicto,
Clarion and Roca Partida islands. The archipelago is
biologically very important because of the high incidence of endemic species that occur on it: 33% of the
plants and 90% of the terrestrial avian fauna are unique
at the leve1 of genus, species or subspecies. The archipelago is located between the parallels 18" 20' and 19"
20' of North latitude and between the meridians 110"
45' and 114" 50' of West longitude. Socorro is the biggest of the four islands, with 140 km2 of surface area
and with a maximum elevation of a 1,040 m. The
vegetation of the island varies from coastal halophytes
to severa1 kinds of shrubs and severa1 kinds of forest
along an elevation gradient. However, because of
overgrazing by introduced sheep, approximately 30%
of the natural vegetation and soil of the island has been
destroyed and eroded (Figure 1).
METHODS. We spent at least five days during each
of October and December 1988 and February 1990,
searching for the lizard in two floristic-physiognomical
Ortega-Rubio, Alfredo, Sergio Alvarez-Cardenas, Patricia Galina-Tessaro and Gustavo Arnaud-Franco. 1991.
Microhabitat spatial utilization by the Socorro Island Lizard Urosauris auriculatus (Cope). Journal of the
Arizona-Nevada Academy of Science 24-25:55-57.
JOURNAL OF THE ARIZONA-NEVADA ACADEMY OF SCIENCE
VOL. 24-25
RESULTS. A total of 506 individuals of Urosaurus
auriculatus were recorded during the three periods of
work. The microhabitats used by these individuals are
shown in Table 1. Also, in Table 1 it is possible to
observe the amounts of microhabitats available in each
ecosystem. The species is more of a generalist in the
use of substrate in the grassland than in the forest.
Also, there exist differences among the kinds of microhabitats used in each ecosystem. Such differences are
statistically highly significant (X2 = 4,136.74; df = 7;
p 0.001).
TABLE 1. Frequency of availability and utilization
of recognized microhabitats by the endemic Socorro
Island lizard in the two studied kinds of vegetation.
Habitat
Microhabitat
SOCORRO ISLANO
FIGURE 1.Location of the Socorro island and present
extent of vegetation types and areas. E = Eroded area;
G = Grassland; F = Ficus forest; C = Croton scrub;
M = Mixed scrub; B = Boumelia-Guetarda forest and
P = Prairie.
associations of the island where it occurs: the BoumeliaGuetarda forest and the grassland.
For each lizard observed, the following data were
recorded: their sex, their size (Tuvenile or Adult; determined by approximate sizes), the habitat, the specific
substrate where it was first sighted, and the hour.
Eight types of microhabitat were recognized: coi1 with
grass, thin trees (<30 cm in trunk diameter), medium
trees (>30 cm), thin logs (<30 cm in trunk in diameter), medium logs (>30 cm), small rocks (<30 cm
in diameter), medium-sized rocks (>30 and <lo0 cm)
and large rocks (>lo0 cm).
Microhabitat niche breadth was calculated using
the diversity measure of Simpson (Levins 1968): B =
( Z Pi 2 ) -1, where pi is the proportion of individuals
found in the i th substrate or habitat. A standardized
measure (Pianka, 1973), B,, was also calculated to
range between O (only one sort of microhabitat used)
and 1 (equal dispersion over al1 the microhabitats):
where N is the number of microhabitats considered.
Soil with grass
Thin trees, < 30cm
Medium trees, >30cm
Thin logs, <30cm
Medium logs, >30cm
Small rocks, <30cm
Medium rocks,
>30cm <l m
Large rocks, > l m
Grassland
Boumelia-Guetarda
FOREST
Avail- Utiliza- Avail- Utilizaability
tion
abillty
tion
0.458
0.038
0.082
0.063
0.179
0.054
0.148
0.013
0.067
0.013
0.181
0.029
0.282
0.120
0.182
0.084
0.128
0.048
0.058
0.005
0.538
0.038
0.347
O
0.062
0.064
0.376
0.173
0.082
0.074
0.014
O
Thus, in the grassland U. auriculatus individuals
used soil with grass, large and medium-sized rocks in
greater frequency than in the Boumelia-Guetarda forest,
showing a preference for the use of medium-sized trees
and medium-sized rocks. On the other hand, these
lizards generally showed a preference for the use of
medium-sized trees and medium-sized logs in the
forest.
DISCUSSION. Urosaurus auriculatus individuals are
able to use different kinds of substrata depending not
only on the availability of various classes of microhabitats, but probably due to the intrinsic plasticity of
the individuals of this species.
Lizards strongly depend upon close substrate
adaptation to avoid predators, to thermoregulate efficiently, and to be successful in mating and defending
territory (Barbault et al. 1985, Ortegaet al. 1982). Thus,
unless recent events have altered evolved relationships, the substrate selection that one particular species
shows must be a response evolved to cope with a com-
MICROHABITAT SPATIAL UTILIZATION
plex combination of various selective pressures and
with morphological and physiological constraints of
the individuals.
Even more, microhabitat selection is, in the case
of lizards, the main niche dimension of a guild where
ecological segregation takes place (Schoener 1974,1977,
Toft 1985). Thus, the species of lizard guilds are often
strongly associated with a very particular kind of
substrate and specialized in its use (Barbault and
Grenot 1977, Mellado 1980, Mellado et al. 1975, OrtegaRubio 1989).
For al1 these reasons, it is very interesting that
Urosaurus auriculatus uses different kinds of microhabitats depending on the ecosystem where it is found.
This ecological plasticity could result from the lack of
pressures exerted by interspecificcompetition, as it is
the only lizard on the island.
However, in spite of their remarkable plasticity in
the use of different substrata, not previously reported
for other similar lizards, at present this species is
seriously threatened. We were not able to find any
Urosaurus on nude soil, the kind of substrata that now
covers one third of Socorro Island and the microhabitat
that expands day by day.
ACKNOWLEDGEMENTS
This study was sponsored by the Centro de Investigaciones Biologicas de Baja California Sur (CIB), the
Secretaria de Programacion y Presupuesto (SPP), the
Consejo Nacional de Ciencia y Tecnologia (CONACyT),
the Fund for Studies and Research Ricardo J Zevada
from Mexico and by The World Wildlife Fund of the
United States of America.
We would also like to thank the Secretaria de
Desarrollo Urbano y Ecologia (SEDUE) and the Armada de Mexico for support for this work.
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