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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. LITERATURE CITED BARBAULT, R. and C. GRENOT. 1977. Richesse specifique et organization spatiale du peuplement de lizards du Bolson de Mapimi (Desert de Chihuahua, Mexique). Compte Rendu Academie Sciences du Paris. 284:2281-2283. BARBAULT, R.A ORTEGA y M. MAURY. 1985. Food partitioning and community organization in a mountain lizard guild of Northern Mexico. Oecologia 65:550-554. 57 BRATTSTROM, B.H. 1955. Notes on the herpetology of the Revillagigedo islands, Mexico. American Midland Naturalist. 54(1):212-229. BRATTSTROM, G.H. 1982. A comparison of the social behavior of Urosaurus auriculatus and U. clarionensis on Revillagigedo islands, Mexico. Herpetological Review. 13(1):11-12. BRATTSTROM, B.H. 1990. Biogeography of the Islas Revillagigedo, Mexico. Journal of Biogeography (1990) 17:177-183. COPE, E.D. 1871. Description of the common lizards of Socorro. Proceedings of the Boston Society of Natural History 14:303. LEVINS, R. 1968. Evolution in changing environments. Princeton Univ. Press. MELLADO, J. 1980. Utilizacion del espacio en una comunidad de lacertilios del matorral mediterraneo en la reserva biologica de Donana. Donana Acta Vertebrata 7:41-59. MELLADO, J., A. PARRENO and F. HIRALDO. 1975. The structure of a mediterranean lizard community. Dona Acta Vertebrata 2:15-160. MITTLEMAN, M.B. 1942. A summary of the iguanid genus Urosaurus. Bulletin of the Museum of Compared Zoology 91(2):103-181. ORTEGA-RUBIO. 1989. Lizard spatial organization and habitat partitioning in la Sierra de Guadarrama, Spain. Herpetology 19(3):85-92. ORTEGA, A., M.E. MAURY and R. BARBAULT. 1982. Spatial organization and habitat partitioning in a mountain lizard community of Mexico. Acta Oecologica, Oecologia Generalis 3:323-33. PIANKA, E.R. 1973. The structure of lizard communities. Annual Review of Ecology and Systematics 4:53-74. SCHOENER, T.W. 1974. Resource partitioning in ecological commuinities. Science 185:27-39. SCHOENER, T.W. 1977. Competition and the niche. ln: GANS, C. and D.W. TINKLE (eds.), "Biology of Reptilia," Academic Press Publ. 7:35-136. SLEVIN, J. 1926. Expedition to the Revillagigedo islands Mexico in 1925. 111. Notes on a collection of reptiles and amphibians from the Tres Marias and Revillagigedo Islands, and West coast of Mexico, with description of a new species of Tantilla. Proceedings of the California Academy of Sciences, 4th Series 15(3):195-207. TOFT, A.C. 1985. Resource partitioning in amphibians and reptiles. Copeia 1:l-21.