Island hopping in a biodiversity hotspot archipelago: reconstructed invasion history and updated status and distribution of alien frogs in the Philippines.
Six alien frogs have been introduced in the Philippines: chronologically, Hylarana erythraea, Rhinella marina, Lithobates catesbeianus, Hoplobatrachus rugulosus, Kaloula pulchra, and Eleutherodactylus planirostris. Here, we collected and synthesized historical and geographical data to reconstruct their history of invasion and to update their current invasion status and distribution in the Philippines. Four pathway categories (falling in 8 subcategories) have facilitated their introduction: (1) intentional 'release' for biological control and hunting in the wild; (2) 'escape' from farms; (3) 'contamination' of agricultural commodities, fish stocks, and ornamental plants/nursery materials; and (4) 'stowaway' on container/bulk and (hitchhiker on) ship/boat - of which the last two were important in most recent introductions. Their spatio-temporal pattern of distribution showed a stratified-diffusion process of spread involving primarily leading-edge and long-distance dispersal. The pathways that facilitated their secondary (post-introduction) long-distance dispersal were either the same as those of their introduction or shifted over time. Estimation of rate of spread showed that H. erythraea, R. marina, H. rugulosus, and K. pulchra have not reached spatial saturation and are conditioning to spread, with the latter spreading fastest. The status of Lithobates catesbeianus, whether it successfully established or not, is undetermined. Meanwhile, the other alien frogs are now considered fully invasive species, of which R. marina is the most widespread, whereas E. planirostris is the least distributed. Our study provides science-based information that can help guide the development and implementation of pathway-specific measures to prevent and control future and current invasions by alien frogs.