Differential oviposition and offspring success of gray treefrogs in the presence of an invasive fish.
Females often decide where to place their eggs or offspring based on the relative risks and benefits of a location. One trade-off may be between ovipositing with predators and ovipositing with competitors. Many amphibians show risk-sensitive oviposition and select oviposition sites based on offspring performance. We examined differential oviposition and offspring success by gray treefrogs (Hyla versicolor) in response to the presence of caged or free-ranging invasive western mosquitofish (Gambusia affinis) using cattletank mesocosms as experimental ponds. Our experiment sought to answer these questions by comparing the number of eggs laid and tadpoles produced among the experimental treatments: (1) Do gray treefrogs exhibit risk-sensitive oviposition? and (2) What is the relative importance of pre-colonization and post-colonization consumptive and trait-mediated effects of western mosquitofish? Gray treefrogs laid more eggs in control and caged predator mesocosms than in free-ranging predator mesocosms. At the end of the experiment, there were more tadpoles in control and caged predator mesocosms than in free-ranging predator mesocosms. Proportional yield was lower in free-ranging predator mesocosms than control and caged predator mesocosms. Eggs were laid 7-8 d earlier in control mesocosms than caged and free-ranging predator mesocosms. Western mosquitofish therefore had a negative effect on the successful colonization of experimental ponds by gray treefrogs, most likely through direct physical interactions. Our results also suggest gray treefrogs shift oviposition preferences as the number of conspecifics reaches a threshold where competition risk outweighs predation risk. Western mosquitofish therefore have great potential to affect the distribution of gray treefrogs through pre- and post-colonization effects.