Direct chemical inhibition responsible for displacement of a native herbivore by an invasive congener

The mechanisms causing native displacement by invasive species are challenging to identify. We used survey data to demonstrate a local extinction of a native gastropod ( Pomacea paludosa ) from a Florida wetland coincident with invasion of a non-native gastropod ( P. maculata ). We then examined the potential for P. maculata to displace P. paludosa . We used two field experiments to examine effects of densities and heterospecific interactions on juvenile growth in situ and we used lab and mesocosm experiments to explore chemically-mediated interactions between the species. Field experiments produced variable results; evidence for reduced P. paludosa growth in the presence of P. maculata was found under higher nutrient conditions with fast snail growth rates, but growth inhibition disappeared under oligotrophic conditions. Resource reduction could not be demonstrated in the field. Juvenile P. paludosa did not avoid mucus of P. maculata and did not reduce feeding in response to cues. Nevertheless, exposure to mucus and waterborne cues of P. maculata reduced P. paludosa growth by 53% relative to conspecific cue exposure. In mesocosms juvenile P. paludosa growth was significantly negatively correlated with adult P. maculata waterborne cues and was particularly affected at exposures ≥60% (~0.6 · m -2 ). Combined mucus and waterborne cues appeared to further reduce growth at low exposure (20%). We documented the first apparent local extinction of P. paludosa by P. maculata. We found no evidence for competition, but the results collectively pointed to direct chemical inhibition of growth that could be important at field scales, even at low densities.