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Microhabitat Use by Introduced Hemidactylus turcicus (Mediterranean Geckos) in North Central Florida
Patricia Gomez-Zlatar, Michael P. Moulton, and Richard Franz

Southeastern Naturalist, Volume 5, Number 3 (2006): 425–434

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2006 SOUTHEASTERN NATURALIST 5(3):425–434 Microhabitat Use by Introduced Hemidactylus turcicus (Mediterranean Geckos) in North Central Florida Patricia Gomez-Zlatar1, Michael P. Moulton1,*, and Richard Franz2 Abstract - We examined the relationship between seven wall-microhabitat features and the occurrence of the nonindigenous Hemidactylus turcicus (Mediterranean Gecko) in north-central Florida using a repeatable technique. We characterized 160 one-story walls by age of the building, cardinal orientation, color, length, presence or absence of a light source, building material, and vegetation level, and recorded the presence or absence of H. turcicus for each wall during two separate nocturnal visits. The occurrence of H. turcicus was only dependent on wall surface color and length. Both the lack of significance of the majority of the microhabitat variables investigated and the fact that H. turcicus was found on all wall types suggest that this gecko is capable of inhabiting a wide variety of wall environments. This habitat flexibility may be a key factor in the prolific expansion of this gecko’s nonnative range. Introduction Since its initial introduction to Florida in 1915 (Stejneger 1922), Hemidactylus turcicus (Mediterranean Gecko) has become widely distributed throughout the southern United States (Conant and Collins 1991). A native of Eurasia and Africa, this nocturnal gecko exhibits a discontinuous nonnative range that suggests dispersal via human movement, particularly along highways (Davis 1974, Meshaka 1995). Although frequently seen on buildings in both its native and nonnative range, detailed microhabitat studies for this species are not available. The information available consists mainly of incidental observations made during other studies (Conant 1955, Davis 1974, King 1959, Meshaka 1995, Punzo 2001, Rose and Barbour 1968, Selcer 1986) or for populations that coexist with other gecko species (Luiselli and Capizzi 1999, Meshaka 1995, Saenz 1996). The few results supported by quantitative data (Luiselli and Capizzi 1999, Nelson and Carey 1993) often lack a rigorous framework making such studies difficult to compare and conclusions regarding habitat use difficult to draw. Further, the data-collection techniques in these few studies are vague and impossible to reproduce. Thus, we embarked on a systematic exploratory study of microhabitat in a locality where H. turcicus exists in isolation from potential competing gecko species in an attempt to contribute quantitative baseline data. A secondary goal of our work was to devise a technique for rapidly assessing microhabitat use that could be repeated easily by future workers. 1Department of Wildlife Ecology and Conservation, Box 110430, University of Florida, Gainesville, FL 32611-0430. 2Florida Museum of Natural History, Box 117800, University of Florida, Gainesville, FL 32611-7800. *Corresponding author - 426 Southeastern Naturalist Vol. 5, No. 3 Methods and Materials This study was conducted on the University of Florida campus and the adjacent Gainesville Veterans Affairs Medical Hospital Center in Gainesville, Alachua County, FL, between March and June 2002. We located 160 one-story buildings, and then randomly selected one wall from each. Walls were restricted to one story in height to facilitate the detection of geckos. Only one wall per building was used in an attempt to reduce the potential bias from sampling the same gecko more than once. We considered a wall to represent accurately the microhabitat use in this species, as H. turcicus has been shown to possess a small home range (Rose and Barbour 1968, Selcer 1986, Trout and Schwaner 1994). We characterized each selected wall by age of building, cardinal orientation, color, length, presence or absence of a light source, building material, and vegetation level. We obtained building-age information (in years) from literature provided by the University of Florida (UF Physical Plant Division 2000) and an unofficial list created specifically for this study by the Veterans Administration Hospital Engineering Department. We ignored the possibility of any current renovations, as none of the buildings in our study had any renovations since 1998 and there were no construction crews present during our study. We used the age of a building