Historical biogeography of theIsthmus ofPanama
Authors: Leigh Egbert G. , O'Dea Aaron , Vermeij Geerat J. 
Published: July 2013

About 3 million years ago (Ma), theIsthmus ofPanama joined theAmericas, forming a land bridge over which inhabitants of eachAmerica invaded the other—theGreatAmericanBioticInterchange. These invasions transformed land ecosystems inSouth andMiddleAmerica. Humans invading fromAsia over 12000 years ago killed most mammals over 44 kg, again transforming tropicalAmerican ecosystems. As a sea barrier, the isthmus induced divergent environmental change off its two coasts—creating contrasting ecosystems through differential extinction and diversification.

Approximately 65 Mainvading marsupials and ungulates ofNorthAmerican ancestry, and xenarthrans of uncertain provenance replaced nearly allSouthAmerica's non‐volant mammals. There is no geological evidence for a land bridge at that time. Together with rodents and primates crossing fromAfrica 42 to 30 Ma,SouthAmerica's mammals evolved in isolation until the interchange's first heralds less than 10 Ma. Its carnivores were ineffective marsupials. Meanwhile,NorthAmerica was invaded by more competitiveEurasian mammals. TheAmericas had comparable expanses of tropical forest 55 Ma; later, climate change confinedNorthAmerican tropical forest to a far smaller area. When the isthmus formed,NorthAmerican carnivores replaced their marsupial counterparts. Although invaders crossed in both directions,NorthAmerican mammals spread widely, diversified greatly, and steadily replacedSouthAmerican open‐country counterparts, unused to effective predators. InvadingSouthAmerican mammals were less successful.SouthAmerica's birds, bats, and smaller rainforest mammals, equally isolated, mostly survived invasion. Its vegetation, enriched by many overseas invaders, remained intact. This vegetation resists herbivory effectively. When climate permitted,SouthAmerica's rainforest, with its bats, birds and mammals, spread toMexico. Present‐day tropicalAmerican vegetation is largely zoned by trade‐offs between exploiting well‐watered settings versus surviving droughts, exploiting fertile versus coping with poor soil, and exploiting lowland warmth versus coping with cooler altitudes.

At the start of theMiocene, a common marine biota extended fromTrinidad toEcuador and westernMexico, which evolved in isolation from theIndo‐Pacific until the Pleistocene. The seaway between theAmericas began shoaling over 12 Ma. About 10 Ma the land bridge was briefly near‐complete, allowing some interchange of land mammals between the continents. By 7 Ma, the rising sill had split deeper‐water populations. Sea temperature, salinity and sedimentary carbon content had begun to increase in theSouthernCaribbean, but not thePacific. By 4 Ma, the seaway's narrowing began to extinguishCaribbean upwellings. By 2 Ma, upwellings remained only alongVenezuela;Caribbean plankton, suspension‐feeding molluscs and their predators had declined sharply, largely replaced by bottom‐dwelling corals and calcareous algae and magnificent coral reefs. Closing the seaway extinguished theEasternPacific's reef corals (successors recolonized from theIndo‐Pacific 6000 years ago), whereas many molluscs of productive waters that once thrived in theCaribbean now survive only in theEasternPacific. The present‐day productiveEasternPacific, with few, small coral reefs and a plankton‐based ecosystem contrasts with theCaribbean, whose clear water favours expansive coral reefs and bottom‐dwelling primary producers. These ecosystems reflect the trade‐off between fast growth and effective defence with attendant longevity. Overfishing with new technologies during the last few centuries, however, has caused population crashes of ever‐smaller marine animals, devastatingCaribbean ecosystems.