Aggression affects distributions and hybrid zone expansion in montane Plethodon salamanders

Ecologists have long been interested in the mechanisms that drive the distribution of closely related species. Often overlapping congeneric species do not differ enough in their uses of space and food resources to prevent competition. Over time, insufficient resource partitioning can result in the evolution of an interference mechanisms such as aggression. Aggressive interactions are a well-documented form of competition among Plethodon salamanders, and has been shown to affect their distributions. The Nantahala Mountain range of western North Carolina is the site of contact between P. shermani and P. teyahalee along an elevational gradient. Hybridization occurs between these two species at intermediate elevations. A long-term study suggests that the hybrid zone is expanding at the expense of both parentals’ ranges.I used field enclosures and laboratory behavioral trials to investigate whether elevation-dependent competition and aggressive behavior are influential in delineating the vertical distributions of the parental species, and provide a mechanism driving hybrid zone expansion. I found that P. shermani was more aggressive than hybrids or P. teyahalee, suggesting that interference competition has maintained the distribution of P. shermani at the high elevations. Hybrids exhibited aggressiveness that was intermediate to the parentals, and hybrids may be competitively superior to P. teyahalee. I submit competitive exclusion as the primary mechanism driving the downward hybrid expansion