as a rough estimate of the number of daytime retreats (cracks and/or crevices), as Luiselli and Capizzi (1999) found the age of a building to be highly positively correlated with the dilapidation condition of the walls. Although the Luiselli and Capizzi (1999) study included 2000- to 25-year-old buildings whose construction materials were not specified, the necessity of using this surrogate for the number of retreats arose when it became apparent that estimating the number of potential retreats with the naked eye was highly unreliable. It is important to note that this retreat approximation needs to be used with caution, as building disintegration is a complex mechanism whereby age is but one factor in a long list that includes but is not limited to material and climate. We determined the cardinal orientation of walls using the 2000 Building Information List for the University of Florida and official maps of the VA Hospital Engineering Department. We classified walls as north, south, west, or east. For walls not clearly oriented in one of these directions, we allowed a 45° angle of leeway on each side of the cardinal direction. The motivation behind including this variable derives from the disparity in sun exposure among the four orientations, which may result in differences in night wall temperatures. Specifically, west and south walls would be expected to retain greater warmth into the night as these directions receive sunlight later in the day. We categorized wall color as dark if we could distinguish a 3-inch by 5- inch index card fastened to the wall at a perpendicular distance of 10 feet (3.048 meters) in daylight. Walls where the index card could not be perceived at a perpendicular distance of 10 feet were appointed to the light level. All the walls we surveyed were uniform in color across their entire surface. Length was measured along the base of the wall in meters. Length was used as a general measure for size, as all walls were one-story high and thus roughly the same height. 2006 P. Gomez-Zlatar, M.P. Moulton, and R. Franz 427 We measured light occurrence with respect to the entire wall in order to account for gecko movement. We classified walls as high light if they possessed at least one light source (any brightness), whereas walls containing no light source were assigned to the low-light category. It is important to mention that we initially attempted to measure light intensity in lumens via a light meter. However, study walls displayed intricate light-intensity mosaics that were too complex to be quantified as one measurement, which would make repeatability of the technique problematic. Further, inconsistency in management increased the variability in light intensity as bulb replacement was never immediate, and when new bulbs were installed, the wattage was often different. Thus, we consider our measurement to be an adequate representation of the basic lighting conditions available to H. turcicus at our study site. Construction material was confined to four types: aluminum, brick, cement, and wood. We restricted the study to walls that featured a predominate material (> 60% of wall). We classified walls into three vegetation levels that were based on the cement/vegetation ratio bordering the wall. The cement level referred to a wall where at least 60% of the length was bordered by cement, the mixed level to a wall whose length was bordered more than 40% but less than 60% by either cement or vegetation, and the vegetated level to a wall whose length was bordered at least 60% by vegetation. We quantified the vegetation in this way, as opposed to plant species diversity or height, to account for the highly unpredictable management program (e.g., pruning or mowing) encountered throughout the study. Additionally, since habitat selection in reptiles is believed to be most effective when controlled by reliable environmental cues that are independent of daily and/or seasonal fluctuations (Heatwole 1977), we considered a wall’s cement/vegetation ratio to be an acceptable basis for vegetation classification as it is constant and therefore evident in all situations. A summary of the variables we used and the different levels is presented in Table 1. Our sampling regime consisted of visiting 10 walls per night on two nights per week. Each visit occurred approximately two hours after sunset, a period of high activity for H. turcicus in Gainesville (Gomez-Zlatar and Moulton 2005, King 1959). The four months we selected for our study coincided with part of the reproductive season of H. turcicus, and thus further ensured gecko activity (Selcer 1986). Sampling