--The introduced house geckos, Hemidactylus turcicus and Cyrtopodion scabrum exist sympatry Galveston, Texas. The nightly perch choices of each species captivity were recorded, first species-segregated enclosures, then with species mixed. While segregated by H. turcicus occupied vertical walls more frequently than ground or rock perches. Cyrtopodion scabrum perched predominantly on rocks. While mixed the enclosures, the proportion of individuals not visible increased independently for both while the proportion of both species occupying their respective preferred perches decreased. Perch heights did not differ between regardless of whether the species were segregated or mixed the enclosures. The observed changes use of space while sympatry may be due to competitive interference between these two and may reflect the situation Galveston, where C. scabrum appears to be gradually displacing H. turcicus areas near the docks, where these two species previously existed sympatry. l f erpetology, Vol. 30, No. 1, p. 46-51, 1996 i t 1996 Society for the Study of Amph bians and Reptiles i al Interference for Perch Sites Two Species of Interspecific competition occurs when two or more species utilize the same limiting resources (Pianka, 1981). Species with overlapping re1 Present Address: Texas Parks and Wildlife Department, 4200 Smith School Road, Austin, Texas 78744, USA. specific competition occurs when two or species utilize the same limiting resources a, 1981). Species with overlapping res urce requirements may compete either by depletion of resources, or by competitive interf ence (Odum, 1971; Pianka, 1981; Schoener, 1982), such as aggressive dominance, mutual predation, or active inhibition, ultimately controlling free access to resources. Both types of competition may be considered negative interactions (Burkholder, 1952), that both species 46 This content downloaded from 157.55.39.58 on Tue, 11 Oct 2016 04:31:24 UTC All use subject to http://about.jstor.org/terms PERCH CHOICE INTERFERENCE IN GECKOS are detrimentally affected. In consideration of positive-negative two-species population interactions, Odum (1971) emphasized that (1) in the evolution and development of ecosystems, negative interactions tend to be minimized favor of positive symbiosis that enhances the survival of the interacting species, and that (2) or new associations are more likely to develop severe negative coactions than are older associations. Ortiz and Jenssen (1982) predicted that syntopic species of recent contact should exhibit particularly high intensities of interactions, because the coexisting species have had little time to undergo niche divergence. A number of studies have addressed the subject of interference competition (Case and Gilpin, 1974; Schoener, 1977; Salzburg, 1984; Jenssen et al., 1984; Bolger and Case, 1992; Klawinski et al., 1994). Salzburg (1984) provided experimental evidence that the lizards Anolis sagrei and A. cristatellus were competing by behavioral interference for habitat resources, and that access to those resources affected the fitness of the two species. Bolger and Case (1992), a study of two species of asexual parthenogenic geckos (Lepidodactylus lugubris and Hemidactylus garnotii) and one sexual species (H. frenatus) found that behavioral interference by the latter, more recently introduced is at least partially responsible for an observed decline the two asexual species on tropical Pacific islands. The present study investigates competitive behavioral interference for perch sites between two species of introduced house geckos, the Mediterranean gecko, Hemidactylus turcicus, and the more recently introduced roughtail gecko, Cyrtopodion scabrum. These species are currently coexisting Galveston, Texas, a recent association (Selcer and Bloom, 1984) that may not yet have reached equilibrium (Klawinski et al., 1994). Hemidactylus turcicus and C. scabrum occur syntopically Galveston, Texas (Selcer and Bloom, 1984; Bloom et al., 1986), where they share the walls and windows of well-lighted stucco, wood, and brick buildings near the entrance to the Port of Galveston. Prior to 1983, C. scabrum was not known to occur the United States, and the association between the two species Galveston is probably of recent origin (Selcer and Bloom, 1984). Little has been reported about the life history and behavior of C. scabrum (Vaughan, 1991); however, this Old World house gecko's Middle East historical range overlaps that of H. turcicus (Conant and Collins, 1991). The two species are ecologically similar; H. turcicus and C. scabrum share similar body sizes and activity periods (Vaughan, 1991). Both species are known to perch upon walls or other vertical substrates while foraging (Conant and Collins, 1991; Klawinski et al., 1994). Both have been observed feeding on insects; spiders and flies comprise about 50% of the diets of both species (Vaughan, 1991). Klawinski et al. (1994) reported that ditary overlap these two species of geckos occupying sympatric areas Galveston was less than that found allopatric areas. In sympatry, a dietary shift to other prey taxa was exhibited by H. turcicus, but not by C. scabrum. The same au hors found that perch heights at capture for the two species Galveston did not differ and concluded that reduced overlap sympatry was not due to perch height differences. Hemidactylus turcicus is being gradually displaced by the more recently introduced C. scabrum areas of Galveston where both previously coexisted (Vaughan, 1991; Klawinski et al., 1994). The observed dietary shift was attributed to possible competition between the two species of geckos sympatry, which may be leading to the competitive exclusion of H. turcicus habitat areas occupied prior to the introduction of C. scabrum. As H. turcicus and C. scabrum are similar several aspects of their ecology (Selcer and Bloom, 1984; Vaughan, 1991), the question of microhabitat partitioning the Galveston populations arose. Of particular interest is whether the foraging perch choices of one species were affected by the presence of the other. The method chosen to evaluate such an interaction was to assess the distribution of nightly perch choices of each species captivity, a species-segregated situation, and to compare those results with results obtained when the two species were forced to share habitat space. MATERIALS AND METHODS Geckos of both species were collected by hand from the walls and windows of buildings near the Port of Galveston, Galveston County, Texas, between March and September of 1989. Upon capture, geckos of both species were transported to the laboratory, where each individual was sexed and weighed. Snout-vent length was measured to the nearest mm. Only adult geckos were used this study. After sevral marking methods proved unsatisfactory, a closeup color photograph was taken of the uniquely patterned and pigmented dorsal surface of each gecko. These prints were enlarged and were used to identify individuals during this study. In captivity, repeated observations of nightly perch occupancy were made for each individual of the two species of geckos, first allopatric groups to gather baseline data, and then sympatry to assess the result of interaction. Perch substrates consisted of vertical walls, cinder 47 This content downloaded from 157.55.39.58 on Tue, 11 Oct 2016 04:31:24 UTC All use subject to http://about.jstor.org/terms R. K. VAUGHAN ET AL.