An invasive predator, the red imported fire ant, alters latitudinal gradients of multiple traits in a native lizard
Biogeographers and evolutionary ecologists have long been fascinated and perplexed by latitudinal gradients and the selective mechanisms that drive their formation and maintenance. Accelerating environmental change, including climate change and the introduction of invasive species, can alter selective pressures across large portions of species’ ranges. Associated changes in traits are generally assumed to be adaptive, but could alter existing latitudinal patterns, potentially imposing costs. We examined how latitudinal patterns of morphology, behavior and physiology of a lizard are affected by invasion of a predatory ant. The red imported fire ant, Solenopsis invicta, was introduced into the southeastern U.S. in the 1930’s and has since spread north to occupy the southern half of the range of the eastern fence lizard, Sceloporus undulatus. We sampled lizards from 11 populations across the latitudinal range of the eastern fence lizard, including fire ant invaded and uninvaded areas, and measured relative hind limb length, antipredator behaviors in response to fire ant attack, and baseline and acute stress responses.
When moving from north to south along a latitudinal gradient outside of the fire ant’s range, lizard populations become less responsive to ants, have relatively shorter legs, and have lower stress responses. However, the direction of these gradients is reversed in the presence of fire ants; in more southern areas invaded by fire ants, lizard populations are more responsive to ants and have longer legs, adaptations to surviving attack by fire ants, and have higher stress responses, likely a response to frequent attacks by these predatory ants. Observed differences in these traits occur via diverse pathways in response to selection, from those with high heritability, such as morphology (leg length), to traits under ontogenetic (behavior) and potentially epigenetic (stress response) control. These changes also represent shifts away from trait values that evolved under natural conditions, suggesting that these adaptations may incur costs. Our results reveal that latitudinal gradients may change via multiple mechanisms over ecological timescales, and suggest that future research should examine the consequences of adaptation to novel environmental change.