Predation Experiment:
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 126 models and 21 control pieces in both habitat types. The 7
sites in Kenedy and Karnes City included yards, vacant lots, alley-ways,
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. Models were secured to the substrate
using 2 inch nails with the nail head painted over to cover the metallic
surface.
We conducted experiments during two time intervals: 9-June-2018 to
29-June-2018 and 4-August-2018 to 21-August-2018. During the first nine
days of each time period, models were placed in the urban environment
and then were subsequently relocated to the natural ranch setting for
nine days. We used these two time periods (early and late summer) to
account for differences in weather and possibly predation. During the
early summer, round models were painted to color-match the substrate and
lizards in the urban environment. During the late summer, models were
repainted to color-match the red soils and lizards that were found on
the ranch in Dimmit County. This experimental design allowed us to test
for any differences in predation rates due to background color-matching
between models and the substrate upon which they were placed.
Upon initial deployment, we recorded the coordinates of each model using
the Collector for ArcGIS app (ESRI) with 1-m accuracy. We also
photographed models upon initial placement and upon discovering a
potential predation event or disturbance with a Nikon d3300 digital SLR
camera with a Tamron 16-300mm lens. We checked models every three days
during the 9 day period and models that had evidence of predation or
disturbance were photographed and removed. We used similar criterion as
Brodie, (1993) and Bittner, (2003) when categorizing predation events.
Predation marks on the models were categorized into 4 categories:
Birds-obvious āVā or āUā shaped peck marks and models that had been
decapitated; Rodents- bite marks with distinguishable chisel teeth
imprints left on the model; Other- bites that left distinct half-moon
shaped impressions on both upper and lower sides of the model; or
Unknown- models that had limbs removed but no clear bite or peck marks
and models that were found in multiple pieces (Fig. 2).