duration was kept under two hours each night in an effort to homogenize environmental conditions among walls. We examined each of the 160 walls twice for completeness, once during March/April and once during May/June. We sampled each wall by passing a flashlight systematically across the entire surface, going from right to left, top to bottom. We recorded the presence or absence of H. turcicus for each wall. Data from the two visits were pooled, and we considered a wall to be inhabited by a gecko if at least one gecko was present during at least one of the two visits. We used chi-square analyses to test for differences in frequencies of gecko occurrence versus cardinal orientation, wall color, construction material, presence of a light source, and vegetation level. We evaluated building 428 Southeastern Naturalist Vol. 5, No. 3 age and wall length with respect to gecko presence with one-way ANOVAs. Means were compared with Duncan’s multiple-range tests. We conducted a preliminary study to elucidate wall-temperature patterns over seven nights during the months of July and August 2001. Each survey night began approximately one to two hours after sunset, and lasted two hours. The number of walls visited per night was dictated solely by the two-hour sampling time permitted, and thus varied as some buildings were more isolated than others. All walls surveyed were located on the University of Florida campus and the Gainesville Veterans Affairs Medical Hospital Center. We characterized walls with respect to construction material, and cardinal orientation as detailed above. We recorded a total of four temperature readings from each sampled wall with a Raytek Raynger ST model temperature gun. The readings were measured from the upper middle, lower middle, right middle, and left middle of each wall. All walls of a building were sampled whenever possible. We compared average wall temperatures across construction material and cardinal orientation with a two-way ANOVA, and used Duncan’s multiple-range tests to compare means. All statistical analyses had a significance level of 5%, and were performed using the SAS system, Version 9. Table 1. Description of wall-characterization variables for microhabitat study # of walls Variables Levels sampled Criteria Age - 158 Age in years* Cardinal orientation North 43 Location of wall on official maps** South 49 Location of wall on official maps** West 34 Location of wall on official maps** East 34 Location of wall on official maps** Color Light 95 The inability to perceive a 3"x 5" white index card at a perpendicular distance of 10ft (3.048m) from the wall; during the day*** Dark 65 The ability to perceive a 3"x 5" white index card at a perpendicular distance of 10ft (3.048m) from the wall; during the day*** Length - 160 Measure along base of wall in meters Light High 38 Presence of at least one light source on the wall Low 122 No light source present on the wall Material Aluminum 46 Physical observation; > 50% of wall surface Brick 33 Physical observation; > 50% of wall surface Cement 59 Physical observation; > 50% of wall surface Wood 22 Physical observation; > 50% of wall surface Vegetation Cement 30 􀂕 60% of wall length bordered by cement Mix 31 > 40% to less than 60% of wall length bordered by cement or vegetation Vegetation 99 􀂕 60% of wall length bordered by any type or height of vegetation *Sources used: 2000 Building Information List for the University of Florida prepared by the UF Physical Plant Division and unofficial list prepared by the VA Hospital Engineering Department. **Sources used: 2000 Building Information List for the University of Florida prepared by the UF Physical Plant Division and official VA Hospital Engineering maps. *** Index cards were provided by AMPAQ, Dallas, TX 75252. 2006 P. Gomez-Zlatar, M.P. Moulton, and R. Franz 429 Results Of the 160 walls that we surveyed, 26 walls had at least one gecko on the first visit only, 20 walls had geckos on just the second visit, and 39 walls had geckos during both sampling visits. Seventy-five walls had no geckos on either visit. Geckos were observed on walls of all categorical variable-level types. The proportion of walls recording at least one H. turcicus observation is listed for each categorical variable level in Table 2. The presence and absence of H. turcicus by wall frequency is displayed for each categorical variable level in Figure 1. We observed geckos more often than expected by chance on dark colored walls, whereas fewer than expected were observed on light-colored walls (p = 0.0023). The results of our chi-square tests indicated that gecko presence was