Asymmetric interference competition and niche partitioning between native and invasive Anolis lizards

Species can compete both directly via aggressive encounters (interference) and indirectly through their shared use of a limited resource (exploitation). Depending on the circumstances interference, exploitation, and their interplay can either lead to competitive exclusion or drive niche partitioning to maintain species coexistence. Thus, understanding species coexistence in nature requires accurately identifying the mechanisms that contribute to competition among the species in question. In the southern United States, the native lizard Anolis carolinensis becomes more arboreal in the presence of the invasive Anolis sagrei, resulting in highly consistent vertical habitat partitioning where the species co-occur. These species have been thought to largely ignore each other and engage only in exploitative competition for shared arthropod prey. To test for the presence and consequences of direct interference, we conducted behavioral trials in the field, introducing a heterospecific male intruder to individuals of both species. We find that interference competition is asymmetric in favor of A. sagrei, which are more likely to display and less likely to retreat than A. carolinensis. Concordant with their arboreal tendencies, male A. carolinensis also trend toward retreating upward more often than expected by chance. These asymmetries are prevalent despite the almost complete absence of physical attacks, suggesting that interspecific signaling and avoidance behavior by A. carolinensis resolve most potential conflicts before they escalate to combat. Our results highlight the potential for direct interference more subtle than frequent outright combat to structure communities, and Anolis assemblages in particular.