Invasive species pose a major threat to global biodiversity. Invasion potential can be influenced by reproductive attributes, such as fecundity, egg production and duration and number of reproductive events. The minimum reproductive size or age at first reproduction can also play a role in the establishment of an invasive species, with breeding at younger ages facilitating establishment. The American bullfrog (Lithobates catesbeianus) is an invasive anuran amphibian introduced in more than 40 countries. In the Willamette Valley, Oregon, USA, this nonnative species has profound effects on native ecosystems. However, we lack information on bullfrog reproductive characteristics in its invaded range that may influence the dynamics of invasion. Therefore, we characterized reproductive characteristics with emphasis on the minimum size at which males and females reach sexual maturity in the Willamette valley.
Results/Conclusions
We collected 70 individuals during summer 2013. We dissected the animals and quantified characteristics of sexual maturity including snout-vent length, total length, sex, tympanum diameter, presence of distended oviducts or eggs for females, and testes length and sperm activity in males. The minimum reproductive size we measured is smaller than predicted and differs from the minimum reproductive size for native ranges. Our results show American bullfrogs in the Willamette valley are able to reproduce at smaller sizes relative to reproductive adults within the native range. Our results are similar to minimum reproductive sizes observed in the South American bullfrog invasion ranges. Reduction in size at reproductive maturity could impact the invasive success of American bullfrogs and give us insight into the history of their invasions.