independent of cardinal orientation, construction material, presence of a light source, and vegetation level. These results are presented in Table 3. Chisquare tables detailing observed and expected frequencies, along with cell chi-square values for each of these variables are presented in Appendix 1. Although insignificant, general trends gleaned from the chi-square tables Table 2. Proportion of walls of each categorical variable level at which at least one H. turcicus observation was recorded. Percentage of walls at which at least one Variables Levels H. turcicus observation was recorded Cardinal orientation North 39.5% South 61.2% West 58.8% East 52.9% Color Light 43.2% Dark 67.7% Light High 65.8% Low 49.2% Material Aluminum 41.3% Brick 60.6% Cement 54.2% Wood 63.6% Vegetation Cement 50.0% Mix 54.8% Vegetation 53.5% Table 3. Chi-square statistics and corresponding p-values for all categorical microhabitat variables Variables Degrees of freedom Chi-square value P-value Cardinal orientation 3 4.9238 0.1775 Color 1 9.3288 0.0023* Light 1 3.2098 0.0732 Material 3 4.3282 0.2281 Vegetation 2 0.1609 0.9227 *Significant at the 5% level. 430 Southeastern Naturalist Vol. 5, No. 3 included lower gecko occurrence than expected by chance on both northern and aluminum walls. Conversely, geckos were present more than expected by chance on walls containing at least one light source. Vegetation demonstrated no discernable trend. Building age was not a factor in determining the presence of H. turcicus on a wall (p = 0.3183). Wall length appeared to influence H. turcicus presence (p less than 0.0001), as walls recording a gecko observation were an average length of 19.6 meters whereas walls with no gecko observations were 13.1 meters long. We also compared average temperatures of a total of 16 brick, 52 cement, 38 wood, and 78 aluminum walls. The average wall temperatures of the four types of material were all significantly different from each other (p less than 0.0001). Brick walls had the highest mean temperatures with an average of 26.8 °C, followed by cement walls at 26.1 °C, wood walls at 25.1 °C, and aluminum walls at 23.8 °C. Average temperatures of 47 southern, 46 northern, 46 eastern, and 45 western walls were assessed. No significance was found among the average temperatures of walls oriented in different cardinal Figure 1. The presence and absence of H. turcicus by wall frequency for cardinal orientation, color, material, presence of a light source, and vegetation. 2006 P. Gomez-Zlatar, M.P. Moulton, and R. Franz 431 directions (p = 0.499). The two-way interaction between material and cardinal orientation also was not significant (p = 0.9998). Discussion Of the seven variables we examined, two were significant. The first was wall color and the second was wall size. We discuss these in turn below. In addition, we address the unexpected insignificance of light sources, and also the potential trends that emerged from the insignificant variables. A greater number of dark walls than expected by a hypothesis of independence were occupied by H. turcicus. This gecko species possesses both light and dark dorsal markings (Bartlett and Bartlett 1999) that allow it to conceal itself against a variety of substrate (i.e., wall) colors and patterns, making camouflage an unlikely motive for this choice. Further study will be necessary to determine the basis for the result. Walls that recorded H. turcicus observations were longer than those with no geckos. Additional space may be an important component of this gecko’s microhabitat, as it is likely to provide a greater number of retreats, a larger foraging area, and an increase in social interactions crucial for reproduction. The occurrence of H. turcicus on a wall was independent of the presence of a light source. This result was unanticipated, as investigators have argued that H. turcicus was always found high on a wall in proximity to a light source, to facilitate the capture of their insect prey (Conant 1955, Conant and Collins 1991, Davis 1974, Nelson and Carey 1993, Punzo 2001). The idea that H. turcicus might not require a light source for prey capture is alluded to in two dietary studies (Capula and Luiselli 1994, Saenz 1996), which found that a large proportion of H. turcicus’ diet consisted of grounddwelling prey rather than flying prey. Further evidence against the need for a light source for prey detection came from our observations of geckos on walls with a wide gamut of light intensities, including complete darkness. Nonetheless, it is prudent to keep in mind the contradicting general trend whereby walls with at least one light