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Either your web browser doesn't support Javascript or it is currently turned off. In the latter case, please turn on Javascript support in your web browser and reload this page. Ecology and Evolution24 Mar11 10 : DOI: Texas horned lizards Phrynosoma cornutum have a number of ways to avoid predation, including camouflage, sharp cranial horns, flattening of the body, mcmillon masters thesis south texas, and the ability to squirt blood from the eyes.
These mcmillon masters thesis south texas and their relatively low survival rates in the wild suggest these lizards are under high predation pressure.
These lizards have been declining in much of their eastern range due to increased urbanization, agriculture, and loss of prey species. We hypothesized that one reason for the high densities observed in these towns may be due to reduced predation pressure. We used model Texas horned lizards to test whether predation levels were lower in two south Texas towns than on a nearby ranch. We constructed models from urethane foam, mcmillon masters thesis south texas material that is ideal for preserving marks left behind by predators.
On the ranch, avian predation attempts appear to be common especially when the models did not match the color of the soil. Texas horned lizards living in small South Texas towns experience a predation release.
This allows them to exist at much higher than normal densities than populations living in more natural environments. Altered community structure is a hallmark of urban environments; urban communities can differ greatly from their natural counterparts with some species able to adapt, while others decline Fischer et al. The role that predation plays in structuring urban communities is not well understood but has been suggested to be similar to what is found in natural areas Shochat et al.
Increasing evidence, however, indicates that predation may act differently in urban environments, leading to what some authors have termed an urban predation paradox Eötvös et al. Studies done largely on mcmillon masters thesis south texas birds and mammals have shown that urban environments have high densities of mesopredators, but paradoxically lower rates of predation Eötvös et al.
Lower predation rates in urban environments have been attributed to predators subsisting mainly on anthropogenic mcmillon masters thesis south texas i. Subsequently, mcmillon masters thesis south texas, prey species in urban areas may experience an ecological mcmillon masters thesis south texas and lower predation rates that can allow them to exist in hyperabundance Fischer et al.
Alternatively, some research has shown that predators are more sensitive to urbanization and are pushed out of urban areas because of a lack of suitable habitat. Consequently, urban environments may act as refugia for some prey species due to the lack of predators in those areas Berger, ; Leighton et al.
With natural habitats being increasingly altered by human development, an important conservation question, now and in the future, will be to determine how predation affects the structure and assemblages of urban communities. Few studies have been conducted on the impact that predation has on reptiles living in urban environments, and results from these are often conflicting French et al. Due to the difficulty in observing predation events, many researchers have turned to the use of clay or foam models to measure predation Bateman et al.
Of these studies, only Mcmillan and Irshick explicitly tested differences in predation rates between urban and natural environments. Their results, consistent with the urban predation paradox, showed significantly lower amounts of predation on green anole models Anolis carolinensis in the urban area. Texas horned lizards are known to have a multitude of predators including snakes, predatory lizards, birds, rodents, canids, and felids Sherbrooke, Their low annual survival rate 8.
Texas horned lizards, an iconic vertebrate of the American southwest, mcmillon masters thesis south texas, have declined, especially in eastern areas of their historic range, mcmillon masters thesis south texas, and are now a threatened species in the state of Texas Donaldson et al. Texas horned lizards are still found in some small Texas towns, including populations occurring in the towns of Kenedy and Karnes City in southern Texas.
Past research has shown that lizards in these towns occur at average densities of Lizards in these towns predominately eat smaller ants Pheidole spp. and termites Tenuirostritermes cinereus rather than their commonly preferred prey of large harvester ants Alenius, Foraging mcmillon masters thesis south texas smaller prey items may increase handling time for Texas horned lizards, which would put them at higher risk of predation.
We hypothesized that predation rates are lower in town than in more natural areas, and that this has facilitated both high densities of lizards and the exploitation of small prey items in towns. In this study, we created foam models of Texas horned lizards and placed them in small towns and a natural rural habitat to test the hypothesis that predation rates would be lower in town.
We made hatchling, juvenile, and adult models to determine whether predation rates varied by size and to potentially sample smaller predators that might preferentially attack a small lizard over an adult. We also varied the coloration of the models to determine whether less cryptic models were predated at higher rates and if this differed between urban and rural sites.
Karnes City and Kenedy are two small towns 3,—3, people known for having Texas horned lizards and are the sites of ongoing studies Figure 1. We have censused 15—17 study plots in Kenedy 3—4 study plots and Karnes City 12—13 study plots since Ackel, ; Alenius, ; Wall, The study plots are irregular in shape, range from 0.
Honey mesquite Prosopis glandulosaanacua Ehretia anacuaand sugar hackberry Celtis laevigata are the most common trees on the study plots Wall, During each survey, we walk linear transects with 2—5 people, spaced 2 meters apart, until we search the entire area of the site.
Surveys typically last 20 min to 2. Texas horned lizard near an elementary school in Karnes City, Texas. Photograph by D. Over the course of six field seasons —predator observations in and adjacent to our study plots include frequent daily or weekly sightings of cats Felis catusdogs Canis lupus familiarisand northern raccoons Procyon lotor. Our census methods for horned lizards should be good at detecting snakes on our study plots e. We have never observed American crows Corvus brachyrhynchosloggerhead shrikes Lanius ludovicianusor American kestrels Falco sparveriusin these towns during the summer.
