Ecology, 90(1), 2009, pp. 208–217 Ó 2009 by the Ecological Society of America Invasive fire ants alter behavior and morphology of native lizards TRACY LANGKILDE 1 Department of Biology, Pennsylvania State University, 417 Mueller Laboratory, University Park, Pennsylvania 16802 USA Abstract. Nonnative species introductions are becoming more common, but long-term consequences of the novel pressures imposed by invaders on native species remain poorly known. The red imported fire ant, Solenopsis invicta, is an invasive species with potential global impact. Comparison of lizards across the invasive range within the United States reveals novel antipredator strategies and altered morphologies that mitigate potentially lethal attack by these ants, within 70 years of their introduction. The likelihood that adult lizards will behaviorally respond to fire ant attack increases with time since invasion, but hatchlings exhibit high levels of antipredator behavior irrespective of their site of origin. Adults and hatchlings from sites invaded longer ago also have relatively longer hind limbs. This trait increases the effectiveness of behavioral strategies for removing ants and is likely an adaptive response to minimize envenomation during attack. The observed changes are not correlated with habitat, exposure to fire ants, or latitude, arguing against phenotypic plasticity and learning as causal mechanisms, and museum specimens show that morphological differences were not evident prior to fire ant invasion. These data contribute to our growing awareness that ecological invasions can prompt adaptive responses, altering the nature of interactions between invaders and the natives they contact. Key words: adaptation; behavior; fence lizard; fire ant; invasive species; morphology; predator; Sceloporus undulatus; Solenopsis invicta. INTRODUCTION Introductions of nonnative species are increasingly common as global connectivity accelerates through human movement. Since 1980, over 205 nonindigenous species have been introduced to the United States, 30% of which are expected to have serious consequences for both the environment and economy (U.S. Congress 1993). These invaders can destroy natural habitats, out- compete, depredate, and poison native taxa, and ultimately cause declines of native species (Mooney and Cleland 2001). Successful management of invaders relies, therefore, on understanding the long-term re- sponse by native species to the novel pressures imposed by invasive species (Stockwell et al. 2003). Historically, research has been focused on the invaders themselves and how they respond to the novel environments they are introduced into or encounter as they spread (Ha¨nfling and Kollmann 2002). More recently, we have begun to realize that native species are also capable of rapid adaptation. The directional selection imposed by invaders can lead to nonrandom changes in morpholo- gy, physiology, and behavior of native species over ‘‘ecological’’ timescales (Freeman and Byers 2006, Phillips and Shine 2006, Strauss et al. 2006). These changes increase the fitness of native taxa, allowing them to take advantage of novel resources provided by the invader or increasing their chances of surviving by avoiding predation or toxic prey. By contributing to our understanding of the scenarios that can lead to rapid adaptation, such studies make a valuable contribution to conservation planning and provide insight into how past interactions can shape contemporary communities (Connell 1980, Stockwell et al. 2003, Sax et al. 2007). Here I examine the adaptive response of a native lizard to invasion by the venomous fire ant, Solenopsis invicta. Originating from South America, this ant has invaded six countries including the United States, and is predicted eventually to colonize .50% of the terrestrial surface of the Earth (Morrison et al. 2004). Fire ants are small (2–6 mm in length), but aggressive, and have a unique attack strategy that they use to defend their mound and obtain prey: tens to hundreds of ants swarm and simultaneously administer multiple stings, allowing them to overwhelm even very large animals (Barr et al. 1994, Holtcamp et al. 1997). The impact of fire ants on vertebrate populations is poorly understood, largely due to logistical difficulties and the lack of baseline or control data for comparison (Allen et al. 2004, Tschinkel 2006). In contrast, their effect on agriculture and public health is well known, with damages estimated in the billions of dollars annually within the United States alone (Morrison et al. 2004). As a result, the spread of fire ants across the United States since their accidental introduction in the 1930s has been well documented, facilitating investigation of their long- Manuscript received 19 February 2008; accepted 29 April 2008. Corresponding Editor: S. J. Simpson. 1 E-mail: tll30@psu.edu 208