Predation release of Texas horned lizards (Phrynosoma cornutum) living
in small towns
Abstract
Texas horned lizards (Phrynosoma cornutum) have a number of ways to
avoid predation, including camouflage, sharp cranial horns, flattening
of the body, and the ability to squirt blood from the eyes. These
characteristics and their relatively low survival rates in the wild
suggests 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. However, they can
be still be found in some small south Texas towns where they can reach
densities that are much higher (~50 lizards/ha) than in
natural areas (~4-10 lizards/ha). 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, a material that
is ideal for preserving marks left behind by predators. Models (n = 126)
and control pieces of foam (n = 21) were left in the field for 9 days in
each location in early and late summer and subsequent predation marks
were categorized by predator taxa. We observed significantly more
predation attempts on the models than on controls and significantly
fewer attempts in town (n = 1) compared to the ranch (n = 60). On the
ranch, avian predation attempts appear to be common especially when the
models did not match the color of the soil. Our results suggest that
human modified environments that have suitable habitat and food
resources may provide a refuge for some prey species like horned lizards
from predators.