The Dimmit County ranch located within the South Texas Plains ecoregion is approximately 32 km North of the Chapparal WMA wildlife management area. The habitat is dominated by honey mesquite Prosopis glandulosa and Acacia thornscrub communities typical of south Texas shrubland.
This relatively wild habitat maintains natural communities of both predators and prey for Texas horned lizards making it an ideal site for monitoring natural predation on these lizards.
The original pewter horned lizard replica was scanned to create an object file. obj that was used to 3D print three size classes of model Texas horned lizards: hatchling 23 mm SVLjuvenile 50 mm SVLand adult 84 mm SVL size models. We used these 3D printed models to create molds capable of producing the different size classes used in this study and multiple models per casting.
Foam iT! The controls were constructed from round pieces of urethane foam left over from casting the lizard models and painted with acrylic paint and otherwise treated exactly like the lizard models to control for predators being attracted to the foam material or to the paint Figure 2.
Two distinct color variations of Texas horned lizard models were used in this study. Gray models a were painted to match gray lizards found in Kenedy and Karnes City, and red models b were painted to match Texas horned lizards found on the Dimmit County ranch.
We used 3 size classes of models: hatchlings, juveniles, and adults c and controls d. Models were created to be as morphologically accurate as possible to mimic actual Texas horned lizards e. The specific colors of the models were determined using photographs of multiple adult Texas horned lizards from each location where models were placed, as well as from photographs taken of the surrounding substrates. After painting, we placed dried models outside and covered them with a loose layer of soil to allow paint fumes to dissipate for a period of 7 days prior to placing them in the field.
To test whether models were successfully painted to color match their surrounding substrates, we photographed each model in the field with a ColorChecker Passport Photo with software version 1. in the frame. Using this color standard and the ColorChecker camera calibration software plugin for Adobe Lightroom Classic, we created digital negative DNG profiles that could then be used to create images that were calibrated to their true colors, making it possible to compare color values across all photographs.
After calibration, a portion of the model lizard's coloration was cropped from the photograph using ImageJ and compared with an exact sized crop of substrate adjacent ~1 cm mcmillon masters thesis south texas the model, mcmillon masters thesis south texas.
RGB red, green, blue color values were obtained from each cropped photograph using the Color Inspector 3D v. We calculated COI scores, indicating the percent color match between substrate and model, for 40 models.
We placed 6 models of each size class hatchlings, juveniles, adults and 3 controls across 7 sites in town and 7 sites on the ranch for a total of models and 21 control pieces in both habitat types. The 7 sites in Kenedy and Karnes City included yards, mcmillon masters thesis south texas, vacant mcmillon masters thesis south texas, alleyways, and school playgrounds in areas that contained horned lizards determined from previous surveys.
The 7 sites of model placement on the ranch included areas with known Texas horned lizard activity as determined by surveys for lizards and their scat. During the first 9 days of each period, models were placed in the urban environment and then were subsequently relocated to the natural ranch setting for 9 days.
We used these two time periods early and late summer to account for differences in weather and possibly predation. During the early summer, models were painted to color match the substrate and lizards in the urban environment. During the late summer, models were repainted to color mcmillon masters thesis south texas the red soils and lizards that were found on the ranch in Dimmit County.
This experimental design mcmillon masters thesis south texas us to test for any differences in predation rates due to background color matching between models and the —substrate upon which they were placed.
We used similar criterion as Brodie and Bittner when categorizing predation events, mcmillon masters thesis south texas. Models that were painted to background color match their environments have significantly higher COI scores than those that were not painted to match. Mcmillon masters thesis south texas total of 61 predation events were recorded over both sampling periods representing Four predation events occurred on the controls over both sampling periods, all of which were located on the ranch.
Models that had evidence of being disturbed by nonpredators i. Predation events recorded in town and on the Dimmit County ranch for the two sampling periods and by model size class in Numbers in parentheses are total number of models used at each site. Our controls during the second round were painted to color match the soils on the ranch and as a result resembled dried prickly pear Opuntia spp. pads and fruits, mcmillon masters thesis south texas, which may have attracted Texas tortoises Gopherus berlandieri.
We frequently encountered Texas tortoises eating both the fruits and pads of Opuntia spp. on the Dimmit County ranch and the bite marks on the controls were similar in size and shape to the tortoise bites on cactus pads. We observed significantly more predation events by avian predators on the ranch when models were not painted to background color match i. We observed no difference in predation events among the remaining predation categories. Our results supported our hypothesis that predation on Texas horned lizards in town would be lower than in the natural ranch environment and are consistent with other studies that have found lower predation in urban areas Fischer et al.
Although we did not explicitly measure predator densities, there is anecdotal evidence that the predator community in Kenedy and Karnes City differs in both the abundance and diversity of predators when compared to the ranch. For instance, birds of prey and snakes are rarely seen in or near our study plots in the towns but were seen daily at the ranch.
Feral and pet cats and dogs are also common in town but do not occur on the ranch. Altered predator communities are a consistent result of urbanization Fischer et al. We did not expect models to be attacked by snakes because snakes rely on thermal, motion, and olfactory cues to sense prey de Cock Buning, ; all of which are not exhibited by model lizards, mcmillon masters thesis south texas.
The lack of movement could also decrease model attacks by predators that use motion to detect prey like birds Antczak et al. For instance, we set up motion detection video cameras in the urban areas for some models and filmed several instances of cats walking by and ignoring the models. Nevertheless, residents have told us that sometimes their pet cats will bring dead horned lizards back to the house. Another limitation of models may be that predators are attracted to the material used to construct the models Bateman et al.
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