source tended to have geckos present more often than would be expected under independence. Although insignificant, this trend emphasizes that the role of light as a microhabitat cue for H. turcicus remains unclear. Future studies, particularly controlled experiments, may be the key to elucidating this relationship. Another general trend we ascertained was that a smaller proportion of north walls contained geckos than all other directions. It is unlikely that thermoregulation is an underlying factor as north walls had an average temperature comparable to all other cardinal directions. Explanations for this trend are difficult to formulate at this time. The last general trend detected was that a smaller proportion of aluminum walls contained geckos. The average summer temperature for aluminum walls was at least 1.3 °C lower than all other materials surveyed, and may explain this trend. With an average summer temperature of 23.8 °C, aluminum is warmer than the average adult substrate for fall (23.2 °C) but cooler than the average for spring (24.9 °C), which is the beginning of the reproductive season (Gomez-Zlatar and Moulton 2005). Thus, it is possible 432 Southeastern Naturalist Vol. 5, No. 3 that lower substrate temperatures deter H. turcicus from aluminum walls, especially during the reproductive season. An alternative explanation for this trend, which was not studied here, is the possibility that aluminum disintegrates at a slower rate than other materials and provides fewer cracks and crevices for H. turcicus to use as retreats. Further study into the thermal field preferences of this species and into the complexities of material decay is necessary before any conclusions can be made. The lack of a significant relationship between building age, material, cardinal orientations, presence of light, and vegetative structure suggests a variety of scenarios. Although the variables we selected were mostly those measured by other observers, it is possible that the variables of this study are simply not important to these geckos. Alternatively, there could be subtle microclimatic factors that we did not measure. However, since H. turcicus was consistently found on all wall types, a more likely interpretation of our results is that these geckos are highly plastic with respect to their microhabitat needs relative to the range of physical parameters present on the walls of buildings. This ultimately permits this species to thrive on a variety of substrates, and thus facilitates its ability to extend its nonnative range. As with all scientific studies, our data and conclusions have limitations. For instance, it is impossible to determine if the absence of H. turcicus on a wall is due to a microhabitat cue or if it is an artifact of this species’ dispersal via human movement. Also, shortcomings in sampling technique, such as not observing a wall throughout the entire night and frightening geckos as we approached a wall, could have resulted in an underestimation of walls containing geckos. In addition, it should be noted that performing seven separate statistical tests resulted in the inflation of the experiment-wise error rate. Although unfortunate, the absence of certain variable combinations in our study site (resulting in zeros in the matrix of all possible variable combinations) greatly limited the number of statistical tests that could be used on this dataset (Gomez-Zlatar 2003). We encourage future studies to consider this problem when selecting a study site so as to avoid a similar predicament. As many as 11 species of introduced geckos have established reproductive populations in south Florida, whereas an additional five species have occasionally been sighted and as such are considered to be of uncertain status (Meshaka et al. 2004). The probability is high that one or more of these species will ultimately expand its range into our study area. Thus, we believe that our results not only provide important baseline data for H. turcicus, but for all future studies of the ecology of invading gecko species. We have made a concerted effort to develop a simple technique for scoring microhabitat use, and we hope that future studies will not only use these methods, but also ultimately refine them so that investigators anywhere in the world can compare results among sites in a meaningful way. Acknowledgments We thank Camilo Gomez, Elza Kephart, Chuck Knapp, Esther Langan, Alex Martin, and Robin Sternberg for their help in the field. We especially thank the University of Florida and the staff at the Gainesville Veterans Affairs Medical Center 2006 P. Gomez-Zlatar, M.P. Moulton, and R. Franz 433 for allowing us access for our study. Finally, we thank two reviewers for their insightful comments, which allowed this manuscript to reach its full potential. Literature Cited Bartlett, R.D., and P.P. Bartlett. 1999. A Field Guide to Florida Reptiles and Amphibians. Gulf Publishing Company, Houston, TX. Capula, M., and L. Luiselli. 1994. Trophic niche overlap in sympatric Tarentola mauritanica and Hemidactylus turcicus: A preliminary study. Herpetolological Journal 4:24–25. Conant, R. 1955. Notes on three Texas reptiles, including an addition to the fauna of the state. American Museum Novitates 1726:1–6. Conant, R., and J.T. Collins. 1991. A Field Guide to Reptiles and Amphibians of Eastern and Central North America. Houghton Mifflin Company, Boston, MA. Davis, W.K. 1974. The Mediterranean Gecko, Hemidactylus turcicus in Texas. Journal of Herpetology 8:77–80. Gomez-Zlatar, P. 2003. Microhabitat preference of the introduced gecko, Hemidactylus turcicus in an urban environment. M.Sc. Thesis. University of Florida, Gainesville, FL. Gomez-Zlatar, P., and M.P. Moulton. 2005. Habitat use by the nonindigenous Mediterranean Gecko (Hemidactylus turcicus) in north central Florida. Florida Scientist 68(3): 206–214. Heatwole, H. 1977. Habitat Selection in reptiles. Pp. 137–155, In C. Gans and D.W. Tinkle (Ed.). Biology of the Reptilia, Vol. 7 (Ecology and Behavior A). Academic Press Inc., New York, NY. King, W. 1959. Observations on the ecology of a new population of the Mediterranean Gecko, Hemidactylus turcicus, in Florida. Quarterly Journal of the Florida Acadamy of Science 21:317–318. Luiselli, L., and D. Capizzi. 1999. Ecological distribution of the geckos Tarentola mauritanica and Hemidactylus turcicus in the urban area of Rome in relation to age of buildings and condition of the walls. Journal of Herpetology 33:316–319. Meshaka, Jr., W.E. 1995. Reproductive cycle and colonization ability of the Mediterranean Gecko (Hemidactylus turcicus) in south-central Florida. Florida Scientist 58:10–15. Meshaka, Jr., W.E., B.P. Butterfield, and J.B. Hauge. 2004. The exotic amphibians and reptiles of Florida. Krieger Publishing Company, Malabar, FL. Nelson, D.H., and D. Carey. 1993. Range extension of the Mediterranean Gecko (Hemidactylus turcicus) along the northeastern gulf coast of the Unites States. Northeast Gulf Science 13:53–58. Punzo, F. 2001. The Mediterranean Gecko, Hemidactylus turcicus: Life in an urban landscape. Florida Scientist 64:56–66. Rose, F.L., and C.D. Barbour. 1968. Ecology and reproductive cycles of the introduced gecko, Hemidactylus turcicus, in the southern United States. American Midland Naturalist 79:159–168. Saenz, D. 1996. Dietary overview of Hemidactylus turcicus with possible implications of food partitioning. Journal of Herpetology 30:461–466. Selcer, K.W. 1986. Life history of a successful colonizer: The Mediterranean Gecko, Hemidactylus turcicus, in southern Texas. Copeia 1986:956–962. Stejneger, L. Two geckos new to the fauna of the United States. Copeia 1922:56. Trout, L., and T.D. Schwaner. 1994. Allozyme evidence for insularity in exotic populations of the Mediterranean Gecko (Hemidactylus turcicus). Journal of Herpetology 28:391–393. 434 Southeastern Naturalist Vol. 5, No. 3 University of Florida Physical Plant Division, Architecture/Engineering Department. 2000. 2000 Building information list for the University of Florida. Gainesville, FL. Appendix 1. Observed and expected frequency followed by individual cell chisquare values for the chi-square test of each categorical variable. Cells that have a large influence on the chi-square table are indicated by an asterisk. A. Cardinal orientation Presence of H. turcicus 􀁲2 parameters East North South West No Observed 16 26 19 14 Expected 15.938 20.156 22.969 15.938 Cell 􀁲2 0.0002 1.6492* 0.6858 0.2355 Yes Observed 18 17 30 20 Expected 18.063 22.844 26.031 18.063 Cell 􀁲2 0.0002 1.4949* 0.6051 0.2078 B. Light Presence of H. turcicus 􀁲2 parameters Light No light No Observed 13 62 Expected 17.813 57.188 Cell 􀁲2 1.3002* 0.405 Yes Observed 25 60 Expected 20.188 64.813 Cell 􀁲2 1.1473* 0.3573 C. Material Presence of H. turcicus 􀁲2 parameters Aluminum Brick Cement Wood No Observed 27 13 27 8 Expected 21.563 15.469 27.656 10.313 Cell 􀁲2 1.3712* 0.394 0.0156 0.5186 Yes Observed 19 20 32 14 Expected 24.438 17.531 31.344 11.688 Cell 􀁲2 1.2099* 0.3476 0.0137 0.4576 D. Vegetation Presence of H. turcicus 􀁲2 parameters Cement Mix Vegetation No Observed 15 14 46 Expected 14.063 14.531 46.406 Cell 􀁲2 0.0625 0.0194 0.0036 Yes Observed 15 17 53 Expected 15.938 16.469 52.594 Cell 􀁲2 0.0551 0.0171 0.0031