Historical biogeography of the I sthmus of P anama

About 3 million years ago ( Ma ), the I sthmus of P anama joined the A mericas, forming a land bridge over which inhabitants of each A merica invaded the other—the G reat A merican B iotic I nterchange. These invasions transformed land ecosystems in S outh and M iddle A merica. Humans invading from A sia over 12000 years ago killed most mammals over 44 kg, again transforming tropical A merican 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  Ma invading marsupials and ungulates of N orth A merican ancestry, and xenarthrans of uncertain provenance replaced nearly all S outh A merica's non‐volant mammals. There is no geological evidence for a land bridge at that time. Together with rodents and primates crossing from A frica 42 to 30 Ma, S outh A merica's mammals evolved in isolation until the interchange's first heralds less than 10 Ma. Its carnivores were ineffective marsupials. Meanwhile, N orth A merica was invaded by more competitive E urasian mammals. The A mericas had comparable expanses of tropical forest 55 Ma; later, climate change confined N orth A merican tropical forest to a far smaller area. When the isthmus formed, N orth A merican carnivores replaced their marsupial counterparts. Although invaders crossed in both directions, N orth A merican mammals spread widely, diversified greatly, and steadily replaced S outh A merican open‐country counterparts, unused to effective predators. Invading S outh A merican mammals were less successful. S outh A merica'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, S outh A merica's rainforest, with its bats, birds and mammals, spread to M exico. Present‐day tropical A merican 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 the M iocene, a common marine biota extended from T rinidad to E cuador and western M exico, which evolved in isolation from the I ndo‐ P acific until the Pleistocene. The seaway between the A mericas 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 the S outhern C aribbean, but not the P acific. By 4 Ma, the seaway's narrowing began to extinguish C aribbean upwellings. By 2 Ma, upwellings remained only along V enezuela; C aribbean 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 the E astern P acific's reef corals (successors recolonized from the I ndo‐ P acific 6000 years ago), whereas many molluscs of productive waters that once thrived in the C aribbean now survive only in the E astern P acific. The present‐day productive E astern P acific, with few, small coral reefs and a plankton‐based ecosystem contrasts with the C aribbean, 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, devastating C aribbean ecosystems.