Book Description

Many environments are changing rapidly due to human impacts including habitat alteration, climate change, and the introduction of invasive species. This global environmental change has numerous direct and indirect effects on species that can exert novel pressures. In some cases organisms must adapt to these changes or face extirpation or extinction. Understanding the diversity and extent of these effects of global change, and the processes by which species adapt to it, is critical to future conservation and can provide important insights into ecological and evolutionary processes. My dissertation addresses questions about the effects imposed by invasive species on native species and how native species adapt to associated novel pressures by using a system of native fence lizards (Sceloporus undulatus) subject to differing levels of exposure to an invasive predatory fire ant (Solenopsis invicta). In the southeastern United States, fence lizard populations have adapted to this novel predator via an increase in anti-ant behaviors, including twitching and fleeing which promote escape from and removal of attacking ants; longer hind limbs which support this behavior; and a heightened stress response. Novel pressures induced by invasive species may include lethal (e.g. predation, injury) and sublethal (e.g. altered energy or habitat availability) effects. The nature of these effects may vary with characteristics of an organism (e.g., sex, age, size) and history of exposure to the invader. I explored how the effects of fire ants on fence lizards varied with life stage of the lizard and their evolutionary exposure (adaptation) to the ants. I first investigated the effects of fire ants on the vulnerable life history stage of fence lizard eggs by creating artificial nests in the field and monitoring them daily. I found that fire ants do prey on fence lizard eggs in the field and may predate up to 60% of fence lizard nests in fire ant-invaded areas. Eggs from lizards from fire ant-invaded and -uninvaded sites were equally susceptible to predation, suggesting a lack of adaptation to this threat. I next quantified the effects of fire ants on juvenile and adult lizards from fire ant-invaded and -uninvaded sites by placing them in large field enclosures with natural and reduced densities of fire ants for two weeks. Fire ants reduced survival of adult but not juvenile lizards, regardless of their evolutionary history with fire ants. Juvenile lizards grew less in enclosures with fire ants than in fire ant-free enclosures, but there was no effect of fire ant density on adult growth. This research shows that fire ants have both lethal and sublethal effects on lizards, and that these vary with lizard life stage. I conducted field surveys and manipulations to examine the consequences of behavioral adaptations of fence lizards to fire ants. I exposed lizards from fire ant-invaded and -uninvaded sites to encounters with predatory fire ants and non-threatening native ants, and to simulated attacks by a native predator (a taxidermied American Kestrel). While lizards responded similarly to the native predator, lizards from fire ant-invaded sites showed a generalized increase in responsiveness to all ants. While this behavior is adaptive against fire ants, it may also attract attention from native predators as evidenced by my findings of higher injury rates of lizards at fire ant-invaded sites. At a geographic scale, I found that the presence of fire ants reverses natural latitudinal gradients in behavior, stress responsiveness, and morphology of fence lizards. This implies that fire ants are driving these traits away from historical values that evolved under fire ant-free conditions and suggests potential costs to adapting to fire ants. As a whole, this work shows that invasive species can have broad impacts on native taxa, and that these impacts may vary with the life stage of affected native species and the degree to which they have adapted to the invader. Adaptive responses by native species may be strong enough to reverse pre-existing latitudinal clines in relevant traits over large portions of an affected species' range. However, while native species can adapt to novel invaders, these adaptations may incur also costs that should be considered when assessing population persistence and management in the face of these increasing perturbations.