North and Middle America (defined as Mexico to the Panama Canal) are connected landmasses that allowed for the movement of species throughout the region. However, how did species of frogs come to settle where they are found today?

Working with the hypothesis that time has allowed for radiation of groups to include many more species, we see that historical contingency may help explain patterns of diversity seen within the two regions considered in this discussion, Middle and North America.

Bufonidae (toads) moved from South America into Middle America during the Paleocene and reached North America by the Oligocene, as determined by finding fossils in the appropriate layers of rock. Hylidae (treefrogs) also moved from South America into Middle America, arriving during the early Paleocene, and then continued to move into North America afterwards. Fossils for this group date back to the Oligocene in North America and movement may have been impeded by the incomplete land bridge between North and Middle America during the Eocene. Members of Leptodactylidae (leaf litter frogs) are expected to have moved north after the closing of the Panama Portal in the Pliocene, allowing the frogs to eventually reach North America. Ranidae (leopard frogs) moved from the North to Middle America during the Late Pliocene. Of the possible families, we find all but Rhinophrynidae (only represented by one species throughout the regions considered here) centered in Middle America, but also ranging into North and South America, albeit to varying extents and abundances. It appears as though at least some members of each family were able to adapt to new areas and keep a foothold in the new region.

The earliest arriving families appear to have driven the present day composition of amphibians. For example, Ranids (leopard frogs) dominant the anuran fauna in North America but are less diverse in Middle America; they arrived in Middle America during the Pliocene. However, by the Pliocene time Leptodactylidae (leaf litter frogs) already were filling many of the forest niches, where many Ranids will move into for cover and foraging during non-breeding seasons. The leaf litter frogs outcompeted the leopard frogs for this habitat. Furthermore, the direct development of many Leptodactylidae eggs directly into froglets has released the group from a dependency on standing water sources. This has allowed for more niches to be exploited, thus leading to further speciation. Bufonid toads provide another example of a higher diversity and abundance in Middle America, where they first arrived from South America. Upon reaching North America, they were in competition with Ranids for breeding pools and non-breeding forest habitat. Therefore, the bufonid toads are less diverse in North America.

A common theme noticed here is that families arriving first were able to fill niches (leading to diversification and speciation), and therefore, were better competitors in relation to later arriving familial groups. In the case of the Middle and North American anurans, historical contingency can be suggested as an explanation for the present composition of fauna.

The vast composition of anurans in Middle and North America, coupled with the still drastically understudied taxonomic groups residing in these areas makes it difficult for definite answers to be drawn. However, based on evidence presented above from the fossil record, natural history and biological information and biogeographical studies, conclusions can be suggested. It appears that the current distribute of frogs in Middle and North America provide a relatively clear idea of how anurans have moved, with first arriving families being the most speciose and biological diverse today. It is likely that with more study and recovery of more fossils, the picture will be made clearer for amphibians. This also suggests the importance of continued natural history and field studies throughout the world so we can continue to understand how and why amphibians live in the areas where they do.

Photo credit: Julie M. Ray