Book Description

Levels of the stress hormone, corticosterone (CORT) in the blood can be a great indicator of stress levels in lizards. This approach reveals that invasive species impose novel pressure on natives, elevating stress levels. Native species often behaviorally adapt to these pressures in order to increase their chances of surviving exposure to these nonnative threats. We tested the hypothesis that eastern fence lizards (Sceloporus undulatus) that have been exposed to predatory invasive fire ants (Solenopsis invicta) for several generations exhibit different behavioral responses to experimentally elevated CORT than do lizards whose habitat has not yet been invaded. Lizards from the uninvaded site respond to elevated CORT levels by hiding more and moving and basking less, whereas those from the invaded site exhibit the opposite pattern; hiding less, moving more, and spend more time on the basking log when CORT levels are elevated. These differences in response may facilitate survival in these different environments. Within natural uninvaded populations, lizards would benefit from becoming less active and hiding in response to a stressful event, as this would reduce their vulnerability to predators and conspecifics. In contrast, within fire ant invaded sites, increased CORT levels occur following encounters with fire ants. Responding to elevated CORT levels by moving away and off the ground would increase the lizards' chances of surviving these encounters in the presence of this invader. The changes in basking behavior following elevated CORT levels appears to be due to changes in the propensity of lizards escaping up off the ground rather than having any thermoregulatory relevance. A second study revealed that lizards body temperatures were not affected by experimentally elevated CORT levels, and this response did not differ between fire ant invaded and uninvaded sites. Together, these reveal that the behavioral response to physiological stress can be altered by the introduction of non-native species, allowing native species to persist in the face of this